counterclockwise p–t path and isobaric cooling of metapelites from brattnipene, sør rondane...

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Precambrian Research 234 (2013) 210– 228

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Precambrian Research

j o ur nal hom epa ge: www.elsev ier .com/ locate /precamres

ounterclockwise P–T path and isobaric cooling of metapelites from Brattnipene,ør Rondane Mountains, East Antarctica: Implications for a tectonothermalvent at the proto-Gondwana margin

otaro Babaa,∗, Yasuhito Osanaib, Nobuhiko Nakanob, Masaaki Owadac, Tomokazu Hokadad,e,enji Horied, Tatsuro Adachib, Tsuyoshi Toyoshimaf

Department of Natural Environment, University of the Ryukyus, Okinawa 903-0213, JapanDivision of Earth Sciences, Faculty of Social and Cultural Studies, Kyushu University, Fukuoka 819-0395, JapanDepartment of Earth Sciences, Yamaguchi University, Yamaguchi 753-8512, JapanNational Institute of Polar Research, Tokyo 190-8518, JapanDepartment of Polar Science, The Graduate University for Advanced Studies, Tokyo 190-8518, JapanGraduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan

r t i c l e i n f o

rticle history:eceived 17 February 2012eceived in revised form 1 October 2012ccepted 7 October 2012vailable online 13 October 2012

eywords:ounterclockwise P–T path

sobaric coolingør Rondane Mountainsntarcticaondwana

a b s t r a c t

In this paper we provide evidence for a counterclockwise P–T path and isobaric cooling for metapeliticrocks from the Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica. The counterclock-wise path was determined using the following mineral textures and relationships: (1) garnet coronae onsillimanite where the sillimanite is associated with spinel in orthopyroxene-bearing granulite; (2) garnetsthat contain inclusions of spinel, sillimanite, and corundum, and where the sillimanite and corundumhave sparse tiny spinel inclusions; (3) the garnet–orthopyroxene–plagioclase–quartz equilibria rela-tionships using the contrasting compositions of high-Mg garnet – plagioclase inclusions and high-Cagarnet coronae – matrix plagioclase point to an increase in pressure. The peak metamorphic condi-tions were determined by thermobarometry and pseudosection analyses, and are consistently withinthe ranges 850–900 ◦C and 8–9 kbar. Orthopyroxene porphyroblasts close to garnet coronae have rel-atively high Al2O3 contents (∼6.1 wt.%), consistent with the highest temperature conditions reportedso far from the Sør Rondane Mountains. Prismatic subidioblastic staurolite formed as a secondarymineral along the margins of the garnets, and this retrograde staurolite indicates a back-reaction dur-ing isobaric cooling. The conditions for staurolite formation are taken to be 680–700 ◦C and 8.5 kbarusing isopleths, and this is consistent with the appearance of secondary kyanite in the matrix. On
the basis of the metamorphic P–T path and the tectonic setting of the precursor rocks, the followingtentative scenarios can be inferred: (1) the heat source for the early sequence of metamorphism wasderived from advective heat flow at an active continental margin, and (2) the subsequent increase inpressure was caused by overthrusting or obduction of either oceanic crust or oceanic island arc onto theBrattnipene region. These tectonothermal events took place along a possible proto-Gondwana margin at ca. 650–600 Ma.
. Introduction

Metamorphic P–T paths of high-grade gneiss terranes giveruitful information for evaluating crustal evolution, and ournderstanding of the geological context and geotectonic develop-ent of ancient orogens has progressed with such information.

ecently, contrasting metamorphic P–T paths with different peakges have been reported from coastal exposures (Schirmacherills) and inland nunataks (Filchenerfjella and Jutulsessen) in

∗ Corresponding author at: Senbaru 1, Nishihara, Okinawa 903-0213, Japan.E-mail address: [email protected] (S. Baba).

301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.precamres.2012.10.002

© 2012 Elsevier B.V. All rights reserved.

central Dronning Maud Land (DML), East Antarctica (Baba et al.,2008, 2010). The Schirmacher Hills is considered to representa nappe and klippe (e.g. Grantham et al., 2008; Jacobs et al.,2008), an isolated exotic terrane, or perhaps an island arc (Babaet al., 2010) in the late Neoproterozoic–Early Palaeozoic EastAfrica–Antarctica Orogen (EAAO) (Jacobs et al., 2008 and refer-ences therein). Grantham et al. (2008) further proposed an EastAfrican–East Dronning Maud Land Orogen (>600 Ma), including theSchirmacher Hills, the northeast Sør Rondane Mountain, and the
Lutzow-Holm Complex, which formed a mega-nappe together withvarious blocks in Mozambique and Sri Lanka. However, this modelhas not been constrained very well by information on the metamor-phism in the Sør Rondane Mountains and the Schirmacher Hills.
dx.doi.org/10.1016/j.precamres.2012.10.002

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S. Baba et al. / Precambrian Research 234 (2013) 210– 228 211

Fig. 1. (a) Geological setting of the southern part of the East African–Antarctic Orogen (EAAO) modified after Jacobs and Thomas (2004). DML, Dronning Maud Land; SØ, SørR , AntaS n Shea

eebb

ondane Mountains. (b) Overview map of eastern and central Dronning Maud Landhiraishi et al., 1997; Osanai et al., 2013). MTB: Main Tectonic Boundary; MSZ: Mai

The Sør Rondane Mountains (SRM; 22–28◦E, 71.5–72.5◦S) in
astern Dronning Maud Land (DML), East Antarctica, are consid-red to represent the central part of an ancient orogen formedy the amalgamation of East and West Gondwana (Fig. 1a and). Therefore, the mountains have attracted interest as a key area
rctica. (c) Simplified geological map of the Sør Rondane Mountains (modified afterr Zone.

for understanding the amalgamation process of the Gondwana
supercontinent, but no one has revisited the area since the 32ndJapanese Antarctica Research Expedition (JARE) 1990–1991, dueto its inaccessible location. As a result of the 26th–32nd JARE, theconditions of metamorphism in the SRM granulites and gneisses

212 S. Baba et al. / Precambrian Research 234 (2013) 210– 228

F i et as ated.

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ig. 2. Simplified geological map of the Brattnipene region (modified after Osanaillimanite–garnet–biotite gneiss, amphibolite and pyroxene–granulite are intercal

ave been reported by numerous workers (Asami and Shiraishi,987; Shiraishi and Kojima, 1987; Grew et al., 1989; Asami et al.,990, 1993, 2007; Makimoto et al., 1990; Ishizuka et al., 1995,996), although a P–T path for the metamorphism has only beenublished by Asami et al. (1992). The inferred P–T path showsn overall clockwise loop, involves an isobaric reheating derivedrom acid plutonic intrusives during a retrograde stage, and wasstablished on the basis of mineralogical observations from a hugerea (ca. 50 km × 150 km) including Brattnipene, Austkampane,utsthamaren, and Balchenfjella (Asami et al., 1992). In order tochieve more fruitful discussions on the geotectonic evolution, pre-ise P–T paths for each area need to be established.

For our study, the focus was on mineral textures observed inetapelites that crop out on the Hitosashiyubi and Koyubi ridges in

he Brattnipene area, revisited during the 49th JARE (2007–2008).everal rocks with distinctive textures and compositions were usedo establish a precise metamorphic P–T path. These textures andssemblages indicate a probable counterclockwise P–T path andsobaric cooling, a metamorphic history not previously reportedrom the SRM. The inferred P–T path is not consistent with that ofhe adjoining terrane, including the Schirmacher Hills, except forts isobaric cooling history. We also briefly discuss the geotectonicmplications of this new information in the light of other availableata from high-grade gneiss terranes characterized by counter-lockwise P–T path. The mineral abbreviations used in equations,ables, and figures are taken from Kretz (1983).

. Geological setting

The Sør Rondane Mountains are inland nunataks, roughly50 km long and 100 km wide, underlain by medium to high-gradeetamorphic rocks that are cut by various granitoid intrusions and

ost-metamorphic dykes (Fig. 1c). They have been divided into twoerranes, separated by the Main Tectonic Boundary (MTB) on the
asis of metamorphic processes, geochronology, geochemistry ofhe protoliths, and magnetic anomalies (Osanai et al., 2013: Fig. 1c).he northeastern terrane consists of amphibolite to granulite faciesocks, whereas the southwestern terrane consists of greenschist to
l., 1996). Bt–Hbl gneiss and Hbl gneiss containing garnet in place. Thin layers of

granulite facies rocks associated with a metamorphosed tonalitecomplex. The general structural features of the metamorphic rocksare controlled by the east–west trend of the foliation and foldaxes (Toyoshima et al., 1995). A large east–west trending shearzone (Main Shear Zone) appears in the southwestern part of themountains (Fig. 1c).

