Journal of Petrology Advance Access published online on November 30, 2007
Journal of Petrology, doi:10.1093/petrology/egm072
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Contrasting Episodes of Regional Granulite-Facies Metamorphism in Enclaves and Host Gneisses from the Aravalli–Delhi Mobile Belt, NW India
1Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721 302, India 2Department of Earth and Planetary Sciences, Hiroshima University, Higashi-Hiroshima, Japan and 3Indian Institute of Science Education and Research, Hc Block, Sector Iii, Salt Lake, Kolkata-700 106, India
Received September 20, 2006; Revised typescript accepted October 16, 2007
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONThe Aravalli–Delhi Mobile Belt in the northwestern part of India demonstrates how granulite enclaves and their host gneisses can be utilized to unravel multistage metamorphic histories of orogenic belts, using three suites of metamorphic rocks: (1) an enclave of pelitic migmatite gneiss–leptynite gneiss; (2) metamorphosed megacrystic granitoids, intrusive into the enclave; (3) host tonalite–trondhjemite–granodiorite (TTG) gneisses associated with an interlayered sequence of garnetiferous metabasite and psammo-pelitic schist, locally migmatitic. Based on integrated structural, petrographic, mineral compositional, geothermobarometric studies and P–T pseudosection modelling in the systems NCKFMASH and NCFMASH, we record three distinct tectonothermal events: an older, medium-pressure granulite-facies metamorphic event (M1) in the sillimanite stability field, which is registered only in the enclave, a younger, kyanite-grade high-pressure granulite-facies event (M2), common to all the three litho-associations, and a terminal amphibolite-facies metamorphic overprint (M3). The high-P granulite facies event has a clockwise P–T loop with a well-constrained prograde, peak (M2, P
12–15 kbar, T 815°C) and retrograde (M2R, 6·1 kbar, T 625°C) metamorphic history. M3 is recorded particularly in late shear zones. When collated with available geochronological data, the metamorphic P–T conditions provide the first constraint of crustal thickening in this belt, leading to the amalgamation of two crustal blocks during a collisional orogeny of possible Early Mesoproterozoic age. M3 reactivation is inferred to be of Grenvillian age. KEY WORDS: Northwestern India; polycyclic granulite enclave; pseudosection; high-pressure metamorphism; P–T path
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INTRODUCTION |
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Enclaves of granulite-facies rocks occurring in amphibolite-facies metamorphosed gneisses are common in many terrains, particularly those experiencing high-pressure metamorphism (Pin & Vielzeuf, 1983
; Chopin, 2003). The apparent metamorphic gap between the enclaves and the host gneisses has been explained by both monocyclic (Chopin, 2003; 
tipská et al., 2006) and polycyclic (Okay, 1989; Wain, 1997) models. However, in none of these models has the possibility of a renewed prograde burial of the granulite enclaves to high-pressure granulite metamorphism been discussed. In this study, we address this issue using granulite-facies rocks in the Sandmata area in the northwestern part of Peninsular India (Fig. 1a), which occur as an enclave within felsic gneisses metamorphosed to amphibolite facies. We have studied three distinct lithotectonic components, namely the granulite enclave, megacrystic granitoids intrusive into the enclave rocks and an interlayered sequence of psammo-pelitic and metabasic schists–gneiss from a new location near Sandmata (Fig. 1b and c). Using key reaction textures, mineral chemistry, geothermobarometry and P–T pseudosection modelling, we demonstrate distinct metamorphic gaps between the granulites and the host gneisses. We also establish that both the enclave and the host gneisses have experienced renewed burial and high-pressure granulite-facies metamorphism during a younger tectonothermal event, which was followed by an amphibolite-facies overprint. The results provide new insight into crustal evolutionary processes in this part of the Indian shield. Mineral abbreviations used are after Kretz (1983).
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GEOLOGICAL BACKGROUND |
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In the Precambrian shield of northwestern India, the
800 km long NNE–SSW-trending Aravalli–Delhi Mobile Belt (ADMB) wraps around the Archaean Bundelkhand craton. The ADMB consists of greenschist-to amphibolite-facies metasedimentary sequences of Palaeo- to Mesoproterozoic and Meso- to Neoproterozoic depositional ages (Aravalli and Delhi Supergroups respectively, Fig. 1b) and an association of amphibolite- to granulite-facies metamorphosed mafic and felsic orthogneisses and sedimentary rocks [the Banded Gneissic Complex of Gupta (1934) and Heron (1953), BGC henceforth]. The BGC in southern Rajasthan (Fig. 1b) is a 3·3–2·45 Ga composite gneiss–migmatite–granitoid complex metamorphosed at amphibolite-facies conditions (Heron, 1953; Gopalan et al., 1990; Wiedenbeck & Goswami, 1994; Roy & Kröner, 1996; Wiedenbeck et al., 1996a). In contrast, in central Rajasthan (the study area), the BGC (designated as BGC-II by Gupta, 1934) consists of high-grade supracrustal rocks and metamorphosed granitoids of Proterozoic age (Buick et al., 2006). The BGC-II has been classified into (1) supracrustal and meta-igneous granulites of the Sandmata Complex (SMC), and (2) the Mangalwar Complex (MC), which consists of amphibolite-facies supracrustals, amphibolites and granitoids or tonalite–trondhjemite–granodiorite (TTG) gneisses (Gupta et al., 1997) (Fig. 1c). Enclaves of granulite-facies SMC rocks occurring within the MC can be traced on a regional scale from Sandmata in the south to Bhinai in the north (Fig. 1b). The granulite enclaves comprise supracrustal granulites (pelitic granulite, calc-silicate granulite, and leptynite (quartzo-feldspathic) gneiss) and intrusions of gabbro (now two-pyroxene granulites) and norite. The supracrustal granulites have been intruded by a megacrystic granitoid suite of charnockitic to enderbitic composition (Guha & Bhattacharya, 1995; Dasgupta et al., 1997). The contacts between the granulite–megacrystic granitoid and the MC are marked by multiply deformed ductile shear zones (Guha & Bhattacharya, 1995; Roy et al., 2005). The magmatic emplacement of charno-enderbite in the SMC has been dated at 1·72 Ga (Sarkar et al., 1989) (Fig. 1b). U–Pb zircon geochronology from the megacrystic granites from the Sandmata area and megacrystic charnockite–enderbite and the granitoids from the Bhinai area, which have undergone ductile deformation, point to a major thermotectonic event at 1·62 to 1·69 Ga (Wiedenbeck et al., 1996b; Roy et al., 2005). Buick et al. (2006), on the other hand, placed both magmatic emplacement of charno-enderbite and granulite-facies metamorphism in the SMC at 1·72 Ga from U–Pb zircon geochronology [laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) method], an age that is indistinguishable from the emplacement ages of the TTG gneisses in the MC (Fig. 1b). They have also dated an amphibolite-facies overprint in the SMC and MC at 0·94–0·95 Ga using the same method.
