Ultrahigh-temperature metamorphism in the Achankovil Zone: Implications for the correlation of crustal blocks in southern India

Gondwana Research 10 (2006) 99 – 114 www.elsevier.com/locate/gr

Ultrahigh-temperature metamorphism in the Achankovil Zone: Implications for the correlation of crustal blocks in southern India Shunsuke Ishii a , Toshiaki Tsunogae a,b,⁎, M. Santosh c a

Graduate School of Life and Environmental Sciences (Earth Evolution Sciences), University of Tsukuba, Ibaraki 305-8572, Japan b Department of Geology, University of Johannesburg, P. O. Box 524, Auckland Park 2006, Republic of South Africa c Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan Received 14 May 2005; accepted 9 November 2005 Available online 9 May 2006

Abstract The Achankovil Zone of southern India, a NW–SE trending lineament of 8–10 km in width and N 100 km length, is a kinematically debated crustal feature, considered to mark the boundary between the Madurai Granulite Block in the north and the Trivandrum Granulite Block in the south. Both these crustal blocks show evidence for ultrahigh-temperature metamorphism during the Pan-African orogeny, although the exhumation styles are markedly different. The Achankovil Zone is characterized by discontinuous strands of cordierite-bearing gneiss with an assemblage of cordierite + garnet + quartz + plagioclase + spinel + ilmenite + magnetite ± orthopyroxene ± biotite ± K-feldspar ± sillimanite. The lithology preserves several peak and post-peak metamorphic assemblages including: (1) orthopyroxene + garnet, (2) perthite and/or anti-perthite, (3) cordierite ± orthopyroxene corona around garnet, and (4) cordierite + quartz symplectite after garnet. We estimate the peak metamorphic conditions of these rocks using orthopyroxene-bearing geothermobarometers and feldspar solvus which yield 8.5–9.5 kbar and 940–1040 °C, the highest P– T conditions so far recorded from the Achankovil Zone. The retrograde conditions were obtained from cordierite-bearing geothermobarometers at 3.5–4.5 kbar and 720 ± 60 °C. From orthopyroxene chemistry, we record a multistage exhumation history for these rocks, which is closely comparable with those reported in recent studies from the Madurai Granulite Block, but different from those documented from the Trivandrum Granulite Block. An evaluation of the petrologic and geochronologic data, together with the nature of exhumation paths leads us to propose that the Achankovil Zone is probably the southern flank of the Madurai Granulite Block, and not a unit of the Trivandrum Granulite Block as presently believed. Post-tectonic alkali granites that form an array of “suturing plutons” along the margin of the Madurai Granulite Block and within the Achankovil Zone, but are absent in the Trivandrum Granulite Block, suggest that the boundary between the Madurai Granulite Block and the Trivandrum Granulite Block might lie along the Tenmalai shear zone at the southern extremity of the Achankovil Zone. © 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Keywords: Achankovil Zone; Cordierite gneiss; P–T path; Ultrahigh-temperature metamorphism; Southern India

1. Introduction The Achankovil Shear Zone is a NW–SE trending major lineament of 8–10 km in width and N 100 km length (Fig. 1; Drury and Holt, 1980; Drury et al., 1984). This zone separates two major crustal blocks in southern India, namely the Trivandrum Granulite Block (TGB) to the south and the Madurai Granulite Block (MGB) to the north, although the kinematic evolution of this crust-scale structure is debated (cf. ⁎ Corresponding author. Graduate School of Life and Environmental Sciences (Earth Evolution Sciences), University of Tsukuba, Ibaraki 305-8572, Japan. E-mail address: [email protected] (T. Tsunogae).

Sacks et al., 1997; Rajesh et al., 1998). We therefore refer to this zone as the Achankovil Zone (ACZ) in this paper. Lithologically, the ACZ consists dominantly of charnockite, leptynite, cordierite gneiss, khondalite, ultramafic granulite, and calcsilicates, punctured by a number of late Proterozoic granitic intrusives (e.g., Santosh et al., 2005). Previous studies grouped the ACZ lithologies along with those of the TGB and considered them to be units belonging to a single metasedimentary belt (e.g., Braun and Kriegsman, 2003). The cordierite gneisses investigated in this study form discontinuous, km-size lenses along the northern flank of the ACZ (Sinha-Roy et al., 1984; Santosh, 1987). Only limited petrological investigations have been carried out on the metamorphic P–T

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Fig. 1. Simplified geological map of the southern part of Kerala and Tamil Nadu (after GSI, 1995a,b) with the sample localities of cordierite gneiss discussed in this paper. DC: Dharwar Craton, MGB: Madurai Block, MSB: Madras Block, NNB: Northern Block, NGB: Nilgiri Block, TGB: Trivandrum Block, NCB: Nagarcoil Block, ACZ: Achankovil Shear Zones, PCSZ: Palghat–Cauvery Shear Zone, P: Phanerozoic cover.

conditions of these rocks. Santosh (1987) estimated peak P–T conditions of 5.5–7.0 kbar at 700–800 °C from these rocks based on geothermometers and mineral equilibrium, and documented moderate to high-density (d = 0.8–0.9 g/cm3) CO2-rich fluid inclusions. Santosh (1987) also reported various reaction textures such as garnet breaking down to cordierite ± orthopyroxene ± quartz, attesting to near-isothermal decompression. Nandakumar and Harley (2000) obtained P–T conditions of 6.5–7.0 kbar and 925 ± 20 °C from charnockite and cordierite gneiss in the ACZ. They concluded that temperatures are significantly higher in the ACZ than in the central part of the TGB. According to their results, the post-peak metamorphic P–T path is characterized by an initial stage of near isobaric cooling followed by decompression from 800 °C and 6 kbar to 750 °C and 4 kbar. Cenki et al. (2002) emphasized the role of melt associated with the cordierite gneisses of ACZ and constructed prograde and retrograde P–T path based on KFMASH partial petrogenetic grid, quantitative pseudosections, and geothermobarometry. They obtained a clockwise P–T path culminating at 6–7 kbar and 900–950 °C. Their peak-temperature estimates are based on the results for garnet–orthopyroxene geothermobarometer of Harley and Green (1982). In this study, we report results from detailed petrological investigation of cordierite-bearing gneisses from two localities within the ACZ (Fig. 1) and a third locality with identical lithology further north, within the southwestern domain of the MGB, providing new evidence for UHT metamorphism in the ACZ. We also re-evaluate the P–T path to understand the nature

of exhumation of rocks along this zone. Our results yield a new model for the evolution of the rocks along the ACZ and revise the current concepts of correlation of the ACZ with the TGB. 2. Geology of the study area Samples of cordierite gneiss and associated rocks were collected from two localities within the ACZ (localities KR2-1 and KR6-5) and a third one (locality KR8-2) from further north. Those within the ACZ are located around Pattanamthitta town, about 27 km north of Kottarakkara. Locality KR2-1 (Punnalathupadi, about 4 km WSW from Pattanamthitta, cf. Santosh, 1987) is an open quarry composed mainly of cordierite gneiss, charnockite, quartzo-feldspathic gneiss, and mafic granulite. The rocks show NNW–SSE to NW–SE foliation, which is concordant to the regional trend of the ACZ. Locality KR6-5 (Vallikodu Kottayam, about 3 km SSE from Pattanamthitta) is a large active quarry composed mainly of cordierite gneiss, charnockite, quartzo-feldspathic gneiss, and alkali-feldspar granite. The rocks also display a NNW–SSE foliation similar to the previous locality. Pink-colored alkali-feldspar granite sheets of about 1 to 2 m in thickness intrude parallel to, or slightly oblique to the gneissic foliation. This locality has not been described in previous studies. The third locality (Vadavathur, locality KR8-2, Fig. 1) at about 10 km east of Kottayam is within the southern part of the MGB. The NW–SE foliation of the rock dipping sharply (70– 80°) toward SW is almost concordant with the two localities