Banded gneisses of felsic to intermediate compositions (Bt–Hbl,Hbl, and Grt–Bt gneiss) are dominant in the area, with layers,blocks, and lenses of amphibolite, mafic granulite, charnockite,pelitic gneiss, marble, and calc-silicate gneisses intercalated. Themetamorphosed tonalite complex (simply portrayed as plutonicrock in Fig. 1c) is exposed in the southern part of the Main ShearZone. Late-stage granitic and pegmatitic intrusions, and high-K mafic dikes (lamprophyre and minette), are also observed aspost-tectonic dykes, throughout the area. The geochemical char-acteristics of meta-basic igneous rocks in the SRM reveal unitswith characteristics typical of oceanic, island arc, and continen-tal margin arc settings (Osanai et al., 1992). Previous work byAsami et al. (1992, 2007) and Ishizuka et al. (1996), for exam-ple, has put the peak conditions of metamorphism in the NEterrane at 860–895 ◦C and 12 kbar, and Asami et al. (1992) sug-gested a clockwise P–T path with isobaric heating derived fromacid plutonic intrusives during a retrograde stage. Shiraishi et al.(2008) summarized the thermal history of the SRM using theavailable geochronological data (e.g., Grew et al., 1992; Shiraishiand Kagami, 1992; Asami et al., 2005), and they concluded thatthe granulite-facies metamorphism occurred at 650–600 Ma, andthat a later amphibolite-facies metamorphism in the NE and SWterranes occurred around 570 Ma. The metamorphosed tonalitecomplex represents the pre-collisional intrusive rocks that wereintruded during the early Neoproterozoic (920–990 Ma; Shiraishiet al., 2008; Kamei et al., 2012).

The Brattnipene region constitutes the northern part of the SWterrane, and it consists predominantly of biotite–hornblende, horn-
blende, and garnet–biotite gneisses, and these gneisses containthin layers or blocks of garnet–sillimanite–biotite gneiss, amphibo-lite, pyroxene-granulite, marble, charnockite, and enderbite (Fig. 2).Orthopyroxene-bearing granulite is sparse and occurs either as thin

S. Baba et al. / Precambrian Rese

Fig. 3. Photographs of field view and sampled outcrop. (a) Large-scale field view oftgr

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inclusions (Fig. 6a and b). The marginal part of them was replacedby secondary biotite and, in places, enclosed by biotite aggregate

he Hitosashiyubi-ridge with sample locality. (b) Field occurrence of garnet–biotiteneiss with intercalation of orthopyroxene-bearing layers in the Hitosashiyubi-idge.

ayers or blocks. Adachi et al. (2010) proposed that the presencer absence of orthopyroxene in the biotite–hornblende and horn-lende gneisses of the Brattnipene region was controlled by theegree of hydration during retrograde metamorphism.

. Petrography

Metapelites in the Brattnipene region occur as thin layers alter-ating with calcsilicate gneiss (Fig. 3a and b), two-pyroxenesranulite, and amphibolite in the biotite–hornblende gneiss. Theetapelite consists essentially of quartz, plagioclase, K-feldspar,

arnet, biotite, and sillimanite, subordinate spinel, corundum,nd ilmenite, and traces of zircon, apatite, rutile, and monazite.rthopyroxene, staurolite, and kyanite are rarely present. In this

ection, we describe four specimens from the granulites andneisses exposed on the Hitosashiyubi and Koyubi ridges in theorthern Brattnipene region (Fig. 2). These specimens display dis-inctive mineral textures and compositions that have enabled us tolucidate the P–T path of metamorphism.

.1. Sillimanite-bearing orthopyroxene–garnet granuliteSil–Opx–Grt granulite: B071217T02A)

The Sil–Opx–Grt granulite consists mainly of garnet, orthopy-oxene, biotite, quartz, and plagioclase. Aggregates of corundum-
earing garnet (Crn-bearing domains) are present locally, and theock is distinctively heterogeneous (Fig. 4a). However, leucocraticein and domain are lacking. Most of the garnet occurs as por-hyroblast which contain inclusions of biotite, spinel, plagioclase,
arch 234 (2013) 210– 228 213

ilmenite, and sillimanite. Rarely, the garnet occurs as reactioncoronae around sillimanite (Fig. 4b). Sillimanite occurs only asinclusions within garnet, and the sillimanite is associated with tinyround grains of spinel, biotite, and plagioclase. Orthopyroxene lacksinclusions, and the margins of orthopyroxene are typically replacedby cummingtonite. Corundum occurs as inclusions within garnetalong with spinel and ilmenite (Fig. 4c). Orthopyroxene porphy-roblasts and corundum do not belong to the same stable mineralassemblage, because they are never in contact with each other inthe matrix, and are always separated from each other by garnet andbiotite. Biotite crystals form a weak foliation spatially associatedwith tabular quartz and plagioclase. Some secondary biotite occursalong the margins of the garnet porphyroblasts. Zircon, rutile, andapatite are present as accessory minerals.

3.2. Staurolite-bearing biotite–garnet gneiss (St–Bt–Grt gneiss:B071217T02C)

This gneiss consists mainly of plagioclase, biotite, garnet, quartz,and sillimanite, together with minor magnetite, ilmenite, spinel,corundum, and staurolite. Biotite and garnet-rich domains occur,where the garnet forms aggregates surrounded by biotite (Fig. 5a).Biotite has also developed along the garnet grain boundaries withinthe aggregates. Corundum and staurolite occur in association withmagnetite and spinel along the garnet grain boundaries. Stauroliteoccurs as prismatic subidioblastic grains, and it commonly containsspinel and magnetite, and rarely corundum at its core (Fig. 5b andc). Corundum that contains inclusions of spinel and magnetite isalso present adjacent to the staurolite. Staurolite is a late-stagemineral that encloses the jagged margins of garnet and corundum.Garnet contains ilmenite, spinel, and sillimanite as tiny inclusions.In places, sillimanite crystals in the matrix and as inclusions includeround grains of spinel. Zircon, monazite, rutile, and apatite arepresent as accessory minerals.

3.3. Sapphirine–spinel bearing garnet gneiss (Spr–Spl gneiss:B07121705C)

This gneiss consists mainly of fine-grained garnet, biotite, pla-gioclase, and quartz, with minor sillimanite and cordierite (Fig. 5dand e). Biotite, together with garnet, forms compositional layering.Fine-grained subidioblastic garnet crystals (up to 1.5 mm in diam-eter) contain tiny inclusions of sillimanite, quartz, biotite, spinel,and sapphirine. A few inclusions of sapphirine and spinel can beseen within garnet in any one thin section, and these two mineralsare in direct contact with each other (Fig. 5f). Cordierite occurs onlyin sillimanite-rich compositional layers. Traces of rutile and apatiteare present in the gneiss.

3.4. Sillimanite–kyanite–garnet gneiss (Sil–Ky–Grtgneiss:O90123101)

This rock is a melanocratic part of the pelitic gneiss layer thatsporadically associated with leucocratic domain in outcrop. It iscomposed chiefly of biotite, quartz, plagioclase, K-feldspar, gar-net, kyanite, sillimanite and spinel. Ilmenite, zircon and apatite areaccessory minerals. Garnet porphyroblast, up to 3 mm in diam-eter, contains sillimanite, biotite, quartz, ilmenite and spinel as

completely (Fig. 6b). Sillimanite grains occur as both inclusion andmatrix of a garnet grain. Kyanite is found in the matrix consistingof plagioclase, quartz and K-feldspar together with biotite at thegarnet margin (Fig. 6a).


Fig. 4. Photomicrographs showing the mineral relationships in the Sil–Opx–Grt granulite. (a) Wide view of the mineral occurrence in thinsection. A domain of garneta arneta m in r(

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ggregate surrounded by biotite is observed. Orthopyroxene can be seen close to glso included by garnet corona. Orthopyroxene also observed in matrix within 5 mCrn-bearing domain). Scale bar represents 1.0 mm.

