Time relationship between metamorphism and deformation in ... fileThe southern margin of the...
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Time relationship between metamorphism and deformation in Proterozoicrocks of the Lunavada region, Southern Aravalli Mountain Belt (India) Ð
a microstructural study
Manish A. Mamtania,*, S.S. Merhb, R.V. Karanthb, R.O. Greilingc
aDepartment of Geology & Geophysics, Indian Institute of Technology, Kharagpur-721302, West Bengal, IndiabFaculty of Science, M.S. University of Baroda, Vadodara-390002, Gujarat, India
cGeologisch-PalaÈontologisches Institut, Ruprecht-Karls-UniversitaÈt Heidelberg, INF-234, D-69120, Heidelberg, Germany
Accepted 2 May 2000
Abstract
The southern margin of the Aravalli Mountain Belt (AMB) is known to have undergone polyphase deformation during the Mesoproter-
ozoic. The Lunavada Group of rocks, which is an important constituent of the southern parts of AMB, reveals three episodes of deformation;
D1, D2 and D3. In this paper, interpretations based on petrographic studies of schists and quartzites of the region are presented and the
relationship between metamorphic and deformational events is discussed. It is established that from north to south, there is a marked zonation
from chlorite to garnet±biotite schists. Metamorphism (M1) accompanied D1 and was progressive. M2-1 metamorphism associated with major
part of D2 was also progressive. However, M2-2 that synchronized with the waning phases of D2 and early-D3 deformation was retrogressive.
Porphyroblast±matrix relationships in the garnet±biotite schists of the region have been useful in establishing these facts. The metamorphic
rocks studied were intruded by Godhra Granite during the late-D3/post-D3 event. The heat supplied by this granite resulted in static
recrystallization and formation of annealing microstructures in rocks close to the granite. It is established that Grain Boundary Migration
Recrystallization associated with dislocation creep and Grain Boundary Area Reduction were the two deformation mechanisms dominant in
rocks lying far and close from the Godhra Granite, respectively. q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
The Southern Aravalli Mountain Belt (SAMB) forms the
southernmost tip of the Aravalli Mountain Belt (AMB)
which is a major Proterozoic orogenic belt in northwestern
India (Fig. 1). The SAMB occupies an area of more than
30,000 km2 extending from southern parts of Rajasthan into
northeastern Gujarat and comprises metasedimentary and
granitic rocks. The metasediments belong to the Lunavada
and Champaner Groups of the Aravalli Supergroup (Gupta
et al., 1992, 1995). Mamtani (1998) and Mamtani et al.
(1999a, 2000) have worked out the structural geology of
the area around Lunavada. In the present paper, various
microstructures observed in schists and quartzites of the
Lunavada region are described. These microstructures
have been used to understand microscale deformation
mechanisms. Moreover, a correlation is established between
metamorphic and deformation events on the basis of
porphyroblast±matrix relationships preserved in garnet±
biotite schists of the region.
2. Geological setting and structural geology
The Proterozoic rocks of the Lunavada region, Panchma-
hals district, Gujarat are assigned to the Lunavada Group
which is the second youngest group of the Aravalli Super-
group (Gupta et al., 1980, 1992, 1995). The Lunavada
Group comprises phyllite, mica schist, calc-silicate,
quartz±chlorite schist, meta-subgreywacke, meta-siltstone,
meta-semipelite, meta-protoquartzite with minor layers and
thin sheets of dolomitic marble, petromict meta-conglomer-
ate, manganiferous phyllite and phosphatic algal meta-dolo-
mite (Gupta et al., 1980, 1992, 1995). It occupies an area of
10,000 km2 in the SAMB and is ¯anked in the northeast and
northwest by the Udaipur and Jharol Groups of the Aravalli
Supergroup (Fig. 2). To its west and south lie the Godhra
granite and gneisses. The Godhra granite has been dated as
955 ^ 20 Ma by Rb/Sr method (Gopalan et al., 1979). These
granitic rocks have an intrusive relationship with the
Journal of Asian Earth Sciences 19 (2001) 195±205
1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S1367-9120(00)00029-8
www.elsevier.nl/locate/jseaes
* Corresponding author.
