The Metamorphic History of the Helags Mountain...

Examensarbete vid Institutionen för geovetenskaper Degree Project at the Department of Earth Sciences

ISSN 1650-6553 Nr 356

The Metamorphic History of the Helags Mountain Area, Scandinavian Caledonides

Den metamorfa historien i Helagsfjällets område, skandinaviska Kaledoniderna

Sara Johansson

INSTITUTIONEN FÖR GEOVETENSKAPER

D E P A R T M E N T O F E A R T H S C I E N C E S

Examensarbete vid Institutionen för geovetenskaper Degree Project at the Department of Earth Sciences

ISSN 1650-6553 Nr 356

The Metamorphic History of the Helags Mountain Area, Scandinavian Caledonides

Den metamorfa historien i Helagsfjällets område, skandinaviska Kaledoniderna

Sara Johansson

Abstract The Metamorphic History of the Helags Mountain Area, Scandinavian Caledonides Sara Johansson The Scandinavian Caledonides formed as a result of collision between the continents Baltica and Laurentia, in Silurian and Early Devonian time. The evolution of the orogen has been a topic of research since before the turn of the last century. However, there are still uncertainties regarding the character and timing of the orogenic processes involved in the formation of the Caledonian orogen. Identification and study of high-pressure terranes are a key to understanding the processes involved, and such terrains are found in Jämtland, central Sweden. The most well-known location is Mt. Åreskutan. This study focuses on the Helags Mountain, a locality potentially equivalent to Mt. Åreskutan. It has combined structural and mineralogical studies, pressure and temperature esti-mates, and monazite geochronology, in an attempt to obtain an overview of the metamorphic his-tory.

The Helags Mt. geology, as on Åreskutan, is dominated by a klippe of high grade gneisses, overlying lower grade schists and amphibolites, both typical of the middle and lower part of the Seve Nappe Complex in the Swedish Caledonides. The gneisses are dominantly felsic and contain garnet. Two episodes of garnet growth, likely separated in time, are observed in the gneisses. The first episode probably took place in the presence of melt, as is evident from the presence of inclusion of so called nanogranites. This is further supported, but not fully confirmed, by observed homo-genization of the garnet core chemistry. Such processes take place at high temperature, above 700°C. Pressure estimates are less well defined and indicate about 1 GPa during this first garnet growth event. This event may be related to the observed migmatisation. The second garnet growth episode took place at lower pressure and temperature conditions, and similarities with garnet observed in studies elsewhere indicate a connection with shearing and emplacement of the Middle Seve unit. However, no garnets were observed in the studied shear zone, and it is with the available data not possible to confirm a relation to a specific event. Monazite geochronology has contributed Caledonian ages (400-480 Ma) but has not yielded any precise results with regard to the timing of the migmatisation and thrusting. Keywords: Seve Nappe Complex, Scandinavian Caledonides, metamorphism, monazite geochrono-logy, garnet Degree Project E1 in Earth Science, 1GV025, 30 credits Supervisors: Jaroslaw Majka and David G. Gee Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala (www.geo.uu.se) ISSN 1650-6553, Examensarbete vid Institutionen för geovetenskaper, No. 356, 2016 The whole document is available at www.diva-portal.org

Populärvetenskaplig sammanfattning Den metamorfa historien i Helagsfjällets område, skandinaviska Kaledoniderna Sara Johansson Den svenska fjällkedjan har en lång historia. Dess nuvarande utformning är ett resultat av att Iapetus-havet, en föregångare till dagens Atlanten, slöts och de tidigare kontinenterna Baltica och Laurentia kolliderade. Trots att fjällkedjan studerats flitigt sedan före sekelskiftet är det mycket som är okänt om de geologiska processerna som varit en del av bergens utveckling. För att bättre förstå fjällkedjans ut-veckling studeras bergarter från områden som varit särskilt kraftigt påverkade. Flera sådana områden påträffas i Jämtlandsfjällen. Denna studie har fokuserat på Helagsfjällets område, beläget i södra delen av Jämtlands län. Studier av det särskilt motståndskraftiga mineralet granat från områdets bergarter har tillsammans med dateringar av mineralet monazit givit ny information om områdets geologiska historia.

Helagsfjällets geologi, liksom den välstuderade Åreskutan, utgörs av en enhet av granatförande gneisser, vilken överlagrar en undre enhet av lägre omvandlingsgrad. Detta är typiskt för den mellersta och lägre delen av det så kallade Sevekomplexet. Två generationer granater tyder på att minst två geologiska processer, skilda åt i tid, påverkat områdets bergarter. Den första av dessa granatgenerationer uppvisar bland annat inneslutningar vilka tolkas som bevarade delar av en tidigare smälta. Det är möjligt att denna granatgeneration är relaterad till den tidiga händelse som orsakat uppsmältning, av vilken spår kan studeras på flera platser i området. Tryck- och temperaturberäkningar visar att detta hände under tryck omkring 1 GPa, och temperaturer på över minst 600°C, kanske över 700°C. Den andra granat-generationen är mer svårtolkad. Tryck och temperatur var lägre, och likheter med granater observerade på andra platser tyder på att denna andra granattillväxt skedde i samband med skjuvning av den övre enheten, över den underliggande enheten. Försök att datera dessa två perioder av granattillväxt gav åldrar mellan 400 och 480 miljoner år. Liksom på Åreskutan tyder detta på en tektonisk historia som sträcker sig från Ordovicium till tidig Devon. Nyckelord: Seveskollan, skandinaviska Kaledoniderna, metamorfos, monazit-datering, granat Examensarbete E1 i geovetenskap, 1GV025, 30 hp Handledare: Jaroslaw Majka och David G. Gee Institutionen för geovetenskaper, Uppsala universitet, Villavägen 16, 752 36 Uppsala (www.geo.uu.se) ISSN 1650-6553, Examensarbete vid Institutionen för geovetenskaper, Nr 356, 2016 Hela publikationen finns tillgänglig på www.diva-portal.org

Table of Contents

1. Introduction................................................................................................................. 1

Previous work .................................................................................................................... 2

Aim of project .................................................................................................................... 2

2. Regional geology ......................................................................................................... 3

The Caledonides in Jämtland ............................................................................................. 5

Metamorphic history of the Seve Nappe Complex............................................................ 8

The area of the Helags Mountain..................................................................................... 10

3. Methods...................................................................................................................... 12

Thin sections .................................................................................................................... 12

Microprobe analysis......................................................................................................... 12

Compositional maps ........................................................................................................ 12

Raman spectroscopy ........................................................................................................ 13

Pressure-temperature estimates........................................................................................ 13

Thermodynamic modelling.............................................................................................. 13

Whole rock chemistry ...................................................................................................... 14

Monazite geochronology ................................................................................................. 14

4. Results ........................................................................................................................ 16

Field work and microscopy observations ....................................................................... 16

Shear zone....................................................................................................................... 18

Garnet growth history ..................................................................................................... 20

Garnet textures ................................................................................................................ 20

Garnet chemistry............................................................................................................. 22

Garnet inclusions ............................................................................................................ 29

Pressure-Temperature conditions.................................................................................... 32

Pressure calculations....................................................................................................... 32

Temperature calculations ................................................................................................ 33

Table of Contents (continued) Thermodynamic modelling......................................................................................... 34

Monazite geochronology ............................................................................................ 36

5. Discussion and conclusions .................................................................................. 42

Garnet growth history ................................................................................................. 42

First garnet growth episode (Grt-I) ............................................................................. 42

Second garnet growth episode (Grt-II) ....................................................................... 44

Shearing ...................................................................................................................... 45

Proposed P-T-path ...................................................................................................... 46

Monazite geochronology ............................................................................................ 48

The comparability of the Helags and Åreskutan mountain areas ............................... 48

6. Summary and final remarks ............................................................................... 50

7. Acknowledgements .............................................................................................. 51

8. References ............................................................................................................. 52

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1. Introduction

To understand how present day mountain chains are forming, we need to understand the

geodynamic processes involved. Comparison to, for example, the Himalaya-Tibet orogen with

older Phanerozoic or Precambrian orogens indicate that these processes have been acting in a

similar manner at least since the Proterozoic, and possibly even further back in the Earth‘s

history (Korenaga 2013). Therefore, much can be learnt about present day orogeny by

studying older, deeply eroded orogens and resolving their evolution.

One mountain chain that has been studied extensively is the Scandes of western

Scandinavia. Its uplift occurred during the opening of the North Atlantic Ocean, but its

dominantly Caledonian orogen evolution dates back to 400-500 million years ago. Forming

the eastern part of the North Atlantic Caledonides, the good exposure of the rocks in this

region provides unique insight into the evolution of a Paleozoic orogen which is thought to be

of Alpine-Himalayan style and dimensions (Dewey 1969). It was formed through a series of

events, including ocean (Iapetus) closing, development of subduction zones, and thrusting of

the palaeocontinent Laurentia over Baltica (Gee et al. 1975).

The Scandinavian Caledonides are made up by several nappe complexes, transported

eastwards far across the margin of Baltica. The nappe stacks are especially well exposed in

the central part of the orogen, and it was already in the 19th

century that Mt. Åreskutan in

central Scandes was put forward as a key locality for the occurrence of thrusting (Törnebohm

1888). Subsequent work provided evidence of transport of rock units several hundred km

eastwards (current coordinates) as a result of continent collision (Törnebohm 1896, Högbom

1909, Asklund 1938, Gee et al. 1975). Recently, the research focus has turned to the high

grade rocks of Jämtland County and particularly the upper part of Mt. Åreskutan, the

Åreskutan Nappe (Arnbom 1980).These rocks belong to the middle part of the Seve Nappe

Complex (SNC), a nappe complex inferred to being transported from the outermost margin of

Baltica. Even today new information about the metamorphic history of the SNC is being

presented, advancing our knowledge of the evolution of the region as well as the evolution of

the Scandinavian Caledonides (Klonowska et al. 2014, 2015).

The Helags Mountain area is situated ca. 50 km southwest of Mt. Åreskutan, in

Jämtland County. According to the bedrock map of Jämtland County (Strömberg et al. 1984),

this area is equivalent to Mt. Åreskutan with respect to lithologies, making it the southernmost

correlation of the Middle Seve Nappe. However, little is known about the metamorphic

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history of this area, and thus the question remains whether or not the Helags Mt. area is a true

equivalent to the key area of Mt. Åreskutan.

1.1 Previous work

In the early 1970‘s, bedrock mapping of Jäntland County was conducted as part of a program

in the Swedish mountain chain, by the Geological Survey of Sweden (SGU). The material

was compiled into a bedrock map of the County by Strömberg et al. (1984), with a

corresponding map description by Karis & Strömberg (1998). On that map, the Helags Mt.

area, including the two peaks Helags and Predikstolen, was presented as an equivalent to Mt.