In the study area (Fig. 1c), the SMC granulites, represented by pelitic migmatite gneiss (PMG) and leptynite gneiss, occur as a mega-enclave, nearly 14 km long and 0·4–3 km wide, within a tectonized intrusive megacrystic granitoid (Figs 1d and 2a). The earliest deformation D1 is recorded only in the enclave granulites in the form of an S1 foliation (Fig. 2b). The granulites additionally record overprints of D2 and D3 deformations (Fig. 2c). The megacrystic granitoids are of two types. Type 1 granitoids (charnockitic to enderbitic) are massive, with megacrystic plagioclase and K-feldspar. The second type, of broadly similar composition is, however, migmatitic, with leucosomes occurring as veins and patches (Fig. 2d) or as bands alternating with melanosomes consisting of garnet, biotite and plagioclase (S2). The migmatitic variety of megacrystic granitoid generally occurs as a 50–100 m wide partial aureole around the granulite enclave (Fig. 1d). The rock is locally extremely deformed, producing a well-developed gneissic foliation (S3), which envelops garnet porphyroblasts (Fig. 2e).
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The interlayered psammo-pelitic schist–garnet-bearing metabasite association of the MC occurs as 10–40 m wide and
100 m long bands and lenses at the contact of granulites and type 2 granitoids (Fig. 1d) and also within type 2 granitoids. Similar occurrences of this rock association are also noted within the TTG gneisses of the MC (Fig. 1c). The psammo-pelitic schist is locally migmatitic with the development of granitic leucosomes defining the S2 foliation. A pervasive foliation (S3) envelops porphyroblastic garnet (Fig. 2f). Narrow shear zones are marked by the development of an S4 foliation in the pelitic migmatite gneiss, the psammo-pelitic schist and the type 2 granitoid. In the granitoid, the shear zone deformation (D4) refolded the S3 planes (Fig. 2g). A summary of the deformational history and the relationship between mineral growth and deformation recorded in different lithologies is presented in Table 1. The locations of the samples studied here are shown in Fig. 1c and d. The complete list of mineral associations in the studied samples is given in the Electronic Appendix, which may be downloaded from http://www.petrology.oxfordjournals.org.
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PETROGRAPHY |
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The SMC lithological association
PMG
An S1 granulite-facies banding is defined by alternate aluminous layers (rich in porphyroblastic garnet and sillimanite) and K-feldspar1–quartz ± plagioclase leucosomes. Porphyroblastic garnet1 (20–30% by mode) contains a folded internal schistosity defined by trails of biotite1, quartz and sillimanite1 (Fig. 3a). S2 is defined by biotite2–kyanite that refracts around garnet1 (Fig. 3b). A second generation leucosome (quartz–K-feldspar2) cuts S1. The S2 foliation is overgrown by garnet2 that contains inclusions of kyanite (Fig. 3c) and biotite2. Garnet2 also occurs as overgrowths on garnet1 (Fig. 3d). The last generation of biotite (biotite3) occurs in coronae between K-feldspar2 and garnet2, kyanite and quartz (Fig. 3e).
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Leptynite gneiss
S1 banding is defined by alternate layers of porphyroblastic garnet1 and plagioclase (locally antiperthitic)–K-feldspar–quartz. Garnet1 contains inclusions of prograde biotite1, plagioclase and quartz. A pervasive S2 foliation defined by oriented biotite2 wraps around garnet1. Biotite2 in turn is rimmed by coronal garnet2 (Fig. 3f), which also occurs as a metamorphic overgrowth on garnet1 (Fig. 3g). The garnet2–K-feldspar2 assemblage develops as coronae around plagioclase, quartz and garnet1 (Fig. 3g). Biotite3, intergrown with quartz, occurs as coronae around K-feldspar and garnet2 (Fig. 3g).
Type 1 megacrystic granitoid
This rock is characterized by a coarse-grained, polygonized mosaic of plagioclase, orthopyroxene, quartz and ilmenite. A mylonite foliation (S2) defined by thin bands of extremely fine-grained plagioclase and quartz aggregates and oriented hornblende is locally developed. Coronal garnet, often intergrown with quartz and/or clinopyroxene, has developed around flattened plagioclase, hornblende (Fig. 4a) and orthopyroxene. Because of its development overgrowing the S2 foliation, this garnet is referred to as garnet2.
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Type 2 migmatitic granitoid
The leucosomes contain antiperthitic plagioclase, perthite and quartz, with minor biotite, titanite and zircon. Porphyroblastic garnet, hornblende, biotite and titanite are the dominant constituents in the mesosomes. Porphyroblastic garnet contains isolated inclusions of prograde biotite, quartz, plagioclase and epidote (Fig. 4b). A textural zonation in the occurrence of plagioclase and epidote inclusions in porphyroblasts is observed. Epidote and plagioclase are present in the core and the inner rim of garnet, whereas plagioclase alone is present in the outer rim of garnet. Epidote is totally absent in the matrix. Garnet is rimmed by hornblende–ilmenite–plagioclase (Fig. 4c) and biotite–plagioclase symplectites (Fig. 4b).