E-mail address: [email protected] (M.A. Mamtani).
M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205196
Fig. 1. Generalized geological map of the AMB. Box in the southern parts marks the area of Fig. 2. Arrow points to the SAMB. Map is after published maps of
Geological Survey of India.
Fig. 2. Lithostratigraphic map of southern parts of AMB (after Gupta et al., 1995). A, B and C marked by asterisk are locations of schist samples for which
CSD studies were done. Q1, Q2, Q3 and Q4 marked by asterisk in circle are locations of quartzite samples which were subjected to CSD measurements. Inset:
L is Lunavada and G is Godhra.
surrounding metasedimentary rocks. The rocks of the south-
ernmost part of SAMB belong to the Champaner Group
which comprises of low grade phyllites and quartzites.
The present investigation was carried out around the
towns of Lunavada, Santrampur and Kadana where the
rocks encountered are quartzites alternating with schists
along with some calc-silicate bands. The quartzites form
long ridges whilst the schistose rocks occur in the low-
lying areas. According to Iqbaluddin (1989), the quart-
zite±schist layers belong to the Kadana Formation of the
Lunavada Group.
Field and satellite imagery studies have shown that
the quartzite ridges have a complex regional scale
outcrop pattern which is characteristic of a history
involving polyphase folding (Fig. 3). The northern part
of the study area shows tight D2 folds, closely spaced
axial plane fractures and joints. Shearing is observed to
have occurred along these axial plane fractures during
D3 deformation (Mamtani et al., 1999a). The southern
part of the study area (around Lunavada, Santrampur
and further south in Fig. 3) is characterized by regional
scale folds. Mamtani (1998); Mamtani et al. (1998,
1999a, 2000) have worked out the structural history of
the region which is summarized below:
1. The Proterozoic rocks of the Lunavada region have
M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 197
Fig. 3. Geological map of the study area. Schists of different metamorphic grades are shown by different symbols. Inset: Arrow points to study area.
undergone three episodes of deformation, viz. D1, D2
and D3.
2. The ®rst two deformational events were coaxial and
resulted in NE±SW trending folds.
3. The third episode of deformation resulted in open folds
with trends varying between E±W and NW±SE.
4. Except for the presence of a few D3 kinks and minor fold
axis, there is no other mesoscopic evidence of D3 folding.
D3 folds have developed on km-scale limbs of the D1±D2
folds.
5. The superposition of the three folds in various combina-
tions has resulted in the development of different types of
large scale interference patterns. Type-III interference
pattern (Ramsay and Huber, 1987) has developed on
account of superposition of D1 and D2 folds while
Type-I interference pattern has developed due to super-
position of D3 on D1±D2 folds.
6. The degree of overturning of D2 folds increases from
north to south. The folds are upright in the northernmost
part of the area (around Ditwas). In the south, they get
overturned with a southeasterly vergence.
3. Microstructures and mechanisms of deformation
Petrographic study of schists from the study area has
revealed that the regional metamorphism progressed up to
lower amphibolite facies. This has resulted in the develop-
ment of porphyroblasts of garnet and biotite. From north to
south, a zonation from chlorite to garnet±biotite schist
through biotite schist is recorded (Fig. 3). In this section,
the various microstructures observed in quartzites and
different types of schists are described and have been used
to decipher deformational mechanisms.
