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Precambrian Research 162 (2008) 128–149

Constraining the metamorphic evolution of a cryptic hot Mesoproterozoicorogen in the Central Indian Tectonic Zone, using P–T pseudosection

modelling of mafic intrusions and host reworked granulites

Amit Basu Sarbadhikari, Santanu Kumar Bhowmik ∗Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur 721302, India

Received 13 June 2006; received in revised form 1 March 2007; accepted 4 July 2007

bstract

In this study, we reconstruct the metamorphic pathways of reworking of a deep crustal granulite terrane in the Central Indian Tectonic Zone,sing Archaean/Palaeoproterozoic (?) polycyclic felsic granulites and two groups of intrusive Mesoproterozoic mafic granulites, a coarse-grainedoritic gabbro and a fine-grained gabbroic norite. The granulite terrane, locally referred to as the Bhandara–Balaghat granulite domain, is boundedy the South Indian Block and a Grenville-aged, younger tectonic domain of the Central Indian Tectonic Zone. The granulites and the maficntrusions were multiply deformed and metamorphosed in the early Mesoproterozoic. Using P–T pseudosection modelling of host felsic granulitesnd the mafic intrusions, two distinct metamorphic events (BM2 and BM3) with contrasting P–T paths have been established.

The P–T path of BM2 metamorphism has a clockwise sense, having an important prograde segment of heating of more than 250 ◦C with pressureall, followed by cooling. The peak BM2 metamorphism has been constrained at ∼6 kbar, ∼725 ◦C. Emplacement of the mafic intrusions wasroadly coincident with the low-pressure metamorphism, and they underwent a phase of subsolidus cooling. During subsequent metamorphism,M3, the mafic intrusions and the host felsic granulites were re-metamorphosed along a counterclockwise P–T path. This led to tectonic burial of

he mid-crust to ∼9.4 kbar, ∼760 ◦C, which was followed by cooling accompanying pressure decline.Although, the clockwise P–T path is generally interpreted in terms of thermal relaxation of crust following thrusting, the scale of heating (∼250 ◦C)

uring initial decompression, documented here, is much larger than modelled for tectonically thickened crust, but consistent with magmaticallyctive extensional zones. Based on this evidence and also the syn-metamorphic mafic intrusions, we present an alternate interpretation in terms ofid-crustal extension for the BM2 metamorphism. In contrast, the counter clockwise P–T path during BM3, recording cooling following prograde

urial is explained by tectonic thickening of a hot mid-crust. Collating available geochronological data, the two metamorphic events appear tondicate tectonic switching from lithospheric extension to contraction in the early Mesoproterozoic. The findings provide the first quantitativeonstraints on an early Mesoproterozoic hot orogen at the craton–mobile belt interface in the Central Indian Tectonic Zone.

2007 Elsevier B.V. All rights reserved.

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eywords: P–T pseudosection modelling; Metamorphic P–T path; Hot orogen;

. Introduction

An important aspect in elucidating the metamorphic evolu-ion of orogenic belts is to recognize their polycyclic domains.hese domains, quite common in craton margins (Rivers et al.,989), and often bearing polymetamorphic rocks, are sensitive
arkers of multistage orogenic processes in a mobile belt. How-
ver, during penetrative high-strain ductile reworking, fabrics inhe older rocks tend to rotate into parallelism with the superim-

∗ Corresponding author. Tel.: +91 3222 283390; fax: +91 3222 255303.E-mail address: [email protected] (S.K. Bhowmik).

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301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2007.07.014

proterozoic; Central Indian Tectonic Zone

osed younger fabrics, effectively erasing early metamorphicarageneses by compositional resetting, recrystallisation andeo-mineralisation. As a result, it becomes extremely difficulto recognize these polymetamorphic terranes and to reconstructheir metamorphic P–T–t paths. Because of the superpositionf two temporally unrelated thermal events in these terranes,oining different segments of a P–T path into a single P–T–toop would lead to an apparent P–T path, having no geo-ogical significance (Hand et al., 1992; Hensen et al., 1995;
ernon, 1996; Goncalves et al., 2004). It becomes important,
herefore, to distinguish and to separate earlier, pre-orogenicvolution from tectonic evolution during overprinting orogenicvent.

