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This file is part of the following reference:
Shah, Afroz (2010) Tectono-metamorphic evolution of Big Thompson Canyon region, Colorado Rocky Mountains, USA.
PhD thesis, James Cook University.
Access to this file is available from:
http://eprints.jcu.edu.au/11911
Tectono-metamorphic evolution of Big Thompson Canyon region,
Colorado Rocky Mountains, USA
Volume I
Thesis submitted by
Afi'oz Ahmad SHAH MTech. (Civil Engg.) IIT Kanpur India
In July 2010
for the degree of Doctor of Philosophy in the School of Earth Sciences James Cook University
PhD Thesis Shah A.A.
CONTENT OF VOLUME 1
STATEMENT OF ACCESS ................... " ............................................................................ ii
STATEMENT OF SOURCES ............................................................................................ iii
ELECTRONIC COPY ......................................................................................................... iv
ACKNOWLEDGMENTS .............................. , ...................................................................... v
INTRODUCTION AND THESIS OUTLINE ................................................................... vii
- SECTION A -................................................................................................................ 1-32
Isogmd migmtion with time during orogetlesis.
-SECTION B -................................................................................................................. 1-32
Pressure-Tempemtllre-time-DefoI'nUltion (PTtD) paths derived from FIAs, P-T
pselldosectiolls allli ZOlled gal'llets
-SECTIONC- .............................................................................................................. 1-33
90 million years of orogenesis, 250 millioll yem's of quiescence and thell further orogenesis with no c//(/1/ge ill PT: significance for the role of deformation ill pOlphyroblast growth
- SECTION D - .............................................................................................................. 1-20
The problem, sigllificallce, allli metamorphic implications of 90 millioll years of
polyphase staurolite growth
CONCLUSIONS ............................................................................................................... 1-7
PhD Thesis Shah A.A.
STATEMENT OF SOURCES
DECLARATION
I declare that this thesis is my own work and has not been submitted in any form for
another degree or diploma at any university or other institution of tertiGlY education.
Information derived ji-om the published and unpublished work of others has been
acknowledged in the text and a list of references is given
Afroz Ahmad Shah July 2010
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PhD Thesis Shah A.A.
ELECTRONIC COpy
DECLARATION
1, the undersigned, the author of this thesis, declare that the electronic copy of this thesis
provided to the James Cook University Libra/Y is an accurate copy of the print thesis
submitted, within the limits of the technology available.
Afroz Ahmad Shah July 2010
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PhD Thesis Shah A.A.
ACKNOWLEDGMENTS
I must stmi by thanking Professor Tim Bell for his brilliant supervision skills
peppered with the effervescent humour, which keeps the research at SAMRI going. Tim
has been instrumental in changing my ways of understanding structural and metamorphic
geology. The research conditions within the SAMRI group at James Cook University are
par-excellence. This international group has more than just research to offer! Social,
intellectual and group culture is what most of the other research institutes around the world
lack and I was lucky enough to get all of it here. This project was funded by International
Earth Science Scholarship (IESS) from the School of Emih and Environmental Sciences at
James Cook University.
The staff at the Advanced Analytical Centre at James Cook University provided
superb technical assistance in data collection. In particular, I would like to thank Dr Kevin
Blake for his support on the electron microprobe. and Darren Richardson for his help
during thin section preparation.
Special thanks Dr. Mike Rubenach for help with monazite dating and for endless
discussions about metamorphic petrology and many other aspects of geology. I sincerely
thank all my colleagues in SAMRI for providing an intellectual, social and friendly
atmosphere, enriched with stimulating ideas on just about anything under the sun! I will
surely miss them all, thanks tons! I wish to express my gratitude to the following, who in
their various ways provided warmth, assistance, inspiration, andlor ideas: Ahmed Abu
Sharib, Asghar Ali, Clement Fay, Chris Fletcher, Hui Cao, loan, Jyotindra Sapkota, Matt
Bruce, Mark Rieuwers, Mark Munrooh, Raphael Quentin de Gromard and Shyam Ghimire.
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PhD Thesis Shah A.A.
My sincere thanks to Ashraf Bhat, Firdoos, Jaiby, Jainky, Julia Miller, Lubna, Oana,
Quidsiya, Rafia, Raj, Shrema and Tajamul Bhat for their encouragement throughout this
research.
Special thanks to Elizaveta Moiseeva for her help during my presentation which
greatly improved a pre-completion talk at JCU. Her help at Beijing (China) for my
presentation is greatly acknowledged.
I am much thankful to Prof. IN. Malik (lIT Kanpur, India), for consistent
encouragements throughout this research.
I wish to acknowledge with love and thanks the support and patience of my parents,
Hafiza Akhtar and Mohammad Naseem, my brothers Gowher Naseem Shah and Bilal
Ahmad Shah and my adorable sister Sabiya Naseem for all their encouragement
throughout my stay in Australia. I must thank all my cousins, especially Aasim, Mohsina,
Mugeesa Qudsia and Sahreena and everybody directly or indirectly related to this research.
In the end, I must thank Sheeba, for her encouragement and discussions that have greatly
improved my ways of expression. Thanks for being such a great motivation during the
crucial stage of this work.
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PhD Thesis Shah A.A.
INTRODUCTION AND THESIS OUTLINE
The geologically continuous nature of relative plate motion, which drives
orogenesis, versus the limited number of deformations preserved in the matrix of rocks,
has always been an anomaly that requires explanation. Porphyroblast studies suggest that a
large amount of orogenic history has been destroyed in the matrix and may reveal that
deformation and metamorphism is continuous, although partitioned both spatially and
temporally, for periods of a few 100 million years (e.g., Bell et aI., 2004; Bell & Newman,
2006). Reactivation (shearing) of the bedding or compositional layering in successive
deformations destroys previously developed foliations in the matrix. However,
porphyroblast growth prior to reactivation generally preserves these foliations, plus the
metamotphic histoty, from destruction. If the P-T and bulk composition conditions are
suitable for porphyroblast growth, this history is generally trapped within porphyroblasts,
which appear to grow during regional metamorphism only where the deformation occurs
(e.g. Bell and Rubenach, 1983; Bell and Johnson, 1989; Bell and Hayward, 1991, Cihan
2004; Cihan and Parsons; 2005).
Foliation intersection axes preserved within porphyroblasts (FlAs) enable linkage
of the structural and metamorphic histories from sample to regional scales through
consistent changes in their trend from the core to median to rim (e.g., Bell et aI., 1998,
1981). Significantly, regionally consistent successions ofFIA trends reflect changes in the
direction of bulk shotiening, which must in tum reflect changes in the direction of relative
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PhD Thesis Shah A.A.
plate motion. The period of time over which each FIA trend developed has been dated by
electron microprobe and ion microprobe techniques using U-Th-Pb in monazite inclusions,
correlated directly with shear sense from inclusion trail asymmetry, as well as with phases
of porphyroblast growth.
Combining this data with the construction of P-T pseudosections (phase diagrams
specific to a palticular sample's bulk composition that provide a map of stable mineral
assemblages in P-T space) and thermobarometric calculations using THERMO CALC
allows P-T-t-d paths to be determined and correlated from sample to sample and region to
region. Thus quantitative integration of metamorphism, structure and tectonics within an
orogenic belt has become an exciting possibility. This contribution provides an example of
such an integrated approach using microstructural, F1As, thermodynamic modelling
(MnNCKFMASH) and chemical relationships to investigate in detail the metamorphic and
deformation processes and the progression in the distribution of various index mineral
phases (garnet, staurolite, andalusite and cordierite). The Big Thompson Canyon region,
Colorado Rocky Mountains, USA forms patt of a Proterozoic orogenic belt in the
southwestern United States and provides a classic example ofa large high-T-low-P terrane
and the problems associated with the tectonic interpretation of such a regime (e.g.,
Williams and Karlstrom, 1996). These rocks have been affected by strongly partitioned
deformation and provide an ideal place for developing new concepts on the role of
deformation pattitioning in deformation, metamorphism and tectonism.
This thesis consists of four sections (A-D), with each written in the matmer of
papers for submission to journals, although it is not intended that section B be submitted in
its present form. A palt of section A has been published in the Acta Geologica Sinica and
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PhD Thesis Shah A.A.
Section C has been submitted to the Precambrian research. Section D has been submitted
to the Journal of Geological Society of India and was co-authored by Dr. loan Sanislav and
myself, and we both agree that conceptually, literally, and graphically we each contributed
50% to that paper. Volume I contains the paper text and Volume II contains the
accompanying tables, figures, and appendices.
SECTION A
This section explains the FIA (foliation intersection/inflection axes preserved
within porphyroblasts) technique and its application to all samples are used in this research.
FIAs were measured in garnet, staurolite, andalusite and cordierite porphyroblasts
preserving older foliations as inclusions trail microstructures. Their relative timing was
established based on microstructural and textural relationships. A progression of FIAs
trending successively NE-SW, E-W, SE-NW and NNE-SSW reveals four periods of
staurolite and garnet growth and two growth phases each of cordierite and andalusite.
Isograds associated with each of these FIAs shift progressively across the orogen. This
migration took place relative to a heat source to the WNW. Early formed staurolite was
altered to cordierite and andalusite during development of the last 2 of these FIA sets
(trending SE-NW and NNE-SSW). The lack of relationship of these isograds to pluton
boundaries to the north and south suggests that these igneous rocks were not the primary
heat source for metamorphism. The youngest of these isograds lie sub-parallel to those
mapped by previous workers
SECTIONB
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PhD Thesis Shah A.A.
This patt of the thesis involves the use of microstructural and FlA evidence to track
down the overall prograde mineral succession in these metasediments. It suggests that
garnet, staurolite, andalusite and cordierite porphyroblasts grew in an overall prograde path,
where the growth of garnet was always followed by the formation of staurolite for each
FlA. For the last 2 periods of FlA development the growth of staurolite was also followed
by the development of andalusite and cordierite. Inclusions of earlier minerals within the
younger phases SUppOlt the porphyroblastic mineral sequence obtained through FIAs.
Thermodynamic modeling in the MnNCKFMASH system reveals that the episodic
growth of these phases occurred over a similar bulk compositional range and PT path for
each FIA in the succession. The intersection of Ca, Mn, and Fe isopleths in garnet cores for
3 samples, containing FIA set 1, set 2 and set 3, trending NE-SW, E-W and SE-NW
respectively, indicate that these rocks never got above 4kbars throughout the Colorado
Orogeny. They remained around the same depth until the onset of younger orogeny at
~ 1400 Ma, when the pressure decreased slightly as porphyroblasts formed with inclusion
trails preserving FIA set 4 and trending NNE-SSW. A slightly clockwise P-T path
occurred for both orogenies.
SECTIONC
In this section in-situ EPMA (electron probe microanalysis) dating of monazite
grains preserved as inclusions within foliations defining FlAs contained within garnet,
staurolite, andalusite and cordierite porphyroblasts is described, interpreted and explained.
Monazite grains contained within matrix foliations were also analysed and compared with
the dates obtained from porphyroblasts where the FIAs were used to determine the relative
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PhD Thesis Shah A.A.
age succession. This data have provided a chronology of ages for extended periods of
deformation and metamorphism in the region. Garnet and staurolite formed episodically
for a period of -1 00 Ma during the Colorado orogeny, ceased and then recommenced -300
million years later during the Belthoud Orogeny
SECTIOND
This part of thesis compares two spatially and temporally separated terranes with
similar histories of porphyroblast development and associated granite emplacement at
similar levels in the crust. These areas are a Paleozoic portion of West Central Maine and
the Proterozoic Colorado Front Range, where multiple periods of staurolite growth
occurred over a lengthy period of time. The metastable preservation of early staurolite
porphyroblasts through successive deformation and metamorphic events highlights the
impOltance of deformation on porphyroblast nucleation and growth. These two areas show
that the reactions involved in the formation of metamorphic minerals are episodic during
prograde metamorphism, starting or stopping, as a function of deformation pattitioning and
strain localization. Accessory minerals such as monazite can be dated and their ages can be
better integrated in the deformation and metamorphic history of a region when they are
well constrained by microstructural measurements. The similarities between the two
regions indicate that such processes may be common for most of the orogenic belts.
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PhD Thesis Shah A.A.
References
Bell, T. H., and Hayward, N., 1991. Episodic metamorphic reactions during orogenesis:
the control of deformation partitioning on reaction sites and reaction duration.
Journal of Metamorphic Geology, 9, 619-640.
Bell, T. H., and Johnson, S. E., 1989. Porphyroblast inclusion trails: the key to orogenesis.
Journal of Metamorphic Geology, 7, 279-310.
Bell, T. H., and Newman, R., 2006. Appalachian orogenesis: the role of repeated
gravitational collapse. In: Butler, R., Mazzoli, S. (Eds.), Styles of Continental
Compression. Special Papers of the Geological Society of America, 414, 95-118.
Bell, T. H., and Rubenach, M. J., 1983. Sequential porphyroblast growth and crenulation
cleavege developement during progressive deformation. Tectonophysics, 92, 171-
194.
Bell, T. H., Ham, A. P., and Kim, H. S., 2004. Partitioning of deformation along an orogen
and its effects on porphyroblast growth during orogenesis. Journal of Structural
Geology, 26, 825-845.
Bell, T.H., 1981. Foliation development: the contribution, geometry and significance of
progressive bulk inhomogeneous shortening. Tectonophysics, 75, 273-296.
Bell T.H., Hickey, K.A, Upton, 0.1.0., 1998. Distinguishing and correlating multiple
phases of metamorphism across a multiply deformed region using the axes of spiral,
staircase and sigmoidally curved inclusion trails in garnet. Journal of Metamorphic
Oeology 16, 767-794.
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PhD Thesis Shah A.A.
Cihan, M., 2004. The drawbacks of sectioning rocks relative to fabricorientations in the
matrix: a case study from the Robertson River Metamorphics (North Queensland,
Australia). Journal o/Structural Geology, 26, 2157-2174.
Cihan, M., and Parsons, A. 2005, The use of porphyroblasts to resolve the history of
macro-scale structures: An example from the Robertson River Metamorphics, north
eastern Australia: Journal o/Structural Geology, 27 p. 1027- 1045
Williams, M.L., Karlstrom, K.E., 1996. Looping P-T paths, high-T, low-P middle crustal
metamorphism: Proterozoic evolution of the southwestern United States. Geology 24,
1119-1122.
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Isogl'ad migration with time during orogenesis ..... SHAHA.A
Isograd migration with time during orogenesis
1.0. ABSTRACT ............................................................................................................................. 1
2.0. INTRODUCTION .................................................................................................................. 2
3.0. GENERAL GEOLOGY AND TECTONICS OF THE REGION .............................. 4
4.0. FOLIATIONS ......................................................................................................................... 6
5.0. METHODS .............................................................................................................................. 6 5.1. FIA measurements ..................................................................................... , ................ , ................... 6
6.0. RESULTS .................................................................................................................................. 7 6.1. Determination of multiple FlAs for relative timtng .............................................................. 8
7.0. INTERPRETATION OF THE FIA SUCCESSION ............................................................... 9
8.0. EVIDENCE OF EARLIER MINERAL INCLUSIONS WITHIN YOUNGER PHASES .10 8.1. Inclusions of garnet within staurolite, andalusite alUl cordierite .......................................... 10 8.2. Replacement of staurolite by andalusite ................................................................ , ........ , ........... 11 8,3, Replacement of staurolite by cordierite .. , .............. , ...................................................... , .. , ...... , .. ,,11 8.4, Replacement of andalusite by cordierite ' .................. , ................................................................ , 11
9.0. DATING OF FIAS AND THE SUCCESSION CONFIRMATION ......................... 12
10.0. FIAS DERIVED ISOGRADS ............. , ........................... " .. , .... , ......... , .......... , .............. , .. 12 10'], Staurolite isogr(f(ls for F1As 1, 2, 3 and 4 ................................................................................. 12 10.2, AlUlalusite alU/ cordierite isogmds ...................................................... , .... , ................................. 12
11.0. LATE ANDALUSITE AND CORDIERITE GROWTH ......................................... 13
12.0. DISTRIBUTION OF SILLIMANITE ...... " ....................... " ....... , ................ , ....... " ....... 13
13.0. AXIAL PLANE TRENDS OF REGIONAL FOLDS ................................................ 14
14.0. INTERPRETATION ........ , ................... , ........... ,,, ... , .. , ............. , ............................. " .... , ..... 14 14.1. Porphyroblast rotation or otherwise, ...... " ................ " .......... , .... ,,, ............ , ............................ " .. 14 14,2, Staurolite Isograds ",,"""""""'''''''''' '" """",,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, "" .. """""",,,,,,,,,,,,,,, """"" "" 15 14,3. Alii/all/site isogradfor FIAs 3 alll/4 """,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, .. ,,,,,,,,,,,,,,,,,,,,,,,,,,,,, .. ,,,,,,,,,, 15 14,4. Cordierite isogradsfor FIAs 3 and 4."""""""""""""""""""""""""""""""""""""""""""" 15 14,5. Rare matrix overprinting cordierite alii/ (lJulalusite """""""".""""""""""""""""""""""" 16 14,6, Sillimaflite"" """""'''''''''''''' """ """"", """'"'''''''''''''''''' """"".,,"" ", """""""''',,'''''',, .. ,,'' ",,,.,," 16 14,7. Relationship between FIA trends alii/ axial traces of folds """""""""""""""""" ... """""" 16 14,8, Foliatiofl trend alii/ Heat Flux """"""""",,,,,,,,,,,,,,,,,,,,,,,,.,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 17
15.0. Discussion .......................................................................................................................... 18 15,1. Multiple phases of staurolite growth """"""""""""'" """""""""""",,",,""""",,"", ""'" 18 15,2. Multiple phases of andalusite and cordierite growth""""""""""""""""""."""""", 19
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Isow'ad mif!ration with time during orogenesis ..... SHAHA.A
15.3. Comparison with earlier work ............................................................................................. 20 15.4. Deformation Partitioning ...................................................................................................... 20 15.5. Relevance to granite emplacement .................................................................................... 21 15.6. Significance ofthe shift in the isograd fl'Om FIA to FIA ............................................... 22 Acknowledgements ................................................................................................................................ 23
16.0 REFERENCES .................................................................................................................... 24
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Isograd migratioll with time during orogenesis ..... SHAHA.A
1.0. ABSTRACT
A progression of FIAs (foliation intersection/inflection axes preserved within
porphyroblasts) in the foothills of the Colorado Rocky Mountains reveals four periods of
staurolite growth and two growth phases each of cordierite and andalusite; these FIA based
isograds migrated ~2.5 kms across the orogen. This progression of mineral development
occurred about FIAs trending successively NE-SW, E-W, SE-NW and NNE-SSW.
Granitoids were emplaced during both periods of orogenesis in the surrounding region but
have no direct relationship to the isograds. Isograd migration took place away from a heat
source to the WNW and combined with the lack of relationship to pluton boundaries to the
north and south suggests that the latter rocks were not the heat source for metamorphism
but rather product of it. A final period of andalusite, cordierite and fibrolitic sillimanite
grew over the matrix foliation and consequently, no FIA was determined for it; the isograds
for this last period of mineral growth lie sub-parallel to those mapped by previous workers.
A strong correlation between the distribution of FIA trends and the axial trace of all
folds present in the area suggests that pockets of low strain occur due to deformation
partitioning, in spite of, or perhaps because of, the 4 successive changes in bulk shortening
direction. These pockets preserve folds in the orientation in which they formed from
subsequent rotation or destruction. They provide remarkable confirmation of the veracity of
the FIA data and indeed reveal that such data can be used to determine successions of fold
development from regional maps at which scale many overprinting criteria cannot be
applied.
Al
Isograd migration with time during orogenesis ..... SHAHA.A
2.0. INTRODUCTION
The stlUctural aud metamorphic evolution of low-pressure, high-temperature
Proterozoic rocks in the Colorado frontal ranges, which contain an abundance of granites, is
similar in character to other orogenic belts around the world. Such belts include the eastern
Lachlan Fold Belt (Collins and Vernon, 1992), the Pyrenees (Mezger and Passchier, 2003)
and a granulite belt in the Ordovician of central Australia (Hand et aI., 1999). A number of
mechanisms have been proposed for the origin of such settings including compression of
previously extended continental crust (Etheridge et aI., 1987; Oliver et aI., 1991), mantle
delamination (Loosveld and Etheridge, 1990), heat advection via intrusions due to high
mantle heat flow (Vernon et aI., 1990; Rubenach, 1992; Sandiford et aI., 1995; Rubenach
and Bakel', 1998), radiogenic heating due to emichment of heat-forming elements in the
upper crust (Mildren and Sandiford, 1995; Sandiford et aI., 1998; McLaren et aI., 1999);
and the burial of heat-producing stratigraphic sequences (Hand and Rubatto, 2002).
The relationship between metamorphism and granite emplacement has always been
somewhat of an enigma (e.g. Scaillet 1995; Brown and Dallmeyer, 1996). The question of
which comes first has always been a point of debate and this, to some extent still remains
umesolved, although many would favour that the heat generated by orogenesis eventually
promotes melting and granite emplacement (Brown 1994a, 2001; Solar et aI., 1998; Hutton
1997; Brown and Solar, 1998). The relationship between isograds and igneous bodies is
one approach that has been used to try and determine their inter-relationship. Previously,
single isograds have been determined for a particular mineral species because quantitative
distinctions in the timing of mineral growth from sample to sample within a particular
A2
Isograd mif!ratioll with lillie during orogenesis ..... SHAHA.A
mineral phase have not been possible. With the advent of techniques for the quantitative
measurement of inclusion trails within porphyroblasts, different periods of growth of a
porphyroblastic phase can be identified and confidently correlated across a region (Bell et
a!., 1998; Bell and Mares, 1999). Consequently, one can potentially distinguish isograds for
different periods of growth of a single mineral phase.
The area described herein forms part of a Proterozoic orogenic belt in the
southwestern United States and provides a classic example of a large high-T-low-P terrane
and the problems associated with the tectonic interpretation of such regimes (Williams and
Karlstrom, 1996). The only indication of deformation events that occur before the peak of
metamorphism, and how they affected the thermal structure of an orogenic belt, comes
from combined quantitative micro and macro structural studies. Microstructural studies are
also important in distinguishing between events at the peak of metamorphism and those
significantly post-dating it (Thompson and Ridley, 1987). This research has used the
combined approach of integrating geochronological, metamorphic and structural studies
(using FlAs).
The technique used requires the measurement of the orientation of foliation
inflection or intersection axes preserved by inclusion trails within porphyroblasts, which
are called FIAs. The measurement of these structures within garnet, staurolite andalusite
and cordierite porphyroblasts from the Colorado foothills of the Rocky Mountains has
revealed a succession of 4 FIAs within these rocks. This has enabled examination of
whether or not the distribution of staurolite, cordierite and andalusite isograds changed with
time. That is, whether they shifted with time as each FIA set developed. This paper reveals
the role of granitic emplacement versus isograd migration with time, and their significance
to the overall interpretation of PTtd paths.
A3
Isograd mirrration with time during orogenesis ..... SHAHA.A
3.0. GENERAL GEOLOGY AND TECTONICS OF THE REGION
Figure I shows the topography of this region and its location on the eastern edge of
the Rocky Mountains in Colorado; a regional geological map in shown in Fig. 2. Figure 3
shows a detailed geological map with sample location. The geological history of Colorado
has been well studied (e.g. Finch, 1967; Peter Bird, 1988; KarIstrom et aI., 1997) because
of its spectacular mountains and mineral wealth (Finch, 1967). The basement is composed
mainly of PaleoProterozoic rocks (2.5-1.6Ga, Sim et ai, 2001) with some rocks having a
MesoProterozoic affinity (1.6-0.9 Ga, Sim et aI., 2001, Reed et ai, 1987). The Central
Plains Orogen forms the eastern limit of the Colorado province (Sims and Peterman, 1986).
To the south it is thought to extend as far as northern New Mexico and probably beyond
(Robettson et aI., 1993; KarIstrom et aI., 1997).