. Mineral chemistry

The minerals in the four specimens from the Brattnipene regionere analyzed using a wavelength-dispersive electron microprobe

JEOL JXA-8800M) at the National Institute of Polar Research (Japan)nd an energy dispersive X-ray spectrometer (SHIMADZU EPMA-6) at Yamaguchi University, Japan, using natural minerals andynthetic oxides as standards, an accelerating voltage of 15 kV,nd a specimen current of 12–15 nA. The average compositionsnd compositional ranges of the analyzed minerals are summa-ized in Table 1. Representative mineral compositions are shownn Appendix.

.1. Garnet

Fig. 7 illustrates the compositional features of the garnet.oward the rims of the garnets there is commonly a decrease inMg and a slight increase in grossular content.

In the Sil–Opx–Grt granulite, there is a wider range of XMg0.38–0.47) in garnet coronae than in garnet associated with corun-um (0.40–0.44). The proportion of grossular in the garnet coronaeanges from 0.03 to 0.04, and it is slightly higher in the rimsithout any significant increase in spessartine (0.02–0.03, Fig. 7).arnets in the corundum-bearing domains have higher grossularontents (0.04–0.05, Fig. 7) than garnets in other textural settings0.025–0.04). This compositional difference may represent a vari-tion of the local bulk composition on the thin section scale. Fig. 8hows a map of element concentrations in the garnet coronaeround the sillimanite. The maximum pyrope content was observed
t the center of the texture, and it decreases gradually toward theims. Almandine and spessartine components increase in the out-rmost rim. However, the zonal pattern of the grossular components preserved in several isolated domains (arrows in Fig. 8d) within
corona. (b) Sillimanite is surrounded by garnet corona. Spinel and plagioclase areange. (c) Garnet contains corundum and spinel inclusions in the aggregate domain

the garnet coronae, and the pattern is different from the patternsfor other components. Grossular zonal structures (low contentsat the core to high contents at the rim) are preserved in severaldomains, and they may be relics of garnet grains that existed priorto the impingement of several grains during the formation of thecoronae texture. The increases in grossular content, from cores torims, is interpreted as primary growth zoning, but we note that anypyrope zoning was lost through a process of homogenization afterthe formation of the coronae texture.

Garnets in the St–Bt–Grt and Sil–Ky–Grt gneisses show a widerrange of XMg (0.26–0.46 and 0.15–0.34, respectively) and spessar-tine (0.02–0.16 and 0.02–0.16, respectively) than the other samples.In the St–Bt–Grt gneiss, grossular increases slightly toward thegarnet rims, although XMg decreases and spessartine increasesabruptly (0.07–0.16; Fig. 7). In the case of the Sil–Ky–Grt gneiss,there is no increase in grossular toward the garnet rims, but anincrease in spessartine is common. In the St–Bt–Grt gneiss, the low-est XMg and highest spessartine values were recorded in a garnetrim adjacent to biotite and staurolite, and in the Sil–Ky–Grt gneissthe lowest XMg and highest spessartine values were recorded ina garnet rim adjacent to biotite and kyanite. These compositionalvariations suggest that garnet has been replaced by secondarybiotite and modified by Fe–Mg exchange reactions during the ret-rograde stage of metamorphism.

Garnet grains in the Spr–Spl gneiss record the highest XMg andlowest grossular values, and there are no significant compositionalvariations except for a decrease in XMg toward the rims.

4.2. Biotite

On the whole, biotite Ti (per formula unit (p.f.u.) of 22 oxygens)and F/(F + OH) values show a positive correlation with XMg values(Fig. 9). Inclusions of biotite in the Spr–Spl gneiss have the highest


Fig. 5. Photomicrographs showing the mineral relationships in the St–Bt–Grt gneiss (a–c) and the Spr–Spl gneiss (d–f). (a) Garnet forms aggregate in associate with corundum.Garnet grain boundaries were replaced by biotite. (b and c) Staurolite occurs as prismatic subidioblastic grain and developed at the garnet margins, together with biotite.Staurolite commonly contains magnetite, spinel and rarely corundum inclusions. Corundum also contains magnetite and spinel. (d) Garnet and biotite forms compositionallayering. In places, sillimanite-rich domain can be seen, and cordierite has been formed there. (e and f) Fine grained garnet (up to 0.7 mm) contains tiny inclusions of spineland sapphirine in direct contact with each other. Rare Mg–Al chlorite (Mg-Chl) also can be seen. Scale bar represents 1.0 mm.


F ss. (a)b x. (b) Gb

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ig. 6. Photomicrographs showing the mineral relationships in the Sil–Ky–Grt gneiiotite. Kyanite appears at the marginal part of garnet together with biotite in matriar represents 1.0 mm.

Mg values. High F/(F + OH) values are also characteristic of the otheriotite grains in the same sample. Biotite in the Sil–Opx–Grt gneissas slightly lower XMg values (0.73–0.79) and higher Ti contents0.20–0.31 p.f.u.). Two analyzed biotite grains associated with silli-

anite in the garnet coronae have higher XMg and lower Ti valueshan biotites in other textural settings. In the St–Bt–Grt gneiss,iotite adjacent to staurolite has lower Ti and higher XMg valueshan biotite core compositions. Biotite in the Sil–Ky–Grt gneiss hashe lowest values of XMg (0.54–0.63) and highest contents of Ti0.12–0.37 p.f.u.).

.3. Spinel

Spinel inclusions within garnet, both in the Sil–Opx–Grtranulite and the Spr–Spl gneiss, have lower Zn contents thanhose within staurolite in the St–Bt–Grt gneiss (Fig. 10). Inhe Sil–Opx–Grt granulite, spinel does not vary significantly in

able 1ineral composition of the granulite and gneisses in Brattnipene.

Sample Sil–Opx–Grt granulite crn-domain

Garnet n = 25 n = 8

XMg 0.42 (0.38–0.47) 0.43 (0.40–0.44)

grossular 0.03 (0.03–0.04) 0.04 (0.04–0.05)

Biotite n = 10 n = 4

XMg 0.76 (0.74–0.79) 0.74 (0.72–0.85)

TiO2 2.28 (1.81–2.75) 2.46 (1.97–2.81)

F 0.82 (0.64–0.89) 0.78 (0.72–0.85)

Orthopyroxene n = 23XMg 0.66 (0.64–0.67) –

Al2O3 5.20 (4.25–6.11) –

Fe3+/Fe total 0.03 (0.00–0.10) –

Spinel n = 4 n = 5

XMg 0.44 (0.42–0.47) 0.44 (0.42–0.49)

XZn 0.03 (0.02–0.03) 0.03 (0.03–0.04)

Fe3+/Fe total 0.09 (0.07–0.11) 0.08 (0.06–0.11)

Plagioclase n = 9 n = 4

XAn 0.33 (0.28–0.36) 0.32 (0.29–0.36)

Sapphirine

SiO2 – –

XMg – –

Fe3+/Fe total – –

Cordierite

XMg – –

Staurolite

XMg – –

XZn – –

Corundum n = 6

Fe2O3 – 0.90 (0.78–1.29)

Sillimanite n = 6

Fe2O3 0.75 (0.64–0.84) –

Mg indicates Mg/(Mg + total Fe) for garnet, biotite, cordierite and staurolite and Mg/(Mg

Garnet contains sillimanite and spinel inclusions, and the margin was replaced byarnet includes fibrous sillimanite, and in turn is surrounded by biotite grains. Scale

composition in different textural settings. The maximum ZnOcontent of 10.8 wt% is found in spinel inclusions within stauroliteof the St–Bt–Grt gneiss.

4.4. Orthopyroxene

Fig. 11a shows orthopyroxene compositions in terms of XMg[Mg/(Mg + Fe2+)] and XAl (Al cations/2 per 6 oxygens). Both XMg andXAl values are low when compared with the high-Al2O3 orthopy-roxene from the Schirmacher Hills in central DML.