3.1. Discrete crenulation cleavage
This has developed in chlorite schists in the northern parts
M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205198
Fig. 4. (a) Photomicrograph of chlorite schist showing presence of S0, S1 and S2 on the microscale. The bedding plane (S0) is de®ned by the contact between
quartz-rich and quartz-poor layers. The schistosity S1 is sub-parallel to S0 and is marked by chlorite and muscovite. The schistosity S2 is a discrete crenulation
cleavage which has developed at high angles to S0 and S1. The occurrence of the discrete crenulation cleavage is restricted to the quartz-poor (phyllosilicate-
rich) layers. (b) Photomicrograph documenting drag effect along discrete crenulation cleavage (S2) in chlorite schist. S1 foliation de®ned by muscovite and
chlorite is observed to have dragged due to movement along the cleavage. (c) Photomicrograph of chlorite schist in PPL showing microscale displacement
along S2. Scale bar is 0.4 mm in (a) and (c), and 0.1 mm in (b). Location: Ditwas (north of Kadana).
of the study area. In these rocks, three planar surfaces
are recognizable, viz. S0 (bedding plane), S1 (®rst
schistosity) and S2 (discrete crenulation cleavage)
(Figs. 4 (a)±(c)). The rocks have preserved primary
lithological layering (S0) which is marked by alternating
layers of quartz-rich and phyllosilicate rich layers. The ®rst
schistosity (S1) is sub-parallel to S0 and comprises of chlorite,
quartz and muscovite crystals aligned parallel to one another.
The second schistosity is the discrete crenulation cleavage (S2)
which was formed on account of crenulation of S1 foliation
during D2. The S2 has developed almost perpendicular or at
high angles to the S1 and is observed to have formed only in the
phyllosilicate rich layers. There is evidence of displacement
along the S2 surface (Fig. 4(c)). Similar evidence has been
linked by Gray (1979) to pressure solution. However, at the
present scale of observation, no signi®cant evidence of recrys-
tallized quartz aggregates and no metamorphic differentiation
in the vicinity of the discrete crenulation cleavages along
which the displacement occurred has been observed. More-
over, Fig. 4(b) shows some microscale dragging along the
cleavages. Therefore the possibility of these discrete
crenulation cleavages being planes of shear cannot be
totally ruled out.
3.2. Differentiated crenulation cleavage
This has developed in the higher grade schists of the
region and is particularly well developed in the garnet±
biotite schists to the south of Lunavada and Santrampur. It
is made up of alternating quartz (Q) and mica (M) domains
(Fig. 5). Two schistosities (S1 and S2) are prominent micro-
scopically. S1 is made up of chlorite, muscovite and biotite
crystals while new generation biotite and muscovite ¯akes
are developed parallel to S2. The M-domains vary in thick-
ness from 0.1 to 0.5 mm. A few of these zones also preserve
a shear band cleavage that lies at a low angle (,458) to the
domain boundary between M and Q domains (Mamtani and
Karanth, 1996a; Mamtani et al., 1999b). All these micro-
structures in the cleavage zones have been used to interpret
the mechanisms of deformation during origin of crenulation
cleavages (Mamtani et al., 1999b). Accordingly it has been
argued that pressure solution is an important deformational
mechanism during the early stages of crenulation and this
imparts the domainal fabric to the rock. However, intracrys-
talline crystal plastic deformation becomes dominant during
the later stages which results in the development of shear
structures in cleavage zones.
3.3. Millipede microstructure
This microstructure is characterized by oppositely
concave microfolds (OCMs) and usually occurs as inclusion
trails (Si � internal foliation) within porphyroblasts in
schists (Bell and Rubenach, 1980). Millipede microstructure
is preserved in some biotite porphyroblasts in garnet±biotite
schists of the study area (Figs. 6(a) and (b)). It is de®ned by
oppositely curving quartz inclusion trails (S1) within the
biotite porphyroblast. S1 is relatively straight in the core
of the biotite and gradually curves towards the rims and
continues to merge into the external schistosity (S2). Similar
M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 199
Fig. 5. Differentiated crenulation cleavage (S2) in garnet±biotite schist.
Scale bar is 0.4 mm. Location: Anjavana area (southeast of Lunavada).