The rocks mainly consist of qUattz-feldspar gneiss, biotite gneiss, amphibolite and
migmatite (Reed et aI., 1987). They were generally deformed under amphibolite facies
conditions (Braddock et aI., 1970). Igneous intrusions at ~1.7, ~1.4 and 1.1 Ga reshaped the
PaleoProterozoic rock distribution (Tweto, 1987). The composition of the oldest 01.7 Ga)
and most dominant of these intrusions is intermediate with a calc-alkaline affinity. It is
generally believed that these igneous bodies were emplaced synchronous with regional
deformation during the Colorado orogeny, but a few formed post-tectonically (Reed et aI.,
1987,1993) at ~1.4 Ga. These intrusives are undeformed except locally on their margins,
and were referred as A-type or "anorogenic" plutons, mainly because of their composition
(Anderson and Cullers, 1999). Recently some workers have questioned their anorogenic
A4
IsogJ'ad mif!ration with time during orogenesis ..... SHAHA.A
classification and have argued that they were formed during deformation associated with
the Berthoud orogeny (Nyman et aI., 1994).
The two major deformation/metamorphic events previously recognized reached
temperatures consistent with sillimanite-grade metamorphism (Sims and Stein, 2003)
although metamorphic conditions were very heterogeneous during these episodes. During
the Colorado orogeny, several areas recorded a transition in metamorphic grade from the
chlorite zone to the onset of migmatization (Braddock and Cole, 1979; Selverstone et aI.,
1997). Thermobarometric analyses by Selverstone et al. (1997) and Shaw et al. (1999)
suggested that there were two intervals of heating during the Proterozoic Colorado and
Belihoud orogenies. These were separated by an interval of cooling and decompression
(Sims and Stein, 2003). In the Big Thomson Canyon area (Fig. 3), thermobarometric
analysis of metapelites by Selverstone et al. (1997) indicated that "early metamorphism at
pressures of 7-10kbar (~24-35 km depth). was followed by continuing porphyroblast
growth (garnet and staurolite) down to pressures of 4-6 kbar 0- 14-21 km)". According to
Selverstone et al. (1997), the whole region near the Big Thomson Canyon was reheated
around 1.4-Ga. Geobarometric calculations on late garnet have shown that these rocks lay
at ~ 10km depth when this reheating occurred, similar to that at the end of the Colorado
orogeny (Selverstone et aI., 1997). The portion of the geological map by Selverstone et al.
(1997) reproduced in Fig. 4 shows a succession of isograds from staurolite through
andalusite, sillimanite (and K-feldspar further west from this area) from SE to NW across
the area.
A5
Isograd mif!ration with time during orogenesis ..... SHAHA.A
4.0. FOLIATIONS
The rocks have been multiply deformed. Within the matrix 3 foliations could be
recognized. The most common is the main matrix schistosity S I that lies parallel to
compositional layering So (Fig. SA). Two younger matrix foliations oblique to bedding (S2
and S3) were also observed (Fig. SB). However, S3 was only recognized locally (Fig. SA).
Foliations preserved as inclusion trails in porphyroblasts are commonly truncated by SI
parallel to So and in such cases generally appear to predate this foliation (see below). Figure
SB (a), shows the first matrix schistosity (SI) parallel to bedding (So). In Fig. SB (b), D2 is
crenulating So/lSI.
5.0. METHODS
5.1. FIA measurements
Hayward (1990) and Bell et al. (199S, 1998) described a technique for analysing the
geometries of inclusion trails within porphyroblasts. It involves the measurement of the
FIA, which is achieved by cutting a minimum of eight vertical thin sections around the
compass. These are required to measure a FIA trend within a 10° range, with one cut every
30° around the compass and two cut 10° apatt between the sections where the asymmetry
of the inclusion trails flip (Fig. 6a and b). Because the blocks for thin sectioning are cut
from a horizontal slab 2.8 cm thick, the thin sections are always vertical with the strike
parallel to the long edge and the way up marked with a single barb, as shown in Fig. 6.
FIA measurements are made relative to both geographic coordinates and a line
perpendicular to the earth's surface. The measurement is thus independent of assumptions,
inferences, or interpretations about the timing of the inclusion trails relative to any other
A6
Isograd migration with time during orogenesis ..... SHAHA.A
structures present in the rock and whether or not the porphyroblast has rotated (Hayward,
1990; Bell et ai., 1995, 1998; Bell and Welch, 2002). Where the FIA trends vary from the
core to the rim of the porphyroblasts, a relative timing and thus a FIA succession can be
established (e.g. Fig. 5a,b; Bell et ai., 1998). The accumulated error associated with
determining the trend of the FIA in each rock is random, and is estimated to be +/ - 8° in
both situations when one uses a COCLAR compass (see Bell et ai., 1998). A very
significant microstructural advantage results from cutting sections this way because the
inclusion trails contained in many porphyroblasts can be observed from a large range of
orientations, providing a much better record of the complete inclusion trail geometry than
available from cutting just one or two thin sections.
5.2. Porphyroblast inclusion trail microstructures
Gamet, staurolite, andalusite and cordierite porphyroblasts preserve earlier
foliations as inclusion trails (e.g. Figs. 7 and 8). Gamet and staurolite porphyroblasts are
most common and generally contain excellent inclusion trails (e.g. Fig. 7). Many samples
preserve a very well developed schistosity (e.g. Fig. 7) or a differentiated crenulation
cleavage (e.g. Figs. 9 and 10). These foliations are most commonly straight with curvature
at their extremities (e.g. Figs. 8 and 9). Porphyroblast inclusion trails are commonly
truncated (e.g. Fig. 7c and d) by the matrix foliations but some are not (e.g. Fig. 8).
6.0. RESULTS
A total of 67 oriented samples were collected from the field and were reorientated
in the laboratory. These rocks contain porphyroblasts of gamet, staurolite, cordierite and
andalusite, which contained inclusion trails well enough developed for FIA measurement.
A7
Isograd mif!ration with time during orogenesis ..... SHAHA.A
800 oriented thin sections were prepared from these samples. A total of 13 8 FIA and
pseudo-FIA trends were determined (Table 1) and are shown as trends on a map for each
sample location in Fig. 11 and as a rose diagram in Fig. 12a. A total of 64 and 53 FIAs
were measured in garnet and staurolite porphyroblasts respectively (Fig. 12b and c). The
combined FIA trend data for garnet and staurolite is shown in Fig. 12d. The other
porphyroblastic phases in which FlAs were measured were andalusite and cordierite, with
fourteen measurements in the former and seven in the latter. Their trends are given in Table
1, and shown in map view in Fig. 11, and on a rose diagram in Fig. 12e and f.
6.1. Determillatioll o/mllltiple FIAs/or relative timillg
Samples C52, C67, C69, C76 and C96B preserve a different FIA in the core versus the rim
in garnet (Table 1). These multi-FIA samples enable the relative timing of successive FIAs
to be determined (e.g., Bell et aI., 1998; Sayab, 2005). For example, a FIA trend determined
from the core of the porphyroblast must be older than a FIA in the rim of the same sample.
Recent work by Bell et al. (1998), Ham (1999), Cihan (2000), Newman (2001) and Sayab
(2005) has shown that in some samples, porphyroblasts inclusion trails (Si) are truncated by
the matrix foliation whereas in others they are continuous with the matrix. This further
strengthens the relative timing criterion established by using core/rim FIAs. Generally,
porphyroblasts containing truncated trails are older compared to those continuous with
matrix foliation (e.g. Johnson & Vernon, 1995). Most of the samples which preserve
inclusion trails within porphyroblast were truncated by matrix foliations (e.g. Figs. 7 and
14), but some do not (e.g. Fig. 8). Many samples preserve differentiated crenulation
cleavages that have been overgrown by the porphyroblasts where the asymmetry of the
crenulated cleavage can be determined (e.g. Figs. 9, 10, 13 and 14). The crenulated
A8
Isograd migration with time during orogenesis ..... SHAHA.A
cleavages consist of quartz and ilmenite grains, while the differentiated crenulation
cleavages predominantly contain ilmenite grains. The intersection between the crenulated
and crenulation cleavages can be determined, when viewed in three dimensions, and is
called a pseudo-FIA (P-FIA). The actual FIA is formed during porphyroblast growth and in
these samples is defined by the curvature of the differentiated crenulation cleavage (Table1
and Figs. 14 and 15).
7.0. INTERPRETATION OF THE FIA SUCCESSION
Figure 2 shows that porphyroblastic samples from which FlAs could be measured
were present relatively unifotmly across the eastern half of the region only becoming
patchily distributed to the west and this is significant for the construction of isograds (see
below). Garnet is present in the easternmost samples but staurolite is not. Furthermore,
garnet porphyroblasts are commonly overgrown by staurolite. Both features suggest that in
this region garnet grew before staurolite and this characteristic (see Table 2) provides one
of foul' criteria that were used to detelmine and test the relative timing of successive FIAs.
The few samples containing changes in FIA trend from the cores to the rims of garnet
porphyroblasts provide a second criterion, a third was provided by samples containing
pseudo (P-FIAs) versus actual FlAs (e.g. Figs. 12 and 13) in garnet and staurolite
porphyroblasts (Table 3) and a fourth was provided by those porphyroblasts with trails
truncated by the matrix foliation versus those whose trails were continuous with it. The FIA
succession indicated by arranging changes in FIA trend from garnet to staurolite in the
same sample in Table 2 so that they do not conflict is supported by that in Table 3 and by
those whose trails are truncated versus continuous with the matrix foliation. This
A9
Isograd migration with time during orogenesis ..... SHAHA.A
succession is from FIA 1 (trending NE-SW), to FIA 2 (trending E-W), to FIA 3 (trending
SE-NW), and FIA 4 (trending NNE-SSW). Table 4 shows all the samples for which FIAs
have been determined and classified into FIA sets. Samples containing only one FIA trend
have been placed on this table based on that trend and secondly, whether the inclusion trails
are truncated (FIA 1) or continuous (FIA 4) with the matrix foliation. FIA 1 and 2 is
preserved only in garnet and staurolite porphyroblasts. FIA set 3 is mainly present in garnet
and staurolite porphyroblasts. However, a few samples contain this FIA set in andalusite
and cordierite porphyroblasts (Table 1). FIA set 4 can be found in each of the
porphyroblastic phases. This indicates a progression in the timing of growth as orogenesis
proceeded of garnet, staurolite, andalusite and cordierite.
8.0. EVIDENCE OF EARLIER MINERAL INCLUSIONS WITHIN YOUNGER
PHASES
8.1. Illelusiolls of gamet with ill staurolite, alU/a/usite (II/{/ cordierite
Several samples preserve remnants of older minerals included within younger phases,
during an overall prograde metamorphic succession. Figure l6a, shows an example of a
euhedral garnet porphyroblast included within a big poikiloblastic staurolite. A few
examples of garnet inclusions preserved within andalusite and cordierite are also observed
(e.g. Fig. 16b and c). These mineral relationships have suggested an earlier appearance of
garnet, and before the formation of staurolite, andalusite and cordierite.
AJO
Isograd mif!ralion with time during orogenesis ..... SHAHA.A
8.2. Rep/(lCemellt of st(llll'olite by (lnd(l/l/site
Staurolite porphyroblast in a couple of examples were found to be partially andlor
completely replaced by either andalusite or cordierite (e.g. Fig. 17). Traces of staurolite are
included in some big poikiloblastic andalusite (e.g. Fig. 17a and b). Inclusions preserved
with in the staurolite are texturally and geometrically distinct from those within andalusite
pOlphyroblasts. FIA measurements were impossible for andalusite in this sample, as very
few slides contain this mineral. However, staurolite preserves FIA set 4 inclusion trails.
8.3. Rep/(lcement of st(llll'olite by cOl'diel'ite
Some samples (e.g. C52 ) preserve evidence of direct replacement of staurolite by
cordierite. Traces of staurolite are included in some big cordierite poikiloblasts. Inclusions
trails preserved with in the staurolite are texturally and geometrically distinct from those
within cordierite (e.g. Fig. 17c and d). Cordierite preserves FIA set 4 and its inclusion trails
completely truncate those of staurolite (e.g. Fig. 17d). FIA measurements were impossible
for staurolite in this sample, as very few slides contain this mineral.
8.4. Rep/(lcemellt of (tlU/(I/lisite by conliel'ite
Inclusions of andalusite were found within the porphyroblasts of cordierite in
sample C77 (Fig 18a and b). Cordierite preserves a crenulated (FIA set 3) and a crenulation
cleavage. Andalusite only preserves one foliation, which is roughly consistent with the
orientation of the crenulated cleavage within the cordierite. This suggests that the
andalusite formed before or contemporaneous with the formation ofFIA set 3. Remnants of
inclusions within andalusite, which are mainly composed of staurolite, quartz, muscovite,
minor opaques and ilmenite, are geometrically and texturally distinct from those found
All
Iso,?rad migration with time during orogenesis ..... SHAHA.A
within the matrix and the cordierite porphyroblast. No staurolite inclusions were observed
within the cordierite.
9.0. DATING OF FIAS AND THE SUCCESSION CONFIRMATION
Monazite grains were dated within the foliations defining each FIA set (Thesis
Section C) with FIAs set 1,2 and 3 forming from around 1760 to 1670 Ma and FlA set 4
around 1420 Ma.
10.0. FIAS DERIVED ISOGRADS
10.1. Staurolite isograds for FIAs 1, 2, 3 and 4
The samples which contain FIA 1,2, 3 and 4 preserved in staurolite porphyroblasts
are shown in Fig. 1. The total distribution of FIAs measured from this mineral (Fig. 12c)
resulted from four periods of staurolite growth that show a consistent succession where
relative timing criteria are available (Tables 2 and 3). Staurolite isograds can be defined for
these FlAs as shown in Fig. 19. A slightly clockwise and eastward shift in the staurolite
isograds from FlA 1 to FIA 4 is apparent.
10.2. Afl(/a/usite and cOl'dierite isograds
Andalusite and cordierite were the last minerals to grow in these rocks. Two FIA
sets (3 and 4) were observed in these mineral phases. Cordierite porphyroblasts
predominantly preserve FIA set 4. Foliations defining the latest FIA sets preserved in both
of these mineral phases were always continuous with those in the matrix and deflections of
these trails from porphyroblast to matrix are very slight (e.g. Fig. 8). The andalusite and
Al2
Isograd migration with time during orogenesis ..... SHAHA.A
cordierite isograds for FIAs 3 and 4 are shown in Fig. 20 (a, b). A slightly clockwise shift
with time in both of their isograds, similar to that for the staurolite isograds, is apparent.
11.0. LATE ANDALUSITE AND CORDIERITE GROWTH
A few samples contain rare porphYl'oblasts of andalusite (C92A, C55B, CIOl) or
cordierite (C66, C68B, C96B, C70). Cordierite or andalusite was only present in one thin
section of 8 to a maximum of 18 that were cut from each of these samples and so a FIA
could not be measured for them. In sample C92Arare andalusite was present in 3 out of 8
slides. In all of these samples the porphyroblasts overgrew the matrix with no deflection of
the foliation from inside to outside of the porphyroblast suggesting that they formed at the
very end of deformation (e.g. Figs. 17 and 22). The location of these samples and the two
isograds that they suggest is shown in Fig. 20 (c, d).
12.0. DISTRIBUTION OF SILLIMANITE
Sample C77 contains fibrolitic sillimanite within andalusite and cordierite that
contain FIA set 4. Fibrolitic sillimanite also occurs in some of the samples (e.g. Fig. 21a
and b) mentioned above that contain rare andalusite and cordierite. The fibrolitic sillimanite
in all of these samples appears to postdate FIA 4 and overprints the matrix foliation (e.g.
Fig. 22a and b). The sillimanite indicated by this distribution is shown in Fig. 20d.
Al3
Isograd migration with time during orogenesis ..... SHAHA.A
13.0. AXIAL PLANE TRENDS OF REGIONAL FOLDS
The axial traces of all folds present in the area were measured from regional
geological maps (Fig. 23). They were then plotted on a rose diagram as shown in Fig. 23.
Note the similarity between this figure and that containing the FIA trends (Fig. 24).
14.0. INTERPRETATION
14.1. Porphyroblast rotation or otherwise
A consistent succession of 4 FIA sets as shown in Fig. 12 would be impossible if
the porphyroblasts had rotated. This is shown in Fig. l3. Set 1 FIA forms almost 45° from
FIA set 2. If FIA set 2 formed by rotation of the porphyroblasts around an axis with this
trend, FIA set 1 would have been rotated on a small circle by up to 90° (the maximum
curvature of inclusion trails defining FIA set 2) as shown in Fig. 25a. FIA set 3 lies 80°
from FIA set 1. An enormous spread of FIA set 1 data results when the spread of FIA 1 due
to rotation about FIA 2 is in tum rotated about FIA 3 (Fig. 25b) by up to 135° (the
maximum inclusion trail curvature about FIA 3). If these axes were further rotated about
FIA set 4, the spread of FIA trends would span the whole stereonet (Fig. 25c). The
succession of FIA trends show in Fig. 12 indicates that this is not the case. This does not
eliminate rotation as a mechanism for the development of FIA set 1 but it makes it highly
unlikely. Furthermore, with the recent computer-modeling discovery of the phenomenon of
gyro stasis, a mechanical explanation for the non rotation of porphyroblasts is now available
(Fay et aI., 2008). They name this for the process driving the phenomenon of lack of
rotation of rigid objects
Al4
Isograd migration with time during orogenesis ..... SHAHA.A
14.2. Staurolite Isograds
The migration of the staurolite isograd eastwards from FIA 1 to FIA 4 (Figs. 20a, b,
c and d) indicates that the source of heat for the metamorphism that generated this
succession of events lay to the west. Each of the four periods of staurolite growth involved
more than one phase of growth. Most porphyroblasts contain a single FIA but some
preserve an earlier formed differentiated crenulation cleavage from which a pseudo-FIA
(that predates porphyroblast growth) could be measured (Fig. 14). Consequently, these
rocks record evidence of more than 4 phases of staurolite growth that could not have been
distinguished without the FIA data preserved by their inclusion trails.
14.3. Am/a/I/site isograd for FIAs 3 and 4
The andalusite isograds for FIAs 3 and 4 shown in Fig. 20a, rotate clockwise as they
migrate eastwards. This is similar to the staurolite isograds, suggesting that they were a
product of migration ofthe heat source ii-om WNW to ENE with time.
14.4. Cordierite isogr(l(/s for FIAs 3 ami 4
The cordierite isograds for FIAs 3 and 4 shown in Fig. 20b, rotate clockwise as they
migrate eastwards. The migration of the cordierite isograds from FIA to FIA mimics that of
the staurolite and andalusite isograds, suggesting that they were a product of migration of
the heat source from WNW to ENE with time.
A15
Isograd migration with time during orogenesis ..... SHAHA.A
14.5. Rare matrix overprilltillg cordierite amI (IIulalusite
Some samples contain cordierite or andalusite in only one thin section out of the 8
to 18 cut per sample (Sample C92A contains andalusite in 3 out of 8 thin sections). These
rare porphyroblasts overgrew the matrix foliation without any deflection of inclusion trails
associated with deformation during their growth. The distribution ofthese porphyroblasts is
significantly different to those for which FIAs could be measured and spreads eastwards
across the area as shown in Fig. 20c. The isograds that they indicate run at a high angle to
those isograds indicated by the FIA succession. They bear a close relationship to the
sillimanite isograd mentioned and so their significance is interpreted in that section.
14.6. Sillimallite
The sillimanite isograd shown in Fig. 20d runs almost olthogonal to the FIA 4
isograds determined for cordierite and andalusite. However, it nllls sub parallel to the
cordierite and andalusite isograds indicated for the youngest period of growth of these 2
mineral phases (Fig. 20c). This suggests that the growth of sillimanite and this youngest
phase of cordierite and andalusite were associated with same heat source. This phase of
metamorphism not only overprinted FIA sets 1 through 4, but the heat source causing it had
migrated significantly relative to that which accompanied development of the earlier
formed porphyroblasts.
14.7. Relatiollship betweell FlA trellds alld axial traces offolds
Comparison of Fig. 24a and b shows a strong correlation between the distribution of
FIA trends and the axial trace of all folds present in the area. The four different peaks in the
trends of axial traces exactly match the four FIA trends. Bell et al. (2004) and Bell and
A16
Isograd miwation with time during orogenesis ..... SHAHA.A
Newman (2006) argue that FIAs forming perpendicular to bulk shortening directions and
successions of FIAs record changes in this direction that occurred during orogenesis. Folds
are a product of the same process but are expected to be rotated with time by the
overprinting effects of successive deformation in contrast with the FIAs, which are
unaffected by subsequent ductile deformation (e.g. Bell et aI., 1995; Fay et aI., 2008). The
correlation of fold axial traces with the FIA trends recorded here is, therefore, quite
remarkable. It suggests that pockets of low strain occur due to deformation partitioning, in
spite of, or perhaps because of, the 4 successive changes in bulk shortening direction. These
pockets preserve folds in the orientation in which they formed from subsequent rotation or
destruction. They provide remarkable confirmation of the veracity of the FIA data and
indeed reveal that such data can be used to determine successions of fold development from
regional maps at which scale many overprinting criteria cannot be applied.
14.8. Foliation trend and Heat Flux
Figures 2 and 3 show that no granite is present on the WNW side of the isograds.
Figures 19 and 20a, b reveal that the staurolite, cordierite and andalusite isograds migrated
eastwards at a high angle to SO.1 with clockwise rotation of the FIA trend 1 to 2 to 3 to 4.
This suggests that heat flux through rock was controlled by the orientation of SO,I rather
than the FIA trend which is controlled by the concurrently developing sub-vertical
foliation. That is, reactivation of the heterogeneity provide by SO,1 provided a faster
pathway for heat than a newly developing cleavage. This also accords with ENE migration
of a heat source that lay to the WSW during orogenesis. The most recently formed
andalusite, cordierite and sillimanite isograds (Fig. 20c and d) lie roughly parallel to each
Al7
Isorp'ad mirp'ation with time during orogenesis ..... SHAHA.A
other and the distribution of the -1400 Ma granitoid plutons to the SW but orthogonal to
isograds defined by FlAs.
15.0. Discussion
Prior to the development of FIA studies, there was no way to distinguish different
periods of growth of the same porphyroblastic phase unless changes in inclusion density,
composition or orientation were very obvious. With the advent of a quantitative approach
using FlAs, many different periods of growth of the same porphyroblastic phase can now
be distinguished (Bell 1998). In the example documented herein, four different periods of
growth of garnet and staurolite have been distinguished and the isograds for four of the
periods of staurolite and two each of cordierite and andalusite growth have been mapped.
15.1. Multiple phases of staurolite growth
Isograds can potentially be obliterated by further heating (e.g. Braddock and Cole
1979). Shaw (1999) and Selverstone (1997; Fig. 4) claimed that the staurolite isograd
resulted from two generations of growth. They had isotopic age evidence for 2 periods of
tectonism separated by - 300 million years but did not attribute these two periods of growth
to periods of tectonism that far apati. Rather, they suggested that andalusite growth took
place at the end of the phase of tectonism that produced the staurolite (-1.7Ga) and then
reoccurred some 300 million years after staurolite growth (-1.4Ga). The ability to
quantitatively measure FIAs and distinguish many periods of growth of the same
porphyroblastic phase has changed this. The recognition of a succession of 4 FlAs
preserved within the staurolite porphyroblasts in this region plus at least 2 phases of
staurolite growth within each FIA set, is significantly different to most current concepts
about how reactions take place during deformation and metamorphism. Quantitative
A18
[sogmd migration with lime during orogenesis ..... SHAHA.A
measurement of inclusion trails within porphyroblasts reveals that the reactions from which
porphyroblasts such as staurolite grow do not simply start and go to completion. Rather
they take place over and over again, once the PT and bulk composition are suitable and are
a function of some other factor that previously has not been considered as significant. Bell
and Hayward (1991), Spiess and Bell (1996) and Bell & Bruce (2007) have shown that the
initiation of crenulation hinges controls where porphyroblast nucleation and growth takes
place during regional metamorphism and that growth ceases as soon as a differentiated
crenulation cleavage begins to develop. They argue that this results from strain softening
and the cessation of micro fracture and thus access of the components need for the reaction
to take place to the porphyroblast crystal faces. Consequently, when another deformation
initiates such that crenulations can begin to form again (which requires shortening near
orthogonal to the previous event) the same porphyroblastic phase can regrow because the
reaction was forced previously to cease. Such regrowth of staurolite took place in this
region during 4 different events, the first and last of which were spaced 250 million years
apart!