Orthopyroxene porphyroblasts in the matrix show lower Al2O3contents (up to 4.7 wt.%) and similar XMg values to the rims oforthopyroxene close to garnet coronae. The Al2O3 contents of
orthopyroxene cores close to garnet coronae reach up to 6.1 wt.%,and XMg values range from 0.64 to 0.67. The Fe3+/(Fe2+ + Fe3+)ratios, estimated stoichiometrically on the basis of 6 oxygens and4 cations, are up to 0.10.
St–Bt–Grt gneiss Spr–Spl gneiss Sil–Ky–Grt gneiss

n = 15 n = 12 n = 110.40 (0.26–0.46) 0.46 (0.44–0.48) 0.23 (0.15–0.34)0.04 (0.03–0.04) 0.02 (0.02–0.03) 0.03 (0.03–0.04)n = 8 n = 4 n = 80.71 (0.68–0.74) 0.84 (0.83–0.86) 0.60 (0.54–0.63)1.08 (0.50–1.43) 2.31 (1.70–3.12) 2.08 (3.24–1.07)0.44 (0.37–0.53) 1.11 (1.00–1.19) n.a

– – –– – –– – –n = 4 n = 90.35 (0.21–0.41) 0.58 (0.57–0.59) n.a0.16 (0.09–0.23) 0.06 (0.05–0.07) n.a0.11 (0.00–0.15) 0.12 (0.09–0.14) n.an = 5 n = 8 n = 80.31 (0.29–0.33) 0.26 (0.24–0.31) 0.26 (0.18–0.33)

n = 4– 12.08 (11.90–12.28)– 0.84 (0.81–0.85) –– 0.26 (0.20–0.30) –

n = 8– 0.87 (0.85–0.88) –n = 100.29 (0.25–0.30) – –0.05 (0.03–0.07) – –n = 30.90 (0.70–1.20) – –n = 80.98 (0.60–1.35) n.a n.a

+ Fe2+) for orthopyroxene, sapphirine, and spinel. n.a: not analyzed.


s XMg

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Fig. 7. Compositional variations of garnet in terms of grossular v

.5. Sapphirine

Fig. 11b shows the compositional features of the sapphirinen our specimens, together with sapphirine compositions previ-usly reported from the SRM. An analyzed sapphirine inclusionn the Spr–Spl gneiss has a composition close to the end-member7(Mg,Fe)O:9(Al,Cr,Fe3+)2O3:3SiO2]. XMg values are 0.81–0.85. Stoi-hiometric calculations assuming 20 oxygens and 14 cations givee3+/(Fe3+ + Fe2+) ratios of 0.20–0.30.

.6. Staurolite

Staurolite has XMg values of 0.25–0.30 (Fig. 10). The ZnOontent (0.56–1.35 wt.%) is lower than that in the accom-anying spinel inclusions (4.55–10.82 wt.%). Low XZn valuesZn/(Fe + Mg + Zn) = 0.03–0.07] indicate staurolite of secondary ori-in that formed at relatively low temperatures.

.7. Cordierite

Cordierite is the most magnesian of the analyzed minerals. Its approximately homogeneous and has XMg values in the range.85–0.88.

.8. Corundum and sillimanite

Analyzed corundum grains from the Sil–Opx–Grt granulitend St–Bt–Grt gneiss contain 0.78–1.29 and 0.70–1.20 wt.% Fe2O3,espectively. Sillimanite contains up to 1.35 wt.% Fe2O3 in the ana-yzed samples.

.9. Plagioclase

In the Sil–Opx–Grt granulite, the anorthite contents oflagioclase are similar in both the corundum- and orthopyroxene-earing domains (XAn = 0.28–0.36 and 0.29–0.36, respectively).

n the Sil–Opx–Grt granulite, plagioclase inclusions adjacent to

. Numbers represent spessartine content of each analyzed point.

sillimanite that is enclosed by garnet coronae have slightlyhigher anorthite contents (0.33–0.36) than the matrix plagioclase(0.28–0.30). Matrix plagioclase grains in the Spr–Spl and St–Bt–Grtgneisses have anorthite contents of 0.24–0.31 and 0.29–0.33,respectively. Plagioclases in the Sil–Ky–Grt gneiss have a widerange of anorthite content (0.18–0.33).

5. Interpretation of metamorphic reactions

5.1. Peak metamorphic stage

Metamorphic reactions can been deduced from textural rela-tionships, so that in the Sil–Opx–Grt granulite, for example,sillimanite and spinel within garnet coronae are inferred to havebeen stable before garnet formation, and the orthopyroxene thatoccurs as porphyroblasts close to this texture may have been stableat an early stage of garnet formation. Possible divariant reactionsin the FeO–MgO–Al2O3–SiO2 (FMAS) system are:

Orthopyroxene + spinel + sillimanite = garnet(as coronae) (1)

Orthopyroxene + spinel + quartz = garnet(as coronae) (2)

Fig. 12 shows composition–assemblage relations for the sug-gested reactions. Both reactions are represented as terminal typesin the S–FM–A diagram. In the Qtz–Hc–Spl projections, the compo-sitions of garnet plot almost within the three-phase field of spinelinclusions, orthopyroxene, and quartz, and reaction (2) thereforeappears to be possible. Fig. 13 shows a partial petrogenetic grid forthe FMAS system under high-fO2 conditions, after Hensen (1986).The relative positions of the divariant reactions (1) and (2) appearon the diagram, and garnet was stable on the low-T and high-Pside. In the A–F–M diagram, garnet compositions plot outside theorthopyroxene–spinel–sillimanite join. This discordance might be
explained in terms of (1) a primary Fe-rich spinel that was con-sumed by the garnet-forming reaction, or (2) garnet compositionsthat were modified by late Fe–Mg exchanges during retrogression.Presumably the analyzed compositions of the spinel inclusions and


Fig. 8. Maps of X-ray intensity for Mg, Fe, Mn and Ca of garnet coronae around sillimanite in the Sil–Opx–Grt granulite. Electron microprobe analytical spots of garnet usedfor P–T estimations are indicated. A and B in (d) represent spots no. 13 and no. 30 in Table 2. Arrows indicate low-Ca domains (see text).


Fao

gFor

Ft

Fig. 11. (a) Compositional variations of orthopyroxene in terms of XAl (Al/2, 6 oxe-gens) vs XMg. Dashed circle represent compositional ranges of orthopyroxene fromUHT granulite of Schirmacher Hills in central Dronning Maud Land (Baba et al., 2006and unpublished data). (b) Compositional variations of sapphirine in terms of Alcations vs Si cations per 20 oxegens in sapphirine; 7:9:3 and 2:2:1 are the molecu-

ig. 9. Compositional variations of biotite in terms of Ti (p.f.u. 22 oxygens) vs XMg

nd F/(F + OH) vs XMg. Note that data from the Sil–Ky–Grt gneiss were not plottedn F/(F + OH) vs XMg diagram.

arnets do not represent the original compositions with regard toe and Mg. On the contrary, if we apply the present maximum XMgf orthopyroxene (0.67) and garnet (0.47), two possible univarianteactions are obtained in relation to the spinel XMg ratios:

3.5 quartz + 2.3 spinel + 0.4 orthopyroxene = 1.3 sillimanite

+ garnet (spinel XMg = 0.4) (3)

6.2 quartz + 3.6 spinel = 0.29 orthopyroxene + 2.6 sillimanite

+ garnet (spinel XMg = 0.5) (4)

ig. 10. Compositional variations of spinel (hercynite) and staurolite in Fe–Mg–Znarnary diagrams. Note that data from the Sil–Ky–Grt gneiss were not plotted.

lar ratios of MgO:Al2O3:SiO2. Sapphirine compositions reported from other localitiesin Sør Rondane Mountains are also plotted.

In Fig. 13, reaction (4) appears as a univariant line with a pos-itive slope (moderate angle), and the reaction products garnet,orthopyroxene, and sillimanite are stable on the low-T and high-Pside. Therefore, the degree of modification of Fe and Mg was notestimated, and the relative P–T conditions are considered to havechanged to a higher pressure. Biotite TiO2 and F contents are nothigh (up to 2.28 wt.% and 0.89 wt.%, respectively). Therefore, weassume most of the matrix biotite in the Sil–Opx–Grt granulite issecondary in origin.

Corundum occurs in the garnet-rich domain both in theSil–Opx–Grt granulite and the St–Bt–Grt gneiss, and sometimesit contains magnetite and spinel inclusions. Garnet also con-tains sillimanite and spinel inclusions in the corundum-presentdomains (Figs. 4c and 5a). Inclusions of spinel and sillimanitewithin garnet are inferred to have been stable before corun-dum and garnet formation. These textures indicate the followingreaction:

Spinel + sillimanite = garnet + corundum (5)

Reaction (5) has been calibrated experimentally by Shulters andBohlen (1989), and defines the spinel and sillimanite as stable underhigh-T and low-P conditions, whereas garnet and corundum arestable under relatively low-T and high-P conditions.