Åreskutan with respect to lithologies, and correlated with the so called Åreskutan Nappe of

the Great Seve Nappe (now the Seve Nappe Complex). Since then, little work has been

published about the area. In the late 1970‘s, drilling in search of copper mineralisations was

conducted in the Seve nappes. However, the aim of these drillings was not to further study the

geological evolution, and thus did not provide further insight into the details of the geological

history of the Helags area.

North of the Helags Mt. are located the mountains of Snasahögarna, where the Seve

Nappe Complex is also exposed. This area was presented as an equivalent to the Åreskutan

Nappe (Arnbom 1980, Sjöström 1983a,b, Strömberg et al. 1984), and has been more

extensively studied. The area includes the so-called Snasahögarna Nappe and studies of this

nappe revealed it to be of higher metamorphic grade (granilite facies) than the underlying

Seve units. Recent studies of Snasahögarna have shown an even higher grade metamorphic

history, similar to that of Åreskutan. It also resulted in the findings of the first metamorphic

diamonds in the Seve Nappe Complex, an evidence of ultrahigh pressure conditions (Majka et

al. 2014b, Rosén 2014).

So far, no complementary studies of the Helags Mt. have been conducted. The question

remains whether this area did experienced high-grade metamorphism, like the rocks of Mt.

Åreskutan and Snasahögarna.

1.2 Aim of project

This study investigates the evolution of the poorly explored Helags Mt. area. The study has

been focused on the klippe on the top of the mountain and the early part of the metamorphic

history, and the possible record of (ultra)high pressure metamorphism in the Helags Mt. area.

The aim has been to construct a model of the geological evolution of the area and to make an

attempt to put that into the perspective of the evolution of the Scandinavian Caledonides.

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2. Regional geology

The Scandes, the mountain chain of western Scandinavia as they appear today, was formed

due to rift-margin uplift in Cenozoic time, related to the opening of the Atlantic Ocean

(Gabrielsen et al. 2010). The rock units building up the Scandes however, have a longer

history. They were formed much earlier, during the Archean, Paleoproterozoic and

Mesoproterozic times, and were thrust together with Paleozoic rocks during the Caledonian

orogeny in Ordovician to early Devonian time. They now constitute the eastern part of the

North Atlantic Caledonides, the Scandinavian Caledonides (Fig. 2.1).

The Scaninavian Caledonides are subdivided into tectonostratigraphic units, emplaced

as nappes during continent collision and subsequent syn- and post-orogenic extension and

collapse. The autochthonous crystalline basement underlying the rock units of the

Caledonides belong to the Baltoscandian platform, and are little affected by the Caledonian

orogeny. Further west, the Precambriam basement found in antiformal windows along the

orogen is more deformed (Gee et al. 2010). It is then referred to as parautochthonous (or

included in the Lower Allochthon). In Jämtland, the units are dominated by Paleoproterozoic

granites, gneisses and dolorites (Gee et al. 2014).

The autochthonous and parautochthonous Precambrian basement is overlain by a nappe-

stack, the higher units of which have been thrust several 100 kilometers over what was the

Baltoscandian margin of palaeocontinent Baltica prior to the Caledonian orogeny. The nappe

units were derived from the margin of Baltica (Gee et al. 1975), the continent-ocean transition

zone (Andréasson et al. 1998), relics of the Iapetus Ocean (ocean-floor and island-arc

affinities; Harland & Gayer 1972, Stephens & Gee 1985) as well as material from the

Laurentian margin (Roberts et al. 2007). In addition, some material may have been derived

from terrains of yet uncertain origin (Corfu et al. 2014b).

The allochthons are divided into Lower, Middle, Upper and Uppermost units. The

Lower Allochthon is dominated by sedimentary rocks deposited in Neoproterozoic to early

Paleozoic basins. These sedimentary rocks are interpreted to comprise shelf and continental

rise successions of the Baltoscandian margin of Baltica. The sedimentary successions

represent the transformation from a continental margin to a foreland basin, and they are also

thought to reflect the episodes of subsidence and uplift during the Ordovician and Silurian

(Corfu et al. 2014b).

4

The Middle Allochthon is of a higher metamorphic grade, compared to underlying units.

However, the Neoproterozoic successions of the Middle Allochthon shows similarities to

those of the underlying nappes, and are assigned to the outer and outermost margin of Baltica.

The rocks of the Upper Allochthon are commonly assigned an origin outboard of

Baltica, belonging to the Iapetus Ocean, and the Uppermost Allochthon is referred to as being

of Laurentian affinity.

Figure 2.1. Interpretation of tectonostratigraphic map over the Scandinavian Caledonides (Gee et al. 1985).

Black line mark sketch profile seen in Fig.2.5, going through the Jämtland-Trøndelag region (modified after Gee

et al. 2010).

5

Figure 2.2. Cross-section through the central Scandes (as marked in Fig. 2.1), showing the fold and thrust belt

formed as a result of the collision (based on Gee et al. 2010). Vertical exaggeration about 5x.

The Scandinavian Caledonides constitutes a large part of the mountain region of

Norway and westernmost Sweden. They stretch from the Stavanger region in southern

Norway to the Barents Sea in northern Norway, a distance of almost 2000 km (Fig. 2.1). The

hinterland of the orogen outcrops on the Norwegian side, whereas the foreland is mostly

located on the Swedish side. To the north, exposure of the Caledonian bedrock narrows to ca.

100 km, and further south the Caledonian bedrock is interrupted by the occurrence of rift-

related rocks of Late Palaeozoic ages (Oslo Permian rift; Larsen et al. 2008).

During the later stages of the Caledonian Orogeny, a set of synforms and anitforms

developed (Fig. 2.2) with axes approximately parallel to the length of the mountain belt,

folding the main allochthons. Today, the Scandinavian Caledonides provides the best

exposure of the North Atlantic Caledonides, thanks to the variety of the thrust sheets and the

Cenozoic uplift and subsequent erosion of the mountain chain. This is the only area in which

rocks of Baltican and Laurentian as well as Iapetus affinities are found.

2.1 The Caledonides in Jämtland

A 300 km long profile through the orogen is exposed in the Jämtland-Trøndelag region,

central Scandes. This makes this region highly interesting for studies of the evolution of the

Caledonian Orogeny.

The bedrock of Jämtland County is divided into two parts. In the east, the Precambrian

rocks belong to the Fennoscandian Shield, while the bedrock in the western part consists of

rocks of the Caledonian Orogen (Fig. 2.3).

6

The Lower Allochthon in Jämtland consists mainly of sedimentary rocks and windows

of Precambrian basement (Gee et al. 2014). The upper part of the Lower Allochthon in

Jämtland was referred to as the Hotagen Nappe by Strömberg et al. (1984; Fig. 2.3). It occurs

in the southern and northern parts of the county and comprises a complex set of units

(Strömberg 1998).

In Jämtland, the Middle allochthon is represented by the Offerdal, Särv and Seve

Nappes. Major ductile shear zones and thick mylonites separate the Lower Allochthon from

the Middle Allochthon. The boundary is also marked by an increase in metamorphic grade

from lower to middle greenschist facies (Gee et al. 2014). The rock units themselves,

however, are believed to have a common origin with the rocks of the Lower Allochthons (e.g.

Stephens & Gee 1985).

Overlying the Hotagen Nappe of the Lower Allochthon is the comparing highly strained

Offerdal Nappe, composed of fine-banded and folded metasandstones. Associate nappes are

made up of Precambrian crystalline rocks and are referred to, for example, as the Granite-

Mylonite Nappe. Being the basal part of Middle Allochthon, transported over a long distance,

this nappe is heavily deformed.

Over-thrusting these lower units of the Middle Allochthon is the Särv Nappe

(Strömberg 1961), dominated by Neoproterozoic feldspathic sandstones, carbonates and

tillites (Kumpulainen, 1980, 2011). Dolorite dike swarms intrudes the sedimentary succession

in several locations (Hollocher et al. 2007). These dikes are referred to as the Baltoscandian

Dike Swarms (BDS), and they are likely of early Ediacaran age, ca. 600 Ma (Svenningsen

2001). Dike-intruded sandstone successions can be followed at the same structural level,

making it likely that the Baloscandian Dyke Swarms intruded along the entire outer

Baltoscandian margin. This event was probably related to the opening of the Iapetus Ocean

(Gee 1975, Andréasson 1994).

The Seve Nappe Complex (SNC) constitutes the uppermost part of the Middle

Allochthon. Originally however, it was placed into the base of the Upper Allochthon, a view

that was revised by Gee et al. (1985). The complex can be traced for at least 1000 km along

the Scandinavian Caledonides (Andréasson et al. 1998). In most areas, the contact between

the Särv and Seve Nappes is a major thrust fault, and also marked by a sharp increase in

metamorphic grade and in ductile deformation (Gee et al. 2014). However, in some areas the

contact is described as more transitional (Andréasson 1994), and the protoliths are similar.

7

Figure 2.3. Distribution of tectonostratigraphic units in Jämtland County. Line A-B indicates the location of the

profile through the Tännfors Synform. Modified from Karis & Strömberg (1998).Division into Lower, Middle,

and Upper Allochthon according to Gee et al. (2008).

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The lithologies of the SNC include metasediments (quartzite, feldspathic and

calcsilicate-bearing psammites, and marbles). Abundant occurrences of amphibolized

dolorites (Svenningsen 2001) and gabbros are noted, sometimes associated with ultramafites

(peridotites, dunites) (Gee et al. 2013, Corfu et al. 2014b). Like the Särv Nappes, most of the

quartzite-dominated sedimentary rocks of the SNC where likely deposited during

development of Neoproterozoic rift basins (Albrecht 2000, Gee et al. 2014). The SNC is

commonly interpreted as representing the outermost part of the rifted Baltoscandian margin

(Gee 1975) and includes the continental-ocean transition zone (Andréasson & Albrecht 1995,

Andréasson 1998).

The Middle Allochthon is overlain by a series of nappes of the Upper Allochthon,

mostly assigned to the Köli Nappe Complex and its equivalents (e.g. Stephens & Gee 1985).

In Jämtland, the Köli Nappe rocks are found in the western and northernmost parts. The rocks

of these units consist of ocean-floor volcanic and sedimentary rocks, as well as volcanic rocks

of island-arc and back-arc basin affinities (Stephens & Gee 1985; Stephens 1988; Grimmer &

Greiling 2012).

2.2 Metamorphic history of the Seve Nappe Complex

The Seve Nappe Complex (SNC) is generally metamorphosed in amphibolite facies.

However, locally it includes eclogites and in a few places there is evidence of ultra-high

pressure (UHP) metamorphism (e.g. Zwart 1974, van Roermund 1985, Janák et al. 2013).

Therefore, it is believed to have been involved in subduction and subsequent exhumation.

