Pressure-Temperature Estimates on the Tjeliken Eclogite from Northern Jämtland, Swedish Caledonides

Självständigt arbete Nr 46

Pressure-temperature Estimates on

the Tjeliken Eclogite from Northern

Jämtland, Swedish Caledonides

Pressure-temperature Estimates on

the Tjeliken Eclogite from Northern

Jämtland, Swedish Caledonides

Barbro Andersson

Barbro Andersson

Uppsala universitet, Institutionen för geovetenskaper Kandidatexamen i Geovetenskap, 180 hp Självständigt arbete i geovetenskap, 15 hp Tryckt hos Institutionen för geovetenskaper  Geotryckeriet, Uppsala universitet, Uppsala, 2013. Eclogites are important in order to understand orogenic processes, since  their  presence  indicates  high­pressure  metamorphism.  In  northern  Jämtland,  Swedish  Caledonides,  eclogites  have  been  found  at  several  places  in  the  Seve  Nappe  Complex  (SNC).  The  mountain  Tjeliken  in  the Lower Seve Nappe is one of them. Dating relates the high­pressure  metamorphism to the Late Ordovican subduction of the Baltoscandian  margin during the closure of the Iapetus Ocean.  

In this study new P­T conditions are presented for the Tjeliken eclogite  based on petrographical and geochemical studies of an eclogite sampled  on  the  top  of  Tjeliken  in  summer  2010.  Peak  assemblage  consists  of  garnet  +  omphacite  +  phengite  +  quartz.  New  peak  conditions  are  calculated  to  c.  2.7  GPa  and  700°C.  These  P­T  conditions  are  in  the  upper  part  of  the  quartz  stability  field,  close  to  the  quartz  ­  coesite  stability line. 

The  new  P­T  conditions  correspond  well  to  other  P­T  calculations  of  eclogites  in  northern  Jämtland  and  indicate  a  deep  subduction  of  the  Baltoscandian margin already in the Late Ordovician.  

Självständigt arbete Nr 46

Pressure-temperature Estimates on

the Tjeliken Eclogite from Northern

Jämtland, Swedish Caledonides

Abstract

Eclogites are important in order to understand orogenic processes, since their presence

indicates high-pressure metamorphism. In northern Jämtland, Swedish Caledonides, eclogites have been found at several places in the Seve Nappe Complex (SNC). The mountain Tjeliken in the Lower Seve Nappe is one of them. Dating relates the high-pressure metamorphism to the Late Ordovican subduction of the Baltoscandian margin during the closure of the Iapetus Ocean.

In this study new P-T conditions are presented for the Tjeliken eclogite based on petrographical and geochemical studies of an eclogite sampled on the top of Tjeliken in summer 2010. Peak assemblage consists of garnet + omphacite + phengite + quartz. New peak conditions are calculated to c. 2.7 GPa and 700°C. These P-T conditions are in the upper part of the quartz stability field, close to the quartz - coesite stability line.

The new P-T conditions correspond well to other P-T calculations of eclogites in northern Jämtland and indicate a deep subduction of the Baltoscandian margin already in the Late Ordovician.

Sammanfattning

Eklogiter är viktiga för att förstå bergkedjebildningsprocesser, då deras närvaro indikerar högtrycksmetamorfos. I norra Jämtland, i de svenska Kaledoniderna, har eklogiter hittats på flera ställen i Seveskollan. Fjället Tjeliken i den lägre delen av Seveskollan är en av dessa platser. Datering kopplar samman högtrycksmetamorfosen med subduktionen av den Baltoskandiska plattan när Japetushavet stängdes under sen ordovicium.

Denna studie presenterar nya tryck- och temperaturförhållanden för Tjelikeklogiten baserat på petrografiska och geokemiska studier av en eklogitstuff insamlad från toppen av Tjeliken sommaren 2010. Mineralsammansättningen stabil vid de högsta tryck- och

temperaturförhållandena är granat + omfacit + fengit + kvarts. Nya högsta tryck- och temperaturförhållanden beräknas till cirka 2.7 GPa och 700°C. Dessa förhållanden ligger i övre delen av stabilitetsfältet för kvarts, nära gränsen till stabilitetslinjen för koesit.

De nya framtagna tryck- och temperaturförhållandena sammanfaller väl med andra tryck- och temperaturbestämningar för eklogiter i norra Jämtland. Ökningen i både tryck och temperatur jämfört med tidigare studier indikerar en djup subduktion av den Baltoskandiska plattan redan under sen ordovicium.

Table of contents

1. Introduction!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"# 2. The Caledonian Orogeny!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"# 3. Eclogites!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!$# 4. Occurrence of eclogites in the Scandinavian Caledonides!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!%# 4.1 Norrbotten County!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!%# 4.2 Jämtland!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!%# 5. Methods!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!&# 5.1 Light microscopy!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!&# 5.2 Electron microprobe analysis!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!&# 5.3 Geothermobarometry!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!&# 6. Results!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'# 6.1 Mineral assemblage!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'# 6.2 P-T conditions!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!""# 7. Discussion!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"(# 8. Conclusions!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"$# Acknowledgement!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"%# References!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"%# Appendix!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!")#

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1. Introduction

Eclogites are important for understanding orogenic processes. Their mineralogy reflects high-pressure to ultra high-high-pressure conditions. Such conditions typically occur in collision and subduction zones that are typical geological settings for orogenesis. Hence by studying the mineralogy and texture of eclogites orogenic processes can be reconstructed.