Fig. 12. Projection of coexisting phases in the Sil–Opx–Grt granulite. (a) S(SiO2)–FM(FeO + MgO)–A(Al2O3) ternary plot showing positions of analyzed minerals; (b) Qtz–Hc–Splp ar. Thr domai

skwo

FHpd

rojection from sillimanite; (c) Thompson AFM diagram projected from K-feldspe-equilibration and consumption (see text). The star marks indicate re-integrated

The mineral assemblage of inclusions of sapphirine, spinel, andillimanite within garnet in the Spr–Spl gneiss must also provide a
ey to understanding the peak metamorphic conditions. However,e could not derive a suitable garnet-forming reaction on the basis
f this observed mineral association.

ig. 13. Partial petrogenetic grid for FMAS sysetem under high fO2 conditions (afterensen, 1986). Heavy lines are univariant equilibria. Thin lines indicate relativeosition of divariant reactions. Lines with abbreviation in bold indicate the reactionsiscussed in the text.

e cross marks represent excepted primary compositions of Grt and Spl prior ton bulk composition.

5.2. Retrograde metamorphic stage

Prismatic to subidioblastic staurolite occurs in aggregates ofsecondary biotite that developed at the garnet grain boundaries.Garnet compositions adjacent to staurolite show extremely lowXMg and high spessartine values – the results of re-equilibrationduring retrograde metamorphism. The staurolite grains commonlycontain spinel and magnetite inclusions, and occur together withcorundum and sillimanite. The staurolite was possibly formed bythe following back-reaction:

Spinel + garnet + corundum + sillimanite + H2O = saturolite

(6)

Fig. 14 shows the suggested reaction on an AFM diagram.Staurolite compositions plot inside the garnet rim–spinel–sillimanite/corundum join (Fig. 14, solid line), but outside this areaif a garnet core composition is used (Fig. 14, broken line). Asamiet al. (1990) proposed the reverse of reaction (6) as staurolite brokedown in a dehydration reaction to form garnet in rocks exposed inthe Balchenfjella region of eastern SRM. In that case, staurolite com-
positions plot inside the garnet core–spinel–sillimanite/corundumjoin. Spinel inclusions have higher ZnO contents and higher XMgratios than the host staurolite, and the ZnO in staurolite may beinherited from the reactant spinel.


FTc

gTstss

bTaM

6

6

mbGaeepg

tB9i(fe(tp(lcasb

omet

ric

resu

lts

for

Grt

–Op

x

and

Grt

–Op

x–Pl

–Qtz

asso

ciat

ion

s.

0712

17T0

2A

Spot

Grt

Op

x

Pl

T

(at

8

kbar

)

(◦ C)

P

(at

800

◦ C)

(kba

r)

P

(at

1000

◦ C)

(kba

r)

XM

gX

grs

Xal

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889

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rix)

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n;

coro

.:

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nae

;

inc:

incl

usi

on.

/Mg)

Grt

/(Fe

/Mg)

Op

x.

ey

(198

4).

tach

arya

et

al. (

1991

).

and

Bh

atta

char

ya

(198

4).

uly

et

al. (

1996

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ns

and

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ton

(198

1).

kin

s

and

Ch

iper

a

(198

5).

rley

and

Gre

en

(198

2).

ig. 14. Projection of coexisting phases in the St–Bt–Grt gneiss on to thehompson AFM diagram projected from K-feldspar. Two cases are shown Grtore–Sil/Crn–Spinel as solid line and Grt rim–Sil–Spinel as broken line.

In the Sil–Ky–Grt gneiss, sillimanite occurs as inclusions inarnet, whereas kyanite occurs in the matrix with the biotite.he textures indicate the stable aluminosilicate has changed fromillimanite to kyanite. Based on these textures and garnet composi-ions, especially the high-spessartine garnet, we can conclude thattaurolite formed during a retrograde metamorphism in the kyanitetability field.

Matrix biotite, in all samples where it forms a foliation, mighte developed by late hydration rather than melt-bearing reactions.his is because there are no symplectites or intergrowths of biotitend quartz that could give evidence of the presence of a melt (e.g.,oraes et al., 2002).

. Metamorphic P–T conditions

.1. Grt–Opx–Pl–Qtz thermobarometry

The equilibrium P–T conditions in the early stages ofetamorphism have been estimated using experimentally cali-

rated geothermobarometers based on the systems Grt–Opx andrt–Opx–Pl–Qtz. The compositions of garnet coronae (high-Mgnd low-Ca domains), garnet porphyroblasts, matrix orthopyrox-ne, matrix plagioclase, and inclusion plagioclase were all used forstimating the conditions during early metamorphism. The tem-eratures and pressures calculated from each set of analyses areiven in Table 2.

Temperatures calculated using the Grt–Opx Fe–Mg exchangehermometer (Harley, 1984; Sen and Bhattacharya, 1984;hattacharya et al., 1991; Ganguly et al., 1996) range from 742 to19 ◦C at 8 kbar (Fig. 15). These temperature estimates are min-

ma because of possible Fe–Mg re-equilibration during coolinge.g., Fitzsimons and Harley, 1994). Pressure conditions calculatedor temperatures of 850–900 ◦C using Grt–Opx thermobarom-try (Harley and Green, 1982) and Grt–Opx–Pl–Qtz barometryPerkins and Newton, 1981; Perkins and Chipera, 1985) are cen-ered around 8 ± 1 kbar. Calculations using the compositions oflagioclase inclusions and high-Mg garnet in sillimanite coronaespot A in Fig. 8d), as well as matrix orthopyroxene, give slightlyower pressures (6.5–6.8 kbar at 800 ◦C) than using matrix plagio-
lase and high-Ca garnet coronae (spot B in Fig. 8d) (7.7–8.1 kbart 800 ◦C). This implies an increase in pressure from the earlytage, when plagioclase inclusions and high-Mg garnet were sta-le, to the later stage when matrix plagioclase and high-Ca garnet Ta
ble

2Th

erm

obar

Sam

ple

:

B

coro

.Grt

–co

ro.G

rt(

d.G

rt(c

or

d:

dis

cret

e

aK

D=

(Fe

bH

:

Har

lc

B:

Bh

atd

SB:

Sen

eG

:

Gan

gf

PN:

Per

gPC

:

Per

hH

G:

Ha


FgT

ct

6

mfSGsetft(

6

g(aOtocFadeSp

ig. 15. P–T plot showing geothermobarometry applied toarnet–orthopyroxene–plagioclase–quartz association. Abbreviations as inable 2.

oronae were stable, presuming no sudden changes of tempera-ure.

.2. Sapphirine–spinel thermometry

In the Spr–Spl gneiss, the temperatures during the early stages ofetamorphism, represented by inclusion minerals, are estimated

rom adjacent crystals of sapphirine and spinel. The empiricalpr–Spl Fe–Mg exchange thermometry proposed by Owen andreenough (1991) and Das et al. (2006) was applied, and the resultshow temperatures that range from 789 to 967 ◦C (Table 3). Asamit al. (2007) proposed a modified version of this thermometry, andheir version gives temperatures that are somewhat lower, rangingrom 764 to 937 ◦C. The calculated conditions are broadly consis-ent with those obtained using Grt–Opx–Pl–Qtz thermobarometry850–900 ◦C).

.3. Petrogenetic grids by experiment

Fig. 16 shows an experimentally constrained petrogeneticrid for different bulk compositions of XMg, either 0.81 or 0.72Das et al., 2003). The stability field of the Opx + Grt + Sil + Kfsssociation is located higher on the pressure side than thepx + Sil + Spl + Kfs stability field for both bulk compositions. In

he Sil–Opx–Grt granulite, garnet coronae that contain inclusionsf sillimanite and spinel should be stable at higher pressureonditions above the Spl = Opx + Sil + Grt + Kfs univariant curve inig. 16. To make an assessment of the minerals in this assemblage,

reintegrated bulk composition, excluding corundum-bearing
omains, was determined using modes of the constituent min-rals (Pl = 42.4, Bt = 32.4, Qtz = 12.4, Grt = 6.4, Opx = 4.7, Cum = 0.8,il = 0.5, and Spl = 0.2 mode%) and their average chemical com-ositions. This reintegrated bulk composition is SiO2 = 75.16,
Fig. 16. Experimentary constrained P–T pseudosections for the two bulk composi-tions (modified after Das et al., 2003).