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A. Basu Sarbadhikari, S.K. Bhowmik /

Marker mafic dykes have been applied in several granulite-acies terranes to resolve this issue (e.g. the Vestfold Hills, Eastntarctica, Passchier et al., 1991; Kuehner, 1992; Kuehner andreen, 1991; Harley and Christy, 1994; Harley and Fitzsimons,995). The mafic dykes, which were emplaced in previouslyetamorphosed crust, commonly retain lithological integrity

nd cross-cutting relationships during the overprinting meta-orphic event, even under conditions of high ductile strain.
etamorphic minerals in the mafic dykes, which may appear
s direct metamorphic overprints on magmatic minerals, e.g. inorona textures, or in granoblastic textures in more thoroughly

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ig. 1. (a) Distribution of the different lithotectonic components in the Central Indiandian Block (SIB). The Sausar Mobile Belt (SMB), which occurs in the southern partRKG) domain in its north and the Bhandara–Balaghat granulite (BBG) domain in iausar Group. Box shows the location of the studied BBG domain in this work. (b)

he distribution of the studied felsic granulites, metanoritic gabbro and metagabbroocations, which are superscripted as follows: (1) Bhowmik et al. (2005); (2) Roy etramework of Peninsular India. EGMB (Eastern Ghats Mobile Belt), SPGC (ShilloDMB (Aravalli–Delhi Mobile Belt) represent contiguous Mobile Belts.

mbrian Research 162 (2008) 128–149 129

ecrystallised examples, have been used to estimate the P–Tonditions of metamorphism during the younger tectonothermalvent.

We address this issue using a polycyclic granulite domain athe craton–mobile belt contact (Bhowmik et al., 2005) in theausar Mobile Belt (SMB) in central India, the latter consti-

uting the southern component of the Central Indian Tectonicone (Fig. 1). The chosen domain, locally referred to as the
handara–Balaghat granulite (BBG) domain (Ramachandra,999; Ramachandra and Roy, 2001), occurs as a thrust-boundedheet between the South Indian Block in the south and low-
n Tectonic Zone (CITZ) between the North Indian Block (NIB) and the Southof the CITZ, contains two granulite domains, the Ramakona-Katangi granulite

ts south, across a central domain of low-medium grade cover sequence of theLocation map of Larsara and Dongariya areas in the BBG domain, showingic norite. Also shown are the published geochronological data in these two

al. (2006); (3) Bhowmik et al. (2006). (c) Location of the CITZ in the tectonicng Plateau Gneissic Complex), CGC (Chhotanagpur Gneissic Complex) and

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edium grade cover sequence of Sausar Group rocks in theorth (Fig. 1). Recently, Bhowmik et al. (2005) recorded theolymetamorphic history of the BBG domain, by establishinghree temporally distinct high-grade metamorphic events. Thearliest one caused an ultrahigh-temperature (UHT) metamor-hism (BM1a, P ∼9 kbar, T ∼950–1000 ◦C), being followedy near isobaric cooling (BM1b, P ∼9 kbar, T ∼700–750 ◦C)Bhowmik et al., 2005; Bhowmik, 2006). The metamorphic P–Tath for the UHT event has a counterclockwise sense. Basedn monazite chemical ages, these workers assigned a Palaeo-roterozoic age (∼2.1–2.0 Ga) for the UHT metamorphism.sing Sm–Nd mineral-whole rock isochron method, Roy et al.

2006) have, however, estimated an Archaean age (∼2.67 Ga)or this early granulite metamorphism. A second event inhe Mesoproterozoic (∼1.52–1.47 Ga, monazite chemical ages;howmik et al., 2005) reworked the UHT granulites (Bhowmikt al., 2005). This caused decomposition of UHT garnet torthopyroxene + plagioclase symplectites (BM2), followed byeappearance of garnet + quartz symplectites that replaced theymplectites (BM3). Applying conventional geothermobarom-ters, these authors retrieved P–T conditions of ∼6.5 kbar,675 ◦C and ∼8 kbar, ∼675 ◦C for BM2 and BM3 metamor-

hisms, respectively, which are too low for the formation ofrthopyroxene + plagioclase and garnet + quartz assemblages.