S. Ishii et al. / Gondwana Research 10 (2006) 99–114

discussed above. The outcrop is composed of cordierite gneiss, charnockite, quartzo-feldspathic gneiss, and alkali-feldspar granite (Fig. 2). As shown in the figure, cordierite gneiss has obvious foliation defined by laths of cordierite and lenses of cordierite-rich domains. Most quartzo-feldspathic gneisses (quartz + perthite or anti-perthite) also occur parallel to the foliation of matrix cordierite gneiss and charnockite (leucocratic gneiss 1 in Fig. 2), although some discordant gneisses (leucocratic gneiss 2 in Fig. 2) are also present. For temperature calculations using ternary feldspar geothermometry discussed in a later chapter, we chose perthite in quartzo-feldspathic gneisses occurring parallel to the foliation of matrix cordierite gneiss because they are likely to preserve the peak-temperature condition. Modal abundance of cordierite gradually decreases toward adjacent charnockite in the outcrop. Pink-colored alkalifeldspar granite, which is apparently a later phase, intrudes the rocks cross-cutting the foliation. 3. Petrography and mineral reaction Below we describe detailed petrography of representative rocks from the three localities. The textural features are shown in Figs. 3 and 4. Mineral assemblages and approximate modal abundances of minerals are listed in Table 1. Representative mineral assemblages and compositions are shown in AFM diagrams (Fig. 5). Mineral abbreviations are after Kretz (1983). As discussed below, the term “cordierite gneiss” has been used for all bluish cordierite-bearing pelitic gneisses after Sinha-Roy et al. (1984) and Santosh (1987), although it might not be appropriate because cordierite is apparently a retrograde mineral in these rocks. 3.1. Cordierite gneiss (orthopyroxene-bearing) The orthopyroxene-bearing cordierite gneiss is characterized by bluish patches or laths of cordierite aggregates in hand specimen. The most common assemblage of the rock type is defined by cordierite, garnet, orthopyroxene, quartz, plagioclase, spinel, ilmenite, and magnetite. Modal abundance of cordierite varies

101

widely, from 70% to 30%. Biotite and K-feldspar are minor constituents. Sillimanite occurs only as rare inclusions in garnet in this rock type. The boundary between the matrix charnockite is gradual and, in general, modal abundance of cordierite decreases and that of biotite and garnet increases toward charnockite. The rock type corresponds to the assemblages (i) and (ii) described by Santosh (1987). Sample KR2-1E, representative cordierite gneiss from the locality KR2-1, is characterized by coarse-grained poikiloblastic cordierite (2–3 mm in length) and garnet (2–4 mm) (Fig. 3a). Garnet contains medium-grained (0.5–1 mm) sillimanite and biotite as well as quartz as inclusions. Cordierite forms poikiloblastic aggregates of subhedral grains and contains numerous inclusions of zircon, monazite, and quartz. It is commonly mantling garnet suggesting the progress of the following reaction (Fig. 3a): Grt þ Sil þ Qtz→Crd

ð1Þ

Some cordierites around garnet are intergrown with irregularshaped orthopyroxene probably formed by the following reaction: Grt þ Qtz→Crd þ Opx

ð2Þ

The reaction textures were reported by Santosh (1987) and Nandakumar and Harley (2000) as evidences of retrograde metamorphism. These textures have been reported from many granulite terranes in the world, suggesting near-isothermal decompression after the peak metamorphism. Strongly pleochroic subhedral orthopyroxene (1–2 mm) occurs commonly with cordierite, perthite, and quartz (Fig. 3b). Symplectitic finegrained aggregate of cordierite and quartz occurs around subhedral to anhedral garnet. In places, cordierite + quartz aggregate is closely associated with magnetite and ilmenite (Fig. 3c). Such a texture suggests the reaction, Grt þ O2 →Crd þ Qtz þ Fe2 O3 in Mag or Ilm

ð3Þ

The reaction texture has been reported by Nandakumar and Harley (2000) from the ACZ as an evidence of retrograde metamorphism. Greenish spinel (1–2 mm) is also a common

Fig. 2. Simplified field occurrences of various gneisses in locality KR8-2. Cordierite gneiss has obvious foliation defined by laths of cordierite and lenses of cordieriterich domains. Most quartzo-feldspathic gneisses also occur parallel to the foliation of matrix cordierite gneiss (leucocratic gneiss 1), although some are discordant (leucocratic gneiss 2). Modal abundance of cordierite gradually decreases toward adjacent charnockite in the outcrop. Pink-colored alkali-feldspar granite intrudes the rocks cross-cutting the foliation.

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Fig. 4. Back-scattered electron image photographs showing magnetite + ilmenite + spinel assemblages in samples KR2-1E (a), KR6-5E (b), KR8-2B (c), and KR8-2O (d). (e) and (f) are perthite (sample KR6-5C) and anti-perthite (sample KR2-1E), respectively.

mineral in the gneiss. It is subhedral to anhedral in shape and closely associated with ilmenite and magnetite (2–4 mm) adjacent to or included in cordierite (Fig. 4a). The texture suggests that spinel formed by exsolution from Al-bearing opaque minerals such as magnetite or ilmenite. Spinel does not show grain contact with quartz in any of the examined samples. Perthite is a dominant mineral scattered around cordierite together with medium-grained (1–2 mm) quartz and plagioclase. Cordierite gneiss in locality KR6-5 has mineral assemblage and textures similar to that in locality KR2-1. Secondary minerals such as pinite, muscovite, and chlorite are less abundant in this locality. A representative sample (sample KR6-5E) is composed mainly of coarse-grained anhedral cordierite (2–

3 mm), subhedral garnet (2–4 mm), and orthopyroxene (3–4 mm) (Fig. 3d). Textures such as sillimanite inclusion in garnet, orthopyroxene with strong pleochroism, cordierite + quartz symplectite, and spinel associated with cordierite and opaque minerals are similar to the rocks from locality KR2-1. Biotite is relatively more abundant in this sample and occurs commonly as medium-grained (0.5–1 mm) inclusions within garnet and plagioclase. Matrix subhedral biotite is relatively coarse-grained (1–2 mm). Cordierite gneiss from the southern part of the MGB (locality KR8-2) also shows textures similar to those from localities in the ACZ. A representative sample (KR8-2B) is composed of cordierite, garnet, orthopyroxene, biotite, plagioclase, quartz,

Fig. 3. Photomicrographs of representative rocks discussed in this study. (a) Garnet–cordierite–orthopyroxene assemblage in orthopyroxene-bearing cordierite gneiss (sample KR2-1E). (b) orthopyroxene–cordierite–perthite assemblage in orthopyroxene-bearing cordierite gneiss (sample KR2-1E). (c) Cordierite + quartz symplectite with Fe–Ti Oxide (magnetite + ilmenite) around garnet suggesting the progress of the reaction (3) in sample KR2-1H. (d) Pleochroic orthopyroxene and cordierite + garnet association in sample KR6-5E. (e) Cordierite–garnet–orthopyroxene association in sample KR8-2B. (f) A typical perthite texture in leucocratic gneiss (sample KR8-2O). (a), (d) and (e) are polarized light photographs. (b), (c) and (f) are crossed-polar photographs. (g) Poikiloblastic garnet including sillimanite and quartz (sample KR2-1D). (h) Poikiloblastic garnet including sillimanite, biotite and quartz (sample KR8-2M) in the matrix of quartz + plagioclase + cordierite. (i) Cordierite + spinel association in orthopyroxene-free cordierite gneiss (sample KR8-2N). (j) A typical mineral assemblage of charnockite comprising orthopyroxene, quartz, plagioclase and K-feldspar (sample KR2-1C) (h) and (j) are polarized light photographs. (g) and (i) are crossed-polar photographs.

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Table 1 Representative mineral assemblages of cordierite gneiss and associated rocks from the studied area Sample no.

Rock type a

Crd

Grt

KR2-1A KR2-1B KR2-1C KR2-1C2 KR2-1D KR2-1E KR2-1F KR2-1G1 KR2-1G2 KR2-1H KR2-1I KR2-1J KR2-1K KR2-1L KR2-1M KR2-1N KR6-5B KR6-5C KT6-5E KR6-5L KR8-2A KR8-2B KR8-2C KR8-2D KR8-2E KR8-2H KR8-2L KR8-2M KR8-2N KR8-2O

1b 1b 2 1b 1b 1a 1a 1b 1b 1a 2 3 2 1b 1a 1b 1a 1a 1a 1a 1b 1a 3 1b 1a 1b 2 1b 1b 1a

+++ +++

+++ ++

++ +++ +++ +++ ++ ++ ++

+++ +++ ++ ++ ++ ++ ++ ++ + ++ ++ ++ ++

Opx

+++

++ +++ +++ ++ ++ ++ +++ +++ +++ ++ ++ +++ +++ ++ +++ ++ +++

++ ++ ++ ++ ++ ++ ++

++ +

++

++ +++ + + ++

++, "++ +, "+ ++ +++ ++, "++ + ++ ++, "+ ++, "++ ++, "++ ++, "+ ++ ++, "++ + ++, "+ ++ + +++ ++ ++