MC lithological association
TTG felsic gneisses
The dominant rock type in the MC is a felsic gneiss. Exposed in the eastern part of the mapped area (Fig. 1c), the rock is migmatitic and consists of leucosomes of granitic/granodioritic composition and mesosomes that contain hornblende, quartz, K-feldspar and plagioclase. Garnet occurs locally in both the leuco- and mesosomes.
Garnet-bearing metabasite
This rock shows considerable variation in mineral assemblage, grain size and fabric from the central to the rim part of the lenses. In the cores of the large lenses, the rock consists of garnet–clinopyroxene–quartz–hornblende with minor rutile, titanite, ilmenite, plagioclase and scapolite. A crude S2 gneissic banding, marked by alternate garnet, clinopyroxene and quartz-rich layers, is locally preserved. Porphyroblastic garnet and clinopyroxene lie scattered in a matrix of hornblende–ilmenite–quartz–titanite. Garnet contains scattered inclusions of hornblende (Fig. 5a), ilmenite, plagioclase and rutile. Rutile and plagioclase are totally absent in the matrix. Hornblende and quartz in the matrix define a pervasive S3 foliation, which envelops the garnet and clinopyroxene porphyroblasts. To differentiate from included hornblende, S3-parallel hornblende is named hornblende3. Garnet and the clinopyroxene porphyroblasts are armoured by symplectic intergrowths of hornblende–plagioclase ± ilmenite (Fig. 5a). The symplectites are radially oriented from the walls of garnet, clinopyroxene and hornblende3, implying post-S3 development of the intergrowths.
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There is intense development of S3-parallel hornblende with absence of clinopyroxene in the rims of the metabasic lenses. Hornblende–plagioclase ± ilmenite symplectites both armour garnet and occur within it, locally leading to complete pseudomorphous replacement (Fig. 5b).
Psammo-pelitic schist
These rocks are dominated by aluminosilicate minerals, garnet and quartz with moderate biotite and minor staurolite, ilmenite, plagioclase and rutile, and with or without K-feldspar. The earliest stabilized assemblage is staurolite–ilmenite–quartz–biotite1–rutile–kyanite, which occurs as inclusions within porphyroblastic garnet (Fig. 5c). The schist is locally gneissic and migmatitic (S2) with garnet–biotite–kyanite mesosomes and quartz–K-feldspar-bearing leucosomes. S3 is the main penetrative foliation and is defined by biotite3 and coarse sillimanite (Fig. 2f). Plagioclase additionally occurs as a corona around quartz and aluminosilicates. Biotite4 intergrown with fibrolite defines the S4 foliation in narrow shear zones.
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MINERAL CHEMISTRY |
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Electron microprobe analysis was carried out with a JEOL-JXA 8810 instrument at the Hiroshima University. Natural mineral standards were used and a ZAF correction procedure was adopted. Representative mineral chemical compositions are given in the Electronic Appendix and the salient compositional features are mentioned below.
Garnet
Garnet1 is Alm68–70Prp25–28Grs02–03Sps01Adr01 in the PMG. Garnet2 is more magnesian (Alm69Prp26Grs04 Sps01) in microdomains poor in biotite than in microdomains rich in biotite (Alm74Prp20Grs05Sps01). In the leptynite gneiss, cores of garnet1 are Alm67Prp27Grs05Sps01. In contrast, garnet2 is significantly enriched in grossular (Alm61–73Prp14–25Grs07–15Sps00–01Adr00–02) showing antithetic relationship with the anorthite content in coexisting plagioclase (discussed below).
In the type 2 granitoid, garnet varies in composition from Alm49–55Prp05–11Grs34–41Sps01–06Adr00–04 to Alm62–73Prp09–15Grs12–23Sps00–04Adr00–04. X-ray compositional images (Figs. 6a–d) and line profile (Fig. 6e) reveal two types of compositional zoning in garnet, in sample R8A(2). The first type is characterized by a rimward increase in Mg and Ca and a concomitant decrease in Mn, typical of prograde growth (Chakraborty & Ganguly, 1990, and references cited therein). The second type is marked by localized Mg enrichment in both the cores and the inner rims of garnet (Fig. 6a). Garnet rims against biotite–plagioclase symplectites, and garnet relics within hornblende–plagioclase symplectites show a marked enrichment in spessartine content.
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In the psammo-pelitic schist, garnet shows compositional zoning from Alm65Prp25Grs07Sps01Adr02 in cores to Alm75Prp13Grs08Sps04 at the rims against biotite3. Garnet shows compositional variations in the range Alm45–56Prp12–23Grs25–32Sps01–03Adr00–04 in the metabasite. X-ray compositional maps (Figs. 7a–d) and compositional profiling (Fig. 7e) show a continuous rimward increase in Mg and Ca and a decrease in Mn and Fe, similar to that seen in the garnet of type 2 granitoid. This type of compositional zoning in garnet in mafic bulk compositions has previously been noted in high-pressure occurrences (Guo et al., 2002
). Additionally, garnet shows localized compositional zonation, characterized by elevated pyrope and spessartine and concomitant lower almandine concentrations around internal hornblende–plagioclase–ilmenite symplectites (Fig. 7a and d). This type of compositional zoning appears to mark post-peak compositional re-equilibration, in response to decomposition of garnet (Loomis, 1983).
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Plagioclase and epidote
Plagioclase is compositionally homogeneous in the PMG (An30–31). In the leptynite gneiss, plagioclase is An37–38 in the rim adjoining the most grossular-rich garnet2. However, it is An42 against low grossular garnet2. The most calcic composition (An45) is noted in rims of plagioclase adjoining biotite3–quartz symplectites. Reintegrated composition of antiperthite is An35Ab58Or07.