Dating of the metamorphism suggests that the subduction occurred in Mid Ordovicium, prior

to the main collision between Baltica and Laurentia. The most recent studies of rocks

belonging to the SNC have resulted in the formulation of new and more detailed models of

the Seve subduction systems (e.g. the ―vacuum cleaner‖ model; Majka et al. 2014b).

Estimates of pressure and temperature reflecting conditions during subduction, collision,

exhumation and emplacement, as well as dating of the metamorphic events, have piece by

piece improved the overall understanding of the evolution of the SNC, as well as the

Caledonian orogeny in Scandinavia.

The SNC is commonly divided into three units; with the names of these units varying

between different regions (e.g. Sjöström 1983a, van Roermund 1989, Zachrisson & Sjöstrand

1990). The most widely used division today is the Lower, Middle and Upper Seve Nappes.

Lower Seve is similar to the Särv Nappes in protoliths, but ductilely deformed in amphibolite

and locally eclogite facies. Middle Seve is more pelitic, with migmatitic gneisses and minor

9

amphibolites, eclogites, and garnet peridotites (van Roermund 1989) showing, in general,

higher metamorphic grade than the Lower Seve unit. Upper Seve is dominated by

amphibolites and schists, and exhibits a lower metamorphic grade than Middle Seve

(Ladenberger et al. 2014).

It is within the Lower and Middle Seve units that highest grade rocks are preserved.

Estimates of pressure and temperature conditions have been made for several SNC localities

in Jämtland. Studies of the Middle Seve units of Friningen, northern Jämtland (Fig. 2.3), have

shown peak metamorphic conditions at 2-3 GPa and 700-800°C (Brueckner et al. 2004; Janák

et al. 2013a). Previous studies of Lower Seve eclogites of Tjeliken, northern Jämtland (Fig.

2.3), yielded metamorphic conditions of 1.6 GPa and 650-680°C (Litjens 2002; Brueckner &

van Roermund 2007) but recently new studies have upgraded these results to peak

metamorphic conditions of 3 GPa and 650-700°C (Majka et al. 2014a).

In the central part of Jämtland, in the Handöl-Storlien area just north of the Helags Mt.

area, studied by Sjöström (1983a,b), the highest metamorphic conditions were found in the

Snasahögarna Nappe granulites (Middle Seve), revealing temperatures of 660-680°C and

pressures of 0.6-0.7 GPa. These estimates were upgraded when closer studies of garnets in

Snasahögarna felsic gneisses showed the first occurrence of microdiamonds in the SNC,

indicating pressures of >3 GPa (Majka et al. 2014b, Rosén 2014).

This evidence of ultra-high pressure (UHP) conditions reflects conditions during

subduction of crustal material to mantle depths. Thus, resolving the time constraints for the

metamorphism also gives insight into the timing of events through important parts of the

orogenic history. In Norrbotten County in northern Sweden, an episode of high grade

metamorphism took place during early Ordovician time, ca. 480 Ma (Root & Corfu 2012,

U/Pb), or perhaps even earlier (ca. 500 Ma, Mørk et al. 1988, Sm/Nd). This event was

followed by emplacement of the high-grade units onto the Baltoscandian margin through later

Ordovician time, before the main Baltian-Laurentia collision. Ultra-high pressure (UHP)

metamorphism in northern Jämtland probably took place later than in Norrbotten. Dating has

yielded ages in mid to late Ordovician time, ca. 460-444 Ma (Brueckner et al. 2004,

Brueckner & van Roermund 2007).

For Åreskutan, in central Jämtland, an even more complete picture of the evolution has

been presented in recent studies. In mid-late Ordovician (ca. 455 Ma), a high pressure event is

thought to have taken place, followed by a peak temperature event at ca. 440 Ma, associated

with migmatisation (Ladenberger et al. 2012, 2014, Majka et al. 2012). The shear zone

separating the Lower and Middle Seve units on Åreskutan has been dated to late Silurian. In

10

connection to later exhumation processes, all Seve units experienced retrogression in

amphibolite and subsequent greenschist facies (Majka et al. 2014a).

The distribution of older ages of high-grade metamorphic events in the north gave

further cause to the discussion about the geometry of the Ordovician subduction and

emplacement, prior to the main continental collision. In contrast to the idea of orthogonal

continent collision, a model of transpressive closure of the Iapetus Ocean was first formulated

by Harland (1979) and further developed by Soper et al. (1992). Sinistral transtension and

transpression dominates the hinterland of the orogen during early Devonian exhumation of the

Scandian UHP rocks (e.g. in the Western Gneiss Region).

2.3 The area of the Helags Mountain

The Helags Mt. area is located in the north-south trending Tännfors Synform (Fig. 2.3;

Sjöström1983a). The southern part of the Tännfors Synform is dominated by Lower Seve

rocks (Strömberg 1998), with the boundary to the Särv Nappe marked by a zone of mylonized

felsic rocks. The main lithologies of the Lower Seve in this area are calcareous schists and

phyllites with occurrences of marbles, metasandstones and amphibolites. The rocks making

up the lowlands surrounding the Helags Mt. are dominated by schists and phyllites. On the

eastern side of the Helags Mt., these rocks are rich in calcite and other carbonates and is also

generally graphite bearing (Strömberg 1998).

In this region, the equivalent to the Middle Seve is the Snasahögarna Nappe (Sjöström

1983a). Rocks of the Snasahögarna Nappe outcrop in the mountains of the central part of the

synform, and include Snasahögarna as well as the Helags Mt. The Snasahögarna Nappe,

including the Helags Mt., is dominated by partly migmatized paragneisses of highest

amphibolite to granulite facies. In addition to the garnet-bearing mylonitic basal contact of

these gneisses, narrow shear zones are found within the nappe (Sjöström 1983a).

On the top of the Helags Mt., as on Åreskutan, felsic gneisses outcrop as a klippe, as

mapped by Strömberg et al. (1984). In the following parts of this thesis this higher grade unit

will be referred to as the Helagsfjället Klippe. The extent of the klippe is somewhat

ambiguous but they constitute both the Helags peak and the lower peak of Predikstolen.

A correlation between the Snasahögarna Nappe and the Åreskutan Nappe of Arnbom

(1980) was made by Sjöström (1983a), and the Helagsfjället Klippe shown on the bedrock

map of Jämtland County to be an equivalent to the Åreskutan Nappe with respect to

lithologies (Strömberg et al. 1984).

11

Figure 2.4. Simplified geological map of the Helags Mt. and its immediate surroundings (based on Strömberg et

al. 1984).

12

3. Methods

This study has involved a number of methods, including studies of mineral textures and

mineral chemistry, Raman spectroscopy, pressure and temperature estimations,

thermodynamic modeling, and monazite geochronology. The methods are briefly outlined

below.

Thin sections

Seven samples were collected in the study area, representative of the observed lithologies.

From these samples, thin sections were made at the Geological Institute of the Slovak

Academy of Sciences in Bratislava, Slovakia. The sections were all made using two steps of

diamond polishing agent, with 1 μm and 3 μm sizes respectively.

These thin sections were all studied under petrographic microscope to identify mineral

assemblages and micro textures.

Microprobe analysis

Mineral identification and chemical characterization of samples of special interest were

conducted using electron microprobe analyses. Selected thin sections were coated with a thin

carbon film. In cases where optical mineral identification was not possible, energy dispersive

X-ray spectroscopy (EDS) was utilized. For chemical characterization, quantitative chemical

analyses used for pressure and temperature calculations, and for step scan garnet profiles,

wavelength dispersive X-ray spectroscopy (WDS) was used.

The EDS and WDS analyses were done using a JXA-8530F JEOL Hyperprobe Field

Emission Electron Probe Microanalyzer (EPMA) at the Centre for Experimental Mineralogy,

Petrology and Geochemistry, Uppsala University, Sweden. Operating conditions were 15 kV

acceleration voltages with 10 nA beam. Backscattered electron (BSE) images were acquired

using the same facility, but with manually adjusted conditions to receive maximum resolution.

Compositional maps

X-ray element maps of garnet were collected to study the spatial distribution of major

elements (Fe, Mg, Mn, and Ca) in a zoned garnet. This was done by moving the electron

beam point by point in a raster over the area of interest, producing a bitmap image based on

element concentrations. The instrument used was a JXA-8530F JEOL Hyperprobe Field

Emission Electron Probe Microanalyzer (EPMA) at the Centre for Experimental Mineralogy,

Petrology and Geochemistry, Uppsala University, Sweden. Operating conditions were as

13

follows: acceleration voltage 15 kV, probe current 40 nA, dwell time 200 ms, resolution

450x280 μm.

X-ray element maps of monazites were constructed with respect to Y, U and Th. This

was done using the same operating conditions as for the garnet maps.

Raman spectroscopy

For identification of (sub)micron inclusions in garnet, Raman spectroscopy was utilized. To

avoid interference of insufficient removal of coating of thin sections previously used for

microprobe analyses, four additional thin sections were made.

The micro-Raman spectroscopy was carried out with a Thermo Scientific DXR Raman

Microscope, at the Institute of Chemistry, Slovak Academy of Sciences, in Bratislava,

Slovakia. The spectra excited at room temperature, using an excitation wavelength of 532 nm

of a Nd:YAG laser, with power emission conditions of 10-50 mW. Objective lenses of 100x

and 250x with a slot of 25μm were used. Each spectrum was collected using two

accumulations of 10-30 s, to eliminate contamination by cosmic radiation. The obtained

Raman spectra were compared to Raman spectra from the RRUFF database (http://rruff.info/)

for mineral identifications.

Pressure-temperature estimates

Pressure and temperature conditions were calculated based on mineral chemistry (garnet,

biotite, plagioclase, and phengite). Two different sets of calibrations were used. The first set is

a compiled excel sheet with seven common garnet-biotite geothermometer calibrations,

provided by Prof. D. Waters, Department of Earth Sciences, University of Oxford

(downloaded from http://www.earth.ox.ac.uk/~ davewa/pt/th_tools.html). Geothermometric

calculations were conducted based on calibration of Fe and Mg distribution coefficients in

garnet and biotite. Seven commonly used calibrations were applied, namely those by

Thompson (1976), Holdaway and Lee (1977), Ferry and Spear (1978), Hodges and Spear

(1982), Perchuk and Lavrent‘eva (1983), Dasgupta et al. (1991) and Bhattacharya et al.

(1992). The second set is an integrated garnet-phengite geothermometer and garnet-phengite-

plagioclase-quartz geobarometer by Wu & Zhao (2006).

Thermodynamic modelling

Using the software Perple_X (version 6.7.1), isochemical phase-diagram pseudosections were

calculated (cf. Connolly 2005). Thermodynamic dataset (hp11ver.dat) of Holland & Powell

14

(1998) was used in NCKFMMnASHT system (Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-

H2O-TiO2). Solution models used are as follows: Gt(HP) for garnet (almandine–grossular–

pyrope–spessartine), Pheng(HP) for potassic white mica (muscovite + paragonite + Mg–Al-

celadonite + Fe2+

-Al-celadonite), Pl(h) for plagioclase, Bio(TCC) for Ti-oxy (Ti and O) and

Fe3+

Tschermak exchange in biotite, melt(HP) for melts of granitic composition, and IlGkPy

for ilmenite. All these solution models were provided in the solution_model.dat file.