In the Scandinavian Caledonides eclogites are well known from Western Gneiss Region and Tromsö Nappe in southern and northern Norway, respectively. Less known are the eclogites of the Seve Nappe Complex (SNC) in southern Norrbotten County and northern Jämtland in Sweden. This study focuses on eclogites of the Tjeliken Mountain in the Lower Seve Nappe in northern Jämtland. Eclogites also occur at near by Lake Friningen and Sipmikk Creek in Middle Seve. The eclogites in the area were first characterised by Van Roermund (1985), who calculated P-T conditions for them. However, recently the P-T conditions for the Friningen eclogite were updated and first evidence of UHP metamorphism in the Swedish Caledonides was presented (Janák et al. 2012). Tjeliken shares a similar geological setting with Friningen and for this reason it is likely that P-T conditions can be updated for the latter as well. This essay presents new P-T conditions for Tjeliken eclogite based on petrographical and geochemical studies of an eclogite sampled on top of Tjeliken in summer 2010. P-T

conditions are determined by geothermobarometric calculations. The results bear implications for understanding of tectonic processes during the subduction along the Baltoscandian margin in the Ordovican.

2. The Caledonian Orogeny

The Caledonian orogeny proceeded from Early Ordovician to Early Devonian as a consequence of the closure of the Iapetus Ocean and subsequent collision between the continents Baltica and Laurentia (Gee et al. 2008). The orogenesis included several high-pressure and ultra-high high-pressure phases. During these phases, slabs of continental and oceanic crust were subducted down to mantle depths. The slabs were later brought up to surface again by eduction (e.g. Brueckner et al. 2004).

In Early Ordovician the Iapetus Ocean started to contract by subduction at both margins of the ocean (Gee et al. 2008). The initial phase of the orogeny, the Finnmarkian phase, at c. 500 Ma is traditionally interpreted as an arc-continent collision between the Virisen volcanic arc and the western margin of Baltica (Stephens et al. 1985). A later model (Brueckner et al. 2004) suggests that the collision occurred between the Virisen arc and a microcontinent or peninsula of Baltica similar to today’s “Baja California” in Mexico. Later at c. 460 Ma the resulting composite terrane collided with mainland Baltica during the “Jämtlandian Orogeny”. The closure of Iapetus Ocean terminated with the Scandian orogeny at c. 415-400 Ma when Laurentia and Baltica finally collided (Brueckner et al. 2004). During the collision, Baltica underthrusted Laurentia and slabs of oceanic and continental crust were transported hundreds of kilometres eastwards and southeastwards over Baltica (e.g. Gee. 1975, Gee et al. 2010). For this reason the Caledonides are formed by allochthons stacked on each other and

separated by imbricate thrusts. Structurally the mountain belt can be divided into four major tectonic units called the Lower, Middle, Upper and Uppermost Allochthon (figure 1). The allochthons are further subdivided into smaller tectonic units. The Lower and Middle Allochthons are made up of sediments of Baltican affinities, whereas the Upper and

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Uppermost Allochthons consist of sediments of oceanic origin and Laurentian origin, respectively (Gee et al. 2010).

The Swedish part of Caledonides is built up of all allochthons, except the Uppermost. The highest metamorphic grade rocks occur within the Middle Allochthon that includes the Seve Nappe Complex (Lindström et al. 2000). The complex is c. 800 km long and consists of imbricate thrust sheets that dip gently towards west (e.g. Root et al. 2012). The sheets are pinch-and-swell deformed and almost thin out in western Sweden. However in Norway the nappes reappear again (Gee et al. 1979). The Seve Nappe Complex (SNC) consists of metamorphosed basaltic volcanics and metamorphosed quartz and feldspar rich sandstones (Lindström et al. 2000). The grade of metamorphism varies within the nappe complex from low to high, with the peak in the middle of SNC. Compared to the underlying Särv Nappe and the overlying Köli Nappe Complex the grade of metamorphism is higher in the Seve Nappe Complex (e.g. Root et al. 2012).

Figure 1. Map of the Scandinavian Caledonides showing the different tectonic units and occurrences

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3. Eclogites

Eclogite is a metamafic rock formed in the upper mantle or the lower crust, typically above the stability field of albite. The main minerals of the rock are omphacite and garnet.

Omphacite is a sodic high-pressure clinopyroxene that mainly consists of a solid solution of the components jadeite (Jd), acmite (Ac), diopside (Di), hedenbergite (Hd), (NaAlSi2O6 -NaFe3+Si2O6-CaMgSi2O6-CaFeSi2O6). Garnet mostly consists of a solid solution of almandine (Alm), pyrope (Prp), grossular (Grs) and spessertine (Sps). Additionally eclogites consist of other minerals too, depending on the formation environment of the rock (Bucher et al. 2011). In a metamorphic facies scheme the field of eclogite facies occupies the widest P-T region. It covers a temperature range of 400-1000 °C. Thus eclogites are formed at many geotectonic settings from the neighbouring fields of blueschist, amphibole and granulite facies.

Depending on geological setting and temperature of formation three general types of eclogites can be distinguished (Bucher et al. 2011):

1. Low temperatureandhigh-pressure (LT/HP) eclogites. This type of eclogite forms

at subduction zones where LT/HP - conditions are typical. In this case eclogite forms from blueschist facies.

2. Intermediate temperature (IT) eclogites. These eclogites form at collision zones from amphibole facies.

3. High temperature (HT) eclogites. Eclogites formed due to crustal extension where the geotherm is very hot.

Due to the differences in temperature, pressure and water pressure, each of the three

geodynamic types of eclogites is characterised by typical mineral assemblages stable for the prevailing conditions. During prograde metamorphism, hydrous minerals continuously are substituted by less hydrous minerals. At high pressures and temperatures the mineral assemblage is mostly made up of anhydrous minerals. However, the mineral assemblage an eclogite displays is also affected by the retrogressive metamorphism that occurs after its formation. The retrogressive metamorphism is connected to how the eclogite is brought up to the surface (exhumed), and occurs via the following facies fields (Bucher et al. 2011):

1. Blueschist field as a consequence of cooling and decompression simultaneously. 2. Amphibole field caused by initial decompression followed by cooling.