In the type 2 granitoid, inclusions of plagioclase in garnet shows slight rimward enrichment in anorthite from An40 to An43. Plagioclase inclusions show compositional variation, which can be related to their location in the growth zoned garnet. The inclusions located in the inner rim portion have the composition An43, whereas those at the boundary between the inner and outer rim are An61. However, plagioclase inclusions in the outer rim are more sodic (An55–57) and symplectic plagioclase has the composition An46. Epidote included in garnet has 20 mol% pistacite.
In the psammo-pelitic schist, matrix plagioclase shows rimward depletion in anorthite (An60 to An44). Coronal plagioclase is more calcic (An51) relative to matrix plagioclase rims (An44). In the garnet-bearing metabasite, plagioclase included in garnet shows rimward enrichment from An46 to An85 in the outer rim, in contact with grossular-poor garnet. Symplectic plagioclase is An64.
Hornblende
Amphiboles in the type 2 granitoid and garnet-bearing metabasite are tschermakite to ferro-tschermakite [after Leake et al. (1997) and considering average Fe3+ content in amphibole], except in one sample of granitoid, where it is ferropargasite. Symplectic hornblende shows the highest concentration of Na in the M4 site [0·13–0·17 cations p.f.u.; 23(O) basis]. Matrix hornblende shows the highest enrichment in (Na + K)A (0·61–0·71 p.f.u.), Al(IV) (1·88–1·92 p.f.u.) and Ti (0·13–0·14 p.f.u.). S3 hornblende in the metabasite is tschermakitic and distinctly more magnesian (XMg = 0·59–0·65) than hornblende in the granitoids (XMg = 0·40–0·54).
K-feldspar
K-feldspar2 in the PMG is a K-feldspar–albite solid solution with XAb = 0·10–0·13. In contrast, the host in perthite from the leptynite gneiss is nearly pure K-feldspar. K-feldspar is uniform in composition (Ab09Or88–89Cel02–03) in all the textural modes in the migmatitic granitoid.
Staurolite, ilmenite and titanite
Inclusions of staurolite in garnet in the psammo-pelitic schist are magnesian (XMg = 0·34) and zincian [Zn = 0·142–0·174 p.f.u.; 24 (O) basis]. Ilmenite inclusions in kyanite in the same rock contain substantial haematite (7 mol%) and geikielite (3 mol%) components. Symplectic ilmenite in the megacrystic granitoid is nearly pure, whereas that in the garnet-bearing metabasite contains 5 mol% haematite and 4 mol% pyrophanite. Matrix titanite in the granitoid is aluminous [Al = 0·076–0·087 p.f.u.; 5(O) basis].
Clinopyroxene
Cores of clinopyroxene porphyroblasts in the garnet-bearing metabasite are highly aluminous [Altot = 0·262 p.f.u.; 6(O) basis], magnesian (XMg = 0·75) and moderately sodic (Na = 0·065 p.f.u.). In binary plots of Altot and Na vs P, clinopyroxene falls within the high-pressure field deduced from experimental data (McCarthy & Patiño Douce, 1998). In XCaTs vs XJd space, clinopyroxene cores plot in the field of high-pressure granulites (White, 1964).
Biotite
Cores of matrix biotite in the PMG and leptynite gneiss are titaniferous (TiO2 3·03–3·85 wt%) and magnesian (XMg = 0·61–0·63). Biotite at the contact with garnet is even more magnesian (XMg = 0·65–0·70).
In the granitoid, matrix biotite is titaniferous (TiO2 3·68 wt%) and ferroan (XMg = 0·40–0·43). Coronal biotite replacing garnet is even more Fe-rich (XMg = 0·33).
Cores of matrix biotite in the psammo-pelitic schist are lower in XMg (0·55) than inclusions in garnet or rims of matrix biotite against garnet (XMg = 0·64–0·74). Irrespective of textural type, F and Cl contents in biotite of the megacrystic granitoids and psammo-pelitic schist (total 0·26–0·47 wt%) are lower than those in the leptynite gneiss (total 0·71–1·11 wt%).
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EVOLUTION OF THE MINERAL ASSEMBLAGES |
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A synthesis of structural, petrographic and mineral chemistry data reveals a multistage metamorphic evolution in the study area. For clarity, we describe these in relation to structural fabrics that are produced in these rocks. Accordingly, four metamorphic stages (M1, M2, M2R and M3), equivalent to the stages of foliation development (S1–S4) are distinguished.
M1 metamorphism
The M1 metamorphism, recorded only in the enclave granulite, is marked by the appearance of garnet1 and K-feldspar1 and by the formation of granitic leucosomes that define S1 banding. Inclusions of prograde biotite1–sillimanite1–quartz in garnet1 are consistent with the following generalized biotite melting reaction in the PMG:
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| (1) |
M2 metamorphism
SMC association
The earliest stage of M2 metamorphism is marked by the stability of the biotite2–kyanite assemblage in the PMG and the biotite2–quartz assemblage in the leptynite gneiss. The main phase of M2 metamorphism is marked by the appearance of the garnet2–K-feldspar2 assemblage. In the PMG, features such as the presence of biotite2 and kyanite inclusions within the garnet2–K-feldspar2 assemblage (Fig. 3c), and of S2-parallel granitic leucosomes suggest the following biotite melting reaction:
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| (2) |
; Vielzeuf & Holloway, 1988; White et al., 2001) and natural rock data (Barbey et al., 1990) suggest that reaction (2) operates at relatively high pressures (P > 10 kbar) and temperatures (T >800°C).