Isopleths for garnet composition and silica content of phengite were calculated using the

werami.exe-application provided with Perple_X. Silica content a.p.f.u. (atoms per formula

unit) in phengite and grossular content (XCa) of garnet are pressure sensitive, and pyrope

content (XMg) in garnet is temperature sensitive. Therefore, these isopleths was especially

considered.

Whole rock chemistry

To enable pseudosection modeling, a sample of gneiss (sample 48) was sent for whole rock

chemical analysis. A piece was cut from the sample and crushed. The crushed sample was

milled into a fine powder of which 15 g were sent to Bureau Veritas Minerals Laboratories

(BVML), Vancouver, Canada. Bulk chemistry analysis was carried out on major elements and

additional minor constituents. The loss of ignition (LOI) was determined with an uncertainty

of 5 %.

Monazite geochronology

The chemical analyses of monazite were performed using a Cameca SX100 Electron

Microprobe, equipped with a four wavelength spectrometer, at the Department of Special

Laboratories, Laboratory of Electron Microanalysis, at the Geological Institute of Dionýz

Štúr, Bratislava, Slovakia. The operating conditions were 15 kV accelerating voltage, 180 nA

beam current, using a beam size of 2 or 3 μm depending on the size of the monazite grains.

The following standards and corresponding spectral lines were used: apatite (P Kα), PbCO3

(Pb Mα), ThO2 (Th Mα), UO2 (U Mβ), YPO4 (Y Lα), LaPO4 (La Lα), CePO4 (Ce Lα), PrPO4

(Pr Lβ), NdPO4 (Nd Lα), SmPO4 (Sm Lα), EuPO4 (Eu Lβ), GdPO4 (Gd Lα), TbPO4 (Tb Lα),

DyPO4 (Dy Lβ), HoPO4 (Ho Lβ), ErPO4 (Er Lβ), TmPO4 (Tm Lα), YbPO4 (Yb Lα), LuPO4

(Lu Lα), fayalite (Fe Kα), barite (S Kα), wollastonite (Ca Kα, Si Kα), SrTiO3 (Sr Lα), Al2O3

(Al Kα), GaAs (As Lα). The counting times on peak/background (in seconds) were as

follows: P 10/10, Pb 300/150, Th 35/17.5, U 80/80, Y 40/20, La 5/5, Ce 5/5, Pr 15/15, Nd 5/5,

15

Sm 5/5, Eu 25/25, Gd 10/10, Tb 7/7, Dy 35/35, Ho 30/30, Er 50/50, Tm 15/15, Yb 15/15, Lu

100/100, Fe 5/5, S 10/10, Ca 10/10, Sr 20/20, Al 10/10, Si 10/10, As 120/120.

In order to retrieve dates from the chemical analyses, statistical analyses was done using

Isoplot version 4, an add-in for Microsoft Excel (available at

http://www.bgc.org/isoplot_etc/isoplot.html).

16

4. Results

In the following section, combined field and microscopy observations, as well as the results of

chemical analyses of collected samples are presented. Emphasis is on observations of

importance for the interpretation of the metamorphic history of the study area.

Field work and microscopy observations

The main objective of the field work was to collect samples from which information about the

pressure-temperature-time (P-T-t) history could be extracted. Thin sections were made of the

collected samples and microscopy studies of these thin sections aided in identification of main

mineral assemblage of the different lithologies. The sample locations are marked in Figure

4.1, and samples are summarised in Table 4.1. All sample locations, as well as locations

where structural observations have been made, are located within the area presented as

Middle Seve (Helagsfjället Klippe) by Strömberg et al. (1984).

The study area is dominated by gneisses (SJ14-037, SJ14-043, SJ14-048), some with a

pronounced schistosity showing resemblance to a mica schist (Fig. 4.2; SJ14-043).

Petrographic studies of the thin sections show the main mineral assemblage of the gneisses is

quartz + plagioclase + biotite + phengite + muscovite + carbonates, with accessory titanite,

rutile, ilmenite, zircon and monazite. Garnet occurs only at a few locations, with individual

grains usually less than 1 mm, and one garnet-bearing gneiss was sampled in the northern part

of the study area (SJ14-048). In a few locations suspected kyanite was also observed,

although microscopy studies did not reveal any kyanite.

In addition to the gneisses, two other lithologies were noted. In the southern part of the

area is a calcareous amphibole-bearing gneiss is exposed (SJ14-042). Microscopy studies

show the mineral assemblage to be quartz + plagioclase + amphibole (hornblende) + garnet +

biotite + white mica. Accessory phases are epidote group mineral (zoisite?) + rutile + ilmenite

+ zircon + carbonates + chlorite. Garnet appears as clusters, up to 1 mm in diameter, and is

only distinguishable under petrographic microscope.

In the eastern part of the area several exposures of paragneiss are noted (SJ14-047).

They show a layered structure, with a general dip direction towards west-northwest. At one

location, a conglomerate structure is preserved (Fig. 4.1f).

Figure 4.1. Compiled map of study area, with field observations from this study. Sample locations are marked with sample number, cf. Table 4.1. Miniatures A-F show

examples of structural observations in field, with photo locations indicated by connecting lines. The approximate extent of the Helagsfjället Klippe as marked on the map is in

accordance with Strömberg et al. (1984).

18

Table 4.1. Summary of collected samples and the lithology they represent. For sample locations, see Figure 4.1.

Sample Lithology

SJ14-037 Gneiss

SJ14-042 Garnet and amphibole bearing calcareous gneiss

SJ14-043 Gneiss

SJ14-047 Paragneiss

SJ14-048 Garnet-bearing gneiss

SJ14-050 Sheared gneiss

SJ14-052 Sheared gneiss

Figure.4.2. Hand specimen of gneisses from study area. In a) a gneiss of the type dominating the study area

(SJ14-037), and in b) a gneiss with pronounced schistosity showing resemblance to a mica schist (SJ14-043).

In addition to lithologies, some general structural observations were made in the study

area, the most notable being the large scale layering. This is especially well visible at the peak

of Predikstolen in the southern part of the area (Fig. 4.1c), and from the location of the

mountain lodge in the eastern part. The general dip direction is towards northwest, but

variations are seen at several locations. At the southern mountain slope of the peak of Helags,

the layering is visible as folded light and dark bands (Fig 4.1a), and in the northern part of the

study area, quartz-rich layers are folded to varying extent (Fig 4.1d).

In some parts of the area, migmatitic structures are observed (e.g. Fig 4.1b,e).

Shear zone

Boulders were found to exhibit shear structures in the northern part of the area. In the same

area, a shear zone is exposed at a steep cliff face in north-south direction. Due to the location,

19

the extent and actual orientation of the shear zone is difficult to assess. Its location within the

marked extent of the Middle Seve unit suggests that this is not the basal thrust, but an internal

structure. Two samples were collected from separate locations along the shear zone (SJ14-

050, SJ14-052), in order to extract information about the direction and timing of shearing. The

location of the shear zone presented difficulties collecting oriented samples. Therefore, both

samples were collected by marking the orientation of an accessible surface. In the laboratory

the sample was re-oriented and cut along lineation. This method has its limitations, and thin

sections of these samples are only roughly oriented.

However, some observations were made regarding the direction of shearing. Kinematic

indicators are preserved in the form of plagioclase porphyroclasts with biotite and quartz

crystallized in the pressure shadows, as well as a developed CS-fabric. As is shown in Figure

4.3, they indicate both dextral and sinistral shearing in this zone.

Figure.4.3. Microscope images (PPL) of kinematic indicators preserved in a-c) sample SJ14-050, and d) sample

SJ14-052. Thin sections cut perpendicular to foliation, and along lineation approximately oriented 335/10 (a-c)

and 005/15 (d) Blue arrows mark interpreted sense of shear as indicated by mineral textures.

20

Garnet growth history

When deciphering the metamorphic history of a rock, studies of impervious minerals such as

garnet are important. Slow element diffusion rates allow for the preservation of compositional

zoning in garnet, and element distribution patterns give information about changing

conditions during the history of the mineral. This information is necessary when estimating

pressure and temperature condition. In addition, the preservation of inclusions gives

information about earlier mineral assemblages.

Two garnet bearing samples were collected for further studies: a gneiss (SJ14-048) and

a calcareous amphibole-bearing gneiss (SJ14-042).

Garnet textures

The garnet-bearing gneiss (SJ14-048) exhibits two types of garnet (Fig. 4.4). The first type

(Grt-I) is approximately 0.2-0.5 mm in size. Only a few observations of garnet of this size

were made in field. This type of garnet is only sparsely occurring in the studied sample, but

commonly in association with biotite. The crystal shape is subhedral to euhedral, and

fracturing is commonly observed. All garnet of this type exhibits a cloudy core of numerous

small (sub-μm) inclusions, and a rim free of such inclusions.

More common in this sample is the occurrence of smaller (< 0.2 mm), euhedral garnets.

They are interpreted as a second type (Grt-II). The small size makes them impossible to

distinguish in hand specimen and was only detected through optical microscopy studies. Like

the first garnet type, these garnets are commonly found in association with biotite and occur

evenly distributed throughout the sample. Though some of these garnets also contain small

inclusions in the core region, they are more commonly inclusion-free.

The second garnet-bearing sample is the calcareous amphibole-bearing gneiss from the

southern part of the study area (SJ14-042). In this sample, garnet commonly appears as

clusters of anhedral to subhedral crystals, up to 1 mm across (Fig. 4.5a-c). There are also a

few occurrences of solitary euhedral garnets (Fig. 4.5d). Unlike garnet observed in the gneiss,

these garnets contain very few inclusions, and lack the submicron inclusions in garnet core

altogether.

21

Figure 4.4. Examples of garnet occurring in garnet-bearing gneiss (SJ14-048). Two types of garnet are

recognized, Grt-I and Grt-II. In a) an overview showing both types of garnet. In b) a close-up of Grt-I seen in a).

In c) and d) subhedral garnets of Grt-I type, and in e) and f) euhedral garnets of Grt-II type.

22

Figure 4.5. Garnet clusters in calcareous amphibole-bearing gneiss (SJ14-042). In a) microscope image of garnet

cluster (PPL), and in b) a BSE-image of the same cluster (orientation 45° anti-clockwise relative to the optical

microscope image). White square marks close-up of cluster seen in c). In d) two garnets occurring separate from

cluster.

Garnet chemistry

In order to calculate pressure and temperature conditions at garnet growth, detailed chemical

analysis was carried out on the two largest garnets of type one (Grt-I).