TiO2 = 0.76, Al2O3 = 18.34, Cr2O3 = 0.02, FeO = 6.30, MnO = 0.06,MgO = 7.90, CaO = 3.02, Na2O = 3.50, and K2O = 2.94 wt.%. The XMgratio is 0.69, implying the stability field of Opx–Grt–Sil–Kfs in thepetrogenetic grid has shifted to the lower pressure side around8–10 kbar. The experiment was performed under high fO2 condi-tions. According to the high-T experiments using M2 as startingmaterial (bulk XMg = 0.72), the Fe3+/Fe total ratios in the orthopy-roxene (0.00–0.16) and spinel (0.16–0.29), and the Fe2O3 content ofsillimanite (0.36–0.55 wt.%) (Das et al., 2003), are broadly compa-rable with our rocks. The Fe3+/Fe total ratios in the orthopyroxene(0.00–0.10) and spinel (0.08–0.11) are slightly lower, but the Fe2O3content of sillimanite inclusions (0.64–0.84 wt.%) is higher than inthe experimental products. Thus, the application of these experi-mental results can be justified, and the assumption of an increase inpressure in relation to the formation of the garnet coronae is valid.

6.4. Petrogenetic grids by pseudosections

To investigate the entire P–T history in the Sil–Opx–Grtgranulite, an NCKFMASH P–T isochemical phase diagram wascomputed based on free-energy minimization using Perple X soft-ware (Connolly, 2005) and end-member thermodynamic data fromHolland and Powell (1998). The model solutions are summarizedin Table 4. We used the bulk chemical composition described inthe previous section for the calculation. The composition and lowvolume of aluminosilicate tells us the precursory rock was not anormal pelite. The water-excess and melt-absent conditions wereapplied on the basis of textural observations. In the phase diagram,the Grt–Opx–Pl–Bt–Qtz association appears over a wide range of
P–T (700–925 ◦C, >7.5 kbar) (Fig. 17a). The spinel and sillimanitestable stability fields are not present, and this is consistent with thelow mode of these minerals (Sil = 0.5, and Spl = 0.2 mode%). A largecordierite-bearing stability field appears on the low-P side in the


Table 3Thermobarometric results for Spr–Spl associations.

Sample: B07121705C Spot Spr Spl T (◦C)

Fe2+ Mg Fe2+ Mg KDa OGb Dc OG-Ad D-Ae

Spr (inc)–Spl (inc) within Grt (198-202) 0.522 2.902 0.379 0.54 2.88 837 967 834 937Spr (inc)–Spl (inc) within Grt (201-203) 0.641 2.811 0.387 0.54 2.17 789 917 764 882

a KD = (Mg/Fe)Spr/(Mg/Fe)Spl.b OG: Owen and Greenough (1991).c D: Das et al. (2006).d OG-A: Asami et al. (2007).e D-A: Asami et al. (2007).

Table 4Solution notation and formulae for phase diagram calculation.

Symbol Solution Formula Reference

Grt Garnet Fe3xCa3yMg3(1−x+y+z/3)Al2−2zSi3+zO12, x + y ≤ 1 Holland and Powell (1998)Bt Biotite K[MgxFe1−x]3−wAl1+2wSi3−wO10(OH)2 Powell and Holland (1999)Opx Orthopyroxene [MgxFe1−x]2−yAl2ySi2−yO6 Holland and Powell (1996)Pl, Kfs Feldspar KyNaxCa1−x−yAl2−x−ySi2+x+yO8, x + y ≤ 1 Fuhrman and Lindsley (1988)Chl Chlorite [Mg Fe ]5−y+zAl Si O (OH) Holland et al. (1998)

U and um

gor

ocpA8oapadagIl

FaO

x 1−x

nless otherwise noted, the compositional variables x and y may vary between zeroinimization.

iven P–T window. We could not make a qualitative assessmentf the changes in P–T conditions using the mineral associationsepresented on this diagram.

Fig. 17b shows compositional isopleths of garnet [Mg/(Mg + Fe)],rthopyroxene [Al (O = 6.0 p.f.u.) and Mg/(Mg + Fe)], and plagio-lase [Na/(Ca + Na)] that are drawn concordant with the isochemicalhase diagram. Applying the highest values for XMg (0.67) andl (0.262 p.f.u.) in orthopyroxene, P–T conditions of 8.2 kbar and50 ◦C were obtained, and these conditions are consistent withur calculations using the assemblage Grt–Opx–Pl–Qtz. However,pplying the highest value of XMg for garnet (0.47) to the garnet iso-leths gives 700–750 ◦C in the Grt–Opx–Pl–Bt–Qtz stability fieldnd 7.5–8 kbar in the Opx–Crd–Pl–Grt–Bt–Qtz stability field. Theiscrepancies in these conditions may also be interpreted to be

result of Fe–Mg re-equilibration. The Na/(Na + Ca) ratios of pla-
ioclase inclusions are lower than those in the matrix plagioclase.sopleths using the compositions of the inclusions (0.64–0.67) areocated on the high-T and low-P side, and can be interpreted in
ig. 17. (a) NCKFMASH P–T isochemical phase diagram for the Sil–Opx–Grt granulite, calcre isopleths of Al cations (6 oxegnens) and XMg [Mg/(Fe + Mg)] in orthopyroxene, XMg in2: orthopyroxene XMg = 0.67; P1: plagioclase inclusions XNa = 0.67; P2: matrix plagioclas

2(1+y−z) 3−y+z 10 8

nity and are determined as a function of the computational variables by free-energy

terms of the conditions during an early stage of metamorphism.For the matrix plagioclase with Na/(Na + Ca) ratios of ca. 0.70 andgarnet rims with XMg values of ca. 0.38, the P–T conditions arearound 680–700 ◦C and 8.0–8.5 kbar, which can be interpreted asthe conditions during retrogression.

7. U–Pb zircon age dating

7.1. Analytical methods

U–Pb analyses of zircon were performed using the sensitivehigh-resolution ion microprobe (SHRIMP II) at the National Insti-tute of Polar Research, Tokyo, Japan. The zircon grains wereseparated from the Sil–Opx–Grt granulite and the Spr–Spl gneiss,
although we could not get sufficient number of zircon grains fromthese samples. Only one zircon grain from the Sil–Opx–Grt gran-ulite was found and mounted in epoxy resin. In order to investigateinternal structures of individual zircons, backscattered electron
ulated using the reintegrated domain composition (see text). (b) Also shown in (a) garnet, and XNa [Na/(Ca + Na)] in plagioclase. O1: orthopyroxene Al cations = 0.26;e XNa = 0.70; G1: garnet core XMg = 0.47; G2: garnet rim XMg = 0.38.

2 n Research 234 (2013) 210– 228

(aNswcim2(uruaPctAfMab(

7

1fzgacccde6(

8

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rt

gran

uli

te.

%20

6Pb

cp

pm

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pp

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Th23

2Th

/238U

±%20

6Pb

/238U

age

207Pb

/206Pb

age

208Pb

/232Th

age

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and

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tion

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ion

was

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%

(not

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24 S. Baba et al. / Precambria

BSE) and cathodoluminescence (CL) images were obtained using scanning electron microscope (SEM; JEOL JSM-5900LV) at theational Institute of Polar Research, Japan. These images guided the

election of analytical spots. The surface of grain mount was washedith 2% HCl to remove any lead contamination, and they were then

oated with gold prior to SHRIMP analysis. The analytical techniques essentially same as that described by Williams (1998). An O2

− pri-ary ion beam of ∼2.1 nA was used to sputter an analytical spot of

0 �m diameter on the polished mount. The standard zircon SL13U = 238 ppm), provided by the Australian National University, wassed as the reference value for U concentrations in zircon. Pb/Uatios were corrected for instrumental inter-element fractionationsing ratios measured on the standard zircon TEMORA2 (206Pb/238Uge: 416.78 ± 0.33 Ma; Black et al., 2004). Corrections for commonb were based on the measured 204Pb and the model for common Pbompositions proposed by Stacey and Kramers (1975). Data reduc-ion and processing were conducted using SQUID 2 (Ludwig, 2009).nalytical uncertainties are shown at the 1-sigma uncertainty level

or each analysis. FC1 zircon (1099 Ma; Paces and Miller, 1993) andud Tank zircon (732 Ma; Black and Gulson, 1978) were routinely

nalyzed with TEMORA2 zircon in order to confirm the U/Pb cali-ration. Further analytical procedures are described in Horie et al.2011) in detail.