In the associated leptynite gneiss, the occurrence of garnet2 as coronae around biotite2 (Fig. 3f), and composite garnet2–K-feldspar2 coronae around plagioclase (Fig. 3g), suggest a similar reaction:
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| (R3) |
). In the studied rocks, the high grossular contents of garnet2, and relatively magnesian composition of garnet2 in felsic bulk composition collectively indicate high-pressure granulite-facies metamorphic conditions (Vielzeuf & Schmidt, 2001) during M2. In the type 2 migmatitic granitoid textural features (the presence of epidote and biotite as inclusions, and the absence of epidote in the matrix) and compositional zoning in garnet and plagioclase suggest at least two stages of prograde metamorphism. In the first stage, the core and inner rim region of garnet were produced along with plagioclase, from decomposing prograde biotite and epidote. With further rise in temperature, as indicated by continuous rise in pyrope and fall in spessartine contents in the garnet outer rim (Fig. 6e) and development of leucosomes, the next stage of prograde metamorphism shifted to the model biotite melting reaction (3) discussed above.
MC association
Psammo-pelitic schist. An early stability of the assemblage staurolite–ilmenite–quartz–biotite is inferred in this rock. This is interpreted to have been followed by the appearance of porphyroblastic garnet, kyanite and rutile. This assemblage can appear through the following Fe-end-member model reactions:
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| (4) |
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| (5) |
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| (6) |
Reactions (5) and (6) have relatively flat slopes in P–T space and can progress to the right with increasing pressure. In contrast, reaction (4) has a relatively steep negative slope and can progress to the right by heating or loading, or by a combination of the two.
Garnet-bearing metabasite–Type 1 granitoid. In the metabasite, a prograde metamorphic stage is recorded by the relict inclusion assemblage of hornblende and plagioclase in garnet and by the growth zoning preserved in the porphyroblastic garnet. In the granitoid, the garnet–clinopyroxene–quartz assemblage occurs as coronae around hornblende and plagioclase. Accordingly, this assemblage was produced by the following generalized NCFMASH reaction:
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| (7) |
Reaction (7) has a negative slope in P–T space and may be crossed with increasing pressure, accompanied by rising temperature (Pattison, 2003). The stability of rutile with garnet and clinopyroxene, coupled with the compositional characteristics of hornblende, garnet, plagioclase and clinopyroxene in the metabasite, listed above, attest to high-pressure granulite-facies metamorphism (O’Brien & Rötzler, 2003; Pattison, 2003).
Retrograde path of M2 metamorphism (M2R)
In the rocks of pelitic bulk composition, M2R is marked by the general stability of sillimanite, locally forming a distinct foliation (S3) (Fig. 2f). Textural features of the development of coronal plagioclase around aluminosilicates and quartz in the pelitic schist, biotite3–quartz intergrowths around garnet2 and K-feldspar2 in the leptynite gneiss (Fig. 3g) and the compositional characteristics of plagioclase and adjacent garnet show that both reactions (3) and (6) moved in opposite directions during M2R.
In the garnet-bearing metabasites, a two-stage retrograde evolution is recognized. In the earlier stage, hornblende3 defining S3, was produced at the expense of garnet and clinopyroxene porphyroblasts. The absence of rutile in the matrix and the titaniferous nature of hornblende3 imply the following generalized hydration reaction:
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| (8) |
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| (9) |
M3 metamorphism
The development of a localized biotite–fibrolite schistosity (S4) in pelitic rocks in narrow shear zones is the only manifestation of M3. Because the D4 deformation has refolded F3 and S3 (Table 1), we tentatively ascribe it to a separate lower amphibolite-facies tectonothermal overprint.
Summarizing, this work reveals a polymetamorphic history in the study area. The PMG and the leptynite gneiss record an earlier granulite-facies metamorphic event (M1) in the sillimanite stability field. A uniform high-P granulite-facies overprint (M2) is recorded in all the lithologies of SMC and MC (in the stability field of kyanite in pelitic bulk compositions). The retrograde path of evolution of the high-P granulites (M2R) is characterized largely by decompression with cooling and hydration at lower pressures. M3 is a lower amphibolite-facies overprint on the partially exhumed granulites.
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GEOTHERMOBAROMETRY |
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We have quantified the metamorphic conditions for the M2 and M2R stages using both conventional geothermobarometers (referred to subsequently as CG) and the internally consistent thermodynamic dataset 5.5 of Holland & Powell (1998
: November 2003 upgrade and with THERMOCALC v.3.26; referred to as TC). For the latter, identical geothermobarometric expressions are calculated, except for the garnet–hornblende–plagioclase–quartz assemblage, where average pressure was calculated at reference temperatures, using the THERMOCALC multi-equilibria approach. The results of the geothermobarometry calculations are given in Table 2.
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In the metabasite, the magnesian outer rim of growth zoned garnet, the aluminous core of porphyroblastic clinopyroxene and the sodic core of included plagioclase in garnet yield ‘peak’ M2 P–T estimates of 15·0 ± 2kbar (2
error) and 875 ± 100°C (2 error) using CG at assumed reference P–T conditions of 800°C (for geobarometry) and 10 kbar (for geothermometry). The assumed P–T conditions for CG calculations are close to those obtained by TC (12·3 ± 2·2 kbar, 875 ± 270°C) (Table 2). The presence of Fe3+ in garnet (calculated by charge balance and stoichiometry) reduces temperature estimates by 20–70°C for different calibrations. In the psammo-pelitic schist, calculations using the compositions of the magnesian cores of garnet, and the cores of coarse biotite and sodic plagioclase in the matrix, give M2 P–T estimates that range from 9·4 ± 1·6 kbar (GASP)–11·4 ± 2·4 kbar (GBPQ), 760 ± 50°C (CG) to 10·0 ± 2·0–9·7±3·6 kbar, 910 ± 200°C (TC). Consideration of Fe3+ in garnet and biotite reduces temperature estimates by 45°C. For this calculation, we have considered 3 mol% of total Fe in garnet and 11·6 mol% of that in biotite to be in the Fe3+ state following Holdaway (2000). In the type 2 migmatitic granitoid, the calculated P–T condition is 14 kbar, 800°C (using CG), which is reasonably close to that calculated from the metabasite. Core compositions of garnet2, biotite2 and sodic plagioclase occurring in the matrix in the PMG yield P–T estimates of 10·9 ± 1·8 kbar, 720 ± 100°C for M2 metamorphism (GASP and GBPQ barometry and garnet–biotite thermometry). This is lower than that deduced from the MC suite and granitoid rocks (P 12·4 ± 3·6 kbar, T 815 ± 110°C).