Compositional profiles of two adjacent garnets in sample SJ14-048 were constructed by

electron microprobe step-scan in order to investigate chemical variations (Fig. 4.6). The

inclusion-rich core region is marked based on microscopy observations. Rim region is marked

based on changes in chemistry seen in profiles.

Both analysed garnets have a composition dominated by almandine and spessartine end-

members, with low pyrope and grossular content. In general the composition varies only

slightly in the general core area, with a composition of Alm59-63Sps21-26Grs5-12Prp7-9. In the

core area carrying the inclusions, the chemistry exhibits very limited variations and retains a

flat profile. The inclusion-free outer region shows an almost flat profile, but with slight

23

changes in chemistry towards the rim. In garnet A of Figure 4.6, this change is a slight

decrease in pyrope content towards the rim, matched by a slight increase in spessartine. In

garnet B of Figure 4.6, the slight increase in spessartine content is also evident, but instead

matched by a decrease in grossular.

In the rim regions of the two garnets, there is a remarkable change in chemistry, with a

general composition of Alm52-63Sps19-25Grs6-23Prp4-7. Compared to the core area this is a

distinct increase in grossular, while almandine, pyrope, and spessartine decreases. In addition,

the Fe/(Fe+Mg) ratio shows increase.

Single point analysis of other Grt-I grains throughout the sample show similar results,

with the core regions exhibiting a composition of Alm61-62Sps22-25Grs7-8Prp7-8, and rims a

composition of Alm54-57Grs21-24Sps17-18Prp5-6.

In addition to the compositional profiles, element maps were constructed for garnet A of

Figure 4.6 to further study the variations in chemistry (Fig. 4.7). The maps show the spatial

distribution of the four end-member elements magnesium, manganese, calcium and iron

respectively. Warm colours indicate relatively high concentrations, compared to areas of cold

colours.

The core area appears homogeneous with respect to magnesium and iron, both of quite

low concentrations. Manganese distribution on the other hand, increases continuously from

outside the inner core area (with submicron inclusions) and towards the border to the rim area.

In the case of calcium distribution, the inner core area outline as a region of slightly higher

calcium content than the outer core. This area corresponds to the inner core carrying

inclusions, and some of the relatively larger inclusions in the cloud-like inclusion pattern can

be seen highlighted as dots of higher calcium content.

All four elements show a distinct change in element distribution in the rim area

compared to the general core area, as is also evident from compositional profile. The higher

resolution of the element maps compared to profiles allows for observation of compositional

variations within the rim. Magnesium and manganese content decreases slightly in from the

inner rim and outwards. Calcium and iron compositions in the rim area are somewhat more

complex than was evident from compositional profiles. Both elements show two zones with

an inner rim zone of higher calcium and lower iron, and an outer rim zone with the opposite

relation.

24

Figure.4.6. Compositional profiles of two adjacent garnets (A and B) in gneiss (SJ14-048), showing variation in

almandine, pyrope, grossular, and spessartine end-members as well as variation in Fe/(Fe+Mg) ratio. Miniatures

to the right shows microscopy images (PPL) of the two garnets, with arrows indicating electron microprobe

stepscan profile. Blue lines in miniatures indicate inclusion-rich inner core region as observed in microscope.

25

Figure 4.7. Element maps of garnet A of figure 4.6, showing the spatial distribution of garnet end-member

elements (SJ14-048). Warm colours (orange, yellow) indicate higher concentration relative to cold colours

(blue). Resolution is 450x280 μm.

In addition to observations of changes in chemistry, both within the core and within the

rim of the garnet, the element distribution maps show that the core region exhibits a well

preserved crystal shape. This crystal shape is not matched by the outer crystal shape. This is

suggestive of the rim being formed during a second episode of garnet growth.

According to the work by Rollinson (2002), it is possible to tell different garnet growth

episodes from each other based on the end-member variation in different chemical domains,

such as core and rim regions. By comparing the Fe/(Fe+Mg) ratio (also known as the iron

number #Fe) to the mole fraction of calcium (XCa) and manganese (XMn) respectively, trends

not visible in a compositional profile can be distinguished. The idea is that if seemingly

different zones belong to the same garnet growth event, they will plot with a linear trend with

regard to XCa, XMn, and #Fe (Rollinson 2002). On the other hand, if there is a break in the

trend, or if there is a sub-parallel trend, this indicates more than one growth event. Rollinson

26

(2002) also argue that any discordant trend reflects changes in PT-conditions for the different

episodes. In order to investigate the possibility of more than one garnet growth event, such

plots were compiled for garnet A and garnet B (Fig. 4.8).

Figure. 4.8. Garnet mole fraction plots of #Fe vs. XCa and XMn respectively. In a) and b) data from garnet A of

figure 4.6, and in c) and d) data from garnet B of figure 4.6. #Fe = Fe / (Fe+Mg), XCa = Ca / (Ca+Mn+Mg+Fe),

and XMn = Mn / (Ca+Mn+Mg+Fe).

Comparing the general core area, including both outer inclusion-free core and the inner

inclusion-rich core for both garnet A and garnet B, no distinction can be made between the

two. Even though the inner core areas show lower #Fe, and slightly lower XMn, than the outer

core areas, there is a clear linear trend seen for the general core area.

Analytical points from the rim area of each garnet however, plot distinctly separate from

the garnet core points. As was noted already in the garnet profiles (Fig. 4.6), the Fe/(Fe+Mg)

(#Fe) is higher in the rims than in the cores. The mole fraction of calcium is higher than in the

core area, and the mole fraction of manganese is lower.

27

Following Rollinson (2002), the non-linear trend indicates that the general core region,

with inner core and outer core, belongs to the same growth event. The rim on the other hand

would then have formed during a separate growth event.

A compositional profile was also constructed for part of one garnet cluster in the

calcareous amphibole-bearing gneiss (SJ14-042), including five individual garnet grains (Fig.

4.9). Similar to garnets in gneiss (SJ14-048), these garnets are almandine-rich. However,

spessartine content is notably lower, matched by higher grossular and pyrope contents. Core

regions show a general composition of Alm55-62Grs15-26Sps12-14Prp6-10, and rim regions Alm57-

61Grs16-23Sps11-14Prp8-11.

Neither of the garnets A-E of Figure 4.9 can be assumed to have grown unaffected by

the close association to other grains. This, combined with the relative small grain sizes of

garnet A and C, makes it difficult to distinguish any general, unambiguous trends in

compositional variations. Still, considering only the three larger grains of garnet B, D and E,

some general observations are made. Pyrope content increases towards the rims in all three

grains, and this is matched by a decrease in Fe/(Fe+Mg) ratio. A trend of decreasing

almandine at the rims, together with grossular increase is seen in garnet B and E. Spessartine

show no clear core to rim trend in either garnet, but instead a weak tendency of both increase

(garnet E) and decrease (garnet B, C, and D)

Garnet D is the largest of the five grains included in the profile, and also the garnet that

shows the largest variations in composition. Notably, the grossular content changes quite

dramatically, and the same tendency is seen for the grossular content of the second largest

garnet E.

28

Figure. 4.9. Compositional profile of garnet cluster in calcareous amphibole-bearing gneiss (SJ14-042),

corresponding to part of garnet cluster seen in Fig. 4.5b. The profile shows variations in almandine, pyrope,

grossular, and spessartine end-members as well as variation in Fe/(Fe+Mg) ratio. BSE-image to the right shows

analyzed garnet cluster with arrow indicating EMP step-scan profile. Individual garnet grains (garnet A-E) are

marked in profile based on compositional variations, and textures seen in BSE image.

Not all garnets in the calcareous amphibole-bearing gneiss have crystalized in clusters.

Some occurences of small (about 40 μm), subhedral garnets are found at some distance from

garnet clusters. Single spot analysis, with analytical spots distributed from garnet core to

garnet rim, were preformed on two such garnets, in order to compare them to the garnets

forming clusters (Fig. 4.10). There are only small compositional variations, with slightly

decreasing almandine and spessartine content, as well as Fe/(Fe+Mg) ratio, and increase in

grossular and pyrope content, from core to rim. The spacing and number of analytical spots do

not allow for any patterns of fluctuating compositional changes to be seen (cf. Fig. 4.9). In

terms of similarites to the cluster garnets of Figure 4.9, there is a consistency of decreasing

Fe/(Fe+Mg) ratio and increasing pyrope content. As for almandine and spessartine content

there is less consistency between profiles, both in garnet cluster and for individual grains.

29

Figure. 4.10. Compositional profiles of garnets, constructed by single spot analysis, in calcareous amphibole-

bearing gneiss (SJ14-042). A and B refer to the profiles marked in miniature BSE-image to the right, with

yellow arrows indicating orientation of profiles. The profiles show variations in almandine, pyrope, grossular,

and spessartine end-members as well as variation in Fe/(Fe+Mg) ratio.

Garnet inclusions

Microscopic observations of garnet in gneiss (SJ14-048) revealed the garnets of Grt-I type to

carry three types of inclusions (Fig. 4.11). The most notable type is the cloudlike, (sub)micron

inclusions located in garnet cores. Visually, these inclusions show similarities with diamond

and graphite inclusions found in garnets from e.g. Snasahögarna (Majka et al. 2014b).

Qualitative microprobe chemical analysis (EDS) showed some of the inclusions of this study

to yield carbon peak. In order to investigate the source of the carbon EDS peaks, Raman

spectroscopy was utilized. Figure 4.12 shows four representative Raman spectra. For

comparison, database spectra (http://rruff.info/) of calcite, almandine, and spessartine are also

shown. The comparisons indicates that the peaks of the analysed inclusions correspond to the

host mineral (garnet: almandine, spessartine), in combination with a strong peak around 1100

30

cm-1

corresponding to the carbonate ion (CO32-

). Peaks around 200-500 cm-1

match both those

of spessartine and calcite, which thus does not allow for an exact identification of the

carbonates. However, it corresponds well to the observations of increased Ca content of the

inner core region as seen in element maps. There is further no correlation with database

spectrum of diamond.

Previous studies (e.g. Majka et al. 2014b, Rosén 2014) have shown that diamond

inclusions are found in association with graphite-bearing inclusions. Therefore, a comparison

to database spectrum of graphite was done. However, no indications of the presence of

graphite were found either.

Figure. 4.11. Three types of inclusions are observed in garnet in gneiss ( SJ14-048). In a) BSE-image of garnet

A of figure 4.6 exhibiting a monocrystalline quartz inclusion. In b) BSE-image of adjacent garnet B of figure 4.6

with white squares indicating location of polyphase inclusions. Cloudlike distribution of submicron in both

garnet cores is marked with blue line.