15.2. Multiple phases of andalusite and cordierite growth
A transition from andalusite to cordierite during FIAs 3 and 4 is suggested by a
concomitant increase in the latter phase and accords with the overall migration of staurolite,
andalusite and cordierite during the development of FlAs 1 tlu'ough 4 from W to E across.
This succession suggests growth of cordierite after andalusite during metamorphism
accompanied by deformation, which matches with the PT pseudosection.
The younger cordierite, andalusite and sillimanite isograds that are nearly
orthogonal to those defined by the FIA succession (compare Fig.2Ia-b and 2Ic-d). This
A19
Isograd mif!ration with lillie during orogenesis ..... SHAHA.A
suggests growth of andalusite was followed by cordierite, confirmed by the PT
pseudosections. They clearly resulted from a heat source to the SW instead of the WNW.
15.3. Comparison with earlier work
The isograds mapped by Selverstone (1997) shown in Fig. 4 resemble the youngest
isograds determined through this research. This is to be expected because they were unable
to distinguish the different phases of growth of staurolite, andalusite and cordierite that the
FIA approach allowed. Multiple isograds of garnet, staurolite, andalusite and cordierite that
migrated with time, suggest a much longer history of deformation and a prolonged
metamorphism than previously reported (e.g. Cavosie and Selverstone, 2003; Selverstone,
1997). The complete absence of kyanite and presence of andalusite and sillimanite isograds
suggests a low pressure metamorphic sequence and illustrates that high pressures were
never attained by these metasediments. One reason for their migration could be the
progressive upwards migration of melt.
15.4. Deformation Partitioning
Rocks are subjected to heterogeneities at all scales (e.g. Bell 1981; Williams 1994;
Bell 2004). In a given outcrop some aspects of the deformation history can have affected
some pOliions while others will be affected by other different events. Microstructural (Bell
and Hayward 1991; Spiess and Bell 1996; Bell and Bruce 2007) and FIA data (Bell 1998,
2008) strongly suggest that porphYl'Oblast nucleation and growth is a function of the
partitioning of deformation at the scale of a porphyroblast. This provides a simple
explanation and allows one to develop an intimate understanding of the role of deformation
in porphyroblast nucleation and growth and an understanding of why reactions should take
A20
Isograd migration with time during orogenesis ..... SHAHA.A
place at different times from sample to sample in the same outcrops. Thus assumptions that
mineral growth is merely a function of pressure, temperature and bulk composition (e.g.
Thompson 1957) need to be revised as deformation partitioning has a strong role to play in
the growth of porphyroblastic mineral phases.
Different distributions of FlAs on a histogram from FIA to FIA for garnet,
staurolite, andalusite and cordierite (Fig. 26) reveal changes in the partitioning of
deformation within a region. Garnet dominates in FIA set 1 and staurolite in FIA set 4 (Fig.
26a). The total distribution in each FIA set reveals that there are more samples which
preserve FIA set 3 trails as compared to those which contain FIA set 1,2 or 4 (Fig. 26b).
Garnet contains the maximum FIAs followed by staurolite, cordierite and andalusite (Fig.
26c). Differences from region to region reveal changes in the role of deformation
partitioning at a large scale (e.g. Bell 2004; Bell and Newman 2006; Sayab 2007). The
disbursement of FIAs 1, 2, 3 and 4 in garnet and staurolite porphyroblasts across the area
strongly suggests that the P, T and bulk composition were suitable for growth of these
mineral phases from FIA 1 onwards. Yet staurolite porphyroblasts containing FIA 1 have
only been found in seven samples (Fig. 11). This is more likely to be the result of a change
in the pattern of deformation partitioning during the development of this FIA set rather than
a change in the P and T conditions (e.g. Be112004).
15.5. Relevance to granite emplacement
No broad first-order spatial relationship exists between granites and medium grade
rocks although granites exposed elsewhere in the region are enriched in radiogenic heat
producing elements. The only plutonic rocks known to have intruded during the peak of
metamorphism in the Colorado orogeny are pegmatite swarms close to the Big Thompson
A2I
Isof!l'ad migration with time during orogenesis ..... SHAHA.A
River (Fig. 3). However, the isograd patterns show no relationship to pegmatite contacts.
The emplacement of pegmatites may have contributed to the thermal budget but were not
the primary cause of the low-pressure metamorphism during the Colorado and Berthoud
orogenies as they lie too far away (e.g. Foster and Rubenach, 2006).
The eastward migration of the staurolite isograds with time revealed in Fig. 20 and
their near orthogonal orientation to SO,l suggests that the flux of heat was controlled by the
orientation of foliation parallel to the compositional layering. Intense shear along
compositional layering, referred to as reactivation, is thought to dominate over the
development of new oblique foliations (Ham & Bell, 2004). Heat transference may occur
by advection due to fluid flow along the layering during this process. However, heat
transference by conduction could be enhanced by diffusion associated with the dissolution
and diffusion of elements that accompanies cleavage seam development and reactivation
(Bell and Cuff 1989).
15.6. Significance of the shift in the isograd fi'om FIA to FIA
Heat flux, like deformation, will be heterogeneous and partitioned through the crust
if foliation development is important to diffusion is evident from the distribution of
staurolite isograds. The eastward shift with time signifies that heat progressively increased
although the overall migration is minimal. The succession of three FIAs has developed over
period of ~ 1 00 Ma for the Colorado orogeny. Other five fold FIA successions that have
been dated lasted 100 (Bell and WeIch 2002) to 200 (Cihan 2006) million years. If such
time lengths are involved, why was the eastward migration of index minerals so limited.
One possibility is that the isograds have steep dips rather than gentle ones. The garnet
isograd in the Homestake mine is sub-vertical through the mine centre and formed that way
A22
Isograd migration with time during orogenesis ..... SHAHA.A
(Bell and Bell unpublished data). Steep dips of the isograds in this region would readily
explain the minimal migration over a potentially a long time period.
Acknowledgements
I would like to thank Prof. Tim Bell for his critical comments. Special thanks to Dr. Mike
Rubenach for help and critical discussion about isograds. I would also like to thank Prof.
Jane Selverstone for sending me the manuscripts of the Colorado region.
A23
Isograd migration with time during orogenesis ..... SHAHA.A
16.0 REFERENCES
Anderson 1. L., Cullers R. L., 1999. Paleo- and Mesoproterozoic granite plutonism of
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A24
Isograd migration with time during orogenesis ..... SHAHA.A
Bell, T. H., Welch, P. W., 2002. Prolonged Acadian Orogenesis: Revelations from FIA
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A25
Isograd mif!ration with time during orogenesis ..... SHAHA.A
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Brown, M., Dallmeyer, R. D., 1996. Rapid Variscan exhumation and role of magma in core
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metamorphism from EPMA dating of monazite in the Proterozoic Robertson River
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the Lachlan Fold Belt, southeastern Australia. Tectonophysics, 214,381 - 400.
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A26
Isograd mif!ration with time during orogenesis ..... SHAHA.A
Duguet, Manuel., Le Breton, Nicole., Faure, Michel., 2007. P T paths reconstruction of a
collisional event: The example of the Thiviers-Payzac Unit in the Variscan French
Massif Central. Lithos, 98, 1-4, 210-232.
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in the Early to Middle Proterozoic of northern Australia. American Geophysical
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Fay, C., Bell, T. H. and Hobbs, B. E., 2008. Porphyroblast rotation versus non-rotation:
conflict resolution! Geology, 30, No 4.
Finch, Warren., 1967. Geology of epigenetic uranium deposits in sandstone in the United
States: U.S. Geological Survey Professional Paper, 538, 121.
Foster, D. R W., Rubenach, M. J., 2006. Isograd pattern and regional low-pressure, high
temperature metamorphism of pelitic, mafic and calc-silicate rocks along an east-west
section through the Mt Isa Inlier. Australian Journal of Earth Sciences, 53, 167-186.
Ham, A. P., Bell, T. H., 2004. Recycling of foliations during folding. Journal of Structural
Geology, 26, 1898-2009.
Hand, M., Fanning, M., Sandiford, M., 1995. Low-P high-T metamorphism & the role of
high-heat producing granites in the northern Amnta Inlier. Geological Society of
Australia Abstracts, 40, 60 - 61.
Hand, M., Rubatto, D., 2002. The scale of the thermal problem in the Mount Isa Inlier.
Geological Society of Australia Abstracts, 67, 173.
Hutton, D. H. W., 1988. Granite emplacement mechanisms and tectonic controls:
Inferences from deformation studies: Royal Society of Edinburgh Transactions. Earth
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A27
Isograd migration with time during orogenesis ..... SHAHA.A
Karlstrom, K. E., Dallmeyer, R. D., Grambling, J. A., 1997. 40Ar/39Ar evidence for 1.4 Ga
regional metamorphism in New Mexico: implications for thermal evolution of
lithosphere in the southwestern U.S. Journal of Geology, 105,205-223.
Loosveld, R. J. H., Etheridge, M. A., 1990. A model for low pressure facies metamorphism
during crustal thickening. Journal of Metamorphic Geology, 8, 257 - 267
Ludwig, K. R., 1998. IsoplotlEx: a geochronological toolkit for Microsoft Excel. Berkeley
Geochronological Center, Special Publication, No.1, 43.
McLarens, S., Sandiford, M., Hand, M., 1999. High radiogenic heat-producing granites and
metamorphism: an example from the western Mount Isa Inlier, Australia. Geology,
27,679 - 682.
Mezger, J. E., Passchier, C. W., 2003. Polymetamorphism & ductile deformation of
staurolite-cordierite schist of the Bossost dome: indication for Variscan extension in
the Axial Zone of the central Pyrenees. Geological Magazine, 140, 595 - 612.
Mildren, S. D., Sandiford, M., 1995. Heat refraction and low-pressure metamorphism in the
northern Flinders Ranges, South Australia. Australian Journal of Earth Sciences, 42,
241-247
Nyman, M., Karlstrom, K. E., Graubard, C., Kirby, E., 1994. Mesozoic contractional
orogeny in western North America: Evidence from ca. 1.4 Ga plutons. Geology, 22,
901-904.
Oliver, N. H. S., Holcombe, R. J., Hill, E. J., Pearson, PJ., 1991. Tectono-metamorphic
evolution of the Mary Kathleen Fold Belt, northwest Queensland: a reflection of
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A28
Isograd migration with time during orogenesis ..... SHAHA.A
Paul, K. Sims., Holly, J. Stein., 2003. Tectonic evolution of the Proterozoic Colorado
province, Southern Rocky Mountains: A summary & appraisal. Rocky Mountain
Geology, 38, 183-204.
Reed, J. C. Jr., Bickford, M. E., Premo, W. R, Aleinikoff, J. N., Pallister, J. S., 1987.
Evolution of the Early Proterozoic Colorado Province: Constraints from U-Pb
geochronology. Geology, 15, 861-865.
Reed, J. C. Jr., Bickford, M. E., Tweto, 0., 1993. Proterozoic accretionary terranes of
Colorado and southern Wyoming. In Reed, J. C., Jr.; Bickford, M. E.; Houston, R S.;
Link, P. K.; Rankin, D. W.; Sims, P. K.; & Van Schmus, W. R, eds. Precambrian:
conterminous U.S. Boulder, Colo. Geological Society of America, 211-228.
Robelison, J. M., 1993. Precambrian geology of New Mexico, in Reed, J.C. & others, eds.,
Precambrian--Conterminous U.S. Boulder, Colorado. Geological Society of America,
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Rubenach, M. J., 1992. Proterozoic low-pressure/high temperature metamorphism and an
anticlockwise P-T -t path for the Hazeldene area, Mount Isa Inlier. Journal of
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Creek Anticline, Eastern Succession, Mt Isa Inlier. Australian Journal of Earth
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Sandiford, M., Fraser, G., Arnold, J., Foden, J., Farrow, P., 1995. Some causes and
consequences of high-T, low-P metamorphism in the eastern Mt Lofty Ranges, South
Australia. Australian Journal of Earth Sciences, 42, 233-240.
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A29
Isograd migration with time dUl'ing ol'ogenesis ..... SHAHA.A
Sayab, M., 2005. Microstructural evidence for N-S shortening in the Mount Isa Inlier (NW
Queensland, Australia): the preservation of early W-E-trending foliations in
porphyroblasts revealed by independent 3D measurement techniques. Journal of
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tectonics of the nOlihern Colorado Front Range. In Bolyard, D.W., & Sonnenberg, S.
A., eds. Geologic history of the Colorado Front Range. Denver. Rocky Mountain
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of Mesoproterozoic metamorphism in the Colorado Front Range. Journal of Geology,
107,49-67.
Spiess, R, Bell, T. H., 1996. Microstructural controls on sites of metamorphic reaction: a
case study of the inter-relationship between deformation and metamorphism.
European Journal of Mineralogy, 8, 165-186.
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structure in the nOlih-central United States. Geology, 14,488-491.
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orogenic belts. Testing a model. Geology, 26, 711-714.
A30
Isograd mifTration with time during orogenesis ..... SHAHA.A
Spear, F. S., 1995. Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths.
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Metamorphic Geology, 19, 645- 660.
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Thompson, A. B., Ridley, J. R., 1987. Pressure-temperature-time (P-T-t) histories of
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Crustal Anatexis, Unwin Hyman, London, 272 - 319.
Williams, M. L., 1994. Sigmoidal inclusion trails, punctuated fabric development, and
interactions between metamorphism and deformation. Journal of Metamorphic
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Williams, M.L., Karlstrom, K.E., 1996. Looping PoT paths, high-T, low-P middle crustal
metamorphism: Proterozoic evolution of the southwestern United States. Geology, 24,
1119-1122.
A31
Isograd migration with time during orogenesis ..... SHAHA.A
Williams, M.L., Jercinovic, M.J., Terry, M.P., 1999. Age mapping and dating of monazite
on the electron microprobe: Deconvoluting multistage tectonic histories. Geology, 27,
1023-1026
A32
Section B
PRESSURE-TEMPERATURE-time-DEFORMATION (PTtD) PATHS DERIVED FROM FIAS, P-T PSEUDO SECTIONS AND ZONED GARNETS
PRESSURE-TEMPERA TURE-lime-DEFORMATION. ............................................. SHAH A.A
PRESSURE-TEMPERATURE-time-DEFORMATION (PTtD) PATHS DERIVED
FROM FIAS, P-T PSEUDOSECTIONS AND ZONED GARNETS
PRESSURE-TEMPERATURE-time-DEFORMATION (PTtD) PATHS DERIVED FROM FlAS, poT PSEUDOSECTIONS AND ZONED GARNETS 1
1.0. ABSTRACT
2.0. INTRODUCTION
3.0. REGIONAL GEOLOGY AND TECTONICS
4.0. QUANTITATIVE MEASUREMENTS OF DEFORMATION USING FIAS
5.0. THE FlA SUCCESSION
6.0. PORPHYROBLAST TIMING 6.1. Via FIAs 6.2. Replacenient oJ staurolite by cordierite and andalusite 6.3. Replacement oJ andalusite by cordierite
7.0. TIMING OF METAMORPHIC INDEX MINERALS NOT PRESERVING FlAS 7.1 Garnet inclusions within staurolite, andalusite and cordie rite 7.2. Staurolite inclusions within andalusite and cordierite 7.3. Garne~ andalusite and staurolite inclusions within cordierite 7.4. Sillimanite timing
8.0. DATA FOR THERMO BAROMETRY 8.1. P-T pseudosections 8.2. Garnet Core Isopleths 8.3. Major elements
9.0. INTERPRETATION 9.1. The role oJ deJormation partitioning in porphyroblastgrowth 9.2. Multiple phases oJindex minerals 9.3. Mineral succession and migrating prograde metamorphic path 9.4. PoT pseudosections and garnet nucleation 9.5. Multi-equilibrium thermobarometry and Average PT 9.6. Pressure-Temperature-time-DeJormation (PTtD) path
10.0. DISCUSSION 10.1. Pseudosection and Textural agreement 10.2. Garnet temperature overstepping 10.3. Metamorphic and tectonic relationships 10.4. FIA data and the AUSWUS model
ACKNOWLEDGEMENTS
REFERENCES
B
2
2
4
5
6
8 8 8 9
9 9
10 10 11
11 11 12 13
16 16 17 18 19 21 22
23 23 24 24 26
26
28
1
PRESSURE-TEMPERATURE-time-DEFORMATION. ............................................. SHAH A.A
1.0. ABSTRACT
FlAs (foliation intersection/inflection axes preserved within porphyroblasts) in gamet,
staurolite, andalusite and cordierite from PreCambrian rocks in the nOlthern Colorado Rocky
Mountains reveal 4 periods of growth about axes trending NE-SW, E-W, SE-NW and NNE
SSW during an overall prograde path. The growth of garnet was always followed by
staurolite for each of the 4 FIA sets with andalusite and cordierite following staurolite during
development of the last 2 sets. Thermodynamic modelling in the MnNCKFMASH system
reveals that well documented episodic growth occurred over a similar bulk compositional
range and PT path for each FIA in the succession. The intersection of Ca, Mn, and Fe
isopleths in gamet cores containing FIA set 1 indicate that growth of this mineral phase
began at 540-550°C and 3.8-4.0 kbars. Similarly, the intersection of Ca, Mn, and Fe isopleths
in garnet cores containing FIA sets 2 and set 3 indicate that these rocks never got above
4kbars throughout the Colorado Orogeny. Indeed, they remained at approximately the same
depth with the onset of the much younger Berthoud orogeny, when the pressure decreased
slightly as porphyroblasts formed with inclusion trails preserving FIA set 4. A slightly
clockwise P-T path occurred for both orogenies.
2.0. INTRODUCTION
The calculation of Pressure-Temperature-time and Deformation (P-T-t-D) information for
different events preserved by key index minerals is an important step towards understanding
the defOl'mation and metamorphic history of orogenic belts. Matrix foliations are routinely
reactivated or re-used during younger deformations (e.g. Bell, 1998; Davis, 1993, 1995; Davis
and Forde, 1994; Ham & Bell, 2004) causing them to re-equilibrate to subsequently developed
B 2
PRESSURE-TEMPERA TURE-time-DEFORMATION .............................................. SHAH A.A
P-T conditions (Hickey and Bell, 1996). Deciphering plausible P-T-t-D paths is commonly
difficult and relative to the history of burial, deformation and uplift that the rocks have
experienced, past microstructural approaches provide little information on the topology of a P
T-t-D path associated with porphyroblast growth. Porphyroblasts commonly preserve structural
and metamorphic history that predates what remains in the matrix and provide a means of
accessing the P-T-t-D path for a region (e.g. Spear, 1995). The construction of a quantitative
path is now possible using successions of foliation intersection axes preserved within
porphyroblasts (FIAs) in combination with thermodynamic modeling. Information on the P-T
conditions for the nucleation of garnet for a particular F1A forming event can be determined. If
garnet has grown during the development of a succession of FIA sets, multiple periods of
growth ofthis phase can be distinguished that could not previously be identified.
Thus a FIA approach enables one to track the distribution of minerals along an overall
prograde path at a level of detail that was not previously possible. It should provide insights
into interactions between structural and metamorphic processes that have not previously been
within reach. For example, where one cannot distinguish multiple phases of growth of a single
mineral phase, isograds marking the first appearance of that phase may include several periods
of growth that occurred at very different times. This would disguise any shift in the isograd
with time. Indeed, this has been shown to happen within the area described herein. This
contribution integrates F1As and thermodynamic modelling (MnNCKFMASH) to investigate in
detail the overprinting effects of such a progression on the distribution of garnet, staurolite,
andalusite and cordierite. The area described herein forms part of a large high-T -Iow-P terrane
within the Proterozoic orogenic belt in the southwestern United States (Williams and
Karistrom, 1996).
B 3
PRESSURE-TEMPERATURE-time-DEFORMATJON. ............................................. SHAH A.A
3.0. REGIONAL GEOLOGY AND TECTONICS
The Colorado province is believed to have formed from addition of juvenile rocks to the
southern margin of the Archean Wyoming craton (Selverstone et ai., 1997; Sims and Stein,
2003). The rocks exposed in the Big Thomson Canyon region, Colorado are mainly
metasediments and granitoids (Figs. 1 and 2). The mature character of the metasediments
suggests deposition in a forearc setting (Condie and Mallell, 1983) although Reed et ai.
(1987) argued for a back-arc setting at around 1.8-1.7 Ma. Recent detrital zircon ages suggest
a maximum deposition age of 1758+26 Ma for the Big Thompson sequence (Selverstone et
ai., 2000). Multiple igneous intrusions at -1.7, -1.4 and 1.1 Ga (Tweto, 1987) are of
intermediate compositions with a calc-alkaline affinity were emplaced synchronous with
regional deformation during the Colorado orogeny, but a few formed post-tectonically (Reed
et ai., 1987).
The metasediments appear to have been repeatedly deformed, metamorphosed and
intruded by plutons in two major orogenic episodes at -1750-1700 Ma and -1400 Ma (e.g.
Karlstrom, et aI., 1997). The rocks show an increase in metamorphic grade towards the west
and north and three stages of folding and cleavage development. The first
deformation/metamorphism is regarded as having occurred at -1750 Ma and resulted in
large-scale isoclinal folds (F I) and a regional axial cleavage S I. The second and third stage of
folding (F2 and F3) occurred around 1726- Ma ago, when these rocks were intruded by the
Boulder Creek granodiorite and related rocks. One period of metamorphism (Ml) has been
associated with these events during which garnet and staurolite grew. A second metamorphic
event (M2), which was stronger than fil'St, resulted in the formation of up to sillimanite grade
mineral assemblages, although metamorphic conditions were very heterogeneous throughout
these episodes. A number of areas record a transition in metamorphic grade from the chlorite
zone to the onset of migmatization during the Colorado orogeny (Selverstone et ai., 1997).
B 4
PRESSURE-TEMPERATURE-time-DEFORMATION .............................................. SHAH A.A
4.0. QUANTITATIVE MEASUREMENTS OF DEFORMATION USING FIAS
In the past geologists have used one, sometimes two and rarely three thin sections from a
sample to study processes of deformation and metamorphism. The measurement of one FIA
requires 8 spatially oriented thin sections to be cut from a single sample. This approach
reveals much more information than just one or two thin sections per sample (e.g., Sayab,
2005) and if this had been done with these rocks, most of the deformation and metamorphic
history would have been never recognized. Samples in close proximity to each other can
preserve a different portion of the history because deformation partitions so heterogeneously
through an outcrop (e.g. Bell et aI., 1998). In many samples, porphyroblasts are absent in
some slides and if these were the only ones cut then the history they contain would have been
lost. The ease with which 3-D information can be obtained on foliation orientations in
porphyroblasts and the matrix and the access via a microprobe and whole rock XRF to PT
and age information is extraordinary once 8 thin sections are available per sample.
A total of 77 oriented samples were examined, 800 oriented thin sections prepared
and 145 FIA and pseudo-FIA trends determined (Table I, Fig. 2). A minimum of eight
vertically oriented thin sections around the compass from each rock sample allows the
measurement of one FIA trend through locating the switch in inclusion trail asymmetry
(clockwise or anticlockwise) within the porphyroblasts (e.g. Bell et aI., 1998). The relative
timing of a consistent FIA succession for a region can be established from samples
preserving FIAs that vary in trend from the porphyroblast core to rim (e.g. Bell et aI., 1998).
Figure 3a shows a rose diagram of all FIA trends measured within four porphyroblastic
phases. Sixty-four FIAs are preserved in garnet (Fig. 3b) and sixty-one in staurolite
porphyroblasts (Fig. 3c). FIA data from both phases are combined in Fig. 4d. FIAs were also
measured in samples containing cordierite and andalusite porphyroblasts; fOUlieen from the
former and six from the latter (Figs. 3e and 4t). Five samples preserve a different FIA in the
B 5
PRESSURE-TEMPERATURE-lime-DEFORMATION. ............................................. SHAH A.A
core versus the rim in garnet porphyroblasts (Table 1 and Fig. 4a, b, c). A few samples
contain porphyroblasts that overgrew a differentiated crenulation cleavage where the
asymmetry of the crenulated cleavage can be determined (Table 1 and Figs. 4d and e). The
crenulated cleavages consist of qUaJ1z and ilmenite grains, while the differentiated
crenulation cleavages predominantly contain ilmenite grains. The orientation of the axis of
the crenulated cleavage can be determined using the asymmetry of its curvature into the
differentiated crenulation cleavage in the same manner in which a FIA is measured and is
called a pseudo-FIA (P-FIA) because it did not form during porphyroblast growth.