In the garnet-bearing metabasites and the granitoids, the P–T estimates for the formation of M2R hornblende–plagioclase symplectites were obtained using the composition of garnet edges and coexisting hornblende and plagioclase symplectite. Application of garnet–hornblende–plagioclase–quartz barometry and garnet–hornblende thermometry yields three P–T clusters: 7·1 ± 3·4 kbar, 830–865°C (granitoid), 6·7–7·0 ± 3·4 kbar, 665–740°C (metabasite) and 4·4–6·1 kbar, 550–605°C (granitoid) (Table 2). The temperature estimates are broadly consistent with the Ti content of hornblende in these rocks and that obtained from hornblende–plagioclase thermometry (Table 2). The presence of Fe3+ in hornblende reduces the temperature estimate by 50–100°C. In the psammo-pelitic schist, the P–T estimate is in the range of 6·5–9·7 kbar, 690–740°C (Table 2) for the formation of coronal plagioclase. Even lower temperatures (T 600–625°C) are recorded in leptynite gneiss for biotite3 replacing garnet2 (Table 2). Based on these results, we argue that the studied rocks collectively record an initial steep decompression to P 8·1 ± 2·4 kbar, T 760 ± 140°C, followed by decompression accompanying cooling to P 6·1 ± 2·0 kbar, T 625 ± 140°C during M2R.
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P–T PSEUDOSECTION MODELLING |
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In this section we investigate further the P–T conditions and P–T path of the M2 metamorphism using pseudosection modelling. Such studies are complicated in the case of polymetamorphic granulites because re-metamorphism occurs in a bulk composition that has changed substantially from that of the protolith as a result of partial melting and melt loss during the earlier event (White et al., 2001
). Some progress has been made in this context through melt modelling studies in pelitic migmatites, applying the THERMOCALC software and using an improved thermodynamic dataset and expressions of activity–composition relationships of minerals and melts in simple (KFMASH) and complex systems (NCKFMASHTO) (White et al., 2001, 2007). Critical to the results of melt modelling is the choice of the protolith composition of the migmatites, for which two general approaches have been followed. In the first approach, modelling has been undertaken with several hypothetical bulk compositions (White et al., 2001, 2007; Johnson & Brown, 2004), primarily because the migmatites themselves have refractory bulk-rock compositions from which an unspecified amount of melt has been lost. In the second approach, the effective bulk composition, estimated through calculated mineral modes and mineral compositions, have been applied to compute P–T pseudosection (Kelsey et al., 2003). In the case of the studied polyphase migmatites, it would be difficult to account for the volume and the composition of melt extracted from the metamorphic system during M1. Even where leucosomes are present, ‘melt re-integration’ becomes problematic given the possibility of a cumulate origin of some of these leucosomes (e.g. Johnson & Brown, 2004).
Keeping in mind these limitations, we make an attempt to address the issue of melt loss and preservation of the M1 assemblage in the PMG and its re-metamorphism (M2) in the changed bulk-rock compositions, using P–T pseudosections in the NCKFMASH system. We begin our analysis with the hypothetical average pelite bulk composition of White et al. (2001). We have evaluated the likely effects of compositional variation on the overall topology of the pseudosection through a series of calculations, involving 10% error in the estimation of XMg, Al2O3, CaO and Na2O contents in the bulk-rock. These calculations allow us to predict the possible ranges of melt extraction required to preserve the current M1 mineralogy in the PMG, and also the P–T conditions for the M2 event. The results, given in Fig. 8a–c, are then compared with similar modelling on a natural rock composition. We have selected sample R14 with the following mineral modes, computed from 10 thin sections prepared from the sample: garnet1 32%, K-feldspar 23%, quartz 20%, garnet2 15%, aluminosilicates 5%, biotite 3% and plagioclase 2%. As a starting point, the bulk composition [analysed by X-ray fluorescence (XRF) at the Wadia Institute of Himalayan Geology, Dehradun] has been adjusted to contain the same H2O content as used for P–T pseudosection modelling in Fig. 8c. The effects of variations in H2O contents on the solidus, biotite-out curves, and garnet and melt modes have been assessed subsequently. The results are presented in Fig. 8d.
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For the monocyclic garnetiferous metabasite, an NCFMASH P–T pseudosection for quartz and H2O-saturated condition has been additionally constructed for the bulk-rock composition of sample Raj22A. The rock is silica-saturated, with an intermediate Mg-number (0·52). Even though the garnet in the sample is compositionally zoned, our calculations of garnet and clinopyroxene fractionation reveal only minor changes in bulk composition, and consequently have little effect on the locations of phase boundaries and geometry of compositional isopleths. The datafile coding of the activity–composition relationships of most minerals were taken from the THERMOCALC documentation (Powell & Holland, 2006
). The mixing models for amphibole (hornblende) and clinopyroxene (Di) are after Dale et al. (2005) and Holland & Powell (1996), respectively. The results are shown in Fig. 9a–c.