mid-crustal granulite-facies melting event has been recentlyated at 1.59 Ga by SHRIMP U–Pb zircon dating of the felsicranulites (Bhowmik et al., 2006). Roy et al. (2006) additionallyated coronal garnet in the two pyroxene granulites (metamor-hosed gabbros) to suggest a major cooling event at ∼1.4 Ga. Aerminal greenschist- to amphibolite-facies metamorphic over-rint caused tectonic juxtaposition of the granulites with theausar Group (Bhowmik et al., 2005).

Although, there is considerable uncertainty over the exactiming of the UHT metamorphic event, available geochrono-ogical data reveal that the BBG domain records major early

esoproterozoic metamorphic history. However, these high-ariance metamorphic reaction processes were not adequatelyuantified (e.g. by P–T pseudosections), to establish the pro-rade segments of BM2 and BM3 metamorphisms. In thebsence of this information, the reconstruction of metamorphicistory in the early Mesoproterozoic remains incomplete. Thealient questions are: (a) Are the BM2 and BM3 phases part ofsingle-cycle metamorphic event as suggested previously, or

o they represent short-lived separate cycles, having contrasting–T paths? (b) If the latter, does it indicate the former presence ofhot, accretionary orogen in the Central Indian Tectonic Zone?

In this work, we seek answers to these questions using–T pseudosection modelling of texturally well-constrainedesoproterozoic metamorphic domains in the host fel-

ic granulites and marker mafic intrusions, following thepproaches of Baldwin et al. (2005). This allows high-ariance mineral assemblages mentioned above, such asarnet + orthopyroxene + plagioclase + quartz, to be explored in
ore detail in order to derive quantitative information on the–T vector during metamorphic evolution. Collating availablend new geochronological and metamorphic data, we recon-truct the P–T paths of two short-lived early Mesoproterozoic
mMbG

mbrian Research 162 (2008) 128–149

ectonothermal events. The findings provide the first quantita-ive constraints on the presence of a hot, accretionary orogenn the Central Indian Tectonic Zone, which in turn have impor-ant implications in unravelling the multistage amalgamationrocesses that were operative in the CITZ.

. Samples and their geologic setting

Samples for this study were collected from the BBG domain,brief geological history of which is presented in Table 1.

he results reveal that subsequent to the PalaeoproterozoicBhowmik et al., 2005) or Archaean (Roy et al., 2006) BM1vent, that resulted in cratonisation of the BBG crust, the Meso-roterozoic history commenced with syn-BD2 emplacement ofwo related suites of mafic intrusions, namely noritic gabbrond gabbroic norites. The intrusions and their host granulitesater underwent multiple phases of deformation and metamor-hism, commencing with BD3 and BM3 respectively. In thisork, we have studied the petrology of the mafic intrusions to

stimate the depth of their emplacement and also to constrainhe imprints of regional metamorphism, BM3. The host felsicranulites additionally preserve key textural and mineral compo-itional data concerning the metamorphic pathways of both BM2nd BM3 metamorphisms. Samples for P–T pseudosection mod-lling are, therefore, taken from these three rocks units: felsicranulites, metanoritic gabbro (metagabbro from now on) andetagabbroic norite (metanorite from now on). For the former,

wo samples were selected, one from the Larsara area (sam-le B223A(2)) and the other from the Dongariya area (sample11D) (Fig. 1b), both recording identical BS2–BS3 deformation

abrics. The samples for the mafic rocks (B25 for gabbro and12 for norite) come from the Larsara area. Samples B223A(2)nd B12 were previously studied by Bhowmik et al. (2005).he former is particularly useful since it provides geochrono-

ogic evidence of microdomainal-scale textural evolution inhe early Mesoproterozoic. Sample B12, which resembles theecently dated mafic intrusions from the BBG domain (cf. Royt al., 2006) provides additional opportunity to constrain inde-endently the early Mesoproterozoic metamorphism. It may beoted that the studied rock units show regional-scale distributionn the BBG domain (Fig. 1b) and that the imprints of these meta-

orphisms are additionally recorded from the associated Ironormation granulites, corundum granulites and garnet-cordieriteranulites (Table 1). This allows the metamorphic conclusionsrawn in this study to be representative of the BBG domains a whole. In the next section, we present key microtextu-al and mineral compositional data used in P–T pseudosectionodelling.