5.0. THE FIA SUCCESSION
Figure 5 shows the location and trend of all the FlAs that have been measured. Garnet is
present in the easternmost samples but staurolite is not. Furthermore, garnet porphyroblasts
are commonly overgrown by staurolite. These features suggest that garnet grew before
staurolite and this characteristic provides one of four criteria that were used to determine the
relative timing of the FIA sllccession. The few samples containing changes in FIA trend from
the core to the rim of garnet porphyroblasts provide a second criterion. A third was provided
by samples containing pseudo (p-FIAs) verslls actual FlAs (e.g. Fig. 4) in garnet and
staurolite porphyroblasts (Table 1). A fourth was provided by those porphyroblasts with trails
truncated by the matrix foliation versus those whose trails were continuous with it (e.g. Figs.
6 and 7). The FIA trends for garnet (Fig. 4b), staurolite (Fig. 4c), and both phases combined
(Fig. 4d) are shown as rose diagrams. Four peaks are apparent. The FIA succession consistent
with all the data ( Shah, Section A) from first formed onwards is FIA 1 (trending NE-SW),
FIA 2 (trending E-W), FIA 3 (trending SE-NW) and FIA 4 (trending NNE-SSW). Figure 5
shows the trends of all F1As and pFlAs across the region grouped according to FIA set and
type and marked according to porphyroblastic phase. FIA sets 1 to 4 are present both in both
garnet and staurolite porphyroblasts. FIA sets 3 and 4 are also present in cordierite and
B 6
PRESSURE-TEMPERA TURE-time-DEFORMATION ............................................. SHAH A.A
andalusite (Fig. 4e and t). This progression accords with the normal prograde
garnet7staurolite7andalusite7cordierite succession of porphyroblastic phases in rocks of
this bulk composition (see below).
5.1. Garnet: Generally, garnet porphyroblasts are euhedral and contain excellent inclusion
trails (Fig. 6a and b). Internal foliations (Si) in these porphyroblasts in most samples are
truncated by the external foliations (Sc) but in some samples they are continuous with the
matrix. FIA sets 1 through 3 in these porphyroblast were always truncated by the matrix
foliations while those preserving set 4 are continuous.
5.2. Staurolite: Staurolite porphyroblasts range in shape from poikilitic euhedral, anhedral to
subhedral and contain excellent inclusion trails that are mainly sigmoidal (Figs. 6c,d and 7).
The relationships between internal (Si) and external foliations (Sc) vary. Most samples
preserving FIAs 1 through 3 contain porphyroblasts where the internal foliations are
truncated by the matrix foliation (Fig. 6c). Some samples contain inclusion trails that are
continuous with the foliations preserved in the matrix with most of these belonging to FIA set
4 (Fig. 7). Some samples preserve evidence of replacement of staurolite by andalusite or and
cordierite (e.g. Fig. 8).
5.3. Andalusite: Andalusite porphyroblast are generally poikiloblastic and extremely large in
some samples (> 2cm to 4 mm). Inclusion trails vary from poorly preserved to excellent (Fig.
6e,t). Shapes of the grains are usually anhedral to subhedral and rarely euhedral. In few
samples andalusite has replaced the micaceous part of the matrix completely and has retained
quartz, opaques, biotite and some graphitic inclusions. Two generations of andalusite growth
are preserved via FIAs with most containing F1A 3. Porphyroblasts containing FIA set 4 have
inclusion trails that look very similar to and are continuous with foliations in the matrix (Fig.
6e). Andalusite porphyroblasts contain sillimanite in a few samples and partially replace
B 7
PRESSURE-TEMPERA TURE-time-DEFORMATION. ............................................. SHAH A.A
staurolite in some others (e.g. Fig. 8a). In some samples andalusite pseudomorphs are well
preserved.
5.4. Cordierite: Cordierite porphyroblasts ranging from euhedral, anhedral to subhedral are
mostly poikiloblastic and contain well-preserved inclusion trails (Fig. 6g,h) that are usually
truncated by the matrix foliation in samples preserving FIA set 3 but continuous in those
preserving FIA set 4 (e.g. Fig. 6g,h). Some cordierite porphyroblasts are corroded and
changed mainly to coarse-grained muscovite. Examples of staurolite being replaced by
cordierite and coarse-grained muscovite are common (e.g. Fig. 9).
6.0. PORPHYROBLAST TIMING
6.1. Via FIAs
Four phases of garnet and staurolite growth can be distinguished using the FIA succession (
Shah, Section A). Staurolite always grows after garnet (Fig. 4). This is shown for samples
(C64 and C78B; Table I), which contain a pFIA (parallel to FIA I) plus FIA 2 within garnet
porphyroblasts and staurolite containing FlA 3. Sample (C65A), contains FIA I in garnet and
FlAs 2 and 3 within staurolite (Table. I). In one sample (C44), FIA 2 is preserved within
garnet and FlAs 3 and 4 within staurolite. One sample (C76) contains FlAs 2 and 3 within
garnet, FlA 3 in andalusite and FIA set 4 in cordierite porphyroblasts. This succession
suggests garnet was followed by staurolite, andalusite and finally cordierite in these rocks.
6.2. Replacement of staurolite by cordierite and andalusite
Both cordierite and andalusite replace staurolite as shown in Figs. 8 and 9. This took place
during FlAs 3 and 4 for andalusite and cordierite. Several samples preserve evidence of
staurolite being replaced by cordierite (Figs. 8c, d and 9 (sample C52)) and coarse-grained
muscovite commonly develops during this process. Staurolite in sample C52, which is locally
partially replaced by cordierite preserves FIA set 2; cordierite that does not replace staurolite
contains FIA set 4. Figure 10 (Sample C98B) shows garnet paltially included within
B 8
PRESSURE-TEMPERATURE-time-DEFORMATJON. ............................................. SHAH A.A
staurolite. The inclusions in garnet are geometrically and texturally distinct from those
preserved within staurolite or cordierite porphyroblasts. FIA measurements were impossible
for garnet in this sample, as very few slides contain this mineral but it is partially overgrown
by staurolite and cordierite porphyroblasts and predates them (e.g. Fig. 10).
6.3. Replacement of andalusite by cordierite
Inclusions of andalusite that extinguish simultaneously under cross polars occur within
cordierite in sample e77. A crenulation cleavage in the cordierite preserves a pFIA parallel to
FIA set 3. The PIA was difficult to determine in the cordierite because of the size and
poikiloblastic nature of the latter phase but the crenulation cleavage was continuous with the
foliation in the matrix. The inclusion trails in the andalusite appear weakly crenulated
suggesting that it formed during FIA set 3; they are composed of staurolite, quartz,
muscovite, minor opaques and ilmenite and are geometrically and texturally distinct from
those found within the matrix and the cordierite porphyroblast.
7.0. TIMING OF METAMORPHIC INDEX MINERALS NOT PRESERVING FIAS
7.1 Garnet inclusions within staurolite, andalusite and cordierite
For a few samples FIAs could not be measured in celtain mineral phases and their relative
timing was achieved microstructurally. Figure 11 a, shows an example of a euhedral garnet
porphyroblast included within a poikiloblastic staurolite. The staurolite grains are optically
continuous and contain FIA set 3 (Table 1). A FIA could not be measured for garnet because
it was only present in a few slides but it is locally overgrown by staurolite. Examples of
garnet porphyroblasts included within andalusite and cordierite were also observed (e.g. Fig.
11 b, c). These relationships indicate the growth of garnet prior to staurolite, andalusite and
cordierite.
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PRESSURE-TEMPERATURE-time-DEFORMATION. ............................................. SHAH A.A
7.2. Staurolite inclusions within anda/usite and cordierite
Some staurolite porphyroblasts were found to be partially and/or completely replaced by
andalusite or cordierite (e.g. Figs. 12 and 13). Traces of staurolite are included in some
poikiloblastic andalusite. The scattered grains of andalusite are optically continuous (e.g. Fig.
12). In sample C68A, I thin section out of 16 contains cordierite that has partially replaced
staurolite; the cordierite grew on the edge of the staurolite porphyroblast where it was
wrapped by a differentiated cleavage. No changes in foliation orientation occur from inside to
outside cordierite. In sample C68B, cordierite occurs in I thin section out of 8 where it has
replaced staurolite. Cordierite overgrows the matrix foliation with no deflection of this
foliation on its boundary. In sample C66, lout of 18 thin sections contains cordierite that
overgrew the matrix foliation with no deflection of this structure at its boundary. In sample
C96B, cordierite has replaced staurolite. FIA measurements were impossible for cordierite in
this sample (e.g. Fig. 13). Sample C92 contains andalusite porphyroblasts in 3 out of 7 thin
sections. In most they overgrew fold hinges in the matrix without any deflection of the
foliation from inside to outside of the porphyroblasts. Sample C55B contains porphyroblasts
of andalusite in 2 out of 18 thin sections. Sericitized masses occur in 5 of these slides that
could have been andalusite or staurolite. These microstructures suggest the above-described
transformation of staurolite into andalusite and cordierite took place very late during the
orogenic history.
7.3. Garnet, anda/usite and staurolite inclusions within cordierite
Inclusions of garnet and andalusite are found within porphyroblasts of cordierite in a few
samples (e.g. C93A Fig. l4a). In C76 (Fig. l4b), staurolite and andalusite inclusions are
preserved within cordierite porphyroblasts. The timing of staurolite versus andalusite could
not be determined. Remnants of inclusions within the andalusite, which are mainly composed
of quartz, muscovite, biotite, minor opaques and some ilmenite grains, are geometrically and
B 10
PRESSURE-TEMPERATURE-time-DEFORMATION. ............................................. SHAH A.A
texturally distinct from those found within both cordierite and the matrix. Staurolite growth
may have been followed by the formation of andalusite and cordierite (see below).
7.4. Sillimanite timing
Sillimanite is always younger than any porphyroblastic phase preserved in these rocks (e.g.
Fig. 15). In sample C77 it grew within andalusite that preserves FIA set 4. The other samples
where sillimanite has been found are those containing rare examples of cordierite (C68A,
C68B, C96B, CliO) or andalusite (CSSB, C67, C66, CIOI, C92; Fig. 2) porphyroblasts.
8.0. DATA FOR THERMOBAROMETRY
8.1. P-T pseudosections
Major element bulk rock analyses for seven samples selected to cover the isograd transition
were measured using the Siemens SRS3000 XRF at the Advanced Analytical Centre (ACC),
of James Cook University. A representative portion of each of these samples was crushed to
obtain SO-100mg of powder for XRF analysis. Table 2 shows the elemental data of these
samples. Pseudosections were then calculated in the MnO-Na20-Cao-K20-FeO-MgO-Ah03-
Si02-H20 (MnNCKFMASH) system using THERMOCALC 3.23 (Holland and Powell,
1985; 1990; 1998; Powell et aI., 1998), the dataset of Holland and Powell (1990) and the
mixing models of Tinkham et al. (2001). MnO was included in the model, since it has long
been recognised as expanding the stability fields for garnet (e.g. Tinkham et aI., 2001; Kim
and Bell, 200S). In all rocks, plagioclase was observed to be present in matrix phases and as
inclusions within other minerals. On the basis of compositional and mineralogical criterion,
MnNCKFMASH system modelling was considered suitable for these rocks (e.g. Fig. 16). In
all calculations, plagioclase and quartz were assumed to be in excess and kyanite was
omitted, as no sample enclosed this mineral. The water activity was assumed to be 1. The
limitations of these procedures have been published in several aIlicles (e.g., Vance and
Mahar, 1998; Tinkham et aI., 2001).
B II
PRESSURE-TEMPERATURE-time-DEFORMATION. ............................................. SHAH A.A
8.2. Garnet Core Isopleths
Compositional zoning patterns in garnet porphyroblasts have been used for decades to
understand and interpret the metamorphic conditions that lead to their growth zoning patterns
(e.g. Tracy, 1982; Spear et aI., 1990; Carlson, 1999). The P-T conditions of garnet core
nucleation can be estimated using a tight intersection in PT space of its XMn (spessaltine), Xc.
(grossular), and XFe (almandine) isopleths (e.g., Vance and Mahar, 1998; Evans, 2004;
Sayab, 2005; Cihan et aI., 2006). X-ray maps for 4 of the above 7 samples (e.g. Figs. 17 and
18), one from each of the 4 FIA sets (Table 3), preserve normal growth zoning (e.g. Hirsch et
aI., 2003). One sample (C83) preserves slight resorption at the outer edge of the garnet rim
(Fig. 17b), shown by its pyrope and spessartine content.
Sample CIl7E (FIA set 1) contains garnet, staurolite, biotite, muscovite, plagioclase and
quartz with minor zircon, chlorite, apatite or monazite. Compositional isopleths for XMn
(0.1260), XFe (0.7555) and Xc. (0.0361) in the garnet core intersect tightly in the chl-bi-g-mu
stability field close to the st-in line (Fig. 19a; the small triangle formed by their intersection
lies well within the area of overlap of the 1 (J errors). It suggests garnet nucleation at 540-
550° C and 3.8-4.0 kbar (Fig. 20).
Sample C84 (FIA set 2) contains garnet, staurolite, biotite, muscovite, plagioclase and quartz
with minor chlorite, zircon, xenotine or monazite. In some thin sections, fine-grained
muscovite pseudomorphs after garnet and staurolite are common. Compositional isopleths for
XMn (0.1150), XFe (0.7458) and Xc. (0.0509) in garnet cores intersect tightly in the chl-bi-g
mu stability field (Fig. 19b; the small triangle formed by their intersection lies well within the
area of overlap of the 1 (J errors). It suggests garnet nucleation at 525-537°C and 3.40-3.65
kbar (Fig. 20).
Sample C82 (FIA set 3) contains staurolite, garnet, biotite, quartz and muscovite and minor
chlorite, ilmenite, zircon, apatite or monazite. Compositional isopleths for XMn (0.1418), XFe
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PRESSURE-TEMPERA TURE-time-DEFORMATION. ............................................. SHAH A.A
(0.7246) and Xc. (0.0458) in the core of a garnet porphyroblast tightly intersect in chl-bi-g
mu stability field (Fig. 19c; the small triangle formed by their intersection lies well within the
area of overlap of the I a errors). It suggests garnet nucleation at 525-535°C and 3.3-3.6kbar
(Fig. 20).
Sample C54C (FIA set 4) contains garnet, cordierite, plagioclase, biotite, quartz, and
muscovite and minor chlorite, ilmenite, zircon, apatite or monazite. Fine-grained muscovite
pseudomorphs after staurolite are common and the latter phase is locally replaced by
cordierite. Compositional isopleths for XMn (0.0905), XFe (0.7918) and Xc. (0.0422) in garnet
cores tightly intersect in the chl-st-bi-g-mu stability field (Fig. 19d; the small triangle formed
by their intersection lies well within the area of overlap of the I a errors). It suggests garnet
nucleation at 537-550° C and 3.3-3.71 kbar (Fig. 20).
8.3. Major elements
For each of the above 4 samples major element compositions were determined using an
average of nine spots with a beam diameter of 31lm at 15kY and 20nA (for garnet rims a
beam diameter of I Ilm was used).
Garnet: The composition and stoichiometry of garnet cores and rims are shown in Tables 3
and 4. These samples were taken from sillimanite free zones. The calculated end-members
and structural formulae of their analyses are also shown. These calculations are based on
cations per 12 oxygen. The oxide totals are close to 100 weight percent with one analysis
value greater than 101 weight percent. The cation sums generally approach the ideal value of
8. The almandine content in their cores is less than in rims. The grossular, pyrope and
spessartine contents are greater in the cores. The overall chemistry of the analysed garnets
suggests a typical prograde zoning (e.g. Hirsch et aI., 2003). All samples preserve prograde
growth zoning signatures. The pyrope and spessartine content at the outer edge of the garnet
rim of sample C83 (Fig. 17b) suggests slight resorption.
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PRESSURE-TEMPERA TURE-time-DEFORMATION. ............................................. SHAH A.A
Plagioclase: Table 5 shows the electron-microprobe derived chemical compositions of
plagioclase with the calculated atomic proportion and end-members. Structural formulae
were calculated on the basis of 8 oxygen atoms. The oxide totals generally are around 100 to
98 weight per cent and the cation sums are always close to the ideal value of 5. Small
deviations from ideal NaSi=CaAI substitution shown by some analyses may depend on
inaccuracy of the analyses and/or reflect the effect of alteration. Plagioclase can be
considered a solid solution of albite and anorthite. The K content is generally very low (0.001
to 0.003 atoms p.f.u.). All samples, have plagioclase as an oligoclase with An content fi'om
An 25 to An 29 (Table 5). Only one sample has plagioclase as andesine (in matrix) with An
content about An 31.220 (Table 5).
Staurolite: Table 6 shows the electron-microprobe derived chemical compositions of
staurolite with the calculated atomic proportions. Structural formulae were calculated on the
basis of 46 oxygen atoms. For the Si-O tetrahedra AIlv was assigned by calculating the
difference between 8 and Si. Si ranges from 7.144 to 7.717 atom p.f.u and the corresponding
increase in AIlv ranges from 0.283 to 0.856 atom p.f.u. The octahedral sites are filled with
AlvI (calculated as the difference between the Aholal and AIlv), Ti and Mg. The octahedral Al
ranges from (17.493 to 18.344 atom p.f.u). The majority of the Fe-O tetrahedra are occupied
by Fe (in average 3.46 atoms p.f.u) followed by Mg. The Mn content usually is low, with the
highest value of 0.09 atoms p.f.u. in one sample. Ca is very low, ranges from 0.001-0.004
atoms p.f.u.
Mica: Tables 7 and 8 show the electron-microprobe derived chemical compositions of
muscovite and biotite from with the calculated atomic proportions. Their structural formula
was calculated on the basis of 22 oxygen atoms. There is no significant variation in Si and
corresponding AJ'v content, calculated as the difference between 8.0 and the number of Si
atoms. All micas are saturated with Ti, because of their coexistence with ilmenite and
B 14
PRESSURE-TEMPERATURE-time-DEFORMATION. .............................................. SHAH A.A
sagenitic rutile. The presence of aluminous phases (garnet, staurolite, andalusite and
sillimanite), has caused Al saturation in these micas. Fe3+ was not calculated although the
presence of ilmenite and graphite would restrict its palticipation to a minimum level (Guidotti
and Sassi, 1998a, 1998b). The end member calculation was performed as described in
Holdaway et al. (1988).
The Alvl content in muscovite grains varies from 3.954 to 4.085 atoms per formula
unit (p.f.u). The octahedral sites are predominantly occupied by Al ions (-95%). The
remainder contain Fe and roughly equal proportion of Ti and Mg components. The Mn
content of all muscovite grains is very low « 0.01 cations p.f.u). Octahedral site occupancy
(ideally 4.00 atoms p.f.u.) varies from 4.259 to 4.328 atoms p.f.u. suggesting minor
divergence from dioctahedral micas in all analyses. The twelve-fold interlayer site is largely
filled by K (varies from 0.873-1.129 atoms p.f.u ) and minor amounts of Na (0.13-0.234
atoms p.f.u). The amount of Ca is negligible in all samples, except sample C75, which
contain 0.002 atoms p.f.u. The total occupancy of the interlayer site ranges from 1.006 to
1.363 atoms p.f.u. The H30Kl and NH4K.l substitutions and vacancies in twelve-fold sites
due to charge balance constraints caused by other substitutions in tetrahedral, octahedral and
twelve-fold sites could explain the deficiencies in twelve-fold sites in muscovite (e.g.
(Guidotti and Sassi, 1998a; Guidotti and Sassi, 1998b).
The octahedral sites in biotite are predominantly filled by Fetot,l (2.94-3.518 atoms
p.f.u.) and Mg (1.879-1.336 atoms p.f.u.). Octahedral Al in biotite (which is defined as the
difference between Altot,l and Allv) is roughly constant with an average of 0.97±0.02 atoms
p.f.u., which suggests operation of AI-Tschermaks substitution (Fe, Mg)SiAIIV.lAllv.l. Biotite
in all samples is saturated in Altotal due to coexistence with aluminous phases (garnet,
staurolite, andalusite, sillimanite). Ti contents in biotite range from 0.071 to 0.097 atoms
p.f.u. with no clear correlation with the contents of Fe and Mg. However, they roughly show
B 15
PRESSURE-TEMPERATURE-!ime-DEFORMATION. ............................................. SHAH A.A
an increasing trend with metamorphic grade. Concentrations of Mn are typically low,
between 0.004 and 0.020 atoms p.f.u. The sum of the octahedral cations analysed is below
the theoretical value of six atoms p.f.u. It is always above 5.70, implying no significant
amount of Fe3+ in the biotites. Supposedly, two atoms p.f.u. are allocated to the twelve-fold
site in biotite. The interlayer site in the analysed biotite is dominated by K (1.677-1.877
atoms p.f.u.) with a smaller amount of Na (0.02-0.086 atoms p.f.u.) and Ca (> 0.05 atoms
p.f.u.).
The impOltant compositional trends in muscovite and biotite can be summarised as follows:
I. Si4+ decreases (hence, AllY increases) systematically with increase in grade.
2. Alv1 increases fairly systematically, except for sample C75, where it decreases.
3. On the average, Ti and :EVl increases with increase in grade.
9.0. INTERPRETATION
The consistent FJA succession ( Shah, Section A) and matching progression in ages ( Shah,
Section C) reveal that none of the porphyroblasts rotated during any of the deformations that
postdated their growth
9.1. The role of deformation partitioning in porphyroblast growth
Multiple phases of growth of these porphyroblasts occurred during each of the different bulk
shortening directions defined by the F1As because:
I. None of the samples preserve all the 4 sets of FlAs. Most of them contain 1 to 2 sets of
F1As and rarely 3 sets within a particular mineral phase. Therefore the growth of these
phases was controlled by the partitioning of deformation across the outcrop and where it
did not partition at a suitably fine scale, no porphyroblast growth occurred.
2. F1As are defined by the foliations that resulted from periods of bulk shortening; if one or
more of the set of 4 F1As measured is missing from a sample then it is most likely that
B 16
PRESSURE-TEMPERA TURE-time-DEFORMATION ............................................. SHAH A.A
foliations did not form for that direction of bulk shortening in that vicinity once the PT
conditions for porphyroblast growth had been achieved
3. Samples preserving inclusion trails of FIA set 1 through 3 are always truncated by matrix
foliations while those preserving FIA set 4 are mostly continuous with the matrix
foliation.
4. C68A and C68B are adjacent samples (Fig. 2) preserving different aspects of the
deformation and metamorphic history. C68B contains FIA sets 1 and 2 in garnet
truncated by the matrix foliation. C68A contains FIA set 4 in staurolite continuous with
the matrix foliation. The early foliation history preserved in C68B was destroyed in the
matrix before the growth of staurolite in C68A.
5. FlAs, which are defined by porphyroblast inclusion trails that are truncated by matrix
foliations in all the following samples except C44, reveal that garnet and staurolite grew
episodically because:-
a. C44 preserves FlA set 1 in the garnet and FIA sets 3 and 4 in staurolite
b. C78B preserves FIA sets 1 and 2 in garnet and FIA set 3 in staurolite
c. C65A preserves FIA set 1 in garnet and FlA sets 2 and 3 in staurolite
d. C64 preserves FIA sets 1 and 2 in garnet and FIA set 3 in staurolite
9.2. Multiple phases of index minerals
The differentiation of mUltiple generations of index mineral phases in these rocks resulting
from the measurement ofFlAs enables the following interpretations:-
Garnet: Multiple phases of garnet growth have long been recognized, as this mineral can
grow due to a variety of reactions as a rock passes through PT space (e.g., Spear, 1995).
These rocks have undergone a minimum offour periods of garnet growth revealed by the FIA
succession and some of these involved at least 2 phases (Shah, Section A). These mUltiple
phases of growth are not due to a reaction pathway involving many different reactions.
B 17
PRESSURE-TEMPERATURE-time-DEFORMATION ............................................. SHAH A.A
Rather, they appeal' to be due to the role of crenulation initiation versus crenulation cleavage
development on porphyroblast growth versus cessation reactions (e.g. Spiess & Bell, 1996;
Bell et aI., 1998, 2003, 2004; Bell & Bruce, 2006, 2007). Figure 3b suggests that the
formation of garnet decreased with time based on the number of samples containing FlAs 1
through 4 across the area possibly due to a reduction in the chemical elements needed for
garnet producing reactions with time (e.g. Fig. 19).