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Figure 8a shows the computed phase equilibria using the average pelite composition. The stability of the M1 peak assemblage of garnet–sillimanite–K-feldspar–quartz–melt, in the absence of cordierite, predicts M1 temperatures to be in excess of 800°C at pressures above 7 kbar. For modelling of melt loss, an average M1 P–T condition of 7·5 kbar, 825°C is considered reasonable based on previous thermobarometric estimates (Dasgupta et al., 1997
). The amount of melt and garnet produced at such a P–T condition is 38 molar% and 28 molar%, respectively. For variations in bulk XMg, Na2O and CaO contents and assumed P–T conditions (e.g. ±0·5 kbar, ±25°C), these parameters vary in the range of 37–40 molar% and 27–29 molar%, respectively (Fig. 8b). Under closed-system conditions crystallization at the onset of cooling would lead to complete retrogression of garnet to biotite. The common presence of porphyroblastic garnet in the M1 assemblages of the PMG and in the co-metamorphosed leptynite gneiss, therefore, implies substantial melt loss during and/or after M1 metamorphism. Our calculations predict that for the estimated modes of M1 garnet in the PMG (in the range 20–30%), this would require melt extraction in the range of 21–30 molar%.
Figure 8c shows phase relationships for a model bulk composition, formed after 30% molar melt loss, so as to retain 30% garnet1 in the sample. The remaining 8% molar melt is assumed to produce retrograde biotite as a consequence of crystallization during cooling, producing an assemblage of garnet–sillimanite–K-feldspar–biotite–quartz, prior to M2 metamorphism. During M2 metamorphism, this assemblage reacts to produce garnet2, K-feldspar2 and melt. For the biotite melting reaction (2) to progress in the stability field of kyanite, leading to complete removal of biotite2 in the majority of the microdomains, P–T conditions in excess of 10 kbar, 800°C are required. The modes of melt and garnet2 generated during M2 are distinctly lower (8–10 molar% for melt and 10–12% for garnet2) than that during M1 (Fig. 8c).
Nearly identical results are obtained by P–T pseudosection modelling of the actual rock composition (Fig. 8d). The model predicts generation of garnet and melt modes in the range of 13% and 7–8% during the high-P M2 metamorphism, broadly consistent with the garnet mode in the rock. These mineral and melt mode estimates as well as the topology of the M2 assemblage field do not show large changes with small variations in H2O contents in the protolith. This convergence in the results from the two approaches suggests that meaningful information on the physical conditions of this phase of metamorphism can be obtained. Thus, the peak M2 temperatures, in excess of 785°C, inferred here are more than 90°C higher than that estimated by Fe–Mg exchange thermometry for the enclave granulites (Table 2), but in agreement with independent geothermometric results from the associated rocks. A schematic prograde P–T path for the M2 metamorphism is shown in Fig. 8d (Path 2a), based on the sequence of deduced mineral reactions. Localized development of a retrograde biotite3–quartz assemblage, replacing garnet2 and K-feldspar2 (Fig. 3e), following reaction (2) in an opposite sense suggests retention of M2 melt in microdomains, allowing melt–crystal interaction to progress during M2R. With muscovite being totally absent in the retrograde M2R assemblage, the pseudosection qualitatively indicates that cooling was accompanied by decompression (Path 2b, Fig. 8d).
A notable feature in Fig. 9a is the stability field of orthopyroxene-free, garnet–clinopyroxene–plagioclase–hornblende–quartz assemblage under amphibolite-facies conditions. The reaction controlling the appearance of the garnet–clinopyroxene-bearing assemblage has a slope close to the T axis and is intersected at P >9 kbar for T = 750–800°C. These findings are consistent with qualitative petrogenetic grids in the system CaO–MgO–Al2O3–SiO2–H2O and experimental investigations on silica-saturated metabasic rocks of intermediate Fe–Mg composition (Pattison, 2003). In Fig. 9b, the mineral assemblage fields are contoured with isopleths of XMg(Grt) [Mg/(Mg + Fetot)], XCa(Grt) [= Ca/(Ca + Mg + Fetot)] and XAn. The stability of observed garnet (XMg 0·33) supports the interpretation of a high-P granulite-facies origin for the studied garnet–clinopyroxene–plagioclase–hornblende–quartz assemblage. The modelled M2 peak pressure of 15 kbar at 815°C is 2·7 kbar higher than the geobarometric estimate. The difference between the two stems from the chosen plagioclase composition (An46) in the barometric computation. The P–T pseudosection predicts that plagioclase at such high-pressure conditions would be more sodic (An15–20). This implies that the studied plagioclase has re-equilibrated significantly during retrogression (see below). Based on this analysis, we are constrained to bracket the M2 pressure between 12 and 15 kbar.
Reaction (7) and the growth zoning in garnet (Fig. 7e) provide better constraints on the nature of the prograde P–T path with increasing dP/dT (Path 2a, Fig. 9c). The P–T path predicts that garnet is likely to grow in two successive mineral assemblage fields, producing compositionally zoned garnet. XMg is likely to show continuous rimward increase, and XCa is expected to show an initial rise, followed by gradual decrease. The inflection coincides with the appearance of clinopyroxene in the rock. The model matches the observed zoning pattern in garnet (Fig. 9c). The stabilization of M2R hornblende–plagioclase symplectites (via reaction (9)] indicates post-peak decompression accompanying cooling (Path 2b, Fig. 9c), which is consistent with geothermobarometric results.
Although the calculated NCFMASH pseudosection is broadly consistent with the metamorphic history of the basic rocks deduced independently on the basis of reaction textures and geothermobarometry, the stability fields of the mineral assemblages and also the mineral compositions are additional functions of the Ti and Fe3+ contents in hornblende (Pattison, 2003) and the H2O content of the bulk-rock composition (tipská et al., 2006). Furthermore, the highest temperature part of some of the pseudosections may overlap with incongruent melting of hornblende. Nevertheless, the pseudosections attest the high-pressure M2 granulite-facies metamorphism of both the SMC and MC lithologies.
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SUMMARY AND IMPLICATIONS |
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Combined structural, textural, mineral compositional, geothermobarometric and P–T pseudosection modelling considerations reveal three metamorphic stages. The earliest tectonothermal event (M1–D1) is recorded only in the PMG–leptynite gneiss enclave of the SMC. A medium-pressure granulite-facies metamorphism is inferred for M1. The later tectonothermal events are recorded in all rocks. The intrusive relationship of the granitoid with the SMC enclave suggests that M1 is temporally unrelated to the later stages M2 and M3.