. Petrography and mineral chemistry

The different textural domains in the felsic granulites,etagabbro and metanorite are shown in Fig. 2a–h. Selected
ineral chemical compositions are presented in Tables 2 and 3.ineral chemical data were determined using a Cameca Came-
ax Microbeam Electron Microprobe at the University of Bonn,ermany (sample nos. 223A(2) and B12) and a CAMECA SX-

A. Basu Sarbadhikari, S.K. Bhowmik / Precambrian Research 162 (2008) 128–149 131

Table 1Summary of tectono-metamorphic and tectono-magmatic events recognized in the southern BBG domain of the SMB

Metamorphism Magmatic, melting and veining events Deformation

Meso–Neo-proterozoicA fifth metamorphic event (BM5) in the upper greenschist–lower

amphibolite-facies regimeEmplacement of amphibolitic dykes Narrow, E–W to ENE–WSW trending DSZ

deformation (BD5). Shear zone folds (BF5)have plane non-cylindrical geometry andsteep northerly dipping axial planar fabric

MesoproterozoicA fourth, low-intermediate P, high-T metamorphism (BM4) formed

Opx + Cpx + Pl symplectites, replacing folial Hbl in metagabbroLocalised partial melting in metagabbro.Emplacement of olivine gabbro andgarnetiferous leucogranite

Pervasive BD4 deformation producedmoderately plunging upright to inclinedfolds. Steep to vertical, NNE–SSW toNE–SW striking axial planar foliations.

A third granulite event (BM3) in response to a tectonic thickeningevent produced second generation coronal garnet, overgrowingmylonitic fabric, BS3 in felsic and I Fm granulites. Coevalcoronal Grt + Qtz ± Cpx symplectites were formed inmetagabbroic norite

BD3 produced a strong high-T DSZ fabricthat variably transposed early foliations. Themylonitic fabric, BS3 occurs as axial planarfoliations to a set of southerly vergingisoclinal folds (BF3)

Second granulite metamorphism (BM2), led to decomposition ofporphyroblastic Grt (Grt1) to Opx-Pl symplectites in felsic and IFm granulites and coronal garnet to Spl-Sil symplectites inaluminous granulites

Emplacement of a sequence of maficintrusions of noritic gabbro and gabbroicnorite in the granulite crust, synchronouswith BM2. Localised partial melting in felsicgneiss and felsic granulites

BD2, which reworked the high-T granulitesat the early stage is broadly synchronouswith BM2

Cratonisation of crustArchaean1/Palaeo-protero-zoic2

Subsequent cooling (BM1b) produced exsolution textures in ternaryfeldsparss and coronal Grt-Crn in aluminous granulites andmultistage exsolutions in ferroaugites in I Fm granulites

Early ultrahigh-T, medium-high-P granulite metamorphism(BM1a), producing Grt–Opx, Grt–Spl-Spr-Rt and ferroaugiteassemblages in felsic, aluminous and I Fm granulites,respectively

Extensive dehydration melting in felsic andaluminous granulites, producing migmatitebanding

BD1 produced granulite-facies gneissicbanding (BS1) in almost all lithoassociations

Formation of protolith of felsic, aluminous and iron formation (I Fm) granulites and Grt–Crd gneiss

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M/BD/BS: metamorphism/deformation/structural fabric in the BBG domain.2005). Mineral abbreviations are after Kretz (1983). DSZ, Ductile Shear Zone

00 electron microprobe at the laboratory of the Geologicalurvey of India at Kolkata (sample no. B11D), both equippedith four WD-spectrometers. The operating conditions were set

s 15 kV accelerating voltage, and 12 nA beam current, with atability less than 1%. The analytical width in point analysesode was 2–4 �m for Camebax and 1 �m for SX-100. Crystalsere LiF, two TAP and PET. Counting time was 20 s per X-ray

ine and background. Natural and synthetic minerals were useds standards. Standards (Camebax) were: elements (Fe, Mn, Cr),xides (Al, Mg, Ti), sanidine (K), and Jd-Di-composition (Na,a, Si). Standards (SX-100) are: oxides (Fe, Mn, Ti, Al, Cr),rthoclase (K), albite (Na and Si), andradite (Ca) and olivineMg). Fe3+ was estimated by normalization to the sum of cationser formula and charge balance.