Staurolite: Prior to the quantitative measurement of FlAs, multiple phases of growth of
staurolite could only be distinguished where rare truncational core rim microstructures were
found. A minimum of 4 periods of growth of this mineral occurred as the succession of FlAs
developed. This cannot be explained by a reaction pathway through several different
staurolite producing reactions. Rathel', the cessation and regrowth of this porphyroblastic
phase resulted from the role of crenulation initiation versus crenulation cleavage development
in statting and stopping the reaction (Bell & Bruce, 2006, 2007). Multiple phases of garnet
growth occurred in some samples prior to staurolite growth (Table I). No examples of
staurolite growth followed by garnet growth have been found.
Timing of andalusite growth relative to cordierite: PT pseudosections generally indicate that
andalusite should grow before cordierite. The FIA analysis suggests that this was the case in
these rocks as FIA 3 is more common than FIA 4 in andalusite and FIA 4 is more common in
cordierite than andalusite. Microstructural relationships, such as andalusite preserved within
and replaced by cordierite (e.g. Fig. 14), strongly support this interpretation.
9.3. Mineral sllccession and migrating prograde metamorphic path
The succession of FIA sets preserved within garnet, staurolite, andalusite and cordierite
porphyroblasts from this area, combined with isograd migration (Shah, Section A) from FIA
to FIA indicate a migrating prograde metamorphic path. The metamorphic grade increases
towards the west from the garnet zone defined by the staurolite, andalusite, cordierite and
B 18
PRESSURE-TEMPERA TURE-time-DEFORMATION. ............................................. SHAH A.A
sillimanite isograds (Shah, Section A). The FIA succession indicates that biotite, gamet,
staurolite, andalusite, cordierite, and sillimanite grew sequentially similar to other Low-
Pressure High-Temperature (LPHT) regions. Supporting this, traces of gamet have been
found in staurolite but not the reverse (e.g. Fig. 14), suggesting that growth of garnet always
predated that of staurolite in these pelites (Spear, 1995). Inclusions of garnet have also been
found within andalusite and cordierite phases. Similarly, staurolite can be seen as inclusions
within cordierite or andalusite porphyroblasts (e.g. Fig. 8). In some samples, inclusions of
andalusite are preserved within the porphyroblasts of cordierite (Shah, Section A), which
suggests that andalusite must have grown earlier than cordierite.
9.4. P-T pseudosections and garnet nucleation
P-T Pseudoseclions THERMOCALC software is an extensively used rock thermodynamic modelling tool
designed to tackle mineral equilibria problems by using mathematical calculations involving
solid solutions and by solving simultaneous non-linear equations which eventually allows the
construction of phase diagrams (e.g. Powell et aI., 1998). This tool allows one to build a
pseudosection using the bulk composition and mineralogy of a rock sample. In such a phase
diagram system, the P-T proj ection shows all the stable invariant points and univariant lines
for a given chemical system (e.g. MnNCKFMASH). The boundaries between different
mineral assemblage fields in such a P-T diagram represent the location where the mode of a
single phase goes to zero. However, this rule is violated when univariant reaction lines are
stable, along which the mode of two phases concurrently goes to zero (e.g. Tinkham et aI.,
2001).
Assumptions 1. It is generally assumed that all mineral assemblages considered for construction of a P-T
pseudosection are in equilibrium. This may not be true, as disequilibrium mineral
crystallization can take place.
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PRESSURE-TEMPERATURE-time-DEFORMATJON. ............................................. SHAH A.A
2. The bulk composition of a rock remains unchanged during metamorphism. This is
generally accepted as a fact during the bulk of the modelling processes although it actually is
not true. Several studies have reported a significant loss of materials (mostly Si02) during
metamorphic processes of pelitic rocks (e.g. Ague, 1991; Williams et aI., 2001). Such an
error can partly be rectified by assuming quartz is in excess during THERMOCALC
modelling (e.g. Vance and Mahar, 1998).
A more concerning issue is that the bulk composition of a rock essentially gets modified
when porphyroblasts such as garnet starts to grow. Such minerals take material into their
strllcturallattice, which becomes isolated from the rock reaction system (Vance and Holland,
1993; Marmo et aI., 2002). Therefore, the effective bulk composition of such a reacting
system may not be suitably modelled by using XRF data (the effective bulk composition is
the composition of the volume of rock in which the equilibration of minerals is achieved and
maintained at a given stage in the metamorphic evolution, e.g. Stiiwe, 1997). The outer rim of
a garnet pOl'phyroblast is considered to be in equilibrium with the matrix phases and therefore
the contribution of its core is essentially removed from the whole rock bulk composition (e.g.
Marmo et aI., 2002). However, most of the garnets studied herein were flooded with
inclusions. Therefore, the material isolated within garnet may not be that different from the
bulk composition. This suggests that the effective bulk composition is not significantly
changed by garnet growth (e.g. Vance and Mahar, 1998).
Since it is difficult to accurately estimate the different effective bulk compositions
experienced by a sample, using the whole rock XRF analyses as a first approximation when
building pseudosections is a suitable approach (Stliwe and Powell, 1995).
3. It is assumed that fluid (H20) is present in excess. It is true during prograde
metamorphism, where the bulk of it is produced by various reactions. However, during
B 20
PRESSURE-TEMPERATURE-lime-DEFORMATION. ............................................. SHAH A.A
retrograde metamorphism, care must be taken while constructing and interpreting a P-T
pseudosection (e.g. Guiraud et aI., 2001).
The P-T conditions of nucleation of garnet cores can be estimated using a tight
intersection on these pseudo sections between XMn (spessartine), Xc. (grossular), and XFe
(almandine) isopleths. Garnet nucleated in samples C117B, C83 and C82, which preserve
FIA sets 1, 2 and 3 at 540-550° C and 3.8-4.0 kbar, 525-537°C and 3.40-3.65 kbar and 525-
535°C and 3.3-3.6kbar respectively. It always nucleated within the chl-bt-mu-g stability field
(e.g. Figs. 20) and allowing for error appears to have followed a slightly clockwise path (Fig.
20), which would explain the growth of andalusite before cordierite. Only during FIA set 4,
which formed ~250 million years later, did garnet nucleate within the chl-bi-mu-g-st stability
field (Fig. 19d; 537-550° C and 3.3-3.71 kbar). Garnet nucleation is possibly overstepped in
temperature because the garnet core isopleths for Ca, Fe and Mn always intersect ~20°C
above the garnet-in line obtained from the sample pseudosection.
9.5. Multi-equilibrium thermo barometry and Average PT
Garnet that grew dlll'ing FIAS I, 2 and 3 always contains inclusion trails that are truncated by
the matrix foliation. Only garnet that grew during the development ofFIA 4 contains inclusion
trails that are continuous with the matrix foliation. Garnet is a very refractory mineral and
diffusion of Ca, Fe and Mg within its rim only takes place at temperatures around 620°C
(Tracy, 1982). Re-equilibration with the matrix of garnet that grew during the development of
FIAs I, 2 and 3 is likely to be difficult at temperatures lower than 620°C. Prior to being able to
distinguish periods of garnet growth using FIAs, garnet rim and matrix mineral compositions
(e.g., staurolite, plagioclase, biotite and muscovite) would have been measured in order to
calculate the P-T conditions of the matrix using the average P-T mode in THERMOCALC
(Powell et al. 1998). So this was attempted to see what an approach without a FIA control
would suggest for PT conditions (Tables 4-6). The activities of these phases were calculated
B 21
PRESSURE-TEMPERATURE-time-DEFORMATION. ............................................. SHAH A.A
with AX-software using simple activity-composition models for natural rock-forming minerals
(Holland and Powell, 1990). For each sample, average P-T conditions with their uncertainty
(+1- sigma) values are derived from an independent set of equilibria. The correction coefficients
"cor" values between the pressure and temperature are positive and higher than 0.7, which
suggests that all uncertainty ellipses are moderately flattened and their main axis has a positive
slope (powell and Holland, 1985; 1988). For each sample, the diagnostic parameter called
"sigfit" is lower than the cutoff value. Therefore it can be suggested that a solution for P-T has
been found, which is consistent with the input data within their unceltainties.
All four samples yield matrix P-T conditions in the range of 560-620 and 3-4kbars
which puts the final mineral assemblages in the stability field of sillimanite (Fig. 21). Yet very
few samples preserve late stage sillimanite growth. Where this did occur it postdated the
development of FIA 4 suggesting a late heating event. The average T of the matrix calculated
by this approach was 40 to 60°C hotter than the core for each FIA and essentially did not
overlap (allowing for error) for FIA 3 versus FIAs 1,2 and 4. The pressure dropped for FIA 1,
rose slightly for FIAs 2 and 4 and significantly for FIA 3. This suggests the refi'actory nature
of garnet causes significant problems with the average PT approach and conditions derived
from them should be used very cautiously.
9.6. Presslire-Temperatlire-time-Deformation (PTtD) path
The P and T changed little (a weak ACW path) through the 100 million year
development of FIAs 1 through 3 and slightly through the ~250 million gap before FIA 4
developed (Fig. 22). The lack of mineral growth, monazite nucleation and FIA formation in
between the two cycles suggests that the unit was exhumed to shallower crustal levels, where
none of this could happen 01' that no deformation partitioned through this region during the
intervening period. In the former case these rocks would have been buried again to the
similar crustal conditions to those recorded at the end of the ~ 1700 Ma orogeny. This is
B 22
PRESSURE-TEMPERATURE-time-DEFORMATION ............................................. SHAH A.A
suggested by the presence of FlA set 4 inclusion trails, PT conditions of garnet cores
preserving this FIA set and is also confirmed by the monazite growth during that period. The
post FIA controlled late growth of andalusite, cordierite and sillimanite suggests an increase
in temperature and drop in pressure as ductile deformation ceased, probably due to uplift and
granite emplacement. This was followed by the exhumation of these metasediments to the
present PT conditions.
10.0. DISCUSSION
General significance ofFIA measurement approach for metamorphism
Without measurement of the FlAs and recognition of the succession of periods of
minel'al growth that they disclose, the mUltiple periods of growth of garnet, staurolite,
andalusite and cordierite that occurred in this region would not have been distinguished.
Previous mapping shows the effects of mapping isograds without such controls; the isograds
simply equate with the youngest period of metamorphism which happened to be the only
time that sillimanite formed and occurred some 350 million years after tectonism began.
Being able to distinguish isograds within successive periods of growth of the same mineral
phase has allowed the migration of isograds across the region with time to be defined and its
tectonic implications assessed (e.g. Shah, Section A).
10.1. Pseudosection and Textural agreement
Figure 16 shows MnNCKFMASH P-T pseudosections for stable mineral assemblages
for 7 samples. Darker shades are used for higher variances. In general, garnet should initially
be produced in these rocks either by a reaction between chl-mu or chl-mu-bt. Staurolite was
initially produced by the consumption of chlorite. It has a smaller stability field than that for
garnet in all samples (e.g. Fig. 16). The appearance and disappearance of the mineral
assemblages shown in these pseudosections agrees with their observed geometrical and
B 23
PRESSURE-TEMP ERATURE-time-DEFORMATION. ............................................. SHAH A.A
textural relationships suggesting a prograde progression from chlorite-biotite-garnet
staurolite andalusite to cordierite.
10.2. Garnet temperature overstepping
The P-T conditions indicated by the intersection of compositional isopleths for the
samples analysed containing FIA sets 1 through 4 never lie on the garnet in-line. Their
intersection lies within the garnet stability field but at a higher temperature, even taking into
account the unceltainty value of the garnet isopleths given by THERMOCALC.
Possible explanations for this could be:-
1. The highest Mn content found within garnet porphyroblasts may not be the exact core in 3-D
(its core; e.g. Fig. 17).
2. The presence of ilmenite, garnet resorption or post growth diffusion influenced the Mn
content ofthe core (Vance and Mahar, 1998).
However, the fact that garnet nucleation was overstepped in every case suggests that growth of
this phase was not initiated until well above the temperature conditions of equilibrium (e.g.
Deguet et aI., 2007, Stowell et aI., 2001). Nucleation of any mineral phase requires that P-T and
bulk composition should be appropriate for that phase to grow. Howevel; deformation is also
known to play a vital role in formation of different minerals, particularly porphyroblastic
phases (Bell et aI., 1998; Williams, 1994, Cihan et aI., 2006) through its control on sites for the
access of nutrients needed for nucleation and growth (Spiess & Bell, 1996). Therefore, another
explanation for this phenomenon is that garnet growth did not occur until deformation
partitioned through the rocks and, because they had reached a higher temperature before this
occurred, overstepping occurred.
10.3. Metamorphic and tectonic relationships
The date obtained for a palticular FIA set averages ages for all samples containing
that FIA trend. This is a powerful approach as it allow one to group ages to tightly constrain a
B 24
PRESSURE-TEMP ERATURE-time-DEFORMATION ............................................. SHAH A.A
period of deformation and metamorphism. However, multiple foliations would have
developing over most of all of the extended period of time during that a particular direction
of bulk shOitening (relative plate motion - e.g., Bell & Newman. 2006) occurred. This must
be kept in mind when considering the relationship between the observations made herein and
the tectonism that was taking place. Magmatic events occurred in this region from 1750 to
1700 and at 1400 Ma (Tweto, 1987). Yet FIAs 1,2 and 3 developed over a period of time
that extended from at least 1760 Ma to 1674 Ma and so the emplacement of granite occurred
after porphyroblast growth began at this crustal level and continued afterwards. Previous
assessments of the age of Colorado Orogeny in this region range from 1780 to 1700 Ma (Sim
et aI., 2003).
The NW-SE directed shOitening required to produce the SW-NE trend of FIA set 1
accords well with the present orientation of the Cheyenne Belt which is regarded as a suture
zone. The presence of shear zones with the same trend as of FIA set 1 suggests these might
have been formed during the initial accretion process. The N-S directed shortening indicated
by the W-E trend of FIA set 2 suggests that the W-E trending axial traces of the regional
folds (e.g. Fig. 2) formed at this time or were rotated from SW-NE trends into this
orientation. Growth of garnet and staurolite during this FIA set, with a monazite inclusion
age around 1726 Ma, overlaps the intrusion of 1726± 15 Ma trondhjemites in the area
(Selverstone et aI., 1997) that lie sub-parallel to bedding. They could have been emplaced
during compressional or gravitational collapse phases during FIA set 2 as subsequent
deformation would have rotated them into parallelism with the compositional layering. The
SW-NE directed shOitening required to produce the NW-SE trend of FIA set 3 probably
formed folds with NW-SE trending axial traces of the folds in the region (Fig. 2).
B 25
PRESSURE-TEMPERATURE-time-DEFORMATJON. ............................................. SHAH A.A
10.4. FIA data and the AUSWUS model
Orogenesis ceased for some 250 million years and then recommenced around 1415±16Ma
when FIA set 4 formed (details of monazite ages in sections C). Cordierite and andalusite
porphyroblasts formed before and after this 250 million year period at similar PT conditions
(Figs. 22 and 23). Indeed, Hui (2009) working in the Arkansa River regin of south central
Colorado has dated a succession of FIAs from 1506 to 1366 Ma that includes the FIA 4
described herein. The lack of a significant change in PT conditions suggests that the
deformation effects of compressive relative plate motion were partitioned elsewhere for 250
million years but without a significant change in the far field deviatoric stress that operated.
Such a scenario could possibly have maintained this region at the same depth. This region
was chosen to test the AUSWUS model for the correlation of PreCambrian in the US and
Australia (Karl strom et aI., 1999). This model was previously proposed my M. Brookfield in
1993 on the basis of matching major lineaments. AUSWUS brings together the remarkably
similar geology of Mojavia, California-Nevada; and the Broken Hill block, Australia (Burrett
and BetTy, 2000)
Our data suggest that the deformations are so different in age to those in NE Australia
that the AUSWUS model appeared unlikely. However, deformation in NE Australia occUl"red
throughout this 250 million year gap from 1670 to 1500 Ma with none occurring for some
100 million years before of afterwards (Rubenach et aI., 2008; T. H. Bell pers. comm., 2010).
Therefore, the lack of change in PT conditions in the Colorado front range may actually
provide support for the AUSWUS model than negating it.
ACKNOWLEDGEMENTS
I would like to thank Professor Tim Bell for his critical comments. Special thanks to Dr Mike
for his help with monazite dating and critical discussions. I would also like to thank Professor
B 26
PRESSURE-TEMPERA TURE-time-DEFORMATION. ............................................. SHAH A.A
Jane Selverstone (University of New Mexico, New Mexico) for sending me the manuscripts
of Colorado region.
B 27
PRESSURE-TEMPERATURE-time-DEFORMATJON. ............................................. SHAH A.A
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that fold younger matrix schistosities: their effect on sites of mineral growth.
Tectonophysics, 367, 253-278.
Bell, T. H., Ham, A. P., Kim H. S., 2004. Partitioning of deformation along an orogen and its
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825-845.
Bell, T. H., and M. D. Bruce., 2006. The internal inclusion trails geometries preserved within
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PRESSURE-TEMPERATURE-time-DEFORMATJON. ............................................. SHAH A.A
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macroscopic fold in the Robertson River metamorphics, north Queensland, Australia.
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collisional event: The example of the Thiviers-Payzac Unit in the Variscan French Massif
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Guidotti, C. V., and Sassi, F P., 1998b. Petrogenetic significance of Na-K white mica
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Ham, A. P., Bell, T. H., 2004. Recycling of foliations during folding. Journal of Structural
Geology, 26, 1898-2009.
B 29
PRESSURE-TEMPERATURE-time-DEFORMATION ............................................. SHAH A.A
Hirsch, D. M., Prior, D. J., Carlson, W. D., 2003. An overgrowth model to explain multiple,
dispersed high-Mn regions in the cores of garnet porphyroblasts. American
Mineralogist, 88, 131-14l.
Holdaway, M. J., Dutrow, B. L. and Hinton, R. W. (1988). Devonian and Carboniferous
metamorphism in west-central Maine: the muscovite almandine geobarometer; the
staurolite problem revisited. American Mineralogist,73, 20-47.
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uncet1atinites and correlations: 2. Data and results. Journal of Metamorphic Geology, 3,
343-370.
Holland, T. J. B., Powell, R., 1990. An enlarged and updated internally consistent
thermogynamic dataset with uncertatinites and correlations: the system K20-Na20-CaO
MgO-FeO-Fe203-Ab03-TiOrSi02-C-H2-02. Journal of Metamorphic Geology, 8, 89-
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Holland, T. J. B., Powell, R., 1998. An internally consistent data set for phases of
petrological interest. Journal of Metamorphic Geology, 16, 309-343.
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solutions via non-linear equations, with examples using THERMOCALC. Journal of
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Karlstrom, K. E., Dallmeyer, R. D., Grambling, J. A., 1997. 40Ar/39Ar evidence for 1.4 Ga
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Kim, H., and Bell, T. H., 2005. Combining compositional zoning and foliation intersection
axes (FlAs) in garnet to quantitatively determine early P-T-t paths in multiply deformed
and metamorphosed schists: north central Massachusetts, USA. Contributions to
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Reed, l C., Jr., Bickford, M. E., Premo, W. R., Aleinikoff, IN., and Pallister, J., 1987.
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geochronology. Geology, 15, 861-865.
Rubenach, M. R., Foster, D. R. W., and Evins. P., 2008. Tectonothermal evolution of the
Eastern Fold Belt, Mt Isa Inlier. Precambrian Research, 163, 81-107.
Sayab, M., 2005. Microstructural evidence for N-S shortening in the Mount Isa Inlier (NW
Queensland, Australia): the preservation of early W-E-trending foliations in
porphyroblasts revealed by independent 3D measurement techniques. Journal 0/
Structural Geology, 27,1445-1468.
Selverstone, l, Aleinikoff, J., Hodgins, M., Fanning, C.M., 2000. Mesoproterozoic
reactivation of a Paleoproterozoic transcurrent boundary in the Colorado Front Range:
Implications for ca. 1.7 and 1.4 Ga tectonism. Rocky Mountain Geology, 35, 136-162.
Selverstone, l, Hodgins, M., Shaw, C., Aleinikoff, J. N., & Fanning, C. M., 1997.
Proterozoic tectonics of the northern Colorado Front Range. In Bolyard, D.W., and
Sonnenberg, S. A., eds. Geologic history of the Colorado Front Range. Denver, Rocky
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Sims, Paul K., Stein, Holly J., 2003. Tectonic evolution of the Proterozoic Colorado
province, Southern Rocky Mountains: A summary and appraisal. Rocky Mountain
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Spiess, R., Bell, T.H., 1996. Microstructural controls on sites of metamorphic reaction: a case
study of the inter-relationship between deformation and metamorphism. European
Journal of Mineralogy, 8, 165-186.
Stowell, H.H., Taylor, D.L., Tinkham, D.L., Goldberg, S.A., Ouderkirk, K.A., 2001. Contact
metamorphic P-T - t paths from Sm-Nd garnet ages, phase equilibria modelling and
thermobarometry:garnet ledge, south- eastern Alaska, USA. Journal of Metamorphic
Geology, 19, 645- 660.
Tinkham, D.K., Zuluaga, C.A., Stowell, H.H., 2001. Metapelite phase equilibria modelling in
MnNCKFMASH: The effect of variable AI203 and MgO/(MgO + FeO) on mineral
stability. Geological Material Research, 3, 1--42.
Tracy, R.J, 1982. Compositional zoning and inclusions in metamorphic minerals. Reviews in
Mineralogy, 10, 355-397.
Tweto, o. L., 1987. Rock units of the Precambrian basement in Colorado. U.S. Geological.
Survey Professional Paper, 1321-A, 54.
Vance and Mahar, E., 1998. Pressure-temperature paths from P-T pseudosections and zoned
garnets: potential, limitations and examples from the Zanskar Himalaya, NW India,
Contributions to Mineralogy and Petrology, 114, 101-118.
Williams, M.L., 1994. Sigmoidal inclusion trails, punctuated fabric development, and
interactions between metamorphism and deformation, Journal of Metamorphic
Geology, 121-21
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metamorphism: Proterozoic evolution of the southwestern United States. Geology, 24,
1119-1122.