+++

++ +++

++ ++

Bt

++

++ ++ +++ ++ ++, "++ +

Pl

Kfs

Qtz

Spl

Opq

Sil

Chl

++ +

+, "++

++ ++ + ++ ++ ++ ++ ++ +, "++ ++ +

++ ++ ++ ++ ++ ++ ++ ++ +, "++ ++ ++

"++ "++

+

+ ++ ++ ++ + ++ ++ ++ ++

++ ++ ++ ++ + ++ ++ ++ ++

"++ "++

++ ++ ++ + ++ ++ ++

++ ++ ++ ++ ++ ++ ++

+++ ++

++

++ ++

++ ++ ++ ++ ++

++ ++

++ +++

++ + ++

++ ++ ++

++ ++

++

+ ++ + +++

++

++ + ++ "+ "++ ++ ++ +++ ++ ++ ++ +++ ++ ++ +++ ++ ++

++ ++

+++ ++ + +++ ++ +++ ++

++ ++ ++ ++

Crn

1, 2 2

"++ "++

"++ "++

2, 3 1, 3 1, 2

+ + + + +

1, 2, 3 1 1, 1, 1, 1,

+ "++ + "+ "+

+ ++

2 2 3 2

1 2, 3 3 1, 2 2, 3

+

+ ++

"++ "++

Reaction b

1, 2 2, 3 2 2, 3 1

+ +

1, 2, 3

+++: abundant, ++: moderate, +: rare, ": inclusion in garnet. a 1a: Cordierite gneiss (orthopyroxene-bearing), 1b: cordierite gneiss (orthopyroxene-free), 2: charnockite, 3: pegmatite. b 1: Crd–Qtz symplectite, 2: Crd corona after Grt + Sil + Qtz, 3: Opx + Crd corona after Grt + Qtz.

spinel, ilmenite, magnetite, and sillimanite (Fig. 3e). It is characterized by coarse-grained pleochroic subhedral orthopyroxene (1–1.5 mm) partly replaced by chlorite. This sample shows significant effect of retrograde metamorphism with thick (2– 3 mm in width) cordierite + quartz symplectite around subhedral coarse-grained garnet (3–4 mm) and orthopyroxene (1–2 mm). Textures of spinel + ilmenite + magnetite aggregate (Fig. 4c) and perthite in quartzo-feldspathic portion of the rock (Fig. 3f) are also identified. The mineral assemblages and textural relations in the orthopyroxene-bearing cordierite gneiss suggests that garnet, orthopyroxene, quartz, plagioclase, perthite, and sillimanite define the peak assemblage, while cordierite ± orthopyroxene (Fig. 3a), cordierite + quartz (Fig. 3c), and spinel + magnetite + ilmenite (Fig. 4a–d) are retrograde assemblages. The assemblages are further discussed in later section dealing with P–T conditions. 3.2. Cordierite gneiss (orthopyroxene-free) The mineral assemblage in orthopyroxene-free cordierite gneiss is principally similar to that of orthopyroxene-bearing cordierite gneiss, except the absence of orthopyroxene. The rock thus contains cordierite, garnet, quartz, plagioclase, spinel, ilmenite, and magnetite and corresponds to assemblage (iii)

described in Santosh (1987). Sample KR2-1D is composed mainly of coarse-grained cordierite (2–4 mm), perthite (2– 3 mm), quartz (2–3 mm), and garnet (2–3 mm). Garnet is subhedral and occasionally includes fine-grained (b0.5 mm) quartz and sillimanite (Fig. 3g). Both these minerals occur also as medium-grained (1–2 mm) laths in the matrix, with elongation parallel to the foliation of the rock. Cordierite is closely associated with garnet and occasionally mantling the mineral (Fig. 3g), suggesting the progress of reaction (1). Cordierite + quartz symplectite adjacent to garnet, formed by reaction (3), also occurs in the rock, although vermicular quartz is more finegrained than that in orthopyroxene-bearing cordierite gneiss. Spinel + ilmenite + magnetite association is also present. The textures of cordierite-bearing rocks in localities KR6-5 and KR8-2 are principally identical to those in sample KR2-1D. Sample KR6-5C contains abundant cordierite–quartz symplectite around garnet, suggesting the progress of reaction (3). Sample KR8-2M is characterized by high modal abundance of biotite (up to 15%) and coarse-grained spinel (up to 3 mm) associated with opaque minerals. Similar to sample KR2-1D from the ACZ, poikiloblastic garnet commonly includes sillimanite as well as quartz and biotite (Fig. 3h). Sample KR8-2N is a quartz-rich (above 50%) portion of the rock and is composed of quartz (4–5 mm), cordierite (2–3 mm), biotite

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Fig. 5. AFM triangular diagrams showing mineral assemblages and compositions in the examined samples. Dotted lines indicate assemblages included in garnet. (a) and (f) are diagrams of orthopyroxene-free cordierite gneiss, while (b) to (e) are those of orthopyroxene-bearing cordierite gneiss.

(0.5–1 mm), perthite (1–2 mm), spinel (1–2 mm), and opaque minerals (ilmenite and magnetite; 1–2 mm). It is less foliated and characterized by granoblastic textures of quartz and perthite. Cordierite and spinel are also granoblastic and occur in Alrich portion of the rock (Fig. 3i).

10 nA sample current, and the data were regressed using oxideZAF correction method. Below, we describe mineral chemistry data of cordierite gneiss. Representative compositions of minerals in the analyzed samples are given in Table 2. Fe3+ of garnet, cordierite, orthopyroxene, spinel and sillimanite were calculated based on stoichiometry.

3.3. Charnockite 4.1. Cordierite Charnockite is a dominant lithology in the localities investigated in this study, and also elsewhere in southern India. The rock is composed mainly of plagioclase, K-feldspar (perthite), quartz, orthopyroxene, biotite, and quartz (Fig. 3j). Accessory minerals are magnetite, ilmenite, zircon, and apatite. Sample KR2-1C represents a typical garnet-bearing charnockite. It contains coarse-grained (∼ 3 mm) anhedral orthopyroxene and minor coarse-grained garnet (∼ 4 mm) which is anhedral and poikiloblastic. The garnet contains inclusions of quartz and biotite. Biotite also occurs in the matrix, and adjacent to orthopyroxene as a secondary mineral. Quartz and feldspars are also coarse grained (0.5–2 mm) and forms the matrix of the rock. Charnockite from locality KR8-2 shows similar mineral assemblages and textures, although modal abundances of garnet, biotite, and orthopyroxene vary depending on samples. The rock textures are very similar to that of garnet charnockite reported in previous studies from southern India (e.g., Santosh et al., 1990). 4. Mineral chemistry Chemical analyses of all minerals were performed by electron microprobe analyzer (JEOL JXA8621) at the Chemical Analysis Division of the Research Facility Center for Science and Technology, University of Tsukuba. The analyses were performed under conditions of 20 kV accelerating voltage and

Cordierite shows a uniform magnesian composition in all samples with XMg (= Mg/[Fe + Mg]) in the range of 0.78–0.85. Cordierite in sample KR2-1D (orthopyroxene-free cordierite gneiss) has a higher XMg of 0.82–0.85 than that of orthopyroxene-bearing cordierite gneiss (0.78–0.81, sample KR2-1E). Similar XMg range (0.79–0.81) was also obtained from cordierite in sample KR6-5B (orthopyroxene-bearing cordierite gneiss). Cordierite in orthopyroxene-bearing cordierite gneiss from MGB (sample KR8-2B) has slightly higher but almost consistent XMg of 0.82–0.85. 4.2. Garnet Garnet in cordierite gneiss is essentially a solid solution of pyrope and almandine (XMg = 0.34–0.44) with low contents of spessartine (b 6 mol%) and grossular (b 4 mol%). Although there is no significant difference in composition among examined samples, the mineral shows a similar rimward increases of almandine content. For example, garnet in orthopyroxene-bearing cordierite gneiss (sample KR2-1E) shows slightly pyrope-rich core (Alm57–58 Pyr37–38 Sps3 Grs3) than the rim (Alm58–59 Pyr35–36 Sps3 Grs3), which is consistent with the occurrence of cordierite corona around the garnet. Coarse-grained poikiloblastic garnet in orthopyroxene-free cordierite gneiss (sample KR2-1D) is slightly Fe-rich as Alm57–58 Pyr37–38 Sps3 Grs2 at the core. Its almandine

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Table 2 Representative electron microprove analyses of minerals discussion in this study Mineral name

Garnet

Sample no.