Fig. 6. (a). Photomicrograph of biotite porphyroblast in garnet±biotite schist showing presence of millipede microstructure characterized by oppositely
concave microfolds (OCMs) of quartz±feldspar inclusion trails (S1) within the porphyroblast. (b) Explanatory line drawing of photomicrograph in (a). Scale
bar is 0.1 mm. Location: Vankdi (south of Anjavana).
structures are known to develop around rotating rigid
objects at low strains in laboratory experiments (Ghosh,
1975, 1977; Ghosh and Ramberg, 1976). Johnson and
Moore (1996) and Johnson and Bell (1996) have stated
that the presence of millipedes indicates a state of low strain
during their genesis. Since the microfolds that make up the
millipedes within the biotite are open compared with those
in the matrix, the biotite porphyroblast is interpreted to have
grown under a low strain state during D2.
3.4. Textures in quartzites
Thin sections prepared from different localities of the
area show that the quartzites comprise of two textural vari-
eties based on grain boundaries Ð either the grain bound-
aries are irregular or they are straight. The irregular grain
boundaries (Fig. 7(a)) are prevalent dominantly in the quart-
zite occurrences that are distant from the Godhra Granite.
According to Urai et al. (1986) and Passchier and Trouw
(1996), the presence of irregular grain boundaries indicates
intracrystalline deformation as the rock underwent dynamic
recrystallization by Grain Boundary Migration (GBM).
Some of the quartz crystals show subgrains (Fig. 7(b)), a
textural feature pointing to recovery during dynamic recrys-
tallization. This also indicates that during deformation,
recrystallization-accommodated dislocation creep was
important (Nicolas and Poirier, 1976; Tullis and Yund,
1985; Tullis et al., 1990; Passchier and Trouw, 1996). Dislo-
cation creep has been recognized as an important deforma-
tion mechanism for quartz aggregates under conditions of
greenschist facies or higher (White, 1976; Mitra, 1978;
Hirth and Tullis, 1992).
Thin sections of quartzites occurring closer to the margin
of the Godhra Granite show a granoblastic texture charac-
terized by straight to smoothly curved grain boundaries,
1208 triple points and sharp extinction (Fig. 7(c)). These
microstructural characteristics clearly point to static recrys-
tallization with Grain Boundary Area Reduction (GBAR) as
the principal mechanism (Passchier and Trouw, 1996). The
presence of 1208 triple points, referred to as foam micro-
structure by Vernon (1976), is indicative of heat outlasting
deformation or annealing. Bons and Urai (1992) and Passch-
ier and Trouw (1996) have stated that GBAR is especially
pronounced at high temperatures after deformation ceases,
M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205200
Fig. 7. (a) Photomicrograph of quartzite showing irregular grain boundaries between quartz crystals implying dynamic recrystallization or GBMR (Grain
Boundary Migration Recrystallization). (b) Photomicrograph of quartzite showing subgrain microstructure in quartz crystals which points to recovery during
dynamic recrystallization. (c) Photomicrograph of quartz crystals in quartzite showing granoblastic texture characterized by straight grain boundaries and 1208
triple points. The quartz crystals have sharp extinction. These microstructures are interpreted to indicate that the rock underwent static recrystallization. See
text for detailed discussion. Location: (a) and (b) Anjavana; (c) Boriya. Scale bar is 0.2 mm in (a) and (c), and 0.1 mm in (b).
i.e., in a static environment. In the present case, the high
temperature for static recrystallization was supplied by the
Godhra Granite that intruded the region. Fig. 7(a) and (c) are
photomicrographs (taken at same magni®cation) of quart-
zites occurring far and close to the granite margin. It is quite
clear that the former has ®ner crystals while the latter has
coarser crystals. This indicates that the heat supplied by the
granite played an important role in microstructure develop-
ment of the latter. Further corroboration of this fact has also
come from Crystal Size Distribution (CSD) study of quartz
crystals in schists and quartzites. Moreover, post-deforma-
tional changes in microstructure are known to occur at the
end of an orogeny when deformation has essentially ceased
and the rocks are at high temperatures (.3008C) or when
deformed rocks are subjected to sustained heating from
post-tectonic plutons (Knipe, 1989) and also in laboratory
experiments with octachloropropane (Ree and Park, 1997).