.2. Results of U–Pb zircon age dating

Analyzed zircon in the Sil–Opx–Grt granulite is oval in shape,00 �m long, with aspect ratios of 1:2 and rounded, subhedral sur-aces. The grain has an internal texture of a sector-like irregularoning core with bright CL overgrowth rims. Two analyses on onerain were performed (Table 5). 206Pb/238U ages of 631 ± 8 (discord-nce +7%) and 624 ± 14 Ma (discordance +6%) were obtained fromore and rim, respectively. There was no age difference betweenore and rim, but Th/U ratio of rim (1.35) were higher than that ofore (0.08). The number of analyses is insufficient for the accurateetermination of a concordia age and further discussion. How-ver the obtained ages of ∼630 Ma are within the age range of50–600 Ma which reported as the timing of main metamorphismShiraishi et al., 2008).

. Discussion

.1. Metamorphic evolution in the Sør Rondane Mountains

A P–T path for the metamorphism of the SRM rocks has beenroposed by Asami et al. (1992), characterized by a clockwise tra-

ectory and secondary re-heating at a late stage (path A in Fig. 18).his path was constructed on the basis of: (1) relic kyanite inlagioclase and sillimanite in the matrix, (2) the breakdown of stau-olite in the sillimanite stability field, (3) the sapphirine–kyanitend gedrite–kyanite–quartz associations, (4) P–T estimates usingarious geothermobarometers (e.g., two-pyroxenes, Grt–Opx, andrt–Cpx), (5) retrograde kyanite-formation, and (6) retrogradendalusite formation. The mineral relationships were judged fromextures observed in a variety of rocks from different parts of theRM, so that the evidence for (1) is from Austhamaren, (2) and (3)rom Balchenfjella, (5) from Brattnipene, and (6) from Austkam-ane. As shown in Fig. 1c, these regions are spread over a widerea of nearly 150 km × 50 km. In this article, we have focused onhe metamorphic textures and compositions present in the Brat-nipene region, and we have obtained a P–T path that differs from
hose previously reported.
The P–T path we have determined for the metamorphic evo-ution of the granulites and gneisses in the Brattnipene region ishown in Fig. 18. The path is constrained by our estimates of the Ta

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Fig. 18. Metamorphic P–T paths of the Brattnipene regions. Arrow deduced from themineral textures and associations in this study. Paths A and B as gray broken lineswere taken from the Sør Rondane Mountains (Asami et al., 1992) and the Schir-macher Hills (Baba et al., 2006, 2008, 2010). Reaction lines are also shown for thereference. (1) Richardson (1968); (2) Asami et al. (1990); (3) and (4) Holdaway andL(

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unlikely that this orthogneiss of granitic composition could rep-

ee (1977); (5) Shulters and Bohlen (1989); (6) calculated after Shulters and Bohlen1989).

–T conditions, and the mineral textures of garnet- and staurolite-orming reactions. The early part of the prograde path could note established from the textural relationships present, but oncehe P–T conditions reached high temperatures (850–900 ◦C) closeo an ultra-high temperature condition (over 900 ◦C), the assem-lages sapphirine–spinel–sillimanite (in the Spl–Spr gneiss) andpinel–sillimanite–orthopyroxene (in the Sil–Opx–Grt granulite)re stable. A slight increase in pressure at high temperatures,hereafter, is deduced from the following: (1) using the differ-nt compositions of high-Mg garnet – plagioclase inclusions andigh-Ca garnet coronae – matrix plagioclase to calculate a change

n the estimated pressure, (2) a slight increase in grossular con-ent from core to rim in the garnets, (3) the formation of garnetoronae replacing spinel and sillimanite inclusions, and (4) theresence of stable garnet and corundum after spinel and sillimanite.hese observations all suggest that the stability field changed fromower to higher pressures under the high-T conditions. Therefore,he early part of the calculated P–T path has a counterclockwiserajectory. The timing of the high-temperature metamorphisms interpreted to be ca. 650–600 Ma by Shiraishi et al. (2008),50–620 Ma by Owada et al. (2013), and 640–600 Ma by Adachit al. (2013), and these ages are consistent with our results whichave U–Pb zircon ages of ∼630 Ma.

The retrograde P–T path was one of isobaric cooling, as indicatedy the formation of staurolite via a back-reaction in the kyanite sta-ility field. The estimated conditions of metamorphism using theompositions of garnet rims and matrix plagioclase, using isopleths,re 680–700 ◦C and 8.0–8.5 kbar (Fig. 17), and these conditionsre consistent with the staurolite-forming univariant reaction. Theetamorphic conditions we have estimated are consistent with theork of Asami et al. (1992) who reported the development of sec-

ndary kyanite in a biotite aggregate from the Brattnipene region.he timing of the retrograde metamorphism is interpreted to be ca.70–520 Ma by Adachi et al. (2013).

Cordierite has not previously been found in the Brattnipene
egion. In the Spr–Spl gneiss, cordierite appears in thin sillimanite-earing layers. In general, the formation and presence of cordieritere interpreted as indicators of low-P high-T conditions and/or
arch 234 (2013) 210– 228 225

decompression. However, we assume that this rare appearance inthe Brattnipene region was the result of expansion of the cordieritestability field on the high-P side due to the high values of bulk XMg.This view is supported by the high XMg ratios of coexisting min-erals (e.g., biotite, garnet, and spinel). Therefore, we assume thatcordierite was stable in all lithologies at an early stage of metamor-phism, and that during the subsequent increase in pressure thecordierite was consumed by reactions in relatively low-Mg rocks,but preserved in high-Mg rocks.

In summary, our P–T estimates and the reactions inferred sug-gest a counterclockwise P–T path that initiated with an isothermalpressure increase from the spinel stability field and reached peaktemperatures of 850–900 ◦C at 8–9 kbar. This was followed byisobaric cooling when garnet–sillimanite–corundum–spinel brokedown to staurolite at around 680–700 ◦C and 8.0–8.5 kbar. Themetamorphism took place under pressures that were too high forthe stability of cordierite except in some Mg-rich rocks wherecordierite survived.

8.2. Tectonic implications

Counterclockwise metamorphic P–T paths have been reportedfrom various Proterozoic terranes (see Bohlen and Mezger, 1989;Harley, 1989). Most are characterized by low-pressure (e.g.,4–7 kbar) and high-temperature conditions, a retrograde historyof isobaric cooling, and they have been interpreted as havingformed in and beneath an area of voluminous magmatic accre-tion, with or without additional crustal extension (Harley, 1989).Since 1990, hybrid types of counterclockwise P–T paths, involvinghigh-pressure conditions with retrograde histories of isothermaldecompression, have been reported, as for example in the Water-man Metamorphic Complex, USA (Henry and Dokka, 1992), theLewisian Complex of South Harris, Scotland (Baba, 1998, 1999,2002, 2003), and the Eupa Complex in Namibia (Brandt et al.,2007). The heat source for these metamorphic terranes is thoughtto have derived from the intense magmatic activity in a continen-tal margin setting. The inferred counterclockwise P–T path for theBrattnipene rocks is assumed to have been recorded during theperiod ca. 650–600 Ma, and it is inconsistent with the clockwiseP–T path typical of continental collision. We can suggest, therefore,that the metamorphism was not related with the main collision ofE and W Gondwana (ca. 550–500 Ma). Furthermore, the calculatedBrattnipene pressures of 8.5 kbar are slightly higher than thoseestimated for granulite terranes formed by crustal shortening andlithosphere thinning. As described above, a counterclockwise pathis usually closely associated with magmatic activity, and we need,therefore, to consider magmatic activity in the SRM as the drivingforce for the calculated increase in pressure.