KR2-1D core

KR2-1D rim

KR2-1E core

KR2-1E rim

KR6-5B core

KR6-5B rim

KR8-2B core

KR8-2M rim

KR8-2M core

KR8-2M rim

No. oxygen

12

12

12

12

12

12

12

12

12

12

SiO2 Al2O3 TiO2 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O ZnO Total

39.55 22.29 0.07 0.00 0.00 26.55 1.51 9.63 0.76 0.00 0.00 0.01 100.36

38.79 22.27 0.01 0.05 0.00 27.97 1.89 8.23 1.24 0.01 0.00 0.06 100.5

39.22 21.79 0.03 0.02 0.00 26.56 1.33 9.44 0.98 0.02 0.01 0.13 99.53

38.85 22.25 0.05 0.03 0.00 26.60 1.41 9.11 1.07 0.03 0.01 0.01 99.40

39.92 22.24 0.06 0.02 0.00 24.46 1.89 10.45 0.90 0.02 0.00 0.00 99.95

39.03 22.25 0.01 0.00 0.00 25.38 1.87 10.44 0.81 0.00 0.00 0.04 99.82

39.23 22.65 0.05 0.00 0.00 23.60 2.03 10.78 1.16 0.00 0.00 0.05 99.55

39.62 23.02 0.02 0.00 0.00 24.14 2.14 11.05 1.10 0.04 0.00 0.01 101.13

39.78 22.69 0.02 0.00 0.00 23.39 1.60 11.06 0.92 0.02 0.00 0.03 99.51

40.57 22.36 0.01 0.00 0.00 24.16 1.65 11.10 0.89 0.00 0.00 0.00 100.73

Si Al Ti Cr Fe3+ a Fe2+ Mn Mg Ca Na K Zn Total

3.018 2.004 0.004 0.000 0.000 1.694 0.098 1.095 0.062 0.000 0.000 0.000 7.976

2.991 2.023 0.001 0.003 0.000 1.803 0.123 0.945 0.103 0.001 0.000 0.003 7.996

3.025 1.980 0.002 0.001 0.000 1.712 0.087 1.085 0.081 0.003 0.001 0.007 7.984

3.002 2.026 0.003 0.002 0.000 1.718 0.092 1.048 0.088 0.005 0.000 0.001 7.984

3.035 1.992 0.003 0.001 0.000 1.554 0.122 1.184 0.073 0.003 0.000 0.000 7.967

2.990 2.009 0.000 0.000 0.000 1.626 0.121 1.191 0.066 0.000 0.000 0.002 8.005

2.992 2.035 0.003 0.000 0.000 1.505 0.131 1.225 0.095 0.000 0.000 0.003 7.988

2.979 2.039 0.001 0.000 0.000 1.517 0.136 1.237 0.089 0.006 0.000 0.000 8.004

3.019 2.029 0.001 0.000 0.000 1.484 0.103 1.251 0.074 0.004 0.000 0.002 7.967

3.048 1.979 0.001 0.000 0.000 1.517 0.105 1.242 0.072 0.000 0.000 0.000 7.962

Mg / (Fe + Mg) Pyr Alm Grs Sps

0.39 0.37 0.57 0.02 0.03

0.34 0.32 0.61 0.03 0.04

0.39 0.37 0.58 0.03 0.03

0.38 0.36 0.58 0.03 0.03

0.43 0.40 0.53 0.02 0.04

0.42 0.40 0.54 0.02 0.04

0.45 0.41 0.51 0.03 0.04

0.45 0.42 0.51 0.03 0.05

0.46 0.43 0.51 0.03 0.04

0.45 0.42 0.52 0.02 0.04

Mineral name

Cordierite

Sample no.

KR21D

KR21E

KR65B

KR82B

KR82M

KR82N

KR2-1E core

KR2-1E rim

KR6-5B core

KR6-5L core

KR8-2B core

No. oxygen

18

18

18

18

18

18

6

6

6

6

6

SiO2 Al2O3 TiO2 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O ZnO Total

49.75 33.86 0.04 0.02 0.40 3.68 0.12 11.07 0.01 0.04 0.00 0.00 99.00

49.61 33.43 0.00 0.00 0.00 4.73 0.08 10.60 0.02 0.01 0.03 0.00 98.49

49.93 33.65 0.02 0.00 0.65 4.13 0.18 10.90 0.03 0.02 0.01 0.01 99.51

49.75 33.48 0.00 0.03 0.20 4.13 0.11 10.95 0.02 0.01 0.01 0.03 98.70

50.29 33.18 0.00 0.00 0.00 4.34 0.09 10.74 0.02 0.03 0.01 0.03 98.74

49.79 33.61 0.01 0.03 0.00 3.89 0.18 10.89 0.04 0.04 0.01 0.00 98.49

48.10 7.30 0.15 0.07 1.28 22.95 0.36 19.16 0.08 0.00 0.02 0.08 99.53

49.93 5.06 0.10 0.06 1.05 23.96 0.37 19.73 0.09 0.02 0.00 0.11 100.47

47.33 9.44 0.22 0.03 2.03 20.54 0.52 19.86 0.08 0.04 0.00 0.06 100.15

48.95 8.22 0.19 0.00 0.56 21.39 0.69 20.39 0.08 0.02 0.02 0.06 100.56

48.52 8.41 0.21 0.01 0.57 21.37 0.56 20.28 0.06 0.00 0.00 0.08 100.05

Si Al Ti Cr Fe3+ a Fe2+

4.986 3.999 0.003 0.001 0.030 0.309

Orthopyroxene

5.012 3.980 0.000 0.000 0.000 0.399

4.993 3.966 0.001 0.000 0.049 0.345

5.007 3.970 0.000 0.002 0.015 0.347

5.057 3.931 0.000 0.000 0.000 0.365

5.014 3.988 0.001 0.002 0.000 0.327

1.815 0.325 0.004 0.002 0.036 0.724

1.871 0.223 0.003 0.002 0.029 0.750

1.760 0.414 0.006 0.001 0.057 0.639

1.809 0.358 0.005 0.000 0.015 0.661

1.802 0.368 0.006 0.000 0.016 0.664

S. Ishii et al. / Gondwana Research 10 (2006) 99–114

107

Table 2 (continued) Mineral name

Cordierite

Orthopyroxene

Sample no.

KR21D

KR21E

KR65B

KR82B

KR82M

KR82N

No. oxygen

18

18

18

18

18

18

6

6

6

6

6

Mn Mg Ca Na K Zn Total

0.010 1.653 0.001 0.008 0.000 0.000 11.000

0.007 1.595 0.002 0.001 0.004 0.000 11.000

0.015 1.623 0.003 0.003 0.001 0.001 11.000

0.009 1.642 0.002 0.001 0.001 0.002 11.000

0.008 1.609 0.002 0.005 0.001 0.002 10.981

0.015 1.634 0.005 0.008 0.002 0.000 10.996

0.011 1.077 0.003 0.000 0.001 0.002 4.000

0.012 1.101 0.004 0.002 0.000 0.003 4.000

0.016 1.100 0.003 0.003 0.000 0.002 4.000

0.022 1.122 0.003 0.002 0.001 0.002 4.000

0.018 1.122 0.002 0.000 0.002 0.002 4.000

Mg / (Fe + Mg) Fe3+ / Fetotal XAl b

0.84 0.09

0.80 0.00

0.82 0.12

0.83 0.04

0.82 0.00

0.83 0.00

0.60 0.05 0.16

0.59 0.04 0.11

0.63 0.08 0.21

0.63 0.02 0.18

0.63 0.02 0.18

Mineral name

Spinel

Sample no.

KR2-1E core

KR2-1D core

KR6-5E core

KR6-5L core

KR8-2M core

KR8-2M with Grt

KR8-2N core

KR21D

KR21H

KR82M

No. oxygen

4

4

4

4

4

4

4

5

5

5

SiO2 Al2O3 TiO2 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O ZnO Total

0.01 60.46 0.06 0.06 1.70 25.88 0.30 7.86 0.00 0.07 0.01 3.24 99.65

0.00 59.18 0.00 0.16 3.63 27.80 0.19 8.36 0.01 0.00 0.00 0.56 99.88

0.06 60.84 0.00 0.12 2.24 23.46 0.40 9.83 0.04 0.07 0.02 2.47 99.54

0.07 61.41 0.05 0.19 1.08 27.53 0.26 8.00 0.00 0.05 0.00 2.00 100.64

0.08 61.97 0.03 0.09 2.25 26.02 0.53 9.07 0.00 0.07 0.00 2.09 102.19

0.01 63.14 0.11 0.19 0.88 21.44 0.06 11.67 0.00 0.10 0.03 2.55 100.17

0.01 58.96 0.01 0.03 4.58 27.43 0.47 8.21 0.00 0.06 0.02 0.85 100.62

KR2-1E core

KR2-1E rim

KR6-5B core

KR6-5L core

KR8-2B core

Sillimanite

Si Al Ti Cr Fe3+ a Fe2+ Mn Mg Ca Na K Zn Total

0.000 1.965 0.001 0.001 0.035 0.597 0.007 0.323 0.000 0.004 0.000 0.066 3.000

0.000 1.922 0.000 0.003 0.075 0.640 0.004 0.343 0.000 0.000 0.000 0.011 3.000

0.002 1.953 0.000 0.003 0.046 0.534 0.009 0.399 0.001 0.004 0.001 0.050 3.000

0.002 1.971 0.001 0.004 0.022 0.627 0.006 0.324 0.000 0.003 0.000 0.040 3.000

0.002 1.951 0.001 0.002 0.045 0.581 0.012 0.361 0.000 0.004 0.000 0.041 3.000

0.000 1.979 0.002 0.004 0.018 0.477 0.001 0.463 0.000 0.000 0.001 0.050 3.000

0.000 1.907 0.000 0.001 0.095 0.630 0.011 0.336 0.000 0.000 0.001 0.017 3.000

Mg / (Fe + Mg) Fe3+ / Fetotal

0.35 0.06

0.35 0.11

0.43 0.08

0.34 0.03

0.38 0.07

0.49 0.04

0.35 0.13

Mineral name

Biotite

Sample no.