3. Granulite field caused by initial heating and decompression, followed by cooling.

Whether an eclogite displays its high-pressure assemblage or more of a retrogressive assemblage is dependent on the access of water during the retrogressive metamorphism and the duration of the retrogressive phase (Bucher et al. 2011).

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4. Occurrence of eclogites in the Scandinavian Caledonides

In the Scandinavian Caledonides, eclogites occur in several different geographical areas and in different tectonic units (figure 1). Only the Lower and Upper Allochthon seem to be free of high-pressure and ultra high-pressure rocks (Brueckner et al. 2004).

In southern Norway, eclogites have been found in the Jaeren Nappe, Western Gneiss Region (WGR) and Lindås Nappe. In northern Norway eclogites occur in the Tromsö Nappe and on Lofoten. Rb-Sr dating of the Lindås eclogites has yielded eclogitization age of c. 430 Ma (Glodny et al. 2008). The Western Gneiss Region eclogites, that have a similar history, have been dated to c. 400 and c. 418 Ma (Brueckner et al. 2004). Finally, the eclogites of Tromsö Nappe Complex in the Uppermost Allochthon have been dated to c. 450 Ma (Corfu et al. 2003).

In Sweden eclogites have been found in the Seve Nappe Complex in the southern Norrbotten County and northern Jämtland. The two localities are described in detail below.

4.1 Norrbotten County

In the Norrbotten County, the Seve Nappe is divided into three tectonic mega lenses named from bottom to top, the Vaimok, Sarek and Tssäkok lenses. High-pressure rocks have been found in the Vaimok and Tsäkkok lenses. The Sarek lens shows no evidence of high-grade metamorphism. The Vaimok lens is subdivided into the subnappes: Lower Seve Nappe, Grapesvare Nappe and Maddåive Nappe. Eclogites have been found in the two latter nappes (Root et al. 2012). The Grapesvare Nappe and Maddåive Nappe consist of quartzite, marble, mica schists, calc-silicates and metavolcanic rocks (Albrecht, 2000). These rocks are

interpreted to be rift - related sediments and volcanic rocks formed at the thinned margin of Baltica (Brueckner et al. 2007).

In the Tsäkkok lens, eclogites have been found both, in the Lower Tsäkkok and the Upper Tsäkkok, respectively. The former mainly consists of quartzofeldspathic schist and the latter mainly of marble quartzite. The eclogites of the Tsäkkok lens contain paragonite, low-T glaucophane and crossite that indicate a cooler metamorphic history in the Tsäkkok lens compared to the Vaimok lens (Root et al. 2012).

4.2 Jämtland

In northern Jämtland, the Seve Nappe Complex consists of three units called from east to west the Lower, Middle and Upper Seve Nappe (also referred to as Eastern, Central and Western Belt in some literature) (Gee et al. 2010). The Upper Seve Nappe is free of eclogites. However, in both the Middle and Lower Seve nappes eclogites, have been found (figure 2). According to Van Roermund (1985), the eclogites in Jämtland represent dikes and sills intruded during the early break up of a continent.

The Middle Seve Nappe consists of partially migmatized gneisses and amphibolites. Peridotites are present, mainly concentrated around tectonic contacts (Root et al. 2012). Eclogites have been found at Sipmikken Creek and at Friningen. The Sipmik eclogite

formation has been estimated to a pressure and temperature of 1.8 ± 0.1 GPa and 780 ± 50 °C and the Friningen eclogite to a pressure of 2 GPa (Van Roermund, 1985). However, recently the P-T conditions for the Friningen eclogite were updated to c. 3 GPa and 800 °C (Janák et al. 2012).

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Figure 2. Occurence of eclogites in the Middle Seve Nappe (Central Belt) and Lower Seve Nappe (Eastern Belt) in Jämtland. (Brueckner et al. 2007)

In the Lower Seve Nappe, eclogites have been found at, amongst others the mountain Tjeliken situated approximately 28 km northeast of the town Gäddede. The eclogite body is almost 2 km wide and occupies the top of the mountain. It overlies a unit of mica schist situated in quartzite (figure 3).

Currently, there are two contradictory tectonic interpretations of the area. An older

interpretation considers Tjeliken as a klippe of amphibolite and periodite detached from the surrounding quartzite unit (Strömberg et al. 1984). In a newer interpretation, Tjeliken is considered as a unit of eclogite in mica schists that belongs to the same unit as the surrounding quartzite (Zachrisson et al. 1990).

Additional mapping made within this project seem to support the older tectonic (klippe) interpretation, but the lithologies found are eclogite and mica schist. However, it has to be added that the extent of the eclogite body seems to be smaller than mapped in the older interpretation. It seems to occupy only the highest areas of the mountain and is not present in the lower areas in between the two peaks.

P-T conditions for the Tjeliken eclogites have been determined to 1.4 ± 0.15 GPa and 550 ± 70 °C (Van Roermund, 1985).

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Figure 3. Geological interpretation of Tjeliken Mountain, northern Jämtland. The map is modified from

Zachrisson et al. (1990) and based on the new mapping done within this project.