31

Figure 4.12. Raman spectra from four analyses of inclusions in garnet core from garnet-bearing gneiss (SJ14-

048). For comparison, database (http://rruff.info/) spectra of calcite (RRUFF ID: R040070), almandine (RRUFF

ID: R040076), spessartine 8RRUFF ID: R050063), diamond (RRUFF ID: R050204), and graphite (RRUFF ID:

R090047) are shown. Grey squares mark peaks in analysed inclusions, and their correlation with database

spectra.

32

The other two types of inclusions found in garnet are relatively larger than the carbonate

inclusions. A few monophase quartz inclusions are found, located both in garnet core and

closer to rim region (Fig. 4.11b).The third type of inclusion are polyphase inclusions located

within the garnet core, together with the carbonate inclusions (Fig. 4.11c).These inclusions

contain plagioclase + alkali-feldspar + phengite + quartz + chlorite + ilmenite + epidote (Fig.

4.13). Some of these inclusions also exhibit a negative crystal shape. Cesare et al. (2009)

assigned similar inclusions to being inclusions of relict melts, naming them nanogranites.

Figure 4.13. BSE images of polyphase inclusions in garnet (SJ14-048). Both inclusions are located within the

inner core region, together with submicron inclusions determined to be carbonates.

Pressure-Temperature conditions

In the light of two garnet growth episodes preserved within the garnet-bearing gneiss (SJ14-

048), pressure and temperature estimates were attempted for these separate events, Grt-I and

Grt-II.

Pressure calculations

The geothermobarometer by Wu & Zhao (2006) uses garnet, mica (phengite), plagioclase and

quartz chemistry to calculate pressure, as well as temperature, conditions. For the first garnet

growth episode (Grt-I) garnet core composition was used, together with plagioclase from

garnet inclusions. It would have been preferable to use the chemistry of phengite in polyphase

inclusions in garnet for these calculations. However, quantitative analyses of these phengites

were not possible. Instead core composition of phengite occurring in matrix was used. For the

second garnet growth episode (Grt-II), chemistry of garnet rims, together with matrix

plagioclase and matrix phengite chemistry was used.

33

The results of pressure calculations are summarized in Table 4.2. This

geothermobarometer contains two models; A and B. In model A it is assumed that there is no

ferric iron in the mica, whereas in model B it is assumed to be 50% ferric iron. For garnet core

(Grt-I) the calculated pressure is just above 1 GPa, model B yielding the higher result of 1.2

GPa. For garnet rim (Grt-II), the calculated pressure is lower, just below 1 GPa.

The utilized thermobarometer also calculates temperatures. For garnet core composition

the resulting temperatures are 605-638°C for inner core (carrying carbonate inclusions), and

slightly lower temperatures of 588-626°C for outer core (inclusion-free region). However,

since these calculations are based on limited analytical points, these results are to be viewed

with caution.

Table 4.2. Thermobarometric calculations using garnet, plagioclase, phengite and quartz (Wu & Zhao 2006).

Values are averages of analyses pairs. Matrix phengite was used in all calculations.

Textural relation No. of pairs

Temperature in °C Pressure in GPa

Garnet Plagioclase Model A Model B Model A Model B

Rim Matrix 3 537 585 0.88 0.94

Core Incl. in garnet 3 605 638 1.02 1.16

Outer core Incl. in garnet 3 588 626 1.05 0.92

Temperature calculations

The exchange of iron and magnesium between garnet and biotite (the reaction almandine +

phlogopite = pyrope + annite) is primarily a function of temperature. Thus it is widely used as

a geothermometer. Calculations are done by pairing analytical points in garnet with analytical

points in biotite. When choosing an approach for selecting position of analytical spots, an

important factor to consider is that of equilibrium between selected mineral. In the case of the

first garnet growth episode (Grt-I), garnet core chemistry was paired with chemistry of biotite

occurring as inclusion in garnet. It is however rare to find biotite inclusions large enough for

quantitative analysis (WDS). Therefore, the calculations are based on very limited data.

For calculations of temperature conditions during the second garnet growth episode

(Grt-II), garnet rim chemistry was matched with matrix biotite adjacent to garnet.

The results of garnet-biotite geothermometry are presented in Table 4.3. Reference

pressure was set at 1 GPa, in accordance with the results from pressure calculations.

34

Table 4.3. Average temperatures achieved from thermobarometric calculations on pairs of rim composition of

four garnets (A-D) and biotite occurring adjacent to these garnet, including one inclusion of biotite in garnet

core. Reference pressure = 1 GPa.

Textural relation No. of pairs

Calibrations with average temperature in °C

Garnet Biotite B92-HW B92-GS Dasg91 FS78 HS82 PL83 T76 HL77

Garnet A (rim) Adjacent to garnet 6 498 446 534 505 568 543 556 531

Garnet B (rim) Adjacent to garnet 5 497 449 521 501 560 541 553 528

Garnet C (rim) Adjacent to garnet 2 511 461 548 503 585 542 555 529

Garnet D (rim) Adjacent to garnet 3 497 446 524 515 564 549 564 538

Garnet C (core) Incl. in garnet 2 520 471 590 567 596 580 607 575

B92 - Bhattacharya et al. (1992); Dasp91 - Dasgupta et al. (1991); FS78 - Ferry & Spear (1978); HS82 - Hodges & Spear (1982); PL83 - Perchuk &

Lavrent‘eva (1983); Thompson (1976); Holdaway & Lee (1977).

The average temperatures resulting from the different calibrations show significant

variations, ranging between 446-585°C. Therefore a consideration of the reliability of these

calibrations for the prevailing conditions of this study is needed. Calibrations of Dasgupta et

al. (1991) and Bhattacharya et al. (1992) rely on the most recent thermodynamic data of the

seven calibrations. In addition, their data consider non-ideal mixing in garnet as well as the

influence of titanium in biotite (interaction between Ti and Al in the octahedral site). This is

an important aspect, considering the high Ti content measured in biotite throughout the

studied sample (SJ14-048).

Wu and Cheng (2006) tested eleven calibrations and came to the conclusion that among

the top four thermometers is the calibration by Perchuck and Lavrent‘eva (1983). Notably,

neither the calibration by Thompson (1976), nor the calibration by Holdaway & Lee (1977)

was included in the study by Wu and Cheng (2006).

For the purpose of further discussion and interpretation of the geothermometric data, an

estimated average of the calibration by Dasgupta et al. (1991), Bhattacharya et al. (1992), and

Perchuck and Lavrent‘eva (1983) will be considered. The two former calibrations yielded

average temperatures of ca. 470-590°C for Grt-I (garnet core and biotite inclusion), and 450-

530°C for Grt-II (garnet rim and matrix biotite). Corresponding temperatures for Perchuck

and Lavrent‘eva (1983) yielded temperatures of ca. 580°C for Grt-I and 540°Cfor Grt-II.

Thermodynamic modelling

In an attempt to evaluate peak pressure and temperature conditions, thermodynamic modeling

was employed for the garnet-bearing gneiss (SJ14-048). The modeling was done in the

35

NCKFMMnASTH system using bulk rock chemistry (wt%: Na2O=1.87 MgO=0.65

Al2O3=8.52 SiO2=82.4 K2O=1.18 CaO=1.71 TiO2=0.42 MnO=0.10 FeO=2.17) and

quantitative mineral chemistry analysis (WDS) of garnet core chemistry and core chemistry of

matrix phengite. H2O value was set to 0.26%. This value was retrieved by systematic

modeling with decreasing H2O amount, starting at bulk rock loss of ignition value (0.60%), so

that free H2O was limited to only the lowest pressure conditions.

Figure 4.14 shows the resulting pseudosection and compositional isopleths of XMg and

XCa in garnet, and Si a.p.f.u. in phengite. The isopleths for XMg in garnet and the Si content of

phengite intersect in the biotite + phengite + plagioclase + garnet + kyanite + zoisite + titanite

+ microcline + quartz stability field. The peak pressure, as indicated by the intersection, is

about 1.2 GPa and occurs at temperatures f 400-450°C. However, there is no intersection with

the isopleths for XCa in garnet at these conditions.

The mineral assemblage indicated by the modeled pseudosection is by and large in

agreement with assumed peak assemblage based on observations in garnet inclusions. Kyanite

was not observed in thin sections of this study. However, observations of suspected kyanite

was made in field, and marked on the bedrock map (Strömberg et al. 1984).

There is an intersection between XCa and XMg at considerable lower pressures of ca. 0.5

GPa, but higher temperatures of ca. 500-550°C. The stable mineral assemblage at these

conditions is biotite + phengite + plagioclase + garnet + ilmenite + kyanite + microcline +

quartz.

An important note on this model is that it does not take into account the presence of

CO2 (i.e. carbonate inclusions in garnet core). Neither does the model allow for XCa of the

concentrations found in garnet core to co-exist with phengite of the compositions found in the

sample. The conclusion is therefore that it is not a fully satisfying model to represent peak

conditions.

36

Figure 4.14. P-T pseudosection calculated for garnet-bearing gneiss (SJ14-048), with compositional isopleths of

XMg and XCa in garnet and Si a.p.f.u. in phengite. Intersection of isopleths marked by yellow ellipses. Red field

marks presence of melt.


In order to assess the timing of the two garnet growth episodes, and to possible assess the

timing of shearing, geochronology on monazite was utilized. Both the garnet-bearing gneiss

(SJ14-048) and the sheared gneiss (SJ14-050, SJ14-052) contained monazite as an accessory

phase. In the garnet-bearing gneiss, the anhedral monazite grains occur randomly distributed

throughout the matrix in association with mainly plagioclase and biotite (Fig. 4.15a). A few

monazites also occur as inclusions within other minerals, such as biotite. The grains vary in

size from 8 µm up to 35 µm, with the majority of the grains being in the range 10-25 µm. One

grain of apatite replacing monazite is found.

In the sheared gneiss, monazite was more abundant in sample SJ14-050 than in sample

SJ14-052 which is why only monazite from that sample was dated. In sample SJ14-050,

subhedral monazites occur almost exclusively within plagioclase porphyroclasts (Fig. 4.15b).

37

The grain size varies from 2-3 µm up to 20-25 µm. Like in the garnet-bearing gneiss, one

apatite replacing monazite is recognized, at quartz grain boundaries in the matrix.

Figure 4.15. Examples of monazites and associated phases from garnet-bearing gneiss (SJ14-048) in a) and in b)

from sheared gneiss (SJ14-050).

A description of two strategies for monazite geochronology is outlined in Williams et al.

(2006). These authors refer to the methods as the ‗top-down‘ and the ‗bottom-up‘ approaches.

In the ‗top-down‘ approach, a large number of analyses are made, in several compositional

domains, and statistical tools are then used to distinguish different age populations. The

‗bottom-up‘ approach on the other hand involves detailed measurements in individual

domains. Textural and compositional observations are then considered when interpreting age

populations and assigning them to geological events.