KR2-1D with Grt

KR2-1D with matrix

KR2-1E with matrix

KR6-5E with matrix

KR6-5E with Opx

KR6-5L with Opx

KR8-2B core

No. oxygen

22

22

22

22

22

22

SiO2 Al2O3 TiO2 Cr2O3 FeO MnO MgO

36.30 15.27 5.85 0.03 11.80 0.04 15.12

37.19 15.61 4.66 0.00 9.34 0.02 16.81

37.05 14.75 5.13 0.01 15.98 0.00 13.20

37.64 15.23 5.15 0.05 10.94 0.16 16.09

37.06 14.78 5.22 0.00 12.87 0.09 15.46

38.03 15.39 4.17 0.00 9.88 0.06 17.50

36.77 62.48 0.00 0.03 1.50

36.48 62.33 0.03 0.01 1.35

36.69 62.41 0.02 0.02 1.67

0.00 0.01 0.00 0.03 0.00 0.05 100.88

0.00 0.04 0.00 0.00 0.00 0.03 100.27

0.09 0.02 0.05 0.02 0.01 0.00 100.99

0.990 1.981 0.000 0.001 0.030 0.000 0.000 0.000 0.000 0.002 0.000 0.001 3.005

0.987 1.987 0.001 0.000 0.027 0.000 0.000 0.002 0.000 0.000 0.000 0.001 3.005

0.987 1.979 0.000 0.000 0.034 0.000 0.002 0.001 0.001 0.001 0.000 0.000 3.006

KR8-2B rim

KR8-2M core

KR8-2M rim

22

22

22

22

37.10 15.49 3.81 0.05 11.09 0.10 16.56

37.25 14.90 3.82 0.03 11.76 0.09 16.82

38.69 16.40 3.70 0.03 7.67 0.02 18.80

39.67 17.00 2.15 0.05 6.27 0.04 21.02

(continued on next page)

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S. Ishii et al. / Gondwana Research 10 (2006) 99–114

Table 2 (continued) Mineral name

Biotite

Sample no.

KR2-1D with Grt

KR2-1D with matrix

KR2-1E with matrix

KR6-5E with matrix

KR6-5E with Opx

KR6-5L with Opx

KR8-2B core

KR8-2B rim

KR8-2M core

KR8-2M rim

No. oxygen

22

22

22

22

22

22

22

22

22

22

CaO Na2O K2O ZnO Total

0.00 0.06 9.71 0.00 94.18

0.05 0.03 9.83 0.02 93.54

0.01 0.05 9.62 0.05 95.85

0.00 0.03 10.00 95.27

0.00 0.01 10.01 0.02 95.51

0.00 0.11 9.62 0.06 94.83

0.00 0.03 10.17 0.08 94.48

0.03 0.07 9.65 0.00 94.41

0.00 0.28 9.47 0.02 95.07

0.00 0.25 9.17 0.06 95.67

Si Al Ti Cr Fe2+ Mn Mg Ca Na K Zn Total

5.430 2.691 0.658 0.003 1.476 0.006 3.369 0.000 0.017 1.852 0.000 15.500

5.521 2.730 0.520 0.000 1.160 0.002 3.717 0.008 0.008 1.861 0.002 15.528

5.540 2.598 0.576 0.001 1.998 0.000 2.939 0.002 0.014 1.833 0.006 15.508

5.534 2.638 0.569 0.005 1.344 0.020 3.523 0.000 0.009 1.875 0.000 15.518

5.496 2.582 0.582 0.000 1.596 0.011 3.415 0.000 0.003 1.893 0.002 15.579

5.570 2.657 0.459 0.000 1.210 0.007 3.819 0.000 0.031 1.798 0.007 15.557

5.516 2.713 0.426 0.006 1.378 0.012 3.668 0.000 0.008 1.929 0.009 15.666

5.544 2.613 0.427 0.003 1.463 0.011 3.730 0.005 0.019 1.832 0.000 15.646

5.573 2.785 0.400 0.003 0.924 0.003 4.035 0.000 0.077 1.740 0.002 15.541

5.617 2.837 0.228 0.005 0.743 0.004 4.432 0.000 0.068 1.655 0.006 15.595

0.70

0.76

0.60

0.72

0.68

0.76

0.73

0.72

0.81

0.86

Mg / (Fe + Mg) Mineral name

PI

Kfs

PI

Kfs

PI

Kfs

PI

Kfs

PI

PI

PI

Sample no.

KR2-1D lamella

KR2-1D host

KR2-1E host

KR2-1E lamella

KR6-5C lamella

KR6-5C host

KR8-2O lamella

KR8-2O host

KR21E

KR65B

KR82B

No. oxygen

8

8

8

8

8

8

8

8

8

8

8

SiO2 Al2O3 TiO2 Cr2O3 FeO MnO MgO CaO Na2O K2O ZnO Total

60.96 24.09 0.01 0.03 0.03 0.00 0.00 6.26 8.25 0.18 0.00 99.80

64.69 18.61 0.06 0.03 0.02 0.00 0.00 0.16 1.31 14.48 0.00 99.35

60.58 24.31 0.03 0.00 0.10 0.00 0.01 6.27 7.80 0.40 0.00 99.49

62.09 18.58 0.10 0.01 0.00 0.00 0.00 0.10 0.66 15.09 0.00 96.62

60.77 24.55 0.01 0.00 0.09 0.01 0.03 5.69 8.29 0.19 0.00 99.63

65.75 18.91 0.05 0.00 0.07 0.00 0.00 0.08 1.33 14.47 0.00 100.66

61.30 24.85 0.02 0.00 0.05 0.00 0.00 6.39 7.27 0.49 0.01 100.37

65.14 18.69 0.06 0.00 0.06 0.01 0.00 0.05 1.17 14.74 0.03 99.94

60.58 24.31 0.03 0.00 0.10 0.00 0.01 6.27 7.80 0.40 0.00 99.49

60.02 25.55 0.01 0.03 0.27 0.01 0.03 6.91 7.69 0.32 0.04 100.88

Si Al Ti Cr Fe2+ Mn Mg Ca Na K Zn Total Ab An Or a b

2.719 1.266 0.000 0.001 0.001 0.000 0.000 0.299 0.713 0.010 0.000 5.009 69.8 29.3 1.0

2.990 1.014 0.002 0.001 0.001 0.000 0.000 0.008 0.118 0.853 0.000 4.986 12.0 0.8 87.2

Fe3+ was calculated based on stoichiometry. XAl = Al / 2.