4.3 Dating of high-pressure events in Norrbotten County and Jämtland

Sm-Nd dating of garnets from Tsäkkok and Vaimok lens has yielded ages of 505 ± 18 Ma and 503±14 Ma for peak metamorphism for the Norrbotten area (Mørk et al. 1988). In Jämtland, Sm-Nd dating of two eclogites and four garnet peridotites from Lower and Middle Seve indicates a later high-pressure event at 457.9 ± 4.5 Ma (Brueckner et al. 2007)

U-Pb dating of same areas yields different ages. Dating of zircon from eclogites in the Tsäkkok and Vaimok lenses yields similar ages of 482 ± 1 Ma. For the Tjeliken eclogite, U-Pb dating of zircons indicates an age of 445.6 ± 1.2 Ma (Root et al. 2012).

Both types of isotope dating methods agree on the relative ages of the two areas, with a c. 45 older high-pressure event in the Norrbotten County compared to Jämtland. However, there is a disagreement in the absolute ages of the two areas. Thus, the exact timing of the high-pressure metamorphism is not known. However, the two areas show two separate Late Ordovician high-pressure events in the Swedish Caledonides. Both are older than the HP metamorphism in the parautochtonous Lindås Nappe and WGR in Norway. The c. 450 Ma high-pressure metamorphism in the Tromsö Nappe is of similar age to the Jämtland HP units. However, the Tromsö Nappe belongs to the outer margin of Laurentia and the metamorphism in that nappe occurred at the Laurentian margin before it collided with Baltica (Brueckner et al. 2004). Alternatively, Janák et al. (2012) suggest that the Tromsö Nappe can be an out-of-sequence thrust and might represent the SNC.

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5. Methods

Three methods were used in the study of the Tjeliken eclogite. These are described separately below.

5.1 Light microscopy

Two thin sections of the eclogite were prepared in a polishing lab in Poland and studied using a petrographic microscope Nikon Eclipse E600 Pol. Mineral assemblages and microstructures were identified by standard microscopy procedures.

5.2 Electron microprobe analysis

Microprobe analysis (EMPA)using a Jeol-JXA8530F Hyperprobe electron microprobe was done to obtain mineral chemistry of assemblages present in the samples. A 10 nA beam current with 15 kV accelerating voltage was used. Calibration of the instrument minerals was performed using the following mineral standards Si, Ca – wollastonite, Na – albite, K – orthoclase, Mn, Ti – pyrophanite and metal oxides Fe – Fe2O3, Cr – Cr2O3, Al – Al2O3, Mg – MgO. Counting times: 10 s on peakand 5 s on +/- background. Only the K! lines were measured for all elements. Data was corrected using PAP routine.

5.3 Geothermobarometry

Geothermobarometry was used to calculate the pressure and temperature conditions for the formation of the eclogite. The idea of the method is to find an equilibrium phase assemblage in the rock for which two reactions can be written. For the equilibrium state of these, the temperature and pressure are calculated. In a P-T diagram two lines represent the pressure and temperature of the equilibrium. The intersection of the two lines represents a condition of simultaneous pressure and temperature equilibrium at which the phase assemblage of the rock was formed (Bucher et al. 2011).

The metamorphic conditions of the formation of the Tjeliken eclogite were calculated using a combination of the thermometer of Fe-Mg exchange between garnet and clinopyroxene (Ravna, 2000) and the barometer based on the following net-transfer reaction:

6 diopside + 3 muscovite = 2 grossular + pyrope + 3 celadonite

The latter was calibrated by Ravna and Terry (2004). The calibrations are based on the thermodynamic database of Holland and Powell (1998), activity model for phengite solid solution model presented by Holland and Powell (1998), the clinopyroxene activity model by Holland (1990) and the garnet activity model by Ganguly et al. (1996).

The thermobarometric calculations were made using phengite with high silica content, omphacite with high jadeite content and garnet with high grossular content.

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6. Results

The results are presented in two parts. The first part comprises the mineralogy and petrology of the Tjeliken eclogite and in the second part the geothermobarometric calculations are presented.

6.1 Mineral assemblage

There are four textural types of garnets in the Tjeliken eclogite. First type (Grt I) is represented by small euhedral exsolution garnets present in omphacite (figure 4a). The chemical composition of these is Alm58-64Grs18-20Pyr17-21Sps1-2.

The second type of exsolution garnet (Grt II, figure 4b) also present in omphacite has a more elongated shape compared to the first one. The chemical composition is Alm61Grs22Pyr14Sps3. Representative chemical analyses of exsolution garnets (Grt I and Grt II) are shown in table 1 in the appendix.

Figure 4a. BSE image of exsolution garnets (Grt I) in omphacite.

Figure 4b. BSE image of exsolution garnet (Grt II) in omphacite.

Figure 4c. BSE-image of garnet with inclusions (Grt III). The arrow shows the chemical profile.

Figure 4d. BSE-image of atoll garnets (Grt IV). The two upper arrows show the chemical profile.

Grt Omp Cpx Pl Rt Cpx Grt Omp Grt Grt Qtz

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The third type of garnet in the Tjeliken eclogite is represented by large euhedral to subhedral garnets (Grt III) in omphacite with numerous inclusions (figure 4c). Minerals included are: apatite, amphibole, rutile, ilmenite, quartz, zircon and plagioclase. The chemical composition along a profile in garnet is fairly homogenous (figure 5). General composition of the rims is Alm60-61Grs21Pyr16-17Sps1-2 and of the cores Alm55-56Grs24-26Pyr17-18Sps1-2, respectively (table 2, appendix).

Figure 5. Chemical composition of a garnet type III.

Atoll garnets (Grt IV) with cores partly or fully replaced by quartz and plagioclase represent the fourth type of garnet in the Tjeliken eclogite (figure 4d). The peninsulas have chemical composition of Alm58-62Grs17-22Pyr19-21Sps1-2 in the cores and Alm57-61Grs15-22Pyr19-21Sps1-2 in the rims, respectively. The chemical composition of the rings is Alm57-60Grs20-22Pyr18-20Sps1 in the inner parts and Alm56-57Grs23-24Pyr18-19Sps1 in the outer parts, respectively. Full chemical analysis is shown in figure 6. Representative chemical analyses of these garnets are shown in table 3 in appendix.