B 32
Section C
90 MILLION YEARS OF OROGENESIS, 250 MILLION YEARS OF QUIESCENCE AND THEN FURTHER OROGENESIS WITH NO CHANGE IN
PT: SIGNIFICANCE FOR THE ROLE OF DEFORMATION IN PORPHYROBLAST GROWTH
90 million years of orogenesis. 250 million years ". ". "". SHAHA.A
90 million years of orogenesis, 250 million years of quiescence and then further orogenesis with no change in PT: significance for the role of deformation in porphyroblast growth 1
1.0. Abstract 1
2.0. Introduction 2
3.0. Regional geology and tectonics 3
4.0. Methods 4 4.1. FIA data 4
5.0. Dating of FIA sets 6
6.0. Sample description, results and age interpretation 7 6.1. Sample Cl17 7 6.2. Sample C43 7 6.3. Sample C84 8 6.4. Sample Cl08 8 6.5. Sample C83 9 6.6. Sample C75 9 6.7. Sample C65A 9 6.8. Sample CllO 10 6.9. Sample C77 10 6.10. Sample C55A 11 6.11. Sample C51B 11
7.0. Dating of matrix foliations 12 7.1. Sample C75 (FIA 1 and 2 in staurolite) 12 7.2. Sample C84 (FIA 2 in garnet and 3 in staurolite) 12 7.3. Sample C65 (FIA 2 in garnet and 3 in staurolite) 13 7.4. Sample C43 (FIA I in garnet and 2 in staurolite) 13 7.5. Sample C83 (FIA 2 in garnet and 3 in staurolite) 13 7.6. Sample C51B (FIA 3 in cordierite) 14 7.7. Sample C77 (FIA 3 in cordierite) 14 7.8. Sample CJ08 (FIA 1 in garnet and 2 in staurolite) 14 7.9. Sample ClIO (FIA 4 in andalusite) 14
8.0. Compositional mapping of monazite grains 15
9.0. Interpretation and Discussion 15 9.1. Combining the age data within FIA sets 15 9.2. The significance of FIAs for determining monazite ages 16 9.3. Assessing the spread of the age data from matrix relative to thatfor the F1A sllccession 17 9.4. The porphyroblast ages versus matrix ages 17 9.5. Limitation of FIAs approach for monazite dating 18 9.6. Role of deformation and its significance for porphyroblast growth 18 9.7. Deformation and metamorphism 19 9.8. Regional tectonic implications of Colorado and Berthoud Orogenies 22 9.9. Episodic but continuous porphyroblastic growth during -1700 Ma orogeny 23 Acknowledgements 24
10. References 25
i
90 million veal's of orogenesis. 250 million years ...... .... . SHAHA.A
90 MILLION YEARS OF OROGENESIS, 250 MILLION YEARS OF
QUIESCENCE AND THEN FURTHER OROGENESIS WITH NO CHANGE IN PT:
SIGNIFICANCE FOR THE ROLE OF DEFORMATION IN PORPHYROBLAST
GROWTH
1.0. ABSTRACT
In-situ dating of monazite grains preserved as inclusions within foliations defining FlAs
(foliation inflection/intersection axes preserved within pOlphyroblasts) contained within
garnet, staurolite, andalusite and cordierite porphyroblasts provides a chronology of ages
that matches the FIA succession for the Big Thompson region of the northern Colorado
Rocky Mountains. FIA sets I, 2 and 3 trending NE-SW, E-W and SE-NW formed at
1760.5±9.7, 1719.7±6.4 and 1674±1l Ma respectively. For 3 samples where garnet first
grew during just one of each of these FlAs, the intersection of Ca, Mg, and Fe isopleths in
their cores indicate that these rocks never got above 4kbars throughout the Colorado
Orogeny. FUlthermore, they remained around approximately the same depth for ~250
million years to the onset of the younger Belthoud Orogeny at 1415±16 Ma when the
pressure decreased slightly as porphyroblasts formed with inclusion trails preserving FlA
set 4 trending NNE-SSW. No porphyroblast growth OCCUlTed during the intervening ~250
million years of quiescence, even though the PT did not change over this period. This
confirms microstructural evidence gathered over the past 25 years that crenulation
deformation at the scale of a porphyroblast is required for reactions to re-initiate and enable
further growth.
c 1
90 million years of orogenesis. 250 million years ...... ... .. SHAHA.A
2.0. INTRODUCTION
Multiply deformed and metamorphosed rocks commonly have ambiguous
information housed within their matrix, since each new deformation usually erases the
earlier history preserved (e.g. Ham and Bell, 2004). This primarily occurs because the
matrix generally decrenulates and rotates the early-formed foliations in parallelism with the
compositional layering (e.g. Bell, 1986; Davis, 1995; Bell et a!., 2004; Cihan, 2004).
Deformation partitioning strongly affects such kinds of processes from regional to
porphyroblastic scales and makes it difficult to correlate such processes from one region to
another (Bell et a!., 2004; Cihan and Parsons, 2005). The preservation of multiple early
matrix events as inclusion trails within porphyroblasts has greatly increased our
understanding of complex inclusion trail relationships, which otherwise could not be
interpreted 01' were misleading (Ham and Bell, 2004).
Accurate measurement of the foliation inflection/intersection axes preserved within
different porphyroblastic phases (FIAs) have made it possible to decode lengthy and
complex histories of deformation and metamorphism in orogens around the world (e.g. Bell
and Bell 1999; Bell and Bruce, 2007 ). More than ten years of research and data have
already been published using this technique from tectonically complex regions around the
world (e.g. Bell et a!., 1998; Bell & Chen, 2002; Bell et a!., 2003; Cihan, 2004; Bell et a!.,
2005; Kim and Bell, 2005; Sayab, 2005; Bell & Bruce, 2007).
The integration of detailed microstructural studies and FIA data with garnet isopleth
thermobarometry/MnNCKFMASH pseudosection constlUction can provide complete
pressure-temperature-time deformational trajectories of an area (e.g., Vance and Mahar,
1998; Kim and Bell, 2005; Cihan et a!., 2006).
c 2
90 million years oforogenesis. 250 million years ...... .... . SHAHA.A
Detailed P-T-D paths significantly improve our understanding of large-scale orogenic
processes but the absolute timing of these events remains a vital tool for unravelling the
tectonic evolution of the region. Geometrically and texturally controlled dating methods are
critical for constraining the ages of deformed and metamorphosed sediments and their
textures and foliations (e.g., Rasmussen et aI., 2001; Foster et aI., 2000; 2002; Williams and
Jercinovic, 2002; Cihan et aI., 2006). In pelites and psammites monazite is omnipresent at
amphibolite facies and above and is the typical mineral of choice for in situ geochronology
in such rocks (Dahl et aI., 2005; Williams et aI., 2007).
Dating of monazite using high precision electron microprobe U-Th-Pb techniques
(EPMA) was chosen to link different metamorphic and deformational events (e.g., Montel
et aI., 1996; Shaw et aI., 2001; Dahl et aI., 2005; Goncalves, 2005; Cihan et aI., 2006)
because the bulk of the monazite grains analysed were 25 ~m or smaller in size. Dating of
monazite inclusions within different FIA sets (Bell and Welch, 2002) can potentially
provide a robust tool for understanding and unravelling lengthy and complex orogenic
histories (Cihan et aI., 2006; Sayab, 2005). Integration of FIAs with this approach could
provide a strong basis for studying the complex pressure-temperature-time-deformation
(PT -t-D) paths that rocks appear to have followed. This paper reports the results obtained
from adapting this approach to the rocks collected in and around the Big Thompson region
of Colorado (Fig. 1).
3.0. REGIONAL GEOLOGY AND TECTONICS
The rocks exposed in the Big Thomson Canyon region, Colorado, USA, are mainly
metasediments and granitoids (Fig. 2). Condie and Martell (1983) suggested the
metasediments represent mature sediments deposited in a forearc setting. Reed et al. (1987)
c 3
90 million years oforogenesis. 250 million years ...... .... . SHAHA.A
argued that they were possibly deposited in a back-arc setting between two ~ 1.8-1.7 Ma
magmatic arc systems. Recent detrital zircon ages suggest a maximum age of 1758+26 Ma
for deposition of the Big Thompson sequence (Selverstone et ai., 2000). These sediments
were repeatedly deformed, metamorphosed and intruded by various plutons (e.g., Braddock
and Cole, 1979; Selverstone et ai., 1997; Sims and Stein 2003) during the Colorado (~1700
Ma) and Belthoud (~1400 Ma) Orogenies (Tweto, 1987; Nyman et ai., 1994; Karistrom, et
ai., 1997). The rocks show an increase in metamorphic grade towards the west and north
and three stages offolding and cleavage development (Cavosie and Selverstone, 2003).
The first deformation/metamorphism occurred before 1750 Ma and resulted in
large-scale isoclinal folds (F 1) and a regional axial cleavage S 1. The second and third stage
of folding (F2 and F3) occurred around 1750 Ma ago, when these rocks were intruded by
the Boulder Creek granodiorite and related rocks. Only 1 period of metamol]Jhism has been
associated with these events (Ml) during which garnet and staurolite grew. The second
metamorphic event (M2), which was stronger than first, resulted in the formation of up to
sillimanite grade mineral assemblages (Sims and Stein, 2003), though metamorphic
conditions were very heterogeneous throughout these episodes. A number of areas recorded
an entire transition in metamorphic grade from the chlorite zone to the onset of
migmatization during the Colorado orogeny (Braddock and Cole, 1979; Selverstone et ai.,
1997).
4.0. METHODS
4.1. FIA data
A total of 67 oriented samples were examined for the present research. 800 oriented thin
sections were prepared and a total of 138 FIA and pseudo-FIA trends were determined
c 4
90 million years of orogenesis. 250 million years ...... ... .. SHAHA.A
(Table 1, Fig 2). These measurements were achieved by cutting a minimum of eight
vertically oriented thin sections around the compass from each rock sample (Fig. 3) and
then locating the switch in inclusion trail asymmetry (clockwise or anticlockwise) within
the porphyroblasts (e.g. Bell et aI., 1998). Garnet, staurolite, andalusite and cordierite
porphyroblasts preserve earlier foliations as inclusion trails. These foliations are most
commonly straight with curvature at their extremities (e.g. Fig. 4a). Porphyroblast inclusion
trails are commonly truncated (e.g. Fig. 4a) by the matrix foliations but some are not (e.g.
Fig.4b).
A relative timing and thus a FIA succession can be established from samples
preserving a FlA trend that varies from core to the rim of the porphyroblast (e.g. Bell et aI.,
1998). All FIA measurements are plotted on rose diagram and are shown in Fig. Sa. A total
of 64 and S3 FlAs were measured in garnet and staurolite porphyroblasts respectively (Fig.
Sb and c). The combined FIA trend data for garnet and staurolite is shown in Fig. Sd .. The
other porphyroblastic phases in which FIAs were measured were andalusite and cordierite,
with seven measurements in the former and fourteen in the latter. Their trends are given in
Table 1 and are shown on a rose diagram in Fig. Se and f. A few samples maintain
differentiated crenulation cleavages that have been overgrown by the porphyroblasts where
the asymmetry of the crenulated cleavage can be determined. The crenulated cleavages
consist of qUattz and ilmenite grains, while the differentiated crenulation cleavages
predominantly contain ilmenite grains. The intersection between the crenulated and
crenulation cleavages can be determined, when viewed in three dimensions, and is called a
pseudo-FIA (pseudo-FlA). The actual FIA is formed during porphyroblast growth and in
these samples is defined by the curvature of the differentiated crenulation cleavage. All
measured trends were plotted on a rose diagram as shown in Fig. S.
c 5
90 million years of orogenesis. 250 million years ". ". "". SHAHA.A
5.0. DATING OF FIA SETS
To determine the age of the four FIA sets measured in the area (Thesis Section A), 30
samples were selected for monazite dating. Polished thin sections were made and carbon
coated for use in the JEOL JXA-8300 Superprobe. Only 11 samples out of the 30 selected
contained monazite grains large enough for precise age calculations. Detailed pre-dating
maps were produced from each polished thin section to accurately locate monazite grain
and their textural setting. The analytical procedure is outlined in Table 2. The samples were
analysed with a 1-2 micron meter diameter beam at 15kv and 200nA. The collimators were
opened to a maximum (3mm) and the PHA settings were optimized as well. In all these
measurements, 1trz matrix corrections were performed using standard Pb, U, Th, and Y
concentrations in combination with the preset values for other elements (P 33.3, La 14.5,
Ce 26, Pr 2.6, Nd 10.3, Sm 1.5, Gd 1.48, Dy 0.82, Si 0.25, Ca 0.55 wt. % oxides).
Interference cOlTections of Th and Y on Pb Ma and Th on U MP were executed as in Pyle
et al. (2002). An internal standard monazite from Manangotry in Madagascar of 545+/-2Ma
(Paquette et aI., 1984) was analysed three times before and after each analytical session.
Chemical ages were calculated as described in Montel et al. (1996). Geologically
significant age information can be derived by assuming low amounts of common Pb (e.g.,
Pan'ish, 1990; Gaidies et aI., 2008) and slow diffusion rates for Th, U and Pb in monazite
(Cherniak et aI., 2004). The samples were chosen based on FIA set and the grains were
isolated and clustered according to their age, textural setting and whether any chemical
zonation was present (Cihan, et aI., 2006). This would potentially reduce any error and
make the age information reliable (Montel et aI., 1996; Pyle et aI., 2005; Gaidies et aI.,
2008). Dates and elTors were determined by mean age with standard errors at the 95%
c 6
90 million years of orogenesis. 250 million years ...... .... . SHAHA.A
confidence level for a cluster of spots analysis within a single age domain or grain. Ages
were then calculated for all the grain populations analyzed and plotted using software
Isoplot (Ludwig, 1998).
Three samples contained monazites grams big enough to extract valuable age
information in garnet porphyroblasts. Six contained suitable monazite grains in staurolite
porphyroblasts. Two contained suitable monazite grains in andalusite plus cordierite.
6.0. SAMPLE DESCRIPTION, RESULTS AND AGE INTERPRETATION
Unless otherwise stated monazite inclusions lie with the foliation defining the FIA set for
that mineral phase. All rocks contain biotite, muscovite, plagioclase and quartz with
accessory phases ilmenite and apatite. Quartz and apatite and rarely muscovite, biotite,
chlorite inclusions are always present within both garnet and staurolite porphyroblasts.
Monazite is always present within staurolite but not necessarily in garnet phases ..
6.1. Sample C117
Garnet (FIA set 1) and staurolite (FIA set 2) inclusion trails are always truncated by the
matrix foliation. Extra minor phases include zircon and xenotine. Two monazite inclusions
within garnet have given a mean age spread of 17S6±22 Ma (see Appendix C; Fig. 6).
Age interpretation: Garnet in this sample preserves the oldest deformation event recorded
in the area. The date obtained ii-om this porphyroblast agrees with the other samples
containing the same FIA sets.
6.2. Sample C43
Garnet and staurolite pOlphyroblasts contain FIA sets 1 and 2, respectively. Foliations in
the matrix always truncate those in the porphyroblasts. Staurolite in some thin sections is
c 7
90 million years oforogenesis. 250 million years ...... .... . SHAHA.A
replaced by andalusite and fine-grained muscovite. One monazite grain in staurolite was
dated at 1724±19 Ma (see Appendix C). No monazite grains were found in garnet.
Age interpretation: The age obtained in staurolite accords with the dates obtained for other
samples containing the same FIA set.
6.3. Sample C84
Garnet and staurolite inclusion trails are always truncated by the matrix foliation. Extra
accessory phases include magnetite, zircon, xenotine and monazite. A total of 2 monazite
grains were dated from this sample. One within garnet with an age spread of 1762±21 Ma
(FIA set 1) and other grain within staurolite (FIA set 3) with an age spread of 1683±36 Ma
(see Appendix C).
Age interpretation: The 1762±21 Ma monazite age in garnet accords with the dates
obtained from other samples and fits well with its FIA (see below). The younger grain
stored within staurolite is ellipsoidal and aligned parallel to the foliation defined by the
inclusion trails and should give a representative age for FIA 3
6.4. Sample CI08
Garnet and staurolite porphyroblasts contain FIA sets 1 and 2 respectively. The foliations
within both phases are tluncated by those in the matrix. Extra accessory phases include
apatite and zircon. Two monazite grains within staurolite are dated at an average age of
1721±14 Ma (see Appendix C). No monazite grains were found in garnet.
Age interpretation: The age obtained in staurolite accords with the dates obtained for other
samples containing the same FIA set.
c 8
90 million years o(orogenesis. 250 million years ...... .... . SHAHA.A
6.5. Sample C83
Garnet and staurolite contain FIA sets 2 and 3 respectively. Their inclusion trails are
truncated by the matrix foliations. Length fast chlorite with light green to colourless
pleochroism and grey to greyish green birefringence, suggesting that it is rich in Mg,
appears to be prograde (e.g., Albee, 1962). One monazite grain within staurolite has an age
of 1681±27 Ma (see Appendix C; Fig. 7). No monazite was detected within garnet.
Age interpretation: The age obtained within staurolite foliation accords with the dates
acquired ii-om other samples for FIA set 3.
6.6. Sample C75
This contains cordierite as an additional matrix mineral phase and zircon as an extra
accessory phase. Garnet contains FIA set 1 and staurolite contains a pseudo-FIA belonging
to set 1 FIA plus FIA set 2. The foliations within staurolite and garnet are truncated by
those in the matrix. Two monazite grains were dated in staurolite with an age spread of
1765±23 (see Appendix C; Figs. 8 and 9) and 1712±25 Ma (see Appendix C; Fig. 10). No
monazite grains were found in garnet.
Age interpretation: The 1712±25 Ma age obtained from the staurolite monazite gram
accord with the ages procured from other samples for this FIA set. The older monazite
grain (1765±23 Ma) lay Olthogonal to the foliation preserved as inclusion trails and is
consistent with the ages for FIA set 1 acquired from other samples (see Appendix C). It is
interpreted to represent relics of an earlier formed foliation.
6.7. Sample C65A
This contains additional matrix (andalusite) and minor phases (xenotine). Garnet preserves
FIA set 1 and staurolite contains a pseudo-FIA belonging to set 2 plus FIA set 3. The
c 9
90 million veal's of orogenesis. 250 million years ...... .... . SHAHA.A
foliations within these porphyroblasts are truncated by those in the matrix. Andalusite has
locally replaced staurolite. Fine-grained muscovite-rich pseudomorphs after staurolite are
common. Four monazite grains analysed from this sample lie in two different foliations
within staurolite (e.g. Fig. 11). Three monazite grains within the crenulated cleavage
foliation defined by inclusion trails have an average age of 1717.6±9.5Ma. The fourth
monazite grain, aged 1665±24Ma, was detected within the main foliation of staurolite.
Age interpretation: The dates obtained agree with the ages obtained from other samples for
FIA sets 2 and 3 (see Appendix C).
6.8. Sample CHO
Also contains andalusite and cordierite porphyroblasts. Extra accessory phases are
dominated by magnetite, xenotine, zircon and baddeleyite. Andalusite preserves FIA set 4
and its inclusions trails are continuous with the matrix foliation. Inclusions within
andalusite include staurolite and cordierite. A single monazite grain found in andalusite
gave an age of 1432+39 Ma (see Appendix C) for the foliation preserved as inclusion trails.
Age interpretation: The age recorded accords with the dates acquired from other samples
with this FIA trend. This is the youngest FIA set observed.
6.9. Sample C77
This sample contains andalusite and cordierite porphyroblasts, but no garnet and staurolite
porphyroblasts, and the extra accessory phases of magnetite and xenotine. Inclusion trails
in cordierite are continuous with the matrix foliation and preserve FIA set 4. Cordierite
contains a pseudo-FIA belonging to set 3 and FIA set 4. Andalusite contains inclusion trails
defining FIA set 3 that are truncated by foliations within both the matrix and the youngest
foliation in cordierite. Inclusions in both porphyroblastic phases include staurolite and
c 10
90 million years of orogenesis. 250 million years ...... "' .. SHAHA.A
garnet although the latter is rare. Three monazite grains enclosed within cordierite (2) and
andalusite (I) porphyroblasts were dated. One monazite grain within a crenulated cleavage
seam gives a pseudo-FIA set 3 age of 1678±17 Ma within cordierite. The andalusite
porphyroblast preserves the same foliation as FIA 3. The 1760±18 and 1726±18 Ma ages
were derived from their monazites (see below).
Age interpretation: The two dates obtained from monazite grains preserved within pseudo
FIA (set 3) and andalusite FIA (set 3) accord with the dates obtained from other samples
for this event. The two earlier ages acquired from monazite grains within the main foliation
of cordierite are relics of an older foliation that lies oblique to the main foliation defined by
the inclusion trails. Their dates are compatible with the ages obtained from FIA sets 1 and
2, preserved within other samples.
6.10. Sample CSSA
This sample also contains cordierite plus minor xenotine. Garnet preserves inclusion trails
defining FIA set 3 that are truncated by the foliation in cordierite and the matrix. Cordierite
contains FIA set 4 trails that are continuous with those present within the matrix. Staurolite
is also included in cordierite. A single monazite dated at 1666±26 Ma from this sample is
located within garnet. No monazite grains were found in cordierite.
Age interpretation: The 1666±26 Ma age in garnet accords with the dates obtained from
other samples bearing this FIA set.
6.11. Sample CS1B
This sample also contains cordierite porphyroblasts with inclusion trails defining FIA set 4
that are continuous with foliations preserved within the matrix. Staurolite and andalusite are
also present as inclusions. Two grains of monazite dated at an average age of 1412± 17 Ma
c 11
90 million years of orogenesis. 250 million years ...... .... . SHAHA.A
lie within the foliation preserved within the cordierite. Another monazite was dated at
1762±32 Ma, within the same foliation.
Age interpretation: The date acquired from two monazites in cordierite accords with the
dates obtained from other samples for FIA set 4 (see Appendix C). An older age from one
monazite is consistent with an earlier foliation aligned to FIA set 1 and represents its relics.
7.0. DATING OF MATRIX FOLIATIONS
The foliations within porphyroblasts are completely truncated by those within the matrix
phases in all samples except C110. Consequently, monazite ages in the matrix cannot be
used to date FIAs. They were dated to see what relics of the deformation history
determined from the FIA succession were preserved in the matrix and whether there was
any evidence for deformation occurring between the Colorado and Belihoud orogenies.
7.1. Sample C75 (FIA 1 and 2 in staurolite)
Three foliations in the matrix (Sea-Sec) are shown in Fig. 8d. A 1664±38 Ma age, was
derived from a monazite grain lying sub-parallel to Se2. Another monazite grain that lay
orthogonal to this foliation gave an age of 1762±35 Ma (Fig. 12). This sample preserves
FIA sets 1 and 2 within staurolite porphyroblasts.
Interpretation: The older monazite grain (1762±35 Ma) accords with the dates acquired for
FIA set 1 (see Appendix C).). The younger matrix age is coeval with dates acquired for FIA
set 3 suggesting that the matrix was reused or reactivated during the development of this
FIA set.
7.2. Sample C84 (FIA 2 in garnet and 3 in staurolite)
The dominant foliation in the matrix (Sel) contains a single monazite grain that lies sub
parallel to it that has an age of 1729±23 Ma.
c 12
90 million years of orogenesis. 250 million years", ... ..... SHAHA.A
Interpretation: The single matrix age accords with the date for FIA set 2 (see Appendix C).
and is interpreted to represent a relic of an earlier-formed foliation.
7.3. Sample C65 (FIA 2 in garnet and 3 in staurolite)
Two monazite grains in the main matrix foliation (Se.) have ages 1 742±29 Ma and 166S±23
Ma (e.g. Figs. 13 and 14). Both lie sub-parallel to Se.'
Interpretation: The matrix ages accord with dates acquired for FIA sets 1 and 3 respectively
(see Appendix C) and are interpreted to represent relics of earlier-formed foliations
preserved within the strain shadows of porphyroblast.
7.4. Sample C43 (FIA 1 in garnet and 2 in staurolite)
A single monazite grain parallel to the main matrix foliation (Sci) has an age of 1724±37
Ma. This sample preserves FIA sets I and 2 within garnet and staurolite porphyroblasts.
Interpretation: The matrix age obtained (l724±37 Ma) accords with the dates acquired for
FIA set 2 (see Appendix C), which suggests reactivation of the matrix during these events.
7.5. Sample C83 (FIA 2 in garnet and 3 in staurolite)
A single monazite grain parallel to the main matrix foliation (Sci) of this sample has an age
of 1723±34 Ma (see Appendix C). This sample preserves PIA set 2 within garnet and set 3
in staurolite porphyroblasts.
Interpretation: The matrix age accords with the dates acquired for FIA set 2 rather than
those for FIA set 3. This suggests that this grain was not deformed and recrystallized during
the development of PIA 3 prior to staurolite growth. Some crystallographic orientations of
grains relative to a developing strain field can make a patiicular mineral phase very
competent and hard to deform (e.g. Mancktelow, 1981). This grain could reflect such a
phenomenon.
c 13
90 million years of orogenesis. 250 million years ...... .... . SHAHA.A
7.6. Sample C51B (FIA 3 in cordierite)
A single monazite grain lying Olthogonal to the main matrix foliation (Sci) in this sample
has an age of \685±29 Ma (see Appendix C). This sample preserves FIA set 3 within
cordierite porphyroblasts.
Interpretation: This age accords with the date acquired for FIA set 3 suggesting the
monazite grew at this time and was preserved through modification of the matrix by a
subsequent deformation event.
7.7. Sample C77 (FIA 3 in cordierite)
A single monazite grain lying in the youngest matrix foliation (Sc3) in this sample has an
age of \677±\9 Ma (see Appendix C). This sample preserves FIA set 3 within cordierite
porphyroblasts.
Interpretation: The matrix age accords with the date for FIA set 3 suggesting it formed
during the same overall period of bulk shortening.
7.8. Sample CI08 (FIA 1 in garnet and 2 in staurolite)
A single monazite grain sub-parallel to the main matrix foliation (Sci) has an age of
\675±24 Ma (see Appendix C). This sample preserves FIA sets 1 and 2 within garnet and
staurolite porphyroblasts.
Interpretation: The matrix age accords with the dates acquired for FIA set 3, which suggests
subsequent reactivation of the matrix foliation at this time.