2.711 1.282 0.001 0.000 0.004 0.000 0.000 0.301 0.676 0.023 0.000 4.997 67.6 30.1 2.3

2.965 1.046 0.003 0.000 0.000 0.000 0.000 0.005 0.061 0.919 0.000 4.998 6.2 0.5 93.3

2.711 1.291 0.000 0.000 0.003 0.000 0.002 0.272 0.717 0.011 0.000 5.007 71.7 27.2 1.1

2.994 1.015 0.002 0.000 0.003 0.000 0.000 0.004 0.117 0.840 0.000 4.975 12.2 0.4 87.4

2.712 1.296 0.001 0.000 0.002 0.000 0.000 0.303 0.623 0.028 0.000 4.965 65.4 31.8 2.9

2.994 1.012 0.002 0.000 0.002 0.000 0.000 0.003 0.104 0.864 0.001 4.982 10.7 0.3 89.0

2.711 1.282 0.001 0.000 0.004 0.000 0.000 0.301 0.676 0.023 0.000 4.997 67.6 30.1 2.3

2.659 1.334 0.000 0.001 0.010 0.000 0.002 0.328 0.660 0.018 0.001 5.013 65.6 32.6 1.8

61.76 24.26 0.02 0.04

5.51 8.39 0.31 0.01 100.28 2.735 1.266 0.000 0.000 0.001 0.000 0.000 0.261 0.720 0.018 0.000 5.001 72.1 26.2 1.8

S. Ishii et al. / Gondwana Research 10 (2006) 99–114

content also increases toward rim as Alm58–61 Pyr 32–35 Sps4 Grs2–4. Sample KR6-5B (orthopyroxene-bearing) is slightly Mgrich and its composition ranges from Alm53 Pyr40–41 Sps4 Grs2 (core) to Alm53–55 Pyr39–40 Sps4 Grs2 (rim). Garnet in biotite-rich cordierite gneiss (sample KR6-5L) principally shows the same garnet composition (Alm53 Pyr40 Sps5 Grs2). Garnet in orthopyroxene-bearing cordierite gneiss (sample KR8-2B) from MGB is almost homogeneous in composition as Alm51–57 Pyr33–42 Sps4–6 Grs3–4. 4.3. Orthopyroxene All orthopyroxenes in the studied samples (KR2-1E, KR65B, KR6-5L, and KR8-2B) show compositional zoning in terms of Al (Fig. 6). Generally, the cores of coarse-grained orthopyroxene show the highest Al2O3 for all the samples. Orthopyroxene in sample KR2-1E (cordierite gneiss) has Alrich core (5.8–7.3 wt.% Al2O3) and slightly low-Al rim (4.5– 5.6 wt.%), although the XMg ratios are consistent (0.59–0.60). Fine-grained retrograde orthopyroxene with cordierite also show low Al2O3 content of 4.4–5.9 wt.%. Orthopyroxene in sample KR6-5B shows the highest Al2O3 content in its core (9.1–9.4 wt.% Al2O3), with slight decrease toward rim (7.6– 7.9%). In contrast, XMg ratio slightly increases from core (0.62 to 0.63) toward rim (0.67). Fine-grained orthopyroxene in sample KR6-5L has almost homogeneous range of Al2O3 (8.1– 8.6 wt.%) and XMg (0.62–0.63). Orthopyroxene in cordierite gneiss from the MGB (sample KR8-2B) also shows Al2O3-rich core (5.7–8.4 wt.%) and less aluminous rim (4.4–6.0 wt.%). Slight enrichment of XMg in the rim (0.64–0.67) as compared to the core (0.61–0.63) is also observed. 4.4. Feldspars Plagioclase in the examined samples is principally albite-rich with minor (less than 2 mol%) orthoclase content. Plagioclase in perthite and anti-perthite has a similar compositional range of Ab67–71 in samples KR2-1D (orthopyroxene-free) and KR2-1E

109

(orthopyroxene-bearing), although mineral assemblages of the samples are different. Composition of isolated plagioclase with no exsolution texture in the same samples is also consistent as Ab67–73. Plagioclase from locality KR6-5 also has similar composition of Ab70–72 for sample KR6-5B and Ab68 for sample KR6-5L (orthopyroxene-bearing). That in cordierite gneiss from the MGB (sample KR8-2B) also has a similar composition of Ab65–70. Slightly higher albite content of Ab81–89 was identified from the quartz-rich cordierite gneiss (sample KR8-2O). K-feldspar in samples KR2-1D (orthopyroxene-free) and KR2-1E (orthopyroxene-bearing) has compositions of Or86–87 and Or93–94, respectively. In locality KR6-5 (orthopyroxenebearing), the mineral shows a similar compositional range of Or84–88. K-feldspar from the MGB also shows consistent range of Or91–93 (sample KR8-2B; orthopyroxene-bearing) and Or81–89 (sample KR8-2O; quartz-rich). 4.5. Biotite Biotite is Mg-rich and characterized by high-TiO2 content (up to 5.9 wt.%). Biotite inclusion within garnet is relatively Mg-poor (XMg = 0.70–0.73) compared to the matrix phase (0.74–0.76), although their TiO2 contents are almost consistent (4.1–5.9 wt.%, e.g., sample KR2-1D; orthopyroxene-free). The lowest XMg (0.60) is recorded in anhedral biotite associated with quartz and feldspars in sample KR2-1E (orthopyroxenebearing), although its TiO2 content is still high (5.1–5.2 wt.%). Biotite with higher XMg (0.76–0.77) and lower TiO2 values (3.7–4.1%) coexists with orthopyroxene and garnet in sample KR6-5L. Biotite from the MGB also shows consistent range of higher XMg (0.72–0.73) and lower TiO2 values (3.7–3.8%) (sample KR8-2B; orthopyroxene-bearing), although that in sample KR8-2M (orthopyroxene-free) has the highest XMg (0.81–0.86) and the lowest TiO2 values (2.2–3.7%). 4.6. Spinel Spinel in the examined rocks, which coexists with cordierite, plagioclase, ilmenite, and magnetite (Fig. 4a to d), is principally solid solution of hercynite and Mg-spinel. XMg values are almost consistent as 0.31–0.49 with a small amount of Cr2O3 (0.01–0.19 wt.%) and ZnO (0.52–2.55 wt.%). Spinel in sample KR2-1E has the highest ZnO of 2.2–3.2 wt.%. A similar Znrich spinel has also been found as an inclusion in garnet in sample KR6-5L (1.9–2.0 wt.% ZnO). 4.7. Oxide minerals Ilmenite composition is close to its ideal formulae as FeTiO3. Magnetite is also close to Fe3O4, with low TiO2 of up to 0.35 wt.% (sample KR2-1E) and up to 0.05 wt.% MgO (sample KR2-1D). 5. Metamorphic P–T conditions

Fig. 6. XMg vs. XAl diagram showing orthopyroxene chemistry. P–T grids are taken from Hensen and Harley (1990). The two arrays indicate cooling paths inferred from core's date and rim's date.

Several geothermobarometers are applicable for the cordierite gneiss from the studied area. However, some of the computations based on conventional Fe–Mg exchange reactions are not suitable

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for UHT rocks to infer the peak P–T conditions due to retrograde thermal effects. As a first step we estimate the metamorphic conditions through the application of cation exchange thermometry, and proceed with application of thermometers that are more suitable for UHT rocks. The results are summarized in Table 3. 5.1. Garnet–orthopyroxene geothermobarometers The garnet–orthopyroxene geothermometer was applied to orthopyroxene-bearing cordierite gneiss. Application of the method suggested by Lee and Ganguly (1988), which is based on experimental calibration of Fe–Mg fractionation between garnet and orthopyroxene in the FMAS system at 20–45 kbar and 975–1400 °C, gave temperature ranges of 950–960 °C (sample KR2-1E), 900–970 °C (sample KR6-5L), 1000–1020 °C (sample KR6-5B), and 900–940 °C (sample KR8-2B) at 8 kbar. Calculated temperature ranges from the method of Harley and Green (1982), which is based on experimental data of the mineral pairs at 5–20 kbar and 800–1200 °C, are 970–980 °C (sample KR2-1E), 970–990 °C (sample KR6-5L), 1000–1020 °C (sample KR6-5B), and 940–960 °C (sample KR8-2B) at 8 kbar. The geothermometer of Ganguly et al. (1996), which is based on experimentally calibrated distribution between garnet and orthopyroxene at the P–T range of 12–20 kbar and 1000 °C, provides the lowest temperature ranges of 900–910 °C (sample KR2-1E), 840–900 °C (sample KR6-5L), 930–960 °C (sample KR6-5B), and 860–880 °C (sample KR8-2B) at 8 kbar. Pressure was estimated by the experimental calibration of garnet–orthopyroxene–plagioclase–quartz barometer by Perkins and Chipera (1985), yielding values of 7.9–9.2 kbar (sample KR2-1E), 8.1–9.5 kbar (samples KR6-5B and L), and 8.9– 9.1 kbar (sample KR8-2B) at 900 °C. Moecher et al. (1988) improved this geobarometer using new thermodynamic and experimental data. Calculated results using their method are slightly lower than but almost consistent with those obtained from