It is envisaged that prior to the intrusion of granite, the
quartz crystals were in a higher strain condition character-
ized by irregular grain boundaries. Such a microstructure is
thermodynamically unstable and would have a tendency to
proceed to a lower energy state. The late to post deforma-
tional granitic intrusion provided the necessary heat energy
required for release of internal strain and achievement of a
thermodynamically stable microstructure. As a result, a
stable granoblastic microstructure developed which is
more pronounced in the rocks close to the granite margin.
It can be argued that a granoblastic fabric can also form
syntectonically by dynamic recrystallization (Means and
Ree, 1988) or in high grade gneisses (Passchier et al.,
1990). However, in the present study, it is clearly seen
that the quartzites close to the granite show a granoblastic
texture, sharp extinction and coarser grain size. Quartzites
farther from the granite show irregular grain boundaries,
sub-grains, a ®ner grain size and absence of a granoblastic
texture. It is therefore logical to assume that the microstruc-
tures in quartzites close to the granite are a result of static
recrystallization by GBAR related to heat supplied by the
granite. This is in accordance with Bons and Urai (1992)
and Passchier and Trouw (1996) who have suggested that
GBAR is pronounced only after the deformation ceases.
4. Porphyroblast±matrix relationships
The mica schists around Lunavada and Santrampur are
characterized by foliations of different generations and
porphyroblastic minerals such as garnet and biotite which
contain foliations as quartz inclusion trails. The relationship
between the internal foliation (Si) within the porphyroblasts
and the matrix foliation (Se) outside the porphyroblast was
used to determine the relative timing of growth of minerals
with reference to foliation of a particular generation. This is
in accordance with the criteria described by Zwart (1962),
Spry (1969), Vernon (1976), Ghosh (1993), and Passchier
and Trouw (1996).
Most of the garnet and biotite porphyroblasts preserving
the microfolded or sigmoidal inclusion trails are identi®ed
as syntectonic with D2 deformation (Figs. 8(a) and (b); also
Fig. 6). The intensity/tightness of folding of the inclusions
with respect to those in the matrix has been further useful in
classifying the porphyroblasts as early-D2 or late-D2. Fig.
8(a) shows a porphyroblast of biotite with quartz inclusion
trails (Si � S1) which show open microfolds. In contrast, the
microfolds outside the porphyroblast are tightly crenulated.
This indicates that the biotite porphyroblast grew during the
early stages of D2 deformation. A few porphyroblasts
preserve relatively tight S1 crenulations and also include
the S2 foliation at the rims (Fig. 8(b)). Such pophyroblasts
are classi®ed as late-D2. Some garnet porphyroblasts
preserve sigmoidal S1 inclusion trails of quartz and feldspar
which gradually curve into S2 while the cleavage domain
outside the porphyroblast has a single homogenized folia-
tion S2 (Fig. 8(c)). It is envisaged that the sigmoidally
curving S1 schistosity along with S2 was included in the
garnet porphyroblast during earlier stages of D2. With conti-
nuing deformation, the matrix foliation further deformed
and rotated into parallelism with the S2 while the sigmoidal
relation between S1 and S2 within the porphyroblast
remained frozen in the same stage at which it was included,
thus remaining unaffected by later deformation (Mamtani
and Karanth, 1997). Such porphyroblasts of garnet are also
classi®ed as syn-D2.
5. Thermal metamorphism
Regional metamorphism in the Lunavada±Santrampur
region was followed by thermal metamorphism related to
the intrusion of the Godhra Granite. The effects of heat
supplied by the Godhra granite are signi®cant in the south-
western part of the study area, i.e., to the south of Lunavada.
Since the granite does not lie in the immediate vicinity of
the study area, common high-temperature metamorphic
minerals like andalusite and sillimanite are not observed.