A brief summary of the Mesoproterozoic mineralogical evo-ution is presented through diagrammatic sketches in Fig. 3.

ineral abbreviations are after Kretz (1983), unless stated oth-rwise.

.1. Felsic granulite

In sample, B223A(2), pre-BM2 metamorphism isepresented by the assemblage porphyroblastic garnetPrp31Alm63Sps01Grs05) + porphyroblastic orthopyroxene

Gn(t

ences are superscripted as follows: (1): Roy et al. (2006); (2): Bhowmik et al.

XMg = 0.53; yOpx = (Si + Altot−2) = 0.029] + perthite + quartz +e-sulphides (Fig. 2a).

During BM2, porphyroblastic garnet [garnet (P)] decom-osed both externally and internally to produce orthopyrox-ne + plagioclase symplectites, which occur either as a collarround garnet or within garnet grains (Figs. 2a–c and 3). Thised to partial fragmentation of large garnet porphyroblastsFig. 3), which occur as relics within the symplectite. Gar-et/orthopyroxene ratio in the symplectite domain is very highFigs. 2a and 3). X-ray element images show that irrespectivef grain size, garnet relics are compositionally homogeneousPrp29Alm64Sps01Grs06) (Fig. 2b–c) and are slightly less mag-esian [XMg = Mg/(Fe + Mg) = 0.31] than pre-decompositionarnet (XMg = 0.33) (Table 2). Plagioclase in the core of theymplectite is relatively calcic (An34). Cores of coarse symplec-ite orthopyroxene are more magnesian (XMg = 0.57–0.58) andluminous (yOpx = 0.052) than orthopyroxene porphyroblasts.

During subsequent metamorphism, BM3, a coronal vari-ty of garnet intergrown with granular quartz replaced therthopyroxene + plagioclase symplectites (Figs. 2a and 3).
arnet is preferentially localized as an overgrowth on gar-et (P) (Fig. 3). Coronal garnet is uniformly grossular-richXGrs = 0.09) (Fig. 2c). Symplectite plagioclase rim adjoininghe grossular-rich garnet is slightly more sodic (An29), while

132 A. Basu Sarbadhikari, S.K. Bhowmik / Precambrian Research 162 (2008) 128–149

Fig. 2. Backscattered electron images (a and d), X-ray element maps (b and c) and photomicrographs (e–h), showing textures of different stages of mineralogicalevolution in the felsic granulites (a–d), gabbro (e) and norite (f–h). (a) Porphyroblastic Grt [Grt(P)], containing inclusions of Qtz and perthite (Pe) is armouredby coarse Opx-Pl symplectites, which also advance into the Grt interior [sample B223A(2)]. Also shown is the coronal garnet [Grt(Cor)] + Qtz assemblage, as ametamorphic overgrowth on Grt(P), and replacing Opx-Pl symplectites. Note the occurrence of a monazite (Mnz) grain in the symplectite domain, in the bottomcentral part of the image. (b)–(c) Mg and Ca, X-ray element maps of Grt(P) and adjoining Opx-Pl symplectite domains of felsic granulite, shown in (a). Brighterand darker shades in the X-ray maps indicate relatively higher and lower concentrations of the elements, respectively. Irrespective of grain size of the relict garnetsin the symplectite domain, Mg map shows uniform distribution of this element in these garnets. Ca map, however, shows conspicuous Ca zoning in Grt, both alongits rim and locally in its interior. The elevated Ca concentration coincides with the location of Grt(Cor). (d) Grt(P) in sample B11D is mostly pseudomorphed bycoarse orthopyroxene, with a thin discontinuous inner rim of Pl locally separating garnet from the coarser Opx. Qtz occurs as relict inclusion within Opx. (e) Relictigneous texture in noritic gabbro, showing criss-cross orientation of Pl laths, the long dimensions of which are shown by dashed lines. Pl(M) refers to Pl megacryst.(f) Layered Grt + Cpx + Qtz corona forming around flattened Opx and Pl(M). (g) Coronal Grt + Qtz symplectite forming around recrystallised Cpx [Cpx(R)] andPl(M). (h) Coronal Grt with idioblastic habit has truncated deformation twinning in Pl(M).