In the SRM, on the basis of U–Pb ages of magmatic zircons inmetamorphic rocks, Shiraishi et al. (2008) proposed the follow-ing episodes of magmatism: in the SW terrane around 1130, 1000,650, and 560 Ma, and in the NE terrane around 800 Ma. Extensivemetatonalites in the SW terrane, produced by subduction-relatedmagmatism (Ikeda and Shiraishi, 1998), are considered to havebeen emplaced at 960–920 Ma (Shiraishi et al., 2008). A ca. 951 MaU–Pb age, obtained from an enderbitic orthogneiss in the northernBrattnepene region, is thought to date the magmatic crystalliza-tion of zircon in the igneous protolith. Therefore, according tothe available chronological data, the voluminous tonalites arenot synchronous with the granulite-facies metamorphism (ca.650–600 Ma). A 650 Ma magmatic event is suggested by zirconcores in biotite orthogneiss from the SW terrane. However, it seems

resent the heat source for the high-temperature granulite faciesmetamorphism (850–900 ◦C). Hence, a new model is required toexplain the Brattnipene metamorphic evolution that involved a

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ounterclockwise P–T path, an isobaric cooling history, and theresence of a substantial heat source.

Osanai et al. (1992) have divided the SRM rocks into severaleological units on the basis of geochemical characteristics of theetamorphosed basic to intermediate igneous rocks: unit I, oceanic

ype; unit II, island-arc type; units III–V, accretionary complexypes; and unit VI, continental margin island-arc type. The northernart of the Brattnipene region belongs to Unit II, and is assumed toave formed in an island arc setting, and such a setting leads us toonsider this as providing a source of intense heat during magmaticctivity in a continental margin setting. Certainly, the heat sourceor the metamorphism we have described is likely to have someelation to such a tectonic setting, but we cannot provide furtheronstraints due to inconsistencies in age data.

Taking all the above into account, we put forward here a numberf evolutionary scenarios for further discussion.

Scenario (1): The protoliths of the metamorphic rocks in therattnipene region formed at a continental margin, and subse-uently underwent a low-P high-T type of metamorphism (e.g.,yoke metamorphic complex, Japan; Miyashiro, 1961). The meta-orphism reached the amphibolite to granulite facies in the SW

errane. An increase in pressure and subsequent isobaric coolingan be explained by overthrusting or obduction of relatively hotceanic crust or oceanic island arc material (oceanic type of Unit?) onto the Brattnipene region. The subsidence of an island arcy tectonic loading is favored rather than a tectonic perturba-ion by continental collision, because the pressures were not highnough for them to be equivalent to an overthickend crust (e.g.,0–50 km). As a result of the subsidence, the Brattnipene regionas buried into deeper crustal levels and subsequently cooled

lowly.Scenario (2): The SRM is the result of an amalgamation of sev-

ral isolated terranes that essentially correspond to the units in thelassification of Osanai et al. (1992). Units I, II, III–V, and VI wereormed separately in different tectonic settings and experiencedifferent metamorphisms. Unit II of the Brattnipene region was inhe middle or deep part of an island arc crust, and its subsequentistory was essentially the same as in Scenario (1). Thus, it was themalgamation of island arc and oceanic arc at a continental marginUnits III–V and VI), not a continent–continent collision, that gaveise to the metamorphism characterized by a counterclockwise P–Tistory and isobaric cooling.

Scenario (3): Similar to granulite terranes elsewhere that showsobaric cooling, the metamorphic rocks in the Brattnipene regionre from the middle or deeper parts of a crust that extended over

wide area of the SRM. Extentional thinning of this crust and aubsequent collision or closure may be the cause of the counter-lockwise and isobaric cooling P–T path. However, this scenario isot really credible, because we are unable to explain the differentectonic settings of Units I to VI.

Consequently, we tentatively propose scenarios (1) or (2) as pos-ible models of the tectonic evolution of the Brattnipene region.t least, we are confident in saying that the heat source for thearly sequence of metamorphism was derived from advective heatow at an active continental margin. Further refinement of theseodels will need to take into account the petrogenesis of distinc-

ive rocks in other parts of the SRM (Ishizuka et al., 1995; Asamit al., 2007; Nakano et al., 2011). Recently, for example, an isother-al decompression P–T path has been proposed for metapelites

n Austkampane (Adachi, 2010; Adachi et al., in press), and thisill be a key issue for consideration when refining the tectonicodels. And it is clear that further dating of igneous activity in

he area is needed if we are to find the heat source that caused theigh-T metamorphism in the Brattnipene region, remembering thatemperatures almost reached those of the ultrahigh-temperature900 ◦C) metamorphic régime.

arch 234 (2013) 210– 228

8.3. Comparisons with other terranes

Counterclockwise and isobaric cooling P–T paths have not beenproposed for the rocks in central DML (Fig. 1b), but an isobariccooling retrograde path has been reported from the coastal partof the Schirmacher Hills in central DML (Baba et al., 2008). Inthe Schirmacher Hills, the estimated peak conditions of meta-morphism are ∼950–1050 ◦C and ∼9–10 kbar (path B in Fig. 8),and the timing of the peak metamorphism is estimated to beclose to 643 ± 6.5 Ma using zircon U–Pb dating (Baba et al., 2006,2010). The P–T paths and the peak conditions are inconsistentwith those of the Brattnipene region, but the timing of the meta-morphism is broadly the same. Baba et al. (2010) proposed thatthe 643 Ma metamorphism of the Schirmacher Hills took placein a back-arc tectonic setting, and predated the main collisionalevent of central DML. However, any direct comparison betweenthe Schirmacher Hills and the Brattnipene area is controversial.For example, it is likely that they originated from different ter-ranes, not only because the metamorphic grades and trajectoriesof the P–T paths differ, but because there are significant differencesin the geochemistry of mafic gneisses (Baba et al., 2011). Furthergeochronological and geochemical studies are also needed beforedetailed comparisons can be made, but we can at least concludethat the arc-related tectonothermal events were enhanced at a pos-sible proto-Gondwana margin at around 650–600 Ma. In view ofthe proposed processes, it will be worth re-evaluating the otherPrecambrian orogenic belts that are characterized by contrastingP–T paths, showing both counterclockwise and clockwise trajecto-ries (e.g., Susar mobile belt, Central India Tectonic Zone: Bhowmiket al., 2005; high-grade rocks in MacRobertson and Kemp Lands,East Antarctica: Halpin et al., 2007).

9. Conclusions

(1) The metamorphism in the Brattnipene region followed a coun-terclockwise P–T path with retrogressive isobaric cooling, asinferred from mineral textures and compositions. The earlierstage of this metamorphism is characterized by the breakdownof spinel and sillimanite to form garnet coronae in high-Alorthopyroxene-bearing granulites, and the formation of garnetand corundum in garnet–biotite gneisses. The garnet coronaewere a reaction product during an increase in pressure up to8.0–9.0 kbar at the near-peak conditions of 850–900 ◦C.

(2) The retrograde isobaric cooling is witnessed by the back-reaction formation of prismatic subidioblastic staurolite as asecondary mineral along the margins of garnet. The condi-tions for this reaction are estimated to be 680–700 ◦C and8.0–8.5 kbar, and these conditions are consistent with theappearance of kyanite in the matrix.

(3) On the basis of the metamorphic P–T path and the tectonicsetting of the protoliths, two tentative scenarios for the tec-tonic evolution of the area can be proposed. The heat source forthe early sequence of metamorphism was derived from advec-tive heat flow at an active continental margin. The subsequentincrease in pressure was caused by overthrusting or obduc-tion of oceanic crust or oceanic island arc onto the Brattnipeneregion. The tectonothermal event was enhanced at a possibleproto-Gondwana margin at around 650–600 Ma.

Acknowledgments

We would like to thank the members of 48th and 49th JapanAntarctic Research Expedition, and the crew of the icebreaker Shi-rase. We also thank A. Hubert, G. Johnson-Amin, and members ofthe Belgian Antarctic Research Station (2007–2008) for supporting

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ur fieldwork. We acknowledge K. Shiraishi, Y. Motoyoshi, Y. Hiroi,. Ishizuka, T. Kawasaki, and K. Sajeev for valuable discussions.onstructive comments by S.K. Bhowmik, D. Prakash, and M. Satish-umar improved this manuscript and are gratefully acknowledged.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.precamres.012.10.002.

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counterclockwise p–t path and isobaric cooling of metapelites from brattnipene, sør rondane...

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