Figure 6. Chemical composition of an atoll garnet. Sample points 1-100 are taken in the peninsula and sample points 101-130 are taken in the ring. Figure 4d shows the position of the profile. *# (*# %*# )*# '*# "**# *# (*# %*# )*# '*# "**#

Chemical composition (%) _{Sample points}

Chemical composition of Grt III

+,-# ./,# 01-# 2345236789# :;<# *# "*# (*# $*# %*# =*# )*# &*# '*# >*# *# (*# %*# )*# '*# "**# "(*# Chemical composition (%) Sample points

Chemical composition of Grt IV

+,-# ./,# 01-# 2345236789# :;<#

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Omphacite occurs in the matrix of the eclogite. The grains are large and anhedral. Jadeite content varies from XJd= 0.48 to XJd= 0.56. Quartz inclusions are present in the omphacite as small needle like grains and as larger grains. The latter textural type is surrounded by radial cracks.

Quite commonly in the matrix, the omphacite has broken down to diopside that forms symplectitic intergrowth (figure 7a) with plagioclase (XNa = 0.81-0.98). Diopside is also present in quartz in garnet (figure 7b),probably as a breakdown product of garnet. This type of diopside is richer in aluminium than the former one. Representative chemical analyses of clinopyroxenes and plagioclases are shown in tables 4 and 5 in the appendix, respectively. Phengite occurs rarely in omphacite, as euhedral grains (figure 7c). The silicon content is about 3.3 a.p.f.u and the chemical composition of the grains is almost homogenous. However, some of the phengites have been retrogressed to biotite and/or chlorite (table 6, appendix). Paragasitic amphiboles are present as euhedral inclusions in garnets and quartz and as retrogressive coronas around the garnets. The two types of amphiboles are similar in the aluminium and sodium content (table, 7 appendix).

Other minerals present in lesser amounts in the eclogite are: rutile, zircon, ilmenite and apatite. Rutile appears as euhedral to subhedral red-brownish inclusions in pyroxenes and garnets. Ilmenite, zircon and apatite occur exclusively as inclusions in garnet (figure 7d).

Figure 7a. BSE image of omphacite decomposed into symplectitic intergrowth of diopside and plagioclase.

Figure 7b. BSE image of omphacite in quartz inclusion in garnet.

Figure 7c. BSE image of phengite in omphacite. Figure 7d. BSE image of inclusions of rutile, amphibole, quartz and ilmenite in garnet.

Omp Grt rim Amp Cpx Omp Ph Grt Qtz Pl Cpx Grt Grt Ilm Rt Amp Qtz

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6.2 P-T conditions

Metamorphic peak conditions were calculated for the assemblage garnet + omphacite + phengite. The thermobarometric calculations generated a pressure and temperature between 2.3-2.7 GPa and 660-701°C, respectively. Maximum pressure and temperature of 2.7 ±0.05 GPa and 701 ± 50°C was calculated. It is shown as the intersection of the lines for the Grt – Phn-Cpx thermometer and Grt-Phn-Cpx barometer (figure 8).

Figure 8. Pressure-temperature diagram for peak pressure and peak temperature of the Tjeliken eclogite.

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7. Discussion

Microstructures and microtextures in the Tjeliken eclogite indicate both prograde and retrogressive metamorphism.

Prograde metamorphism is indicated by exsolution garnets (Grt I) and garnets rich in

inclusions (Grt III). The euhedral exsolution garnets in omphacite are first generation garnets exsolved from omphacite when temperature and pressure increased. The inclusions of apatite, amphibole, rutile, ilmenite, quartz, zircon and plagioclase in garnets (Grt III) are interpreted as remnants from lower grade and peak metamorphism. As the metamorphism progressed these became incorporated in the growing garnets.

The peak-pressure assemblage of the Tjeliken eclogite consists of garnet + omphacite + phengite + quartz. This assemblage was stable under the calculated peak pressure and peak temperature of 2.7 GPa (±0.05 GPa) and 701°C (± 50°C), respectively. The conditions are just below the quartz-coesite stability line. Radial cracks around quartz in omphacite might indicate that coesite has been present in the Tjeliken eclogite during peak conditions. The chemical compositions of the different phases are generally homogenous. This indicates that the high grade metamorphic phase is well preserved and that the rock reflects mineral assemblage present at the peak conditions.

Retrogressive metamorphism, caused by exhumation, is indicated by atoll garnets (Grt IV), symplectites and amphibole coronas around garnets. The alteration of atoll garnet cores is caused by fluid infiltration of the garnets during decreasing P-T conditions, generating element exchange between garnet core and matrix. As a result of the reactions, quartz and plagioclase have formed and replaced the garnet cores. Retrogressive alteration of garnets is also visible in their rims, where amphibole coronas have formed as a response to lower P-T conditions. Chemical composition along profiles also shows a slight decrease of pyrope and grossular content in the rims. The symplectitic intergrowth of diopside and plagioclase

produced by partial breakdown of omphacite during decreasing P-T conditions indicates rapid exhumation.

The second type of exsolution garnets (Grt II) might either belong to an early progressive assemblage or a late retrogressive assemblage. Their lower pyrope content (XPrp=0.14) compared to exsolution garnets type I (XPrp=0.17-0.21) indicates lower temperature during their formation. However, it is not possible to say whether this type of garnet formed at early prograde or late retrograde metamorphic stage.