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Table 2Representative electron microprobe analyses of selected phases in felsic granulite

Rock type: Felsic granulite

Sample no.: B223A(2) Sample no.: B11D

Grt (site no.:C-2, texturalsite: P)

Grt (site no.:C-18, texturalsite: P)

Grt (site no.:C-35, texturalsite: Cor)

Opx (site no.:A-27, texturalsite: P(C))

Opx (site no.:A-14, texturalsite: S(C))

Opx (site no.:A-26, texturalsite: S(R))

Opx (site no.:C-64, texturalsite: S(R))

Opx (site no.:C-68, texturalsite: S(C))

Pl (site no.:C-54, texturalsite: S(C))

Pl (Site no.:A-19, Texturalsite: S(R))

Grt (site no.:1/71, texturalsite: C)

Opx (site no.:2/96, texturalsite: S(C))

Pl (site no.:14/1, texturalsite: S(C))

SiO2 38.49 38.95 37.94 51.18 51.90 51.59 52.05 51.93 59.91 60.6 37.73 49.13 59.97TiO2 0.04 0.04 0.03 0.10 0.06 0.06 0.05 0.08 0.01 0.03 0.00 0.08 0.06Al2O3 21.8 21.78 21.87 1.41 2.00 1.57 1.93 1.55 24.84 24.77 21.51 4.68 23.92Cr2O3 0.09 0.08 0.18 0.07 0.12 0.04 0.13 0.11 – – – – –FeO 29.86 30.11 29.04 28.20 25.67 26.96 26.1 26.58 0.04 0.40 31.30 26.24 0.18MnO 0.32 0.51 0.45 0.18 0.16 0.12 0.14 0.12 0.03 – 0.62 0.18 0.00MgO 8.10 7.68 7.29 18.15 19.39 19.03 20.56 20.42 – 0.23 7.31 18.77 0.00CaO 1.88 2.04 3.27 0.35 0.41 0.25 0.26 0.30 7.40 6.03 1.38 0.09 6.08Na2O – – – 0.02 0.02 – 0.04 – 7.84 8.05 – 0.02 8.12K2O – – – 0.01 0.13 – 0.05 – 0.18 0.17 – – 0.32Total 100.58 101.19 100.07 99.67 99.86 99.62 101.31 101.09 100.25 100.28 99.85 99.19 98.65Oxygens 12 12 12 6 6 6 6 6 8 8 12 6 8Si 2.972 2.997 2.949 1.965 1.963 1.967 1.937 1.941 2.67 2.689 2.954 1.873 2.708Ti 0.002 0.002 0.002 0.003 0.002 0.002 0.001 0.002 – 0.001 0.000 0.002 0.002Al 1.984 1.976 2.004 0.064 0.089 0.071 0.085 0.068 1.305 1.296 1.986 0.210 1.274Cr3+ 0.005 0.005 0.011 0.002 0.004 0.001 0.004 0.003 – – – – –Fe3+ 0.000 0.000 0.000 0.040 0.041 0.001 0.015 0.040 0.007Fe2+ 0.905 0.812 0.860 0.773 0.790 – – 0.796 –Fe* 1.928 1.937 1.887 2.050Mn 0.021 0.033 0.03 0.006 0.005 0.004 0.004 0.004 0.001 – 0.041 0.006 –Mg 0.932 0.881 0.844 1.039 1.093 1.081 1.140 1.138 – 0.015 0.853 1.067 –Ca 0.156 0.168 0.273 0.014 0.017 0.010 0.010 0.012 0.353 0.287 0.116 0.004 0.294Na – – – 0.001 0.001 – 0.003 – 0.678 0.693 – 0.001 0.711K – – – 0.000 0.006 – 0.002 – 0.010 0.010 – – 0.018Sum 8.000 7.999 8.000 3.999 3.992 3.996 3.999 3.999 5.018 5.006 8.000 3.999 5.014XPrp 0.307 0.292 0.278 0.279XAlm 0.635 0.641 0.622 0.670XSps 0.007 0.011 0.010 0.013XGrs 0.051 0.056 0.090 0.038XMg(Opx) 0.535 0.574 0.557 0.596 0.590 0.573yOpx 0.029 0.052 0.038 0.022 0.009 0.083XAn 0.342 0.293 0.293