In this study, a ‗bottom-up‘ approach was attempted. In Figure 4.16, BSE images of

representative monazite grains are shown together with corresponding element distribution

maps constructed with respect to Y, U and Th. Both BSE-images, and element maps, show

variations in zonation patterns with the monazites from the garnet-beraing gneiss exhibit

examples of Th-enriched cores and monazites rom sheared gneiss exhibiting concentric

zoning with respect to Th. Based on these observations, an attempt to date these

compositional domains was made. However, Figure 4.16 also shows the analytical spot size

(2-3μm) in relation to monazite grain size. This illustrates the difficulty of avoiding mixing

between compositional domains during analysis.

38

Figure 4.16. Monazite zonation in BSE images, together with X-ray element maps showing the distribution of

Y, U, and Th. In a-c) representative images of monazites from garnet-bearing gneiss (SJ14-048), and in d-f)

representative images of monazites from sheared gneiss (SJ14-050). Circles in BSE-images represent position

and size of analytical spots (2-3μm).

39

In the following section, ‗date‘ refers to the number obtained from the measured

concentrations of Th, U and Pb in single analytical spots, and ‗age‘ refers to the numbers

obtained from calculations combining the data from several analytical spots. In Figure 4.17,

dates are presented as a Gaussian (normal) probability distribution. A wide range of dates is

obtained, with all dates being Caledonian. For the garnet-bearing gneiss (SJ14-048) the dates

are spanning over a range of 400-480 Ma, with a weighted average age of 426+4.5 Ma (2σ

error). For the sheared gneiss (SJ14-050) the dates span from 410 to 470 Ma, with a weighted

average age of 438+6.9 Ma (2σ error).

Figure 4.17. Distribution of dates from dating of monazite in a) garnet-bearing gneiss (SJ14-048), and b)

sheared gneiss (SJ14-050). Weighted average age with 2σ error, and number of analytical spots, is shown in the

boxes.

In both samples, the span of obtained dates is large, with no obvious indication of age

groups in the form of clearly separated peaks. With such large span of dates, using the

weighted average age for the full dataset is a too crude approach to give a meaningful age.

Distinction of age groups based on position of analytical spots was not possible, for reasons

outline above. However, previous work (e.g. Majka et al. 2012) has shown that it is possible

to discriminate between different monazite generations based on the presence of patterns in

distribution of Y, U and Th in relation to age. For example, high Th/U ratio can be correlated

to monazite growth in equilibrium with melt, connected to partitioning of U into melt (Bea &

Montero, 1999), whereas a group of relatively lower ratio would be indicative of monazite

crystallization without presence of melt. Further, the incorporation of Y in monazite is closely

connected to the stability of other Y bearing phases, such as garnet (cf. Kohn et al. 2005). The

40

chemical distribution of Y may therefore be informative when it comes to distinguishing

different growth stages of monazite with or without stable garnet.

Figure 4.18 shows variations in Y, U, and Th relative to dates in monazites from the two

investigated samples. Looking at the Th/U, U/Pb, and Th/Pb ratios, no clear trend can be seen

in either sample. For the garnet-bearing gneiss there is a weak trend of older dates

corresponding to slightly lower Y concentrations than the younger dates. No such trend is

distinguishable for the sheared gneiss. Based on this, no clear distinction between age groups

is however possible.

41

Figure 4.18. Variations in chemistry relative to dates of analytical spots in monazite in garnet-bearing gneiss

(SJ14-048) in red, and in sheared gneiss (SJ14-050) in blue.

42

5. Discussion and conclusions

The aim of this study was to contribute to the understanding of the metamorphic history of the

Helags Mt. area. The results presented here, while not giving any definite answers, still give

some important indications about the metamorphic evolution of the studied area. The

discussion that follows is focused on the record of changing metamorphic conditions, and the

geological processes possible related to these changes. In order to address the hypothesis that

the Helags Mt. is an equivalent to Mt. Åreskutan, a brief comparison between the areas is

finally presented.

Garnet growth history

Within the studied garnet-bearing gneiss (SJ14-048), evidence of two episodes of garnet

growth (Grt-I and Grt-II) are interpreted. This is based on garnet textures and variations in

composition. In Figure 5.1, the interpreted evolution of these episodes is shown, as they are

observed in larger garnets throughout the studied sample.

Figure 5.1 Proposed model of garnet growth episodes, as observed in garnet-bearing gniess (SJ14-048). The first

garnet growth episode (Grt-I) took place in the presece of carbonate rich fluids. With temperatures remainig

high, difussion homogenization could take place, followed by retrograde resorption affecting the garnet rim

during cooling. At a later stage a second garnet growth event took place (Grt-II), resulting in nucleaton of Grt-II

on preserved, older garnet, as well as the formation of numerous smaller garnets.

First garnet growth episode (Grt-I)

The first episode of garnet growth (Grt-I) took place in the presence of carbon-rich fluids.

This resulted in the formation of numerous inclusions in the inner core. Similar inclusions

(and inclusion patterns) have been observed in garnet from (ultra)high-pressure rocks in Mt.

Åreskutan and Snasahögarna (Majka et al. 2014b, Rosén 2014). In those studies, some of the

inclusions have been found to contain microdiamonds. However, Raman spectroscopy of the

43

(sub)micron inclusions in this study showed that the inclusions are composed of carbonates.

Considering both comparisons of obtained Raman spectra to database spectrum of calcite, and

the high concentration of Ca seen in element maps in the inner core region, these carbonate

inclusions are likely dominated by calcite.

The reason for carbonate inclusions to be restricted to the inner core area remains

unclear. One possible explanation is that decrease of fluid activity, or else changing

conditions, restricted the inclusions to the inner core region, and left the outer core region

inclusion free. Another possibility is that the growth rate of garnet shifted within the same

growth episode. Yang and Rivers (2001) consider the growth rate of a mineral as a possible

factor in the tendency to incorporate inclusions. Fast growing garnets would, according to

these authors, be more prone to incorporate inclusions than slower growing garnets. In the

case of this study, this would indicate a difference in growth rate between the inner and outer

core regions of the garnets. As there is not unambiguous data supporting either hypothesis,

this question remains open.

As to pressure and temperature conditions during the first garnet growth event, the

pressure calculations should be viewed as an approximation, taking into account the low

number of analytical points. The calculations place this first garnet growth at around or just

above 1 GPa. Thermodynamic modeling, however not providing a fully satisfying model,

gives the same indications. Thus the conclusion is that the pressure was, minimum, 1.2 GPa,

in accordance with the highest calculated pressure.

Further, there are several indications of these first garnets having formed at high

temperature conditions. During the early stage of garnet growth, it is likely that an original

growth zoning formed. This growth zoning is however not preserved. It is evident from the

flat compositional profiles, as well as the element maps, that the core regions are to a large

extent homogeneous. A common explanation for this is diffusion homogenization, taking

place at a mean temperature of 640 + 30°C, with a complete homogenization occurring above

700°C. The homogenization would also cause geothermometric calculations to result in

anomalous low temperatures (Yardley 1977). That could possibly explain why calculations in

this study only yields temperatures of at most just above 630°C for Grt-I, though these are in

any case considered minimum estimates. Further, homogenization would, together with

multiple garnet growth episodes, is likely the reason why thermodynamic modeling of this

rock proved difficult.

The question of timing of the homogenization with respect to the garnet growth must be

addressed. Two possibilities exist: (1) either the garnet growth and homogenization are two

44

separate events, with one growth stage and one re-heating stage, or (2) this would be the result

of temperatures remaining high even after garnet formation.

A possible answer to that question lies in the polyphase inclusions observed in garnet

core. Cesare et al. (2009) showed that inclusions similar to those found in this study are

remnants of former anatectic melt, so called ‗nanogranites‘. The formation of nanogranites

indicates high melting temperatures, above 700°C, and slow cooling (Cesare et al. 2009).

Slow cooling would also indicate that the homogenization was the results of temperatures

remaining high, and would not be a two-stage phenomenon.

After growth of the first generation of garnet, pressure and temperature decreased

causing retrogression. Studying the compositional profiles and element maps of larger

garnets, it is possible to distinguish a slight change in chemistry in the outermost part of the

core region, just inside the rim. There the manganese content increases slightly, and such

increase in Mn is commonly interpreted as the result of garnet dissolution, i.e. net-transfer

reaction (Kohn & Spear 2000), taking place during retrogression. Assuming that the

homogenization of the garnet core was complete before the retrogression took place, the

resorption would have occurred at a temperature below 610°C. That is, at the lowest estimated

temperature at which homogenization can occur (Yardley 1977).

In summary, this first garnet growth episode was initiated at high temperature, possibly

in the presence of a melt, and was followed by a period of slow cooling. After sufficient

cooling, this first generation of garnet was partly resorbed before the next generation of garnet

started to crystallize.

Second garnet growth episode (Grt-II)

A substantial change in garnet chemistry between the general core area and the rim is evident

from chemical profiles and element maps. This is interpreted as an overgrowth of a second

generation of garnet, Grt-II.

The calcium enrichment in the rim area, especially in the inner part of the rim, could be

indicative of the involvement of plagioclase (or other Ca-bearing phase). According to Tracy

(1982) this indicates a pressure increase at the time of the initiation of the second garnet

growth episode. As seen in the element map, the calcium content in the rim is first increasing,

reaching a maximum in the central rim area, and then decreasing towards the edge of the

garnet crystal. Following the argument by Tracy (1982), this indicates a pressure increase

during initial growth of Grt-II and then decreasing pressure as the growth continued. With the

data available in this study, the calculated pressures are around 0.9-1.0 GPa but should be

45

viewed with caution since it is based on very limited data. In these calculations, matrix

plagioclase has been used, but equilibrium between this plagioclase and the second garnet

generation might not be a valid assumption. Therefore, the conclusion is simply that after

retrogression of the first garnet generation (Grt-I), some geological event caused an increase

in pressure from an unknown level.

As for temperature conditions during the second garnet growth episode, calculation

show temperatures of 450-540°C, significantly lower than for the first episode. The trend of

Fe/(Fe+Mg) ratio increasing from the inner rim and outwards, is commonly interpreted as the

result of decreasing temperature. The conclusion is therefore that the temperature increased,

which initiated garnet growth during a second stage, and thereafter the temperature decreased

again during garnet growth.

Apart from overgrowth on already existing garnet, the second generation of garnet also

resulted in the formation of the small subhedral to euhedral garnets observed throughout the

gneiss (SJ14-048). These garnets bear similarities to the second generation garnets noted in

the Mt. Åreskutan gneisses by Majka et al. (2012), and to garnet observed in the

Snasahögarna Nappe by Sjöström (1983b). In both these studies, the garnets are ascribed to a

post-thrusting garnet growth event. The similarities to observations made in this study leads to

a possible conclusion being that the increase in pressure (and temperature), initiating the

second stage of garnet growth, was caused by the process of shearing, and that garnet growth

continued through the shearing event.