Perkins and Chipera's (1985) geobarometer, yielding pressure ranges of 7.5–8.8 kbar (sample KR2-1E), 7.7–9.1 kbar (samples KR6-5B and L), and 8.5–8.7 kbar (sample KR8-2B) at 900 °C. The experimental garnet–orthopyroxene geobarometer of Harley and Green (1982) gave slightly lower pressures of 5.5 to 6.4 kbar at 900 °C for all the samples. The estimated P–T conditions using the methods of Lee and Ganguly (1988) and Perkins and Chipera (1985) for the assemblage are shown in a P–T diagram (Fig. 7). 5.2. Garnet–cordierite geothermobarometers The garnet–cordierite geothermometer of Bhattacharya et al. (1988) was applied to estimate the retrograde conditions. Application of this method to cordierite corona and adjacent garnet (rim) yielded temperature ranges of 740–750 °C (sample KR2-1E), 770–790 °C (sample KR6-5 B), and 670–740 °C (sample KR8-2B) at 5 kbar. Temperature ranges obtained from the methods of Thompson (1976) and Holdaway and Lee (1977) are about 10 to 40 °C lower than those from Bhattacharya et al. (1988), but the calculated results using the three methods are almost consistent. Retrograde pressure at the corona formation stage was estimated from garnet + cordierite + sillimanite + quartz barometry of Harris and Holland (1984). The calculated results are 4.1– 4.2 kbar (sample KR2-1E), 4.4–4.6 kbar (sample KR6-5 B), and 3.6–5.1 kbar (sample KR8-2B) at 700 °C. The estimated retrograde P–T conditions using the methods of Bhattacharya et al. (1985) and Harris and Holland (1984) (3.5–4.5 kbar and 660– 780 °C) for the assemblage are shown in a P–T diagram (Fig. 7). 5.3. Cordierite–orthopyroxene geothermometer Sakai and Kawasaki (1997) proposed a geothermometer based on Fe–Mg exchange reaction between cordierite and orthopyroxene. We applied the geothermometer to orthopyroxene + cordierite

Table 3 Calculated P–T conditions for cordierite gneisses from ACZ and MGB using conventional geothermobarometers Sample KR2-1E KR2-1D1 KR2-1D3 KR6-5B KR6-5L KR8-2B

Range Average Range Average Range Average Range Average Range Average Range Average

T (HG) a

T (LG) a

T (G) a

T (B) b

T (T) b

T (HL) b

T (SK) b

P (PC) c

P (M) c

P (HG) d

P (HH) d

P (B) d

970–980 975

950–960 955

900–910 905

7.5–8.8 8.3

5.5–5.6 5.6

930–960 950 840–900 880 860–880 870

700–720 710 680–690 685 600–660 625 760–800 780

7.9–9.2 8.5

1000–1020 1005 900–970 940 900–940 920

730–750 740 700–710 705 610–680 640 800–840 820

570–690 650

1000–1020 1010 970–990 980 940–960 950

740–750 750 720–730 725 670–720 690 770–790 780

640–700 670

600–740 720

600–710 690

510–650 560

7.7–9.1 8.8 8.3–8.8 8.5 8.5–8.7 8.6

4.6–4.8 4.7 5.3–5.8 5.6 6.2–6.4 6.3

6.0–6.2 6.1 6.3–6.3 6.3 6.3–6.9 6.6 5.8–6.0 5.9

670–740 740

8.1–9.5 9.1 8.7–9.2 8.9 8.9–9.1 9

4.1–4.1 4.2 4.0–4.1 4.1 3.5–3.7 3.6 4.4–4.6 4.5

3.6–5.1 4.2

4.5–6.9 6.3

T (HG): Harley and Green (1982), T (LG): Lee and Ganguly (1988), T (G): Ganguly et al. (1996), T (B): Bhattacharya et al. (1988), T (T): Thompson (1976), T (HL): Holdaway and Lee (1977), T (SK): Sakai and Kawasaki (1997), P (PC): Perkins and Chipera (1985), P (M): Moecher et al. (1988), P (HG): Harley and Green (1982), P (HH): Harris and Holland (1984), P (B): Bhattacharya (1986). Temperature in °C, pressure in kbar. a Calculated at 8 kbar. b Calculated at 4 kbar. c Calculated at 900 °C. d Calculated at 700 °C.

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5.5. Ternary feldspar geothermometer

Fig. 7. P–T diagram showing peak and retrograde conditions of the examined samples. Peak conditions were calculated from geothermobarometers of Lee and Ganguly (1988) and Perkins and Chipera (1985). Retrograde conditions were from those of Bhattacharya et al. (1988) and Harris and Holland (1984). Dashed lines are isopleths of Al2O3 in orthopyroxene (as % MgTs component) in the FMAS assemblages after Harley (2004). Abbreviations in the figure are the same as Table 3.

corona after garnet+ quartz to infer the temperature of garnet breakdown. The results yielded temperature ranges of 570–690 °C (sample KR2-1E), 640–700 °C (sample KR6-5B), and 510– 650 °C (sample KR8-2B) at 4 kbar. The ranges are lower than those estimated from garnet–cordierite geothermometer (660–780 °C). The estimate is therefore considered to be the lower limit of garnet breakdown to form cordierite + orthopyroxene. 5.4. Al-solubility in orthopyroxene The Al content and XMg of orthopyroxene coexisting with garnet, sillimanite (or cordierite), and quartz are potential indicators of P–T conditions. Fig. 6 shows an XMg vs. XAl plot for selected orthopyroxenes in the gneisses. In this study, we adopted core compositions of porphyroblastic coarse-grained orthopyroxene because they are likely to preserve the near-peak compositions. The isotherms and isobars in the figure are adapted from the theoretical study by Hensen and Harley (1990). Cores of coarse-grained orthopyroxene in sample KR6-5B gave the highest P–T conditions of T N 1000 °C at 8 kbar. However, other samples of cordierite gneiss yielded slightly lower values of 950 °C at 8 kbar (sample KR6-5L), 900 °C at 6.5 kbar, and 850 °C at 6 kbar. The implications for this would be discussed in a later section. Harley (1998) revised the isopleths of Hensen and Harley (1990) using new experimental results. The estimated results using Harley's (1998) isopleths are about 30–50 °C lower than those given by Hensen and Harley's (1990) method. Application of the grid of Harley (2004), which is summarized in Fig. 7, yielded almost consistent result with that of Hensen and Harley (1990). Orthopyroxene in sample KR8-2B from the MGB shows P–T conditions of 950 °C at 8 kbar, which is consistent with the estimates from sample KR6-5B from the ACZ. Rim of the orthopyroxene in sample KR8-2B shows slightly lower P–T of 850 °C at 7 kbar (Fig. 6).

We applied ternary feldspar geothermometry using perthite and/or anti-perthite from the three localities to compare their metamorphic temperatures. Perthite and anti-perthite as well as quartz and orthopyroxene in the examined samples define quartzo-feldspathic layers within the rock parallel to the rock foliation (e.g., Fig. 2). The texture implies that perthite and antiperthite formed during a prograde stage and experienced highgrade metamorphism. Therefore, we consider that ternary feldspar geothermometer is a useful tool to estimate the temperature of the peak metamorphism in these rocks. The calculation technique involves the determination of early singlephase feldspar and the applications of the thermometer following the method described by Hokada (2001). Fig. 8 shows a ternary An–Ab–Or diagram with plots of the compositions of host and lamella feldspars, together with early single-phase feldspar inferred from host-lamella volume proportions estimated from back-scattered images (e.g., Fig. 4e,f). Using the thermodynamic model of Elkins and Grove (1990), we obtain a temperature range of 900–1050 °C at 8 kbar for samples from the ACZ (samples KR2-1D, KR2-1E, and KR6-5C). Perthite in sample KR8-2O from the MGB also shows high temperature of 900 °C. Application of other thermodynamic models such as Fuhrman and Lindsley (1988), Lindsley and Nekvasil (1989), and Nekvasil and Burnham (1987) to anti-perthites (sample KR21E) principally gave similar results. In the case of perthites (samples KR2-1D, KR6-5C, and KR8-2O) the calculated temperatures using models of Fuhrman and Lindsley (1988) and Lindsley and Nekvasil (1989) are about 70 °C lower, and that of Nekvasil and Burnham (1987) are about 100 °C higher than the results of Elkins and Grove (1990). The results from ternary-feldspar geothermometer are consistent with temperature ranges from Al-rich orthopyroxenes and confirm the UHT conditions of metamorphism in all the three localities. 6. Discussion Our study documents ultrahigh-temperature metamorphic conditions with T = 940–1040 °C and P = 8.5–9.5 kbar in cordierite gneisses from the ACZ in southern India (Fig. 7). The computed P–T conditions are significantly higher than those previously obtained from this area by Santosh (1987) (710– 790 °C, 5.5–7 kbar) and Nandakumar and Harley (2000) (860– 920 °C, 6.5–7.5 kbar). In an earlier study, Cenki et al. (2002) obtained high-temperatures in the range of 900–950 °C (at 6– 7 kbar) for a similar rock type from the ACZ based on Harley and Green's (1982) computation of the garnet–orthopyroxene geothermobarometer. Their results are slightly lower than our peak P–T data but almost consistent with our retrograde part of the path estimated from conventional geothermobarometers (Fig. 7), although we did not consider the presence of melt phases as done by Cenki et al. (2002). The application of Al-inorthopyroxene thermometry of Hensen and Harley (1990) based on the Al2O3 isopleths and XMg of orthopyroxene in the present study provides the first conclusive evidence for UHT conditions