7.9. Sample CllO (FIA 4 in andalusite)
A single monazite grain sub-parallel to the main matrix foliation (Sel) has an age of
1438±30 Ma (see Appendix C). This sample preserves FIA set 4 within andalusite
porphyroblasts.
c 14
90 million years oforogenesis. 250 million years ...... .... . SHAHA.A
Interpretation: The matrix age accords with the dates acquired for FIA set 4. The foliations
preserved within the andalusite porphyroblast are very similar to and continuous with the
matrix.
8.0. COMPOSITIONAL MAPPING OF MONAZITE GRAINS
Samples containing monazite were compositionally mapped for Th, Y, U, Pb and Ce using
the JEOL JXA-8300 Superprobe. Most were devoid of any apparent chemical zoning (e.g.
Figs. 9 and 10). One monazite in the matrix of sample C65 showed chemical zoning in both
Th and Y (Fig. 14). Dating of 1742±29 and 1668±48 Ma suggests this was a product of
FIAs 1 and 3 (see below). Sample C75 showed a single example of a monazite with slight
zoning in Thpreserved within a staurolite porphyroblast (Fig. 10). A mean age of 1712±25
Ma was analyzed from this grain.
9.0. INTERPRETATION AND DISCUSSION
9.1. Combining the age data within FIA sets
Monazite grains are common within staurolite, cordierite and andalusite porphyroblasts but
rare in garnet. Of the eleven samples investigated, five contained inclusion trails defining
FIA set 1; six monazite grains were identified and 93 analyses completed defining an age of
1760.5±9.7 Ma (Table 3; Fig. 15). Five samples contained inclusion trails defining FIA set
2; eight monazite grains were identified and 136 analyses completed defining an age of
1719.7±6.4 Ma (Table 3; Fig. 16). Five samples contained inclusion trails defining FIA set
3; five monazite grains were identified and 56 analyses completed defining an age of
I 674± 11 Ma (Table 3; Fig. 17). Three samples contained inclusion trails defining FIA set
4; three monazite grains were identified and 28 analyses completed defining an age of
c 15
90 million years of orogenesis. 250 million years ...... .... . SHAHA.A
1415±16 Ma (Table 3; Fig. 18). These ages, (1760.5±9.7, 1719.7±6.4, 1674±11 and
1415±16 Ma), respectively, confirm the FIA 1,2,3,4 succession established using core/rim
criteria plus the previously recognized (Tweto, 1987; Nyman et aI., 1994; Karlstrom, et aI.,
1997) separation of orogenesis into 2 distinct periods 250 million years apart (Shah, Thesis
Section B).
9.2. The significance ofFIAs for determining monazite ages
Texttu'ally controlled dating of monazite inclusions has recently been used by many
petrologists around the world to date foliation ages (Williams et aI., 1999, Shaw et aI.,
2001, Dahl et aI., 2005). A range of ages will always be present in the matrix due to the
potential for the preservation of monazite grains within the strain shadows of successively
grown porphyroblasts and this is exemplified by Table 3. Depending on the timing of
porphyroblast growth a similar range can be preserved ii-om the influence of younger
events. The most critical phase in using an absolute microstructtu'al dating method is to
acctu'ately identify monazite grains within a particular textural and structtu'al setting. FIA
provide such a setting and offer a robust opportunity to extract in situ information from
individual monazite grains preserved within an independently determined relative
timeframe.
The accord between FIA set and age recorded herein is remarkable (Table 3). Only
one sample (e77) contains "anomalous" older ages which can be attributed to the earlier
events as just mentioned. The recognition of pseudo-FlAs and FIAs provided tight control
over what FIA sets were preserved in each sample. Without this level of control on the
distribution of FlAs, the - 100 million year range in ages obtained (not including the far
younger -1400 Ma ages) would have been attributed to noise. Instead it accords perfectly
with the independently obtained succession ofFIA sets!
c 16
90 million years o(orogenesis. 250 million years." ". "". SHAHA.A
9.3. Assessing the spread of the age data from matrix relative to that for the FIA
succession
Monazite grains are common within the matrix and randomly distributed. They are
generally preserved within muscovite and biotite grains (e.g. Figs 12 and 13). A total of 9
samples out of 30 investigated contained monazite crystals in the matrix. Two samples each
contain a monazite grain, from which a total of 14 analyses were completed defining an age
of 1749±23 Ma (Table 3; Fig. 19). Four samples contained four monazite grains from
which 28 analyses were obtained defining an age of 1726±17 Ma (Table 3; Fig. 20). Six
samples contained six monazite grains from which 59 analyses were completed defining an
age of 1674±11 Ma (Table 3; Fig. 21). One sample contained a single monazite grain from
which 7 analyses were completed defining an age of 1438±29 Ma (Table 3; Fig. 22). These
ages accord to some degree with dates obtained from the porphyroblasts preserving FlA
sets 1, 2, 3 and 4 and clearly reflect those events in spite of the fact that there is no real
control on the significance.
9.4. The porphyroblast ages versus matrix ages
In all the samples that were dated foliations defined by inclusion trails in porphyroblasts are
tlUncated by matrix foliations except in sample ClIO. Therefore, monazite ages in the
matrix have no relevance to the dating of FlAs. In most samples monazite grains in the
matrix foliation gave the same or younger ages than those within the porphyroblasts. Ages
range from 1749±23 to 1674±11 Ma (Table 3) with one sample preserving a monazite with
an approximate FIA 1 age of 1762 Ma and another containing a relic from a foliation
within the matrix that predated porphyroblast growth. The younger ages were always a
product of reuse or reactivation of old foliations (e.g. Bell et aI., 2003) or the development
of new ones. Consequently, only the ages obtained from monazite grains preserved within
c 17
90 million years of orogenesis. 250 million years ". ". "". SHAHA.A
porphyroblasts where a FIA control on the significance of that age were used to time
deformation and metamorphism. However, as mentioned above, it is apparent that most if
not all of the ages associated with the succession of FIA development are preserved within
the matrix. Yet an approach that involves dating monazite grains within the matrix can only
ever provide an average age that does not distinguish when deformation commenced or
when porphyroblast growth ceased.
9.5. Limitation of FIAs approach for monazite dating
The monazite grains within a foliation overgrown by a porphyroblast can have
formed when that schistosity formed or be relics from earlier formed deformation events
(e.g. Sample C5IB). However, if they are small « 20 micron long), ellipsoidal and aligned
parallel to the foliation preserved within the porphyroblasts that defines the FIA, then they
should give the age of that foliation. Since each collapse phase produces a gently dipping
foliation (Bell & Newman, 2006), FlAs are actually controlled by the steeply dipping
foliation forming events. Consequently, the possibility must be kept in mind that if the
foliation containing monazite is gently dipping, the age obtained could be older than the
when the FIA measured from that sample formed.
9.6. Role of deformation and its significance for porphyroblast growth
Nucleation of any mineral phase requires that P-T and bulk composition should be
appropriate for that phase to grow. However, deformation is also known to playa vital role
in formation of different minerals, patiicularly porphyroblastic phases (e.g. Bell et a!.,
1986; Williams, 1994, Cihan et a!., 2006) through its control on sites for the access of
nutrients needed for nucleation and growth (Spiess and Bell, 1996). The FIA controlled
monazite dating described herein reveals -90 million years of continuous
deformation/metamorphism followed by -250 million years of quiescence before
c 18
90 million years of orogenesis. 250 million years ...... .. ". SHAHA.A
orogenesis recommenced for ~ 20 million years with little or no change in PT conditions
(Shah, Thesis Section B). What kept this region at similar crustal levels during the 250
million years of quiescence?
The PT conditions and the bulk composition were clearly suitable for the growth of
porphyroblasts during the ~250 Ma between the development of FlAs 3 and 4. Yet no
porphyroblastic phases grew during this time and there is no microstructural evidence for
any foliations developing. The latter fact is confirmed by dating of monazite grains within
the matrix. They reflect the FIA succession and provide no evidence for any deformation
between at least 1665 and 1438 Ma. It is now well established that deformation and
concurrent metamorphism form cleavage seams by dissolution as well as provide a large
range of components essential for the nucleation and growth of porphyroblasts (Bell &
Cuff, 1989; Spiess & Bell, 1996). Deformation provides sites for nucleation and growth,
and a means of overcoming the energy barrier for nucleation, in the form of the energy
removal from, for example, crenulation hinges (Bell et aI., 1986). The lack ofporphyroblast
growth for this extended ~250 million year period can be attributed to the lack of
crenulation development (Bell et aI., 2003). When deformation recommenced around ~
1415Ma, porphYl'oblasts also began to grow again forming FIA set 4 inclusion trails within
garnet, staurolite, andalusite and cordierite porphyroblasts strongly supporting the role of
crenulation deformation in porphyroblast growth (e.g. Bell et aI., 1986; Cihan et aI., 2006).
9.7. Deformation and metamorphism
The three stages of folding and two stages of metamorphism reported previously from this
region were determined from matrix foliation relationships. A much longer history of
deformation and metamorphism is preserved by the porphyroblasts. The preservation of
four FIA trends with changing directions of shortening from (NE-SW to E-W to SE-NW to
c 19
90 million years oforogenesis. 250 million years ...... .... . SHAHA.A
NNE-SSW), which range in age from 1760.5±9.7 to 1415±16 Ma has considerable
implications for tectonics of this region. The first regional folding episode initiated during
FIA set 1 at 1760.5±9.7 Ma and trended NE-SW and approximately coincides with the
regional trend of the Cheyenne belt. This is regarded as the suture zone along which the
rocks of Colorado and Wyoming province accreted about 1790-1650 Ma ago (Sims and
Stein, 2003). Pressure and temperature at this time was about 540-550° C and 3.8-4.0 kbars,
as proposed by the garnet isopleth geothermobarometry. Intersections of Ca, Mg, and Fe
isopleths in garnet core, which preserved FIA set 1, indicate that these rocks never got
above 4kbars during the Colorado Orogeny (Shah, Thesis Section B).
During the formation ofFIA set 2 (centred around 1719.7±6.4 Ma), the pressure and
temperature changed slightly to 3.40-3.65kbar and 525-537°C. F2 regional folds were
formed during this stage. Foliation evidence from this period or orogeny is rarely preserved
as crenulations in the matrix, due to the effects of rotation and reactivation in the many
subsequent deformations. Trondhjemite dikes, which now have a sill like character, were
emplaced at ~ 1726±15 Ma (Selverstone et ai., 1997). Intrusion froming W-E trending
dikes could occur along the W-E trending vertical foliation that generated this FIA set
during gravitational collapse stages of orogenesis when this vertical foliation would have
created a plane of weakness that failed (e.g., Bell & Newman, 2006). Subsequent
reactivation of the compositional layering would have progressively rotated most of them
into sub-parallelism with the bedding and disguised any crosscutting relics of the dikes
along which they intruded.
Development of the FIA set 3 also during the Colorado Orogeny. The SE-NW trend
of this FIA set was created by NE-SW shortening. Previously formed folds were refolded
and a new F3, generation were created (Fig. 3). The pressure-temperature conditions
c 20
90 million years of orogenesis. 250 million years ...... .... . SHAHA.A
indicated by garnet isopleths conditions for this period were 3.3-3.6kbar and 525-535°C.
More staurolite porphyroblasts also grew at this time. In particular, it marks the first
appearance of andalusite and cordierite porphyroblasts. Some staurolite porphyroblasts
containing FIA set 3 have been partially replaced by andalusite or cordierite suggesting a
decrease in pressure accompanied their development. Absolute dating of monazite grains
enclosed within the inclusions of these four porphyroblastic phases provide an age of
1674±1l Ma for this period of FIA development which appears to end the Colorado
Orogeny. These rocks remained undisturbed for about 250 Ma.
Monazite grains enclosed within the foliations of andalusite and cordierite
porphyroblasts containing FIA set 4 give an age of 1420±14 Ma for this period of
orogenesis. The tight intersection of Ca, Mn and Fe isopleths in garnet cores indicate that
the pressure conditions during this period of orogenesis were similar to those observed at
the end of the Colorado orogeny. Large-scale heating event associated with granite
emplacement and some deformation at this time is regionally known as the Berthoud
Orogeny (Sims and Stein, 2003). FIA set 4 trends NNE-SSW and resulted from NNW -SSE
directed shortening. Garnet, staurolite, cordierite and andalusite also grew during this
period of orogenesis. Slightly higher temperatures were recorded during this period of
orogenesis than previously. Some andalusite and cordierite formed by replacing staurolite
porphyroblasts but most grew as completely new grains. After this period of orogenesis
these rocks were retrogressed, presumably during exhumation. This is revealed by
pseudomorphs after staurolite, garnet, cordierite and andalusite (Shah, Thesis Section B).
c 21
90 million years of orogenesis. 250 million years ...... .... . SHAHA.A
9.8. Regional tectonic implications of Colorado and Berthoud Orogenies
The metasediments exposed in and around the Big Thompson region of Colorado represent
mature sediments deposited in a fore-arc (Condie and Mattell, 1983) or back-arc setting
(Reed et ai., 1987). Detrital zircon ages suggest a maximum age of 1758±26 Ma for
deposition of the Big Thompson sequence (Selverstone et aI., 2000). Previous workers
(e.g., Selverstone et ai., 1997; Chamberlain, 1998; Shaw et ai., 1998; 2001; Williams et ai.,
1999) suggested protracted metamorphism and deformation associated with the ~ 1700 Ma
orogeny and local effects due to the ~ 1400 Ma orogeny.
The results obtained by electron microprobe monazite dating (Shaw et ai. 2001) in the
Homestake shear zone (Fig. 1), revealed that it preserves a pre-1400 Ma deformation
history. They explained that a northeast-striking strongly foliated region was transformed
into a mylonite zone around 1680-1630 Ma. Deformation and concentrated shearing
occurred under high temperature conditions. This orogenic phase was later overprinted by
high temperature conditions associated withthe 1400 Ma episode. A low pressure-high
temperature metamorphic sequence dominated by late staurolite, andalusite, cordierite and
garnet porphyroblastic phases was identified by Shaw et ai. (1999). They argued that these
were formed during the cooling stage (using 40Ar/39Ar geochronology of muscovite and
biotite) of the ~ 1400 Ma orogenic history and thus overprint earlier events allied to
Colorado orogenesis. Stein and Sims (2001) dated PreCambrian-hosted metamorphic
molybdenites from the Colorado mineral belt in the central Front Range. They have
interpreted Re-Os isochron age results of 1430-23 Ma as the time of the peak thermal
episode in the area.
c 22
90 million years of orogenesis. 250 million years ". ". "". SHAHA.A
The evidence presented here for deformation and metamorphism around 1758.8±9Ma
suggests orogenesis commenced around the time of sedimentation. This correlates with the
beginning of contractional deformation along the Cheyenne belt, during which time the
Colorado province was accreted onto the Archean Wyoming province (Chamberlain, 1998).
Orogenesis was essentially continuous for about -100 Ma and then ceased. FIA data
reveals that deformation during 1420±14 Ma Betthoud Orogeny was pervasive, with well
preserved foliations that are continuous with the matrix foliation. However, not a single
monazite grain of Betihoud age was found within garnet or staurolite porphyroblasts that
could be associated with this event. They were only found in andalusite and cordierite
suggesting a slight change in T, P conditions from those forming staurolite was required for
further growth of this phase.
9.9. Episodic but continuous porphyroblastic growth during -1700 Ma Orogeny
The histOlY of relative plate motion in the Alps shows that it changes on average
every 15 million years (e.g., Platt et aI., 1989). Since FIAs directly reflect the direction of
relative plate motion (Bell et aI., 1995; Bell & Newman, 2006), each one will develop over
a period averaging 15 million years but ranging in length from 5 to 30 million years.
Throughout these periods of time, foliations develop alternately in sub-vertical and sub
horizontal orientations. A FIA can be defined by a succession of at least 7 foliations or as
few as just 2 (Bell & Newman, 2007) as the measurement techniqueis independent of the
number of foliations that define it. Consequently, FIAs are correlated from sample to
sample and outcrop to outcrop rather than foliations eliminating the natural heterogeneity in
foliation development resulting from the pattitioning of deformation. Monazite dating
averages the age of some of the foliations from the few samples that are dated that define a
FlA. Clearly, these can have formed over a period of time ranging anywhere from 5 to 30
c 23
90 million years of orogenesis. 250 million years ...... .... . SHAHA.A
million years and all samples containing a FIA would have to be dated to fully define the
range of foliation ages present, which is not a practical approach. Consequently, the age
determined for a FIA will lie anywhere within the range of time that it developed over,
rather than within the middle of that period. This averaging of the ages makes the
deformation involved look more spaced or episodic than what it actually was. Although
plate motion driving orogeny is continuous, its impact on the rocks affected is episodic in
the upper crust because deformation periodically switches from foliations developing sub
vertically to sub-horizontally. This episodic character is increased when changes in
direction of plate motion take place because that upsets for a time the balance between
crustal thickening and collapse. Combined with the spread of possible foliations that define
a FIA, the episodicity appears greater when the ages of successive FlAs are determined
than what it actually is.
Acknowledgements
I would like to thank Prof. Tim Bell for his critical comments. Special thanks to Dr. Mike
Rubenach for help with monazite dating and critical discussions. I would also like to thank
Prof. Jane Selverstone for sending me the manuscripts ofthe Colorado region.
c 24
90 million years of orogenesis. 250 million years ...... .... . SHAHA.A
10. REFERENCES
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90 million years of orogenesis. 250 million years ". ". "". SHAHA.A
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90 million years of orogenesis. 250 million years ...... ... .. SHAHA.A
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c 33
Section D
THE PROBLEM, SIGNIFICANCE, AND METAMORPHIC IMPLICATIONS OF 90 MILLION YEARS OF POLYPHASE STAUROLITE GROWTH
PhD Thesis SHAHA.A.
- SECTIONC-
THE PROBLEM, SIGNIFICANCE, AND METAMORPHIC
IMPLICATIONS OF 60 MILLION YEARS OF POLYPHASE
STAUROLITE GROWTH
1. ABSTRACT ................................................................................................................................................. 4
2. INTRODUCTION ....................................................................................................................................... 4
3. LOCATION AND SETTING ..................................................................................................................... 6
3.1. NORTHERN ApPALACHIANS .................................................................................................................... 6
3.2. COLORADO FRONTAL RANGE ................................................................................................................. 6
4. DATA ............................................................................................................................................................ 8
4.1. FlA ......................................................................................................................................................... 8
4.2. MONAZITE DATA ..................................................................................................................................... 8
4.3. THERMOCALC ......................................................................................................................................... 9
5. INTERPRETATION ................................................................................................................................... 9
6. DISCUSSION AND SIGNIFICANCE ..................................................................................................... 11
ACKNOWLEDGEMENTS ................................................................................................................................ 14
7. REFERENCES .......................................................................................................................................... 15
D2
PhD Thesis SHAHA.A.
1. ABSTRACT
Microstructural measurements of FIAs (foliation intersection/inflection axes preserved in
porphyroblasts) in staurolite reveal at least 3 periods of growth in the Proterozoic Colorado
frontal range and 5 in Palaeozoic Western Maine. Monazite inclusions in staurolite have an
absolute age of 1 765±23 Ma (FIA I), 1718.8±6.9 Ma (FIA 2), and 1674±16 Ma (FIA 3) in
Colorado, and 408±10 Ma (FIA 2), 388±8.8 Ma (FIA 3), 372.1±5.5 Ma (FIA 4), and
352.7±4.2 Ma (FIA 5) in Maine, confirming the multiple periods of deformation and
metamorphism indicated by the FIA succession in each region. Thermodynamic modeling
in the NCMnKFMASH system reveals that this episodic growth of staurolite occurred over
a similar bulk compositional range and PT path. Multiple phases of growth by one reaction
in the same and adjacent rocks in both regions strongly suggest that PT and X are not the
only factors controlling the commencement and cessation of metamorphic reactions. The
FIAs preserved by the staurolite porphyroblasts indicate that each stage of growth in both
areas occurred during deformation and indicate that the local partitioning of deformation at
the scale of a porphyroblast was the controlling factor on whether or not the reaction took
place.
2. INTRODUCTION
The basis of quantitative metamorphic petrology is the inference that the prograde
mineral assemblages within a rock are in chemical equilibrium (Bickle and Baker, 1990).
D4
PhD Thesis SHAHA.A.
This conclusion, though difficult to prove absolutely, seems a logical consequence of the
enormous amounts of geological time available for individual deformation/metamorphism
events. The extraordinary lengths of time available for reactions means that for a cel1ain
bulk composition and PT environment, once local equilibrium is achieved, it ought to go to
completion. Thus bulk composition, PT, fluids and time are generally assumed to be the
only controls on whether or not a reaction will take place. Workers such as Carlson et ai.
(1995), Baxter and DePaolo (2000, 2002 and 2004) and Baxter (2003) have shown that
several problems exist with this approach. However, newly developed approaches to
studying the metamorphic and structural interrelationships have revealed another
significant problem.
It is known that deformation affects fluid and heat flow (Lasaga et ai., 2000) and
can control porphyroblast nucleation. It does this by reducing the nucleation energy barrier
to a reaction (Carlson et ai., 1995; Hirsch, 2007) and providing rapid access and removal of
materials to a reaction site (Bell at ai., 1986). However, existing models for most orogenic
terranes commonly lack detailed time constraints on direct links between deformation and
metamorphism. The measurement of foliation intersection/inflection axes in
porphyroblasts (FlAs) allows detailed access to the timing of growth of the same as well as
different mineral phases. Multiple generations of garnet growth over lengthy time periods
have been recognized and dated via monazite grains preserved within the inclusion trails
that have been used to define a succession ofFIAs (Bell and Welch, 2002).
Garnet is a mineral that can grow from a variety of reactions over a large range of
TP conditions and so the recognition of multiple phases of growth of this phase did not
cause any conceptual problems. However, staurolite is not, since staurolite forming
D5
PhD Thesis SHAHA.A.
reactions typically have a lower vanance than those producing garnet. This study
demonstrates that the same behaviour that occurs for garnet can occur for staurolite in
rocks where there is little or no change in bulk composition. This has considerable
implications for the current assumptions used by most metamorphic petrologists. To
demonstrate the widespread nature of this problem, two differently aged and located fold
belts in the PreCambrian rocks in the Rocky Mountain Foothills in Colorado and the
Palaeozoic rocks of the Appalachians in western Maine are used
3. LOCATION AND SETTING
3.1. Northern Appalachians
The western Central Maine Belt (Fig. 1) was deformed and metamorphosed at
lower amphibolitic facies during the Acadian orogeny. At least three different
metamorphic events affected the region with peak metamorphic conditions occurring
around 400 Ma (Guidotti and Johnson, 2002). The first event produced chlorite, the second
And+St+BtGli, and the third St+Grt+Sill plus pro and retrograde pseudomorphs of earlier
formed porphyroblasts (Guidotti and Johnson, 2002). The Siluro-Devonian metasediments
were intruded syn-tectonically by the 389 to 370 Ma Mooselookmeguntik pluton. A strong
pervasive matrix foliation striking NE is crosscut by an axial plane crenulation cleavage.
3.2. Colorado Frontal Range
The Colorado front (Fig. 2) contains Early Proterozoic schists and gneisses that
record multiple episodes of deformation and metamorphism that varied on a regional scale
D6
PhD Thesis SHAHA.A.
during two main periods of orogeny around 1700 and 1400 Ma with associated igneous
intrusions (Selverstone et aI., 1997). In the northern Front Range most of the rocks reached
the amphibolite facies and extensive migmatization occurred at higher grades. Peak
metamorphic conditions produced Glt+St+And±Sill and have been broadly constrained
between 1.75-1.70 Ga (Nesse, 1984). A second metamorphic event was associated with
regional heating throughout the Front Range and locally produced new growth of garnet,
andalusite, and staurolite (Shaw et aI., 1999). The metasedimentary sequence was affected
by two main plutonic events, one around 1.7 Ga and the second one around 1.4 Ga
(Cavosie and Selverstone, 2003). The rocks show an increase in metamorphic grade
towards the west and nOlth and three stages of folding and cleavage development. The first
deformation/metamorphism is regarded as having occurred before 1750 Ma and resulted in
large-scale isoclinal folds (Fl) and a regional axial cleavage S 1. The second and third stage
of folding (F2 and F3) occurred around 1750 Ma ago, when these rocks were intruded by
the Boulder Creek granodiorite and related rocks. Only 1 period of metamorphism has
been associated with these events (Ml) during which garnet and staurolite grew. The
second metamorphic event (M2), which was stronger than first, resulted in the formation of
up to sillimanite grade mineral assemblages, though metamorphic conditions were very
heterogeneous throughout these episodes. A number of areas recorded an entire transition
in metamorphic grade from the chlorite zone to the onset of migmatization during the
Colorado orogeny (Selverstone et aI., 1997).