The calculated peak pressure and temperature of the Tjeliken eclogite in this study is much higher than earlier estimations of c.1.4 GPa and c. 550 °C. The big difference between the two estimations can partly be explained by the usage of different calculation methods. In the earlier calculation, the P-T dependence of jadeite content in omphacite was used to calculate the pressure. In this study, a net-transfer reaction between garnet, phengite and omphacite is used. The barometer that was used in the earlier study of the Tjeliken eclogite was developed in 1980 and the one used in this study in 2004. Thus there has been a lot of time for

improvements of the accuracy of barometers between the two P-T estimations. For the temperature calculations, a similar method has been used in both studies.

The updated P-T conditions for the Tjeliken eclogite correspond well to the updated P-T conditions by Janák et al. (2012) for the Friningen eclogite. Friningen is situated 25 km northwest of Tjeliken in the Middle Seve that is of a higher metamorphic grade than the Lower Seve, where Tjeliken is situated.

13

The Sm-Nd and U-Pb datings, c. 445 Ma and c. 460 Ma, respectively, do not agree in time of the peak metamorphic conditions of the Jämtland area. Although no exact timing of the high-pressure event can be given, both dating methods indicate a late Ordovician subduction of the Jämtland terrane. Taken the P-T estimations of this study and the study by Janák et al. (2012) into consideration, the subduction of the Baltoscandian margin ought to have been down to depths of at least 80 km, i.e down to the upper mantle. In other words the Ordovician subduction of Baltoscandian margin seems to have happened in the collisional setting. However, more research is needed to confirm that this initial collision between Baltica and Laurentia has commenced in the Ordovician i.e. earlier than previously accepted.

8. Conclusions

• Peak P-T conditions for the Tjeliken eclogite is estimated to 2.7 ± 0.05 GPa and 701 ± 50°C. These conditions correspond to a subduction of the Baltoscandian margin down to the mantle (c. 80 km). Peak-pressure assemblage that was stable during these conditions consists of garnet + omphacite + phengite + quartz.

• Prograde metamorphism is indicated by euhedral exsolution garnets (Grt I) and garnet (Grt III) with inclusions of apatite, amphibole, rutile, ilmenite, quartz, zircon and

plagioclase.

• Retrogressive metamorphism is indicated by atoll garnets (Grt IV), symplectitic

intergrowth of diopside and plagioclase, amphibole coronas around garnets and a slight decrease of grossular and pyrope content in garnet cores

14

Acknowledgement

First and foremost, I want to express my gratitude to my supervisor Dr. Jaroslaw Majka (Uppsala University) for giving me the opportunity to work with this interesting project. I also want to thank him for always taking time to answer my questions and his commitment in my project, which has been to a great help.

Secondly, I also want to thank Iwona Klonowska (Uppsala University) for help with the electron microprobe analysis, fieldwork and editing of figures. I am also grateful to Senior Professor David G. Gee (Uppsala University) for providing me with the eclogite sample from Tjeliken and his contribution of knowledge in general. Furthermore I also want to thank Åke Rosén (Uppsala University) and Karolina Kosminska (AGH-UST, Kraków) for the help they have provided me with during this project.

Finally, I also wish to thank Professor Alasdair Skelton (Stockholm University) in charge of the project “Metamorphic Map of Sweden” (funded by SGU) for the financial support.

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16

Appendix

!

Table 1. Chemical composition of exsolution garnets type I and type II. Analys. point Grt9 Grt10 Grt11 Grt44 Grt43 Tex. type Grt I Grt I Grt I Grt I Grt II SiO2 40.538 39.614 38.874 38.444 38.826 TiO2 0.069 0 0 0 0.058 Al2O3 21.312 22.029 21.744 21.829 21.405 Cr2O3 0.022 0.025 0 0.061 0.051 FeO 26.455 27.141 26.94 29.104 27.391 MnO 0.731 0.576 0.696 0.83 1.416 MgO 5.218 5.349 5.026 4.299 3.418 CaO 7.132 6.327 6.993 6.332 7.906 Na2O 0.512 0.197 0.109 0.036 0.052 K2O 0.012 0.003 0.012 0 0.027 Total 101.489 101.064 100.285 100.899 100.498 Si 3.12 3.05 2.99 2.96 2.98 Ti 0 0 0 0 0 Al 1.93 2 1.97 1.98 1.94 Cr 0 0 0 0 0 Fe2+ 1.7 1.75 1.73 1.87 1.76 Mn 0.05 0.04 0.05 0.05 0.09 Mg 0.6 0.61 0.58 0.49 0.39 Ca 0.59 0.52 0.58 0.52 0.65 Na 0.08 0.03 0.02 0.01 0.01 K 0 0 0 0 0 Total 8.06 7.99 7.91 7.88 7.84 XAlm 0.58 0.60 0.59 0.64 0.61 XSps 0.02 0.01 0.02 0.02 0.03 XPyr 0.20 0.21 0.20 0.17 0.14 XGrs 0.20 0.18 0.20 0.18 0.22

17

Table 2. Chemical composition of garnets with inclusions (Grt III).