The older granulites along with the intrusive charno-enderbite and host gneisses experienced a second granulite-facies metamorphic event (M2) at high pressures in the kyanite facies (P 12–15 kbar, T 815°C). Reaction modelling of the prograde mineral assemblages in all these lithologies reflects prograde burial to depths in excess of 40 km. A qualitative prograde P–T loop for this burial event is shown in Fig. 10. Such ‘prograde’ HP granulite-facies rocks of non-xenolith types can reflect crustal thickening or subduction during short-lived orogenic cycles (O’Brien & Rötzler, 2003). The latter is indicated by preservation of growth zoning in garnet. Simultaneous burial and heating during prograde metamorphism was possibly caused by a combination of crustal stacking on top and thermal relaxation at depth. Geothermobarometric data and reaction textures constrain the retrograde path of evolution of the M2 granulites (M2R) as one of high dP/dT at an early stage, followed by a lower dP/dT path (Fig. 10). Such two-stage exhumation of high-P rocks is common in collisional orogens (de Sigoyer et al., 2000). Based on the prograde and retrograde segments of the P–T loop, we infer a clockwise P–T evolution for this high-P granulite event. M3 is a weak lower amphibolite-facies overprint largely restricted to narrow shear zones, which is tentatively assigned a separate status.
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The pervasive lower P–T metamorphic overprint led to near-complete erasure of the signatures of high-P granulite metamorphism, particularly in the host gneisses and megacrystic granitoids, and produced a uniform amphibolite-facies mineral assemblage. Seen against this backdrop, the present occurrence of the SMC granulite enclave cannot be fully explained in the context of any of the models mentioned above. The monocyclic Model 1 is not applicable because of the polycyclic nature of the granulite enclave. Even the polycyclic Model 2 is not strictly applicable because of in situ late high-P granulite metamorphism and identical prograde and retrograde metamorphic histories in both the enclave and host gneisses. This makes the studied granulite enclave, which has features partially common to both these models, unique. The occurrence of the older granulites within amphibolite-facies gneisses is inferred to be due to differential thermal re-equilibration of the granulite-facies composite lithological pile during exhumation (M2R). The host, because of its dominantly quartzo-feldspathic mineralogy, was preferentially transformed into amphibolite-facies gneisses. Exhumation was also accompanied by copious hydrous fluid infiltration, resulting in near-complete transformation of mafic granulites into amphibolites, except in the cores of the lenses.
Collating available geochronological data we now attempt to provide a timeframe for the different tectonothermal events deduced in this study. The emplacement age of the megacrystic granitoid (1·72 Ga, Sarkar et al., 1989; Buick et al., 2006) intrusive into the SMC enclave constrains the age of M1 metamorphism as >1·72 Ga. Buick et al. (2006) obtained similar emplacement ages for the megacrystic granitoids of the SMC and the TTG gneisses of the MC. The high-P M2 granulite facies metamorphism affecting both the SMC and MC lithologies must have occurred at <1·72 Ga. A spread of single zircon age data from 1·67 to 1·62 Ga have been reported from the SMC granitoids and MC gneisses (Wiedenbeck et al., 1996b; Roy et al., 2005). Therefore, the M2 high-P event and its retrograde evolution (M2R) can be bracketed between 1·72 and 1·62 Ga. Because M2 has affected both the SMC and MC rocks, juxtaposition of the enclave granulite against the surrounding MC gneisses must have taken place at around 1·72 Ga, coinciding with voluminous emplacement of felsic magma represented by the megacrystic granitoids of the SMC and the protoliths of TTG gneisses of the MC. The age of M3 metamorphism can be constrained at 0·95–0·94 Ga based on the geochronological data of Buick et al. (2006).
The superimposed high-P granulite-facies metamorphic event along a clockwise metamorphic P–T path recognized in this study provides new data to help us understand the geodynamic evolution in the ADMB. The metamorphic P–T path provides the first tight constraint of a collisional orogeny of possible Early Mesoproterozoic age in the ADMB, reflecting the amalgamation of two unspecified crustal blocks. The recognition of a linear gravity high over the entire stretch of the Aravalli–Delhi Mobile Belt, and especially over the granulite enclaves (Mishra et al., 2000) raises the possibility that this early Mesoproterozoic orogenic front, marking a protracted period of convergent plate boundary processes, is a regional-scale feature. Convergent plate boundary processes leading to crustal amalgamation in the early Mesoproterozoic have also been recently documented in the Central Indian Tectonic Zone (Bhowmik et al., 2006; Basu Sarbadhikari & Bhowmik, 2007) and the Singhbhum Mobile Belt (Mahato et al., 2007), the two Proterozoic mobile belts adjoining the ADMB. These findings suggest that a substantial phase of crustal assembly leading to the growth of the Indian peninsula occurred in the Pre-Grevillian time period. However, detailed geochronological study is required to strengthen this contention.
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SUPPLEMENTARY DATA |
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Supplementary data for this paper are available at Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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L.S., S.K.B. and S.D. acknowledge research grant from the Department of Science and Technology, Government of India (Grant No. ESS/16/210/2004). S.D. also acknowledges support received through J. C. Bose Fellowship. A. Basu Sarbadhikari and S. Garai are thanked for their support during fieldwork. Discussions with A. Bhattacharya and I. Buick greatly helped to improve the manuscript. We appreciate the help received from I. Buick to improve the presentation of the manuscript. We thank Y. Shibata of Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University for EPMA analysis, and H. K. Sachan of Wadia Institute of Himalayan Geology for XRF analysis. Reviews by D. R. M. Pattison, A. Tomkins, N. Kelly and N. Jons have greatly helped to improve the manuscript. We thank G. Clarke for competent editorial handling.
*Corresponding author. E-mail: santanu{at}gg.iikgp.ernet.in
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