D7
PhD Thesis SHAHA.A.
4. DATA
4.1. FIA
Garnet and staurolite FIAs were measured by the asymmetry method described in
Hayward (1991) and Bell et al. (1995, 1998). A relative succession through time was
obtained using both garnet and staurolite core to rim changes in FIA trend that were
consistent for each area. The FIAs and monazite ages in staurolite porphyroblasts from the
Western Maine (Table I) were measured by Sanislav (2009) as part of his PhD thesis at
James Cook University. Table 2 shows all the samples in which FIAs have been measured
from the Colorado Front Range. A detailed description is provided in Section A and B of
this thesis (e.g. Fig. 3). The trends for each staurolite FIA in the succession are shown on
the rose diagram in Fig. 4.
4.2. Monazite data
A minimum of two samples containing one of each of the FIAs in staurolite from
the succession was used to measure the U, Th, Pb and Y contents of monazite grains. The
ages were determined using a JEOL JXA8200 microprobe housed at the Advanced
Analytical Centre, James Cook University. Table 3 show the trace element analyses of the
monazite grains included in staurolite porphyroblasts from Central Maine used to calculate
the ages. Section C of the thesis provides a detailed description of the monazite dating of
the staurolite porphyroblasts from the Colorado Front Range (Appendix-D). Figure 5a-f
and Fig. 6a-f show the probability curve and calculated age, for West Central Maine and
Colorado Front Range respectively, as weighted averages for each FIA set using ISOPLOT
D8
PhD Thesis SHAHA.A.
3 (Ludwic, 2003).
4.3. Thermocalc
A pseudosection was modeled in the NCMnKFMASH (Figs. 7 and 8) system for
each area by using the computer software Thermocalc (Holland and Powel, 1998) and the
average composition of the pelites from both regions determined from XRF analyses. The
bulk chemishy was determined by XRF at the Advance Analytical Centre, James Cook
University for samples from West Central Maine (Table 4) and Colorado Front Range
(Table 5).
S. INTERPRETATION
The rocks from each area have gone through a similar reaction history, as shown by
the pseudo sections, in reaching the regional PT conditions. In rocks from the Colorado
Front Range staurolite stability extends to lower pressures, about 0.3 kbars less than in
Western Maine, while the St+And stability field is slightly larger. Garnet and staurolite in
both areas appear to be in textural and chemical equilibrium. They have sharp intimate
contacts with one another and when garnet is included within staurolite it has euhedral
boundaries with no signs of retrogression prior to being included. In some samples,
cordierite and andalusite have locally replaced and/or partially pseudomorphosed staurolite.
In those few samples where sillimanite is present, staurolite porphyroblasts are replaced by
coarse muscovite and biotite. This suggests that staurolite mainly grew at the expense of
chlorite and that little biotite was consumed during the reaction. Thermodynamic modeling
D9
PhD Thesis SHAHA.A.
in the NCMnKFMASH system confirms this, with the reactions being:
Ms+Chl+Qtz = St+Bt+H20 (1),
if Grt was not consumed during St growth, or:
Ms+Chl+Grt+Qtz = St+Bt+H20 (2)
if Grt was pattially consumed during St growth. Synchronous growth of Grt and St can be
explained by the following reaction:
Chl+Ms = Mg-St+Mg-richer Bt+Mg-richer Chl+Grt+Na-richer Ms+H20 (3),
proposed by Guidotti (1974) which describes the petrological observations.
At 3-4kb and -520°C these reactions imply the coexistence of the metastable
mineral assemblage Grt+St+Bt+Chl (Figs. 7 and 8). In many satnples staurolite
porphyroblasts preserve chlorite as inclusions suggesting that this reaction did not go
entirely to completion.
Monazite dating shows that the succession of FIA sets accords with a progression
in ages from 1765 to 1674 Ma in the Rockies and 410 to 350 Ma in the Appalachians. The
consistency of this progression with the microstructurally determined FIA succession
confirms the value of FlA successions for accessing the combined structural and
metamorphic history of an orogen (e.g., Aerden, 1994; Bell et ai., 1998; Sayab, 2005; Yeh,
2007). Thus, 5 periods of staurolite growth occurred in the Palaeozoic rocks of Western
Maine and 3 in the Archaean rocks of Colorado.
It appears that many rocks in both regions did not grow staurolite during the first or
even second period of growth of this phase. Furthermore, some grew staurolite at one time
and then grew it again up to 60 million years later. This does not accord with current
concepts of reaction progress whereby once the P-T conditions for mineral growth occur
DlO
PhD Thesis SHAHA.A.
for a particular bulk composition, the reaction goes to completion. The only variables
outside of P-T -x that can explain this are the timing of growth relative to deformation and
the role of deformation in nucleation and growth itself. Bell et al. (2003) showed that
porphyroblasts commonly nucleate and grow into sites of progressive coaxial shortening
surrounded by anastomosing zones of predominantly non-coaxial shortening. When the
growing porphyro blasts reach the surrounding shear zones their growth ceases. It does not
recommence until a new deformation occurs at high angle to the earlier one and
repartitions around the porphyroblast margins, allowing new growth to occur in the strain
shadow (e.g., Spiess and Bell, 1996). This suggests that staurolite growth did not occur
unless the succession of deformation events that formed a specific FIA had taken place.
That is, partitioning of deformation through an outcrop at the scale of a porphyroblast is a
controlling factor on determining sites of metamorphic reaction and whether it occurs (Bell
et aI., 2004).
6. DISCUSSION AND SIGNIFICANCE
This is the first quantitative demonstration that staurolite can grow episodically by
the one reaction in rocks of similar bulk composition. Interpretations of metamorphic
reactions are commonly based on laboratory experiments, which suggest that reaction rates
at high temperature are very fast (Walther and Wood, 1984) and that local equilibrium is
achieved (Bickle et aI., 1997). However, field measurements ofreaction rates indicate that
the speeds of reactions in metamorphic rocks are orders of magnitude lower (Baxter and
DePaolo, 2000; 2002). They also suggest that disequilibrium is a common phenomenon
during mineral growth even for reactions that went to completion (Carlson, 2002). These
Dll
PhD Thesis SHAHA.A.
results require the existence of a factor that controls porphyroblast nucleation and growth
that is distinct from bulk chemistry, PT, fluids and time. A number of studies have pointed
to the role of deformation in determining sites of metamorphic reaction and whether or not
a patticular reaction will take place (e.g., Spiess & Bell, 1996). Bell (1981) showed that
deformation is partitioned into zones of lower strain, dominated by coaxial deformation,
surrounded by anastomosing, higher strain zones, where the non-coaxial component is
dominant. Porphyroblasts preferentially nucleate in zones of lower strain (e.g., crenulation
hinges) prior to crenulation cleavage development (Bell et aI., 2003). A similar conclusion
was reached by Baxter and DePaolo (2004), who pointed out that certain reactions may
occur within discreet portions of the rock, while others react little or not at all. These
studies reveal that the reactions involved in the formation of metamorphic minerals are
episodic during prograde metamorphism, starting or stopping, as a function of deformation
pattitioning and strain localization (e.g., Etheridge and Hobbs, 1974; Wintsch, 1985; Bell
and Hayward, 1991; Dempster and Tanner, 1997;Whitmeyer and Wintsch, 2005). It has
been proposed that chemical and textural equilibrium during metamorphism can occur
locally (e.g., Bickle and Baker, 1990; Cartwright and Valley, 1991), while the overall rock
matrix was not in equilibrium (Kretz, 1973). This would allow reactions to go to
completion locally, but overall, the potential for new reactions to take place could be
preserved until the deformation repartitioned and new reactive sites were created.
Until recently porphyroblast matrix relationships were mainly used to define the
timing of porphyroblast growth relative to the matrix fabric (e.g., Zwart, 1962; Passchier
and Coelho, 2005). Such an approach is simplistic and ignores the fact that the matrix can
be re-used and reactivated many times during orogenesis (Bell et aI., 2003; Worley and
D12
PhD Thesis SHAHA.A.
Wilson, 1996). It also only allows one or two generations of growth of the same
porphyroblastic phase to be recognized. The FIA method provides a quantitative tool to
distinguish and con-elate multiple generations of porphyroblasts over large distances (Bell
and Mares, 1999). Combined with in situ dating of the inclusion trails that define a FIA set,
this provides detailed insights into tectono-metamorphic histories that were previously
inaccessible. Similar monazite ages to those recorded in Maine have been determined by
Pyle et aI., (2005) in the south western New Hampshire region of the NOlthern
Appalachians. Their ages (400±10; 381±8; 372±6; 352±14), combined with our data from
the Western Maine, confirm the -60 Ma of continuous tectono-metamorphism for this
portion of the Appalachians. Peak metamorphic conditions have commonly been regarded
as being contemporaneous along an orogen and late polymetamorphism was interpreted to
result from local pluton driven thermal activity (e.g., De Yoreo et aI., 1989). Orogeny is
not the result of a single event that culminates with peak metamorphic conditions, but
rather, a series of continuous but episodic events driven by tectonic plate motion (e.g., Bell
and Newman, 2006).
Available age data from metamorphic terranes is commonly derived from accessory
minerals that are present in the matrix. Such ages provide limited information and are
difficult to precisely integrate with the tectono-metamorphic history because of
reactivation of compositional layering plus local protection of grains in the strain shadows
of large competent minerals. More recent studies using other approaches (Bell and Welch,
2002; Pyle et aI., 2005) have revealed that accessory minerals such as monazite can grow
episodically during metamorphism. They showed that distinct populations yielding
significantly different ages can be separated, and this work confirms their approach. The
D13
PhD Thesis SHAHA.A.
episodic growth of staurolite porphyroblasts revealed by this study is significant for
metamorphism and reveals a significant role for deformation that needs to be taken into
account.
Acknowledgements
1 would like to thank Prof. Tim Bell for his critical comments. Special thanks to Dr. Mike
Rubenach and loan Sanislav for their help with monazite dating and critical discussions. 1
would also like to thank Prof. Jane Selverstone for sending me the manuscripts of the
Colorado region.
D14
PhD Thesis SHAHA.A.
7. REFERENCES
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Baxter, E. F., and DePaolo, D. J., 2000. Field measurement of slow metamorphic reaction
rates at temperatures of 500·600°C. Science, 288, 1411· 1414.
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orogenic arcs. American Mineralogist, 84,1727-1740.
Bell, T. H., and Newman, R., 2006. Appalachian orogenesis: the role of repeated
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controlled monazite dating of foliations in porphyroblasts and matrix. American
Journal of Science, 302,549-581.
Bell, T. H., Fleming, P. D., and Rubenach, M. J., 1986. Porphyroblast nucleation, growth
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synforms that fold younger matrix schistosities: their effect on sites of mineral
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Bell, T. H., Ham, A., and Kim, H. S., 2004, Partitioning of deformation along an orogen
and its effects on porphyroblast growth during orogenesis. Journal of Structural
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Bell, T. H., Hickey, K. A., and Upton, G. J. G., 1998. Distinguishing and correlating
multiple phases of metamorphism across a multiply deformed region using the axes
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Metamorphic Geology, 16, 767-794.
Bickle, M. J., and Baker, J., 1990. Advective-diffusive transpOlt of isotopic fronts: An
example from Naxos, Greece. Earth and Planetary Science Letters, 97, 78-93.
Bickle, M. J., Chapman, H. J., Feny, J. M., Rumble, D., and Fallick, A. E., 1997. Fluid
flow and diffusion in the Waterville Limestone, south-central Maine: Constraints
from strontium, oxygen and carbon isotopic profiles. Journal of Petrology, 38,
1489-1512.
Carlson, W. D., 2002. Scales of disequilibrium and rates of equilibration during
metamorphism. American Mineralogist, 87,185-204.
Carlson, W. D., Denison, S., and Ketcham, R. A., 1995. Controls on the nucleation and
growth of porphyroblasts: Kinetics from natural textures and numerical models.
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Cartwright, I., and Valley, J. W., 1991. Steep oxygen-isotope gradients at marble
metagranite contacts in the northwest Adirondack Mountains, New York, USA:
products of fluid-hosted diffusion. Earth and Planetary Science Letters, 107, 148-
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Cavosie, A., and Selverstone., J., 2003. Early Proterozoic oceanic crust in the northern
Colorado Front Range: Implications for crustal growth and initiation of basement
faults. Tectonics, 22, 10-1-10-23.
De Yoreo, J. J., Lux, D. R., Guidotti, C. V., Decker, E. R., and Osberg, P. H., 1989. The
Acadian thermal history of western Maine. Journal of Metamorphic Geology, 7,
169-190.
Dempster, T. J., and Tanner, P. W. G., 1997. The biotite isograd, Central Pyrenees: a
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deformation-controlled reaction. Journal of Metamorphic Geology, 15, 531-548.
Etheridge, M. A., and Hobbs, B. E., 1974. Chemical and deformational controls on
recrystallization of mica. Contributions to Mineralogy and Petrology, 43,111-124.
Guidotti, C. V., and Johnson, S. E., 2002. Pseudomorphs and associated microstructures of
western Maine, USA. Journal of Structural Geology, 24, 1l39-1156.
Hayward, N., 1991. Orogenic processes, deformation and mineralization history of
portions of the Appalachian orogen, USA, based on microstructural analysis.
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Hirsch, D. M., 2007. Controls on porphyroblast size along a regional metamorphic field
gradient. Contributions to Mineralogy and Petrology, 155,401-415.
Holland, T. J. B., and Powell, R, 1998. An internally consistent thermodynamic data set
for phases of petrological interest. Journal of Metamorphic Geology, 16, 309-343.
Kretz, R, 1973. Kinetics of the crystallisation of garnet at two localities near Yellowknife.
The Canadian Mineralogist, 12, 1-20.
Lasaga, A. C., Luttge, A., Rye, D. M., and Bolton, E. W., 2000. Dynamic treatment of
invariant and univariant reactions in metamorphic systems. American Journal of
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Ludwic, K. R, 2003. Isoplot 3.0 - A geochronological toolkit for Microsoft Excel.
Berkeley Geochronology Center Special Publication No.4
Nesse, W. D., 1984. Metamorphic petrology of the nOliheast Front Range, Colorado: The
Pingree area. Geological Society of America Bulletin, 95,1158-1167.
Passchier, C.W., Coelho, S., 2006. An outline of shear sense analysis in high-grade rocks.
Gondwana Research, 10, 66-76.
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Pyle, J. M., Spear, F. S., Cheney, J. T., and Layne, G., 2005. Monazite ages in the
Chesham Pond Nappe, SW New Hampshire, U.S.A: implications for assembly of
central New England thrust sheets. American Mineralogist, 90, 592-606.
Sayab, M., 2005. Microstructural evidence for N-S shortening in the Mount Isa Inlier (NW
Queensland, Australia): the preservation of early W-E-trending foliations in
porphyroblasts revealed by independent 3D measurement techniques. Journal of
Structural Geology, 27, 1445-1468.
Selverstone, J., Hodgins, M., Shaw, C., Aleinikoff, and J. N., Fanning, C. M., 1997.
Proterozoic tectonics of the northern Colorado Front Range. In Geologic history of
the Colorado Front Range (Bolyard, D.W., and Sonnenberg, S. A, eds.). Denver,
Rocky Mountain Association of Geologists, 9-18.
Shaw, C. A, Snee, L. W., Selverstone, J., and Reed, J. C., Jr., 1999. 40Arr Ill'
thermo chronology of Mesoproterozoic metamorphism in the Colorado Front Range.
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Spiess, R., and Bell, T. H., 1996. Microstructural controls on sites of metamorphic reaction:
a case study of the inter-relationship between deformation and metamorphism.
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Whitmeyer, S. J., and Wintsch, R. P., 2005. Reaction localization and softening of
texturally hardened mylonites in a reactivated fault zone, central Argentina. Journal
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0/ Metamorphic Geology, 23, 411-424.
Wintsch, R. P., 1985. The possible effects of deformation on chemical processes in
metamorphic fault zones. Advances in Physical GeochemistlY, 4, 251-268.
Worley, B. A., and Wilson, C. J. L., 1996. Deformation partitioning and foliation
reactivation during transpressional orogenesis, an example from the Central
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020
CONCLUSIONS Shah A.A.
CONCLUSIONS
This contribution has demonstrated that by adapting a mUltidisciplinary
approach to deformation and metamorphism via combining microstructural
measurements, geochronology, thermo barometry and phase diagram modeling,
complex tectono-metamorphic histories can be precisely revealed and interpreted. It
provides an example of such an integrated approach to investigate in detail the
metamorphic and deformation processes and the progression in the distribution of
various index mineral phases (garnet, staurolite, andalusite and cordierite) within the
Big Thompson Canyon region, Colorado Rocky Mountains, USA, which forms part
of a Proterozoic orogenic belt in the southwestern United States and provides a
classic example of a large high-T-low-P terrane and the problems associated with the
tectonic interpretation of such a regime (e.g., Williams and Karlstrom, 1996). These
rocks have been affected by strongly partitioned deformation and provide an ideal
place for developing new concepts on the role of deformation partitioning in
deformation, metamorphism and tectonism. The principal conclusions of each section
are summarized below:
SECTION A
A progression of FlAs (foliation intersection/inflection axes preserved within
porphyroblasts) in the foothills of the Colorado Rocky Mountains reveals four periods
of staurolite growth and two growth phases each of cordierite and andalusite; these
E 2
CONCLUSIONS Shah A.A.
FIA based isograds migrated -2.5 kms across the orogen. This progression of mineral
development occurred about FlAs trending successively NE-SW, E-W, SE-NWand
NNE-SSW. Granitoids were emplaced during both periods of orogenesis in the
surrounding region but have no direct relationship to the isograds. Isograd migration
took place away from a heat source to the WNW and combined with the lack of
relationship to pluton boundaries to the nOlth and south suggests that the latter rocks
were not the heat source for metamorphism but rather product of it. A final period of
andalusite, cordierite and fibrolitic sillimanite grew over the matrix foliation and
consequently, no FIA was determined for it; the isograds for this last period of
mineral growth lie sub-parallel to those mapped by previous workers.
A strong correlation between the distribution of FIA trends and the axial trace
of all folds present in the area suggests that pockets of low strain occur due to
deformation partitioning, in spite of, or perhaps because of, the 4 successive changes
in bulk shOltening direction. These pockets preserve folds in the orientation in which
they formed from subsequent rotation or destruction. They provide remarkable
confirmation of the veracity of the FIA data and indeed reveal that such data can be
used to determine successions of fold development from regional maps at which scale
many overprinting criteria camtot be applied.
SECTIONB
FlAs (foliation intersection/inflection axes preserved within porphyroblasts)
in garnet, staurolite, andalusite and cordierite from Precambrian rocks in the northern
Colorado Rocky Mountains reveal 4 periods of growth about axes trending NE-SW,
E 3
CONCLUSIONS Shah A.A.
E-W, SE-NW and NNE-SSW during an overall prograde path. The growth of garnet
was always followed by staurolite for each of the 4 FIA sets with andalusite and
cordierite following staurolite during development of the last 2 sets. Thermodynamic
modelling in the MnNCKFMASH system reveals that well documented episodic
growth occurred over a similar bulk compositional range and PT path for each FIA in
the succession. The intersection of Ca, Mn, and Fe isopleths in garnet cores
containing FIA set 1 indicate that growth of this mineral phase began at 540-550°C
and 3.8-4.0 kbars. Similarly, the intersection of Ca, Mn, and Fe isopleths in garnet
cores containing FIA sets 2 and set 3 indicate that these rocks never got above 4kbars
throughout the Colorado Orogeny. Indeed, they remained at approximately the same
depth with the onset of the much younger Berthoud orogeny, when the pressure
decreased slightly as porphyroblasts formed with inclusion trails preserving FlA set 4.
A slightly clockwise P-T path occurred for both orogenies.
SECTIONC
In-situ dating of monazite grains preserved as inclusions within foliations
defining FlAs contained within garnet, staurolite, andalusite and cordierite
porphyroblasts (foliation inflection/intersection axes preserved within porphyroblasts)
has provided a chronology of ages for extended periods of deformation and
metamorphism in the Big Thompson region of the Colorado Rockies in the USA. The
first period of tectonism, revealed by FIA set 1, trends NE-SE, and occurred around
1758.8±9 Ma. Garnet nucleated at 540-550°C and 3.8-4.0 kbars. The intersection of
Ca, Mg, and Fe isopleths in garnet cores for 3 samples, containing FIA set 1, set 2
E 4
CONCLUSIONS Shah A.A.
(l758.8±9, 1720.5±6.1 Ma) and set 3 (l674±7.6 Ma), trending NE-SW, E-W and SE
NW respectively, indicate that these rocks never got above 4kbars throughout the
Colorado Orogeny. They remained around the same depth until the onset of younger
orogeny at 1420±14 Ma when the pressure decreased slightly as porphyroblasts
formed with inclusion trails preserving FIA set 4 and trending NNE-SSW.
SECTIOND
The lengthy time of tectono-metamorphism experienced by Paleozoic Western
Maine and by the Proterozoic Colorado Front Range resulted in multiple periods of
staurolite growth during subsequent deformation events. Five FIA sets were measured
from staurolite porphyroblasts in Western Maine and three from the Colorado Front
Range. Monazite inclusions in staurolite have an absolute age of l760±12 Ma (FIA 1),
1720.4±6.8 Ma (FIA 2), 1682±18 Ma (FIA 3) in Colorado and 408±10 Ma (FIA 2),
388±8.8 Ma (FIA 3), 372.l±5.5 Ma (FIA 4), 352.7±4.2 Ma (FIA 5) in Maine
confirming the multiple periods of deformation and metamorphism indicated by the
FIA succession in each region.
The metastable preservation of early staurolite porphyroblasts through
successive deformation and metamorphic events highlights the imp011ance of
deformation on porphyroblast nucleation and growth. These two areas show that the
reactions involved in the formation of metamorphic minerals are episodic during
prograde metamorphism, starting or stopping, as a function of deformation
partitioning and strain localization. Accessory minerals such as monazite can be dated
E 5
CONCLUSIONS Shah A.A.
and their ages can be better integrated in the deformation and metamorphic history of
a region when they are well constrained by microstructural measurements.
Recommendations for further investigations
Further work will involve linking metamorphic and deformation processes with the
various pluton emplacement events. There are currently no detailed geochronological
data to confirm that metamorphic episodes were progressive over a span of -1 00 Ma
during the -1700 Ma Colorado orogeny, except those mentioned above (Shah, 2009,
2010). This research reveals that this orogeny took place over SO million years and
resulted from 3 distinct changes in the direction of horizontal bulk shortening over
that time. Orogenesis ceased and recommenced 260 million years later. These 4
periods of tectonism and metamorphism involved more than eight individual
deformation events that were partitioned by affected the breadth of the are studied.
Further work will be undertaken using a laser ablation-inductively coupled plasma
mass spectrometry (LA-ICP-MS) U-Pb zircon/monazite geochronological approach
on both metasediments and plutonic rocks to link deformation, metamorphism and
pluton emplacement mechanisms (e.g. Gabudianu et al.,2009; Cesare et aI., 2002;
Lombardo et aI., 2002; Rubatto et aI., 2001). The results so far suggest that
deformation and metamorphism began well before the plutons arrived at
17S9.7±9.SMa during the Colorado orogeny. However, just as deformation and
metamorphism pmtition, we now know that melting partitions (Rubatto et aI., 2001,
2001a. 2001b; Bauer et aI., 2007). Dating granite along strike from zones where one
FIA set dominates may reveal similar variations in age to those discovered by
Rubatto et al (2009; 2007) in partial melts horizons at depth in the Alps. Such
E 6
CONCLUSIONS Shah A.A.
discoveries would significantly improve interpretation of causal and feedback
relationships between deformation, metamorphism events and pluton emplacement.
Furthermore, the overprinting foliation asymmetries preserved by the porphyroblasts
will provide previously unused quantitative orientation and shear sense data for
determining the mechanism of pluton emplacement.
References
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