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Fig. 8. An–Ab–Or diagram showing compositions of host and lamella feldspars forming perthite and anti-perthite, together with early one-phase feldspar compositions inferred from host-lamella volume proportions (see text for details). Ternary miscibility gaps are estimated for 8 kbar using the method of Elkins and Grove (1990) using computer program “SOLVCALC” coded by Wen and Nekvasil (1994).

in the ACZ with temperatures of 1000 °C at pressures around 8 kbar (Fig. 6). The very high temperature conditions attained by these rocks are also corroborated by garnet–orthopyroxene thermometry from our study which provides an upper estimate of 1020 °C. Ternary feldspar geothermometry of perthite also suggests T N 1000 °C peak metamorphism. Application of the recently revised Al2O3 isopleths of Harley (2004) to Al-rich orthopyroxene in sample KR6-5B (up to 9.4 wt.% Al2O3) provides evidence for UHT metamorphic temperatures exceeding 1020 °C, the highest estimate so far for extreme crustal metamorphism from the ACZ. Such Al-rich orthopyroxene with more that 9 wt.% Al2O3 has been reported from only few localities in southern India such as Ganguvarpatti (up to 10.7 wt.%, Sajeev et al., 2004) and Rajapalaiyam (up to 9.7 wt.%, Tateishi et al., 2004), both located within the MGB. Both these localities are also characterized by T N 1000 °C peak metamorphism. The slightly lower P–T conditions yielded by orthopyroxenes in some samples is considered to be a reflection of the modification of mineral compositions subsequent to peak metamorphism. We also estimated the retrograde conditions in these rocks from cordierite-bearing assemblages formed by reaction (1). Application of garnet–cordierite–sillimanite–quartz geothermobarometers yielded 3.5–4.5 kbar and 720 ± 60 °C, which is almost consistent with the results from previous studies (e.g., Santosh, 1987). Although available P–T data in Fig. 7 are grouped into categories implying simple decompression–cooling path, a more detailed evaluation of the results suggest a possible multiple exhumation history for this crustal block, with initial isobaric cooling from N1000 °C to 950 °C at above 8 kbar followed by rapid decompression down to 4–5 kbar (Figs. 6 and 7). Such initial near-isobaric cooling in UHT rocks from southern India was reported from Ganguvarpatti in the MGB by Tamashiro et al. (2004) based on varying Al2O3 content in orthopyroxene. Sajeev

et al. (2004) and Tamashiro et al. (2004) suggested that the retrograde path of the Ganguvarpatti rocks is not a simple decompression–cooling path, but has an initial isobaric cooling component after peak metamorphism, which was subsequently followed by an early stage of decompression, a second stage of cooling and further decompression. The first category of orthopyroxenes reported in these studies occurs as inclusions in garnet showing Al2O3 up to 10.7 wt. % (Sajeev et al., 2004) and indicating UHT metamorphic temperatures exceeding 1100 °C (Tamashiro et al., 2004). The second category of orthopyroxene, which corresponds to the core of orthopyroxene porphyroblast, shows slightly lower Al2O3 values (8.5–9.4 wt. %). The results are almost consistent with those of porphyroblastic orthopyroxene in sample KR6-5B (9.1–9.4 wt. %) reported in the present study on ACZ rocks. Similar near-isobaric cooling from UHT followed by decompression was also reported by Tateishi et al. (2004) from Rajapalaiyam in the southernmost part of the MGB. Obviously, multiple cooling–decompression history is commonly preserved by Mg–Al granulites from widespread localities within the MGB. In contrast, such a complex retrograde history has not been identified from the TGB. In Fig. 9, we show some of the available P–T paths for these granulite terranes. As the figure clearly demonstrates, our P–T path for rocks from the ACZ is similar to those from the MGB rather than the paths for the TGB rocks. Thus, the UHT peak metamorphic conditions of T N 1000 °C followed by early isobaric cooing and subsequent decompression in the MGB rocks (Sajeev et al., 2004; Tamashiro et al., 2004) are almost identical with the path from the ACZ obtained in this study, except for the slight difference in peak pressure estimates. The available P–T paths from the TGB, on the other hand, are characterized by simple clockwise uplift (e.g., Chacko et al., 1987; Morimoto et al., 2004). Although Chacko et al. (1996) reported UHT peak metamorphic conditions from rock types similar to those of present study, and Morimoto et al. (2004) reported spinel + quartz assemblage in khondalites from the TGB, the temperature estimates in both these reports are lower than the results obtained in the present study. Initial isobaric cooling followed by decompression has been reported by Nandakumar and Harley (2000) from the ACZ, although their estimated temperatures are about 100 °C lower than our results. Cenki et al. (2002) discussed the difference in the nature of P–T path between ACZ and TGB and pointed out the higher P– T conditions along the ACZ as compared to TGB. Nandakumar and Harley (2000) also obtained similar results. However, in all these studies, ACZ was assumed to be an integral unit of TGB, and no efforts were made to compare in detail the nature of the P–T path with that from the MGB towards north. In the present study, we report a new cordierite gneiss locality (KR8-2; Fig. 2) near Kottayam in the MGB, with lithologic characteristics and mineral assemblages identical with those of the other two locations within ACZ, as well as comparable with similar rocks reported from ACZ in previous studies. Importantly, the peak and retrograde P–T conditions of rocks from this locality estimated in this study (cf. Fig. 7) are also identical with those from the ACZ. Additionally, orthopyroxene in cordierite gneiss from this locality also shows consistently high-Al2O3 content of

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thus be either the southern flank of the MGB, or an exotic block that got sutured between the MGB and TGB, and which underwent varying extent of tectonic reworking during the PanAfrican orogeny associated with the final assembly of the Gondwana supercontinent. Acknowledgments

Fig. 9. P–T trajectories estimated from cordierite gneiss samples from the ACZ and MGB discussed in this study. Available P–T paths from the southern India granulite terranes are also plotted in the figure. T1: Chacko et al. (1987), T2: Morimoto et al. (2004), A1: Santosh (1987), A2: Nandakumar and Harley (2000). A3: Cenki et al. (2002), M1: Sajeev et al. (2004) and Tamashiro et al. (2004), M2: Tateishi et al. (2004). T1, T2: Trivandrum Granulite Block, A1-A3: Achankovil Zone, M1, M2: Madurai Granulite Block.

up to 8.4 wt.%, which suggests peak temperatures of ∼950 °C at 8 kbar based on the isopleth of Harley (2004). Compositional data of core and rim orthopyroxenes in Fig. 6 also support initial near-isobaric cooling after the peak metamorphism. Thus, it is evident that the cordierite gneisses along the ACZ are equivalent to those occurring along the southern domain of MGB. An evaluation of available geochronologic and petrologic data from the ACZ and MGB reveals several similarities. PanAfrican metamorphic ages of c. 530–550 Ma have been reported from both terrains (Santosh et al., 2003 and references therein). An array of post-tectonic alkali-feldspar granites occur along the southern margin of MGB as well as within the TGB (e.g., Soman et al., 1983; Tateishi et al., 2004; Santosh et al., 2005). While these “suturing plutons” which intruded during the latest Neoproterozoic–Cambrian times (cf. Santosh et al., 2005) are characteristic of the ACZ, they are notably absent within the khondalite belt to the south in TGB. Also, while orthopyroxene, spinel, garnet, cordierite, sillimanite, and quartz bearing UHT assemblages are commonly present in both MGB (e.g., sample KR8-2B) and ACZ (e.g., samples KR2-1E and KR6-5B), such lithologies have not so far been reported from the TGB, where cordierite mostly occurs in association with sillimanite, garnet and spinel within khondalites, in the absence of orthopyroxene. Such differences in mineral assemblage and lithological character are probably due to different bulk rock chemistry, suggesting different protolith. These features taken together prompt us to propose that the lithological units along the ACZ are more similar to those in the southern MGB, rather than within the TGB. If this is true, then the existing concept that ACZ is a unit of the TGB (the Achankovil Unit of the Kerala Khondalite Belt, e.g., Braun and Kriegsman, 2003) needs to be revised. We propose that the Tenmalai shear zone, which marks the southern extremity of the ACZ, may define the crustal boundary between the MGB and TGB. The ACZ would

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