Nevertheless, the effect of the thermal event is quite obvious
from the CSD studies on rocks of the region. The method of
measuring CSDs using thin sections of rocks has been
described by Marsh (1988) and Mamtani and Karanth
(1996b). CSD studies provide statistical data for crystals
(of a particular mineral) of different sizes within a unit
area of a thin section. Based on this data, CSD plots such
as size (L mm) vs. normal log of population density [ln�n�]can be prepared. The shape of a CSD plot represents the
extent to which a rock underwent annealing.
In the present investigation, CSD of quartz crystals were
calculated in thin sections of three schist and four quartzite
samples collected at varying distance from the boundary of
the Godhra Granite. Fig. 2 shows the location of the schist
and quartzite samples. Figs. 9(a) and 9(b) are CSD plots for
schist and quartzite samples, respectively. Both rock types
indicate that, in comparison with samples away from the
M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 201
Godhra Granite boundary, samples close to the granite
possess (i) quartz crystals which have crystallized over a
wider size range, (ii) CSD plots with a lower slope, and
(iii) fewer quartz crystals within a unit area. Moreover,
the CSD plots for schists lying close to the granite have a
bell shape (A and B in Fig. 9(a)) while the plot for sample
away from the granite is near linear (C in Fig. 9(a)). This
indicates that all the rocks initially underwent continuous
nucleation and growth. Subsequently, rocks closer to the
granite underwent annealing such that smaller crystals
were resorped at the expense of larger crystals (see Cash-
man and Ferry, 1988; Cashman and Marsh, 1988 and
Mamtani and Karanth, 1996b for details of CSD plot inter-
pretations). The heat for annealing was supplied by intru-
sion of the Godhra Granite.
6. Discussion
On the basis of the present petrographic study, the time
relationship between deformation and metamorphism can
be established. The metamorphic history of chlorite schists
occurring in the northern parts of the study area is rather
simple. As mentioned earlier, these rocks show three promi-
nent planar surfaces (S0, S1 and S2). S1 and S2 developed
during D1 and D2 respectively. Chlorite and muscovite crys-
tals formed during D1. These underwent rotation along S2
and some recrystallization during D2. No evidence of
growth of new minerals cutting across D2 is observed in
the chlorite schists, thus implying that D3 was generally
devoid of any metamorphism. The chlorite schists therefore
only record a single metamorphic event. The paragenesis
observed is chlorite 1 muscovite 1 quartz which is typical
of a chlorite zone within the greenschist facies (Yardley,
1989; Spear, 1993). The garnet±biotite schists of the region
are most important in determining the various metamorphic
events that accompanied different deformation episodes.
These possess differentiated crenulation cleavage character-
ized by alternating Q and M domains. Garnet, biotite, chlor-
ite, muscovite and quartz are the major minerals present.
Chlorite and biotite crystals occur along foliations of differ-
ent generations and are accordingly classi®ed. Chlorite(1)
M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205202
Fig. 8. (a) Syn-D2 biotite porphyroblasts in garnet±biotite schist. (b) Photomicrograph of biotite porphyroblast with microfolded quartz inclusion trails. The
biotite is interpreted as late-syn-D2. (c) Photomicrograph of garnet porphyroblast which has grown over a crenulation cleavage zone (S2). Both S1 and S2 are
present within the garnet and the inclusion trails of S1 curve sigmoidally into S2. However, the cleavage zone in the matrix (outside the garnet) is characterized
by only a single schistosity (S2). This implies that the garnet grew over the crenulation cleavage during D2 deformation. Scale bar is 0.4 mm in (a), 0.2 mm in
(b), and 0.1 mm in (c). Location: (a), (b) and (c) Vankdi (south of Anjavana).
and biotite(1) occur along the S1 schistosity and are syn-D1.
The metamorphic event which accompanied D1 is referred
to as M1. Biotite(2) crystals, which occur with their (001)
planes parallel to S2, have grown during D2 deformation.
Biotite(2) porphyroblasts with spiral (helictic) inclusion
trails of quartz (e.g., Fig. 8(a) and (b)) are also syn-D2.