Anal.

point Grt42 Grt3 Grt47 Grt4 Grt80*

Text.

type Grt III core Grt III rim Grt III core Grt III rim Grt III core

SiO2 37,281 38,489 37,575 38,609 37,461 TiO2 0,101 0,034 0,052 0,03 0,043 Al2O3 21,381 22,171 21,319 21,851 21,294 Cr2O3 0,102 0,011 0,006 0,02 0 FeO 25,497 27,986 25,913 28,058 25,369 MnO 0,64 0,692 0,725 0,64 0,557 MgO 4,424 5,579 4,568 5,614 4,32 CaO 9,307 5,97 8,8 6,292 10,017 Na2O 0,12 0,101 0 0,024 0 K2O 0,017 0 0,011 0,014 0 Total 98,87 101,033 98,969 101,152 99,061 Si 2,92 2,96 2,94 2,97 2,94 Ti 0,01 0,00 0,00 0,00 0,00 Al 1,97 2,01 1,97 1,98 1,97 Cr 0,01 0,00 0,00 0,00 0,00 Fe2+ 1,67 1,80 1,70 1,80 1,67 Mn 0,04 0,05 0,05 0,04 0,04 Mg 0,52 0,64 0,53 0,64 0,51 Ca 0,78 0,49 0,74 0,52 0,84 Na 0,02 0,02 0,00 0,00 0,00 K 0,00 0,00 0,00 0,00 0,00 Total 7,93 7,96 7,93 7,96 7,97 XAlm 0,55 0,60 0,56 0,60 0,55 XSps 0,01 0,02 0,02 0,01 0,01 XPyr 0,17 0,21 0,18 0,21 0,17 XGrs 0,26 0,17 0,24 0,17 0,28

Structural formulae calculated on the basis of 12 oxygens. * Analysis used in P-T calculation.

18 Table 3. Chemical composition of atoll garnets (Grt IV). Anal. point Grt40 Grt2 Grt47 Grt96 Grt48 Grt98 Grt102 Grt126 Grt104 Grt128 Grt106 Grt130 Text. type Grt IV Pen core Grt IV Pen rim Grt IV Pen core Grt IV Pen rim Grt IV Pen core Grt IV Pen rim Grt IV Inner ring Grt IV Outer ring Grt IV Inner rng Grt IV Outer ring Grt IV Inner ring Grt IV Outer ring SiO2 37,537 38,203 36,909 38,586 36,63 37,156 37,676 36,822 38,297 38,329 37,938 38,002 TiO2 0,12 0 1,326 0,029 0,036 0,01 0,002 0,011 0,028 0,025 0,035 0,022 Al2O3 22,463 22,408 21,39 22,397 21,583 21,555 22,297 21,479 22,674 22,441 22,329 22,32 Cr2O3 0,022 0,006 0 0,003 0,004 0,057 0 0 0,035 0,05 0,021 0,023 FeO 26,417 28,359 31,201 26,434 27,838 26,58 26,337 28,019 25,947 28,658 26,299 28,49 MnO 0,642 0,693 1,121 0,669 0,734 0,617 0,634 0,717 0,661 0,728 0,616 0,721 MgO 4,743 5,453 3,114 4,932 3,708 4,848 4,706 5,309 4,816 5,69 4,799 5,407 CaO 7,887 5,82 6,157 7,877 7,835 7,484 8,274 5,764 8,678 5,434 8,628 5,871 Na2O 0,05 0,101 0,172 0,042 0,082 0,09 0,05 0,017 0 0,008 0,011 0,046 K2O 0 0,014 0,039 0,014 0 0,02 0,007 0,035 0,019 0,03 0,039 0 Total 99,881 101,057 101,429 100,983 98,45 98,417 99,983 98,173 101,155 101,393 100,715 100,902 Si 2,97 2,90 2,92 3,02 2,90 2,91 2,96 2,85 3,01 2,96 2,98 2,94 Ti 0,01 0,00 0,08 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 Al 2,09 2,01 1,99 2,07 2,01 1,99 2,06 1,96 2,10 2,04 2,07 2,03 Cr 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 Fe2+ _{1,75 1,80 2,06 1,73 1,84 1,74 1,73 1,81 1,70 1,85 1,73 1,84} Mn 0,04 0,04 0,08 0,04 0,05 0,04 0,04 0,05 0,04 0,05 0,04 0,05 Mg 0,56 0,62 0,37 0,58 0,44 0,57 0,55 0,61 0,56 0,66 0,56 0,62 Ca 0,67 0,47 0,52 0,66 0,66 0,63 0,70 0,48 0,73 0,45 0,73 0,49 Na 0,01 0,01 0,03 0,01 0,01 0,01 0,01 0,00 0,00 0,00 0,00 0,01 K 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 Total 8,10 7,87 8,05 8,10 7,92 7,89 8,04 7,76 8,15 8,02 8,10 7,98 XAlm 0,58 0,61 0,68 0,57 0,62 0,59 0,57 0,61 0,56 0,62 0,57 0,61 XSps 0,01 0,02 0,02 0,01 0,02 0,01 0,01 0,02 0,01 0,02 0,01 0,02 XPyr 0,19 0,21 0,12 0,19 0,15 0,19 0,18 0,21 0,19 0,22 0,18 0,21 XGrs 0,22 0,16 0,17 0,22 0,22 0,21 0,23 0,16 0,24 0,15 0,24 0,16