Shearing

This study has been focused on the early part of the metamorphic history, and the possible

record of (ultra)high pressure metamorphism in the Helags Mt. area. However, during field

work in the study area, a shear zone was observed in the northern part. Since the shearing is

possibly connected to the emplacement of the Middle Seve unit of Helags Mt. and

Predikstolen, the shear zone was further studied.

Only limited information can be drawn from studies of the two collected samples (SJ14-

050, SJ14-052). Only a few reliable kinematic indicators have been identified, and they show

both sinistral and dextral sense of shear. This indicates movement both towards northwest and

southeast, though the studied thin sections are only roughly oriented. Movement like that

possibly also suggests that this shear zone have been active and later reactivated.

The attempt to date the shearing event(s) resulted in a range of dates. Given that it is not

possible to distinguish different monazite generations the final conclusion is that the shear

46

zone was active during the Caledonian orogen. The location of the shear zone well within the

inferred extent of the Middle Seve unit (as presented by Strömberg et al. 1984), further

indicates that the shear zone is an internal structure within the unit.

Based on the two inferred shearing directions, and the range of monazite dates, it is

possible that this shear zone was first formed during the emplacement of the Middle Seve unit

of Helags Mt. During the later stages of the orogen, extensional collapse could have

reactivated the shear zone, causing movement in the opposite direction. There is however too

little data in this study to draw any definite conclusions about this matter.

The studied shear zone is located within the area presented as the Middle Seve unit

(Helagsfjället Klippe). Future studies of the emplacement of the Helags Mt. area should rather

be focused on the basal contact between the Middle Seve unit and the surrounding Lower

Seve rocks.

Proposed P-T-path

The presented results give an overview of the metamorphic history, though not a detailed

account of the geological events responsible. In Figure 5.2, the results are summarized in a

proposed pressure-temperature path for the garnet-bearing gneiss (SJ14-048). Garnet growth

occurred in two episodes. Combining studied garnet inclusions, changes in garnet chemistry,

and geothermobarometric calculations, the following scenario is suggested.

Initial garnet growth (Grt-I) took place at pressure around 1 GPa and temperatures of at

least 610°C, in the presence of carbonate rich fluids. The result was inclusion of carbonates,

mainly calcite, in the inner core region of the garnets. It is possible that the temperature at

initiation of garnet growth was even higher, above 700°C, and that (anatectic) melt was

present. This is indicated by the inclusions of polyphase inclusions, so called nanogranites, in

garnet core.

The temperature remained high and the garnet composition was homogenized through

diffusion. Slow cooling (also allowing for the possible preservation of melt inclusions) might

have slowed down garnet growth rate, which could explain the absence of carbonate

inclusions in the outer core region. When the slow cooling proceeded, retrograde resorbtion of

garnet took place. This is indicated by the increase in Mn observed at the outermost part of

the garnet core region.

At a later stage, a second garnet growth episode (Grt-II) was initiated due to increasing

pressure and temperature. During garnet growth, the pressure and temperature decreased.

This is indicated by decrease in Ca, at the outermost garnet rim, seen in the Ca element map,

47

and the successive increase in Fe/(Fe+Mg) ratio through the rim. Garnet of a second

generation nucleated on preserved, older garnets, but also formed new grains.

Figure 5.2. Proposed P-T path for the garnet-bearing gneiss (SJ14-048).

The two events of garnet growth can, on a speculative basis, be connected to two

geological processes observed in field: migmatisation and shearing. Growth of the first

generation of garnet (Grt-I) at high temperature, possibly in the presence of melt, suggests a

connection to the migmatisation. The heated crust cooled slowly and the garnet was

subsequently affected by resorption and retrogression, when the pressure and temperature

conditions had dropped.

During the second garnet growth episode (Grt-II), the pressure and temperature

increased. Similar findings in rocks of the Snasahögarna Nappe and Mt. Åreskutan have been

connected to thrusting, and thus the second garnet generation of this study can possibly be

related to shearing. However, no garnet was for example observed in the shear zone to

confirm this hypothesis.

48


All dates are Caledonian, ranging from 400-480 Ma, and with average ages of 426+4.5 Ma

(2σ error) for the garnet-bearing gneiss and 438+6.9 Ma (2σ error) for the sheared gneiss.

This places the garnet growth episodes, as well as shearing, within the time frame for the

Caledonian orogeny. It suggests a history that span from the early Ordovician to the end of

early Devonian. However, it is not possible, based on the available data in this study, to be

more precise regarding the timing of geological events.

The variations in monazite zonation patterns seen in the two samples suggest that the

crystallization of monazite in these samples is not a simple one, related to specific events.

Rather the crystallization history appears to be influenced by processes throughout the

metamorphic history of these rocks. One example is the weak trend of increasing Y in

monazite with time, observed in the garnet-bearing gneiss (SJ14-048). Such trends are

commonly interpreted as the result of changing garnet stability. For example, in this case, that

early monazite growth took place in the presence of stable garnet but that garnet successively

became unstable while monazite continued to crystallize. However, this trend spans over

almost 100 myrs. Therefore the conclusion is that this is not a simple result of changing garnet

stability, but that of a long and more complex crystallization history.

The result of the monazite geochronology of this study suggests that a more direct

dating approach might be favourable for future studies. One suggestion is garnet dating

through the 176

Lu/176

Hf or 147

Sm/143

Nd methods. This would shed light on the timing of the

garnet growth relative to the observed migmatisation, as well as in relation to shearing.

However, it is noted that the observed submicron inclusions in garnet inner core, and the

effect of homogenization on garnet chemistry, can make direct dating of the first garnet

growth event problematic. Further, the absence of garnet in the studied shear zone requires

another dating method for the shearing event.

However, both the dated samples of this study also carry zircon as an accessory phase.

These grains are larger than the monazite grains (>100µm), and BSE-imaging reveal them to

be zoned, and dating of these zones could shed light on the timing of metamorphic events.

The comparability of the Helags and Åreskutan mountain areas

What is clear from previous investigations is that both areas are dominated by a klippe of high

grade gneisses that overlies schists and amphibolites. This study has in addition provided new

information regarding the metamorphic history of the Helags Mt. area. It has also

49

demonstrated the difficulties in retrieving a full account of the history in an area affected by

multiple metamorphic events. The results of this study are not, however, precise enough to

present a model of the metamorphic history of the study area, comparable to that of Mt.

Åreskutan.

Based on the results of this study it is possible to compare pressure and temperature

conditions of the two areas. This study has shown the highest pressure conditions of the

Helags Mt. to be 1.2 GPa at most. Temperature calculations point towards conditions reaching

above 600°C, with indications of peak temperature around 700°C. The peak metamorphic

conditions appear to have reached upper amphibolite facies, perhaps approaching eclogite

facies. On Mt. Åreskutan on the other hand, peak pressures are reported to be 2.6-3.2 GPa and

temperatures of 800-820°C (Klonowska et al. 2014), well within eclogite facies. In addition,

microdiamonds have been reported recently (Klonowska et al. 2015). Similar (ultra)high

pressure conditions are reported from other SNC localities. In the area of Stor Jougdan,

northern Jämtland, peak conditions are estimated to be 2.8-4.0 GPa and 750-900°C

(Klonowska et al. 2015).

However, the metamorphic conditions of Mt. Åreskutan were, by these recent studies,

upgraded from lower conditions, with a peak pressure of 0.5-0.6 GPa and temperatures not

exceeding 740°C (Arnbom 1980, Arnbom &Troeng 1982, Ladenberger et al. 2010), matching

the results of this study. It is noteworthy that Mt. Åreskutan has long been known to have

experienced granulite facies metamorphism. Still it was only recently that the area was shown

to have had an earlier high-pressure and even ultra-high pressure history (Klonowska et al.

2014, 2015). The upgrade was due to more detailed studies of mineral textures, in

combination with thermodynamic modeling. It is therefore possible that more detailed studies

of the Helags Mt. area could result in similar upgrade. A follow-up study, aiming at

investigating how representative the results of this study are of the whole area, is therefore

advised before a definite answer to the question of whether or not the Helags Mt. area and Mt.

Åreskutan are true correlatives.

Even if this study has not shown the two areas to be true equivalents, this makes a good

foundation for interesting future work. Further studies of the Helags Mt. would probably be

highly interesting. Mt. Åreskutan diamonds have been found only as inclusions in the cores of

garnet. The chance of finding them in the few thin sections from Helags Mt. was small; more

comprehensive investigations may well be rewarding.

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6. Summary and final remarks

Examination of mineral textures and compositions, combined with monazite geochronology,

has provided new information about the metamorphic history of the Helags Mt. area. The

felsic gneisses of the Helagsfjället Klippe are comparable to those of the Middle Seve

elsewhere in central Jämtland. In addition, this study has revealed two garnet growth episodes

to have occurred. The first event took place at high temperatures, possibly reaching above

700°C. Some indications, though not conclusive, place this event in connection with

migmatisation. The second garnet growth episode took place at lower pressure and

temperature conditions, but cannot with any certainty be connected to a certain geological

event. Similarities to garnet found in other studies of similar areas suggest a possible

connection to shearing. A studied, flat-lying shear zone, located within the Helagsfjället

Klippe, reveal only limited information. Based on both sinistral and dextral kinematic

indicators it seems that this zone has been active in both directions, and possibly at two

stages.

Monazite geochronology has not resulted in clear distinction of metamorphic events, but

gives a Caledonian age range (400-480 Ma) comparable with the SNC in northern Sweden

(Norrbotten). It is possible that future geochronological work on these rocks, focused on other

minerals (garnet, zircon), could aid in a more precise dating of events.

Future studies of the Helags Mt. area will show whether or not the results of this study

are representative of the whole area. It is not unlikely that more detailed studies of mineral

textures from different parts of the Helags Mt. could upgrade the metamorphic conditions of

this study, and thus confirm that the Helags Mt. and Mt. Åreskutan are indeed correlatives.

51

7. Acknowledgements

This project has been challenging, and it could not have been completed without the support

from my colleagues and friends. First of all I am eternally grateful to Jarek Majka and David

Gee, who did not only propose this project but has been a tremendous support along the way.

I also want to extend my warmest thanks to Iwona Klonowska, who has devoted much of her

time to assist me in my work, from sample collection to modeling. I am also thankful to

Håkan Sjöström, who during the early stages of this study provided insight and expertise that

greatly assisted the work. I am equally thankful to Marian Janák, for assistance with analyses

in Bratislava and for taking his time to guide me through the methods as well as the wonderful

city. If it was not for my fellow student and college Barbro Andersson, this thesis could not

have been completed. Through many fruitful discussions she has helped to developed and

improved my work. Karin Högdahl and Max Jensen are thanked for their valuable input and

comments during the completion of this thesis. Last, but not least, I want to thank my family.

Without their love and devotion, as well as valuable input into the final report, I could not

have succeeded as I have.

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