Similarly the garnet porphyroblasts with sigmoidal inclu-
sion trails of quartz (e.g., Fig. 8(d)) are also syn-D2. The
metamorphic event which accompanied a major part of D2
deformation is referred to as M2-1. This was a progressive
metamorphic event during which biotite(2) crystals grew
along S2. That these progressive events (M1 and M2-1)
were followed by retrogressive metamorphism (M2-2) during
the waning phases of D2/early D3 is evident by the presence
M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 203
Fig. 9. (a) CSD plot for schist samples, viz. Sample A, B and C collected at 3, 4 and 22 km distance from margin of Godhra Granite. (b) CSD plot for quartzite
samples, viz Q1, Q2, Q3 and Q4 collected at 4, 10, 22 and 30 km distance from margin of Godhra Granite. Location of each sample with reference to contact of
Godhra Granite is shown in Fig. 2.
of (a) chlorite(2) crystals that overgrow S2 foliation, (b)
chlorite around syn-D2 garnet and (c) chlorite along frac-
tures in garnet that penetrate from core to the rim.
The last metamorphic event to affect the rocks was a late-
D3/post-D3 thermal metamorphism. In rocks that lie close to
the granite, this event resulted in annealing, coarser crystals
and the development of granoblastic microstructure in
quartzites. It is concluded that the thermal event led to static
recrystallization of quartz on the microscale due to the heat
supplied by intrusion of the Godhra Granite. Emplacement
of the granite may have been initiated during the waning
phases of D3. However, the ®eld evidence for granite and
related pegmatites intruding the foliation in schists indicates
that the intrusion continued even after D3. This further
supports the interpretation that the development of grano-
blastic texture, annealing and grain growth in quartzite
occurred due to static recrystallization on the microscale.
It is also observed that muscovite crystals in garnet±biotite
schists lying close to the granite are large and free from the
effects of intracrystalline deformation such as undulose
extinction. This indicates that the thermal event also
resulted in recrystallization and grain growth of muscovite.
Fig. 10 summarizes the time relationship between
crystallization and deformation of various minerals in
garnet±mica schists.
7. Conclusions
The present study has provided considerable insight into
the metamorphic history and deformation mechanisms of
the Proterozoic rocks around Lunavada, SAMB (India).
The following conclusions are evident:
1. The rocks of the Lunavada region have undergone meta-
morphism up to lower amphibolite facies. There is a
progression from chlorite grade in the northern parts to
garnet grade in the southern parts.
2. Progressive regional metamorphism M1 and M2-1 accom-
panied D1 and a major part of D2 respectively. M2-2 was a
retrogressive event that accompanied the waning stages
of D2 or early D3 deformation.
3. A thermal event related to late-D3/post D3 Godhra Gran-
ite intrusion followed regional metamorphism. This led
to static recrystallization on the microscale and grain
growth in rocks close to the granite.
4. GBM associated with dislocation creep is suggested to
have been an important deformation mechanism in
quartzites lying far from the granite margin.
5. Annealing by GBAR has been discerned in quartzites
close to the granite.
Acknowledgements
The present paper is an outcome of the doctoral
research on Precambrian rocks of Lunavada region
(India) carried out by M.A.M. Financial support to
M.A.M during various stages of the study was provided
by M.S. University Research Scholarship, ®eldwork grant
from the Association of Geoscientists for International
Development (Brazil), Senior Research Fellowship of
the Council of Scienti®c and Industrial Research, New
Delhi (No. 9/114/(92)/96/EMR-I) and DAAD-Fellowship
of the German Academic Exchange Service, Bonn (No.
A/97/00792). We are grateful to Bruce Marsh and
Michael Zeig (Johns Hopkins University, USA) for carry-
ing out CSD measurements in quartzites using a ªOmni-
met Analyzerº. Comments by Jordi Carreras and an
anonymous reviewer were found useful.
M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205204
Fig. 10. Diagram showing the time relationship between crystallization and deformation in garnet±mica schists of the study area. (a) shows the various
minerals that crystallized during the different deformation events, and (b) shows the correlation between metamorphic and deformation events.
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