19 Table 4. Chemical composition of clinopyroxenes. Anal.

point Omp7* Omp19 Omp30 Cpx17 Cpx6 Cpx32

Tex. type Matrix Matrix Matrix Symp Pl Symp Pl Symp Pl

SiO2 56.206 56.444 55.501 53.848 54.084 53.962 TiO2 0.166 0.115 0.076 0.151 0.222 0.199 Al2O3 12.32 12.213 12.041 2.369 5.123 2.787 Fe2O3 0.0 0 0 0 0.0 0 Cr2O3 0.012 0 0.091 0.025 0.072 0.024 FeO 4.589 4.385 4.775 6.157 7.071 6.599 MnO 0.05 0.067 0 0.016 0.088 0.053 MgO 7.163 7.289 7.231 13.855 10.701 13.292 CaO 11.356 10.912 12.215 20.766 18.233 20.988 Na2O 7.094 7.668 7.326 1.632 2.624 1.528 K2O 0.02 0.028 0 0.025 0.007 0 Total 98.952 99.121 99.256 98.819 98.153 99.408 Si 2.019 2.014 1.984 1.994 2.023 1.994 Ti 0.004 0.003 0.002 0.004 0.006 0.006 Al 0.522 0.514 0.507 0.103 0.226 0,121 Fe3+ 0 0 0.025 0.018 0 0 Cr3+ _{0 0 0.003 0.001 0.002 0.001} Fe2+ _{0.138 0.131 0.117 0.172 0.221 0.204} Mn 0.002 0.002 0 0.001 0.003 0.002 Mg 0.384 0.388 0.385 0.765 0.597 0.732 Ca 0.437 0.417 0.468 0.824 0.731 0.831 Na 0.494 0.53 0.508 0.117 0.19 0.109 K 0.001 0.001 0 0.001 0 0 Total 4.00 4.00 4.00 4,00 4.00 4.00 XAe 0.0 0.0 0.026 0.02 0.0 0.0 XJd 0.53 0.56 0.49 0.10 0.21 0.12 XDiop 0.47 0.44 0.48 0.88 0.79 0.88

Structural formulae calculated on the basis of 6 oxygens and 4 cations. * Analysis used in P-T calculation.

20 Table 5. Chemical composition of plagioclases.

!

Table 6. Chemical composition of phengite. Anal.

point Pl16 Pl33 Pl35

Tex. type Sym

Cpx Sym Cpx Intergr. amp

SiO2 68.047 67.656 61.307 TiO2 0 0.058 0.085 Al2O3 19.556 19.365 24.79 FeO 0.167 0.246 0.857 MnO 0 0.003 0.021 MgO 0.051 0.12 0.036 CaO 0.448 0.946 4.285 Na2O 11.112 10.007 8.311 K2O 0.023 0.033 0.676 Total 99.40 98.438 100.37 Si 2.989 2.994 2.718 Ti 0 0.002 0.003 Al 1.012 1.01 1.295 Fe2+ _{0.006 0.009 0.032} Mn 0 0 0.001 Mg 0.003 0.008 0.002 Ca 0.021 0.045 0.204 Na 0.946 0.859 0.714 K 0.001 0.002 0.038 Total 4.979 4.928 5.008 XNa 0.977 0.948 0.747 XK 0.001 0.002 0.04 XCa 0.022 0.05 0.213

Structural formulae calculated on the basis of 8 oxygens. Anal. point Ph37 Ph38 Ph39* Ph57 Ph58 Ph59 SiO2 50.219 49.714 49.878 51.511 51.76 50.94 TiO2 0.836 0.731 0.795 0.757 0.805 0.943 Al2O3 30.819 28.058 28.347 29.772 29.495 29.822 FeO 1.425 1.63 1.651 1.688 1.594 1.562 MnO 0.002 0.012 0.019 0 0 0.022 MgO 2.338 2.773 2.838 2.697 2.774 2.489 CaO 0.121 0.035 0.065 0.162 0.166 0.162 Na2O 0.834 1.106 0.883 0.873 0.664 0.787 K2O 8.281 9.515 9.552 8.208 7.916 7.856 Total 94.88 93.57 94.03 95.67 95.17 94.58 Si 3.306 3.359 3.351 3.363 3.385 3.355 Ti 0.041 0.037 0.04 0.037 0.04 0.047 Al 2.391 2.234 2.245 2.291 2.273 2.315 Fe2+ _{0.0078 0.092 0.093 0.092 0.087 0.086} Mn 0 0.001 0.001 0 0 0.001 Mg 0.229 0.279 0.284 0.263 0.27 0.244 Ca 0.009 0.003 0.005 0.011 0.012 0.011 Na 0.106 0.145 0.115 0.111 0.084 0.101 K 0.696 0.82 0.819 0.684 0.66 0.66 Total 8.858 8.97 8.953 8.851 8.811 8.821

Structural formulae calculated on the basis of 11 oxygens. *Analysis used in P-T calculations

21 Table 7. Chemical composition of amphiboles.

!

Anal.

point Am14 Am20 Am29 Am34 Am45 Am54

Text.type Corona around Grt III Inclusion in Grt III Corona around Grt IV Inclusion in Grt III Corona around ameboidal Grt II Matrix SiO2 41.93 41.48 42.25 41.63 40.56 45.83 TiO2 0.19 0.20 0.08 1.60 0.06 0.62 Al2O3 16.12 15.83 16.30 18.31 15.78 13.12 FeO 13.71 14.33 13.16 12.98 16.45 11.51 MnO 11.51 10.26 11.43 8.92 9.36 12.78 MgO 0.06 0.05 0.08 0.07 0.06 0.04 CaO 10.60 10.50 10.33 8.04 9.81 6.88 K2O 4.05 3.68 3.33 4.86 3.96 4.90 Na2O 0.15 0.11 0.06 0.41 0.05 0.49 Total 98.31 96.42 97.01 96.81 96.09 96.18 Si 6.16 6.22 6.23 6.14 6.17 6.72 Ti 0.02 0.02 0.01 0.18 0.01 0.07 Al 2.79 2.80 2.84 3.19 2.83 2.27 Fe2+ _{1.68 1.80 1.62 1.60 2.09 1.41} Mn 0.01 0.01 0.01 0.01 0.01 0.01 Mg 2.52 2.29 2.51 1.96 2.12 2.79 Ca 1.67 1.69 1.63 1.27 1.60 1.08 K 0.03 0.02 0.01 0.08 0.01 0.09 Na 1.15 1.07 0.95 1.39 1.17 1.39 Total 16.02 15.91 15.82 15.82 16.00 15.82

Structural formulae calculated on the basis of 23 oxygens.