Fe*, All Fe as Fe2+; P, Porphyroblastic; S, Symplectite; Cor, Corona; C, Core; R, Rim; XPrp, Mg/(Mg + Fe + Ca + Mn); XAlm, Fe/(Mg + Fe + Ca + Mn); XSps, Mn/(Mg + Fe + Ca + Mn); XGrs, Ca/(Mg + Fe + Ca + Mn); XMg(Opx), Mg/(Mg + Fe2+); yOpx, (Al + Si-2); XAn, Ca/(Ca + Na).

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Table 3Representative electron microprobe analyses of selected phases in metanorite

Rock type: Metanorite (sample no. B12)

Grt (site no.:B-8, texturalsite: Cor)

Opx (site no.:B-13, texturalsite: MG(C))

Opx (site no.:B-16, texturalsite: MG(R))

Cpx (site no.:A-21, texturalsite: M(R))

Cpx (site no.:B-14, texturalsite: S)

Cpx (site no.:B-7, texturalsite: S)

Cpx (site no.:A-7, texturalsite: Re(R))

Pl (site no.:B-3, texturalsite: MG(C))

Pl (site no.:B-6, texturalsite: MG(IR))

Pl (site no.:B-5, texturalsite: MG(OR))

SiO2 38.69 52.18 52.48 53.45 53.55 53.32 53.17 53.45 50.39 58.64TiO2 0.04 0.05 0.04 0.11 0.12 0.11 0.10 – – –Al2O3 21.89 0.89 0.67 1.16 1.33 1.27 1.94 30.48 32.46 27.18Cr2O3 0.04 – – 0.00 0.03 0.03 0.00 – – –FeO 27.13 27.00 27.32 9.08 9.13 8.04 9.21 0.05 0.04 0.04MnO 0.54 0.21 0.18 0.14 0.07 0.07 0.05 – 0.03 0.07MgO 5.28 18.89 19.10 13.73 13.57 13.85 14.70 – – 0.01CaO 7.18 0.59 0.36 22.94 22.56 22.84 20.59 12.51 14.97 8.71Na2O – 0.01 – 0.39 0.40 0.38 0.34 4.56 3.07 6.69K2O – – 0.01 – – 0.01 0.04 0.11 0.07 0.20Total 100.79 99.82 100.16 101.00 100.76 99.92 100.14 101.16 101.03 101.54Oxygens 12 6 6 6 6 6 6 8 8 8Si 2.997 1.987 1.992 1.972 1.982 1.983 1.969 2.392 2.273 2.586Ti 0.002 0.001 0.001 0.003 0.003 0.003 0.003 – – –Al 1.999 0.040 0.030 0.051 0.058 0.056 0.085 1.608 1.727 1.413Cr3+ 0.002 – – 0.000 0.001 0.001 0.000 – – –Fe3+ – 0.000 0.000 0.027 0.000 0.000 0.000 0.002 0.002 0.001Fe2+ 0.860 0.867 0.253 0.282 0.250 0.285 – – –Fe* 1.758Mn 0.035 0.007 0.006 0.004 0.002 0.002 0.002 – 0.001 0.003Mg 0.610 1.072 1.081 0.755 0.748 0.768 0.812 – – 0.001Ca 0.597 0.024 0.015 0.907 0.894 0.910 0.817 0.600 0.724 0.412Na – 0.001 – 0.028 0.029 0.027 0.024 0.396 0.269 0.572K – – 0.000 – – 0.000 0.002 0.006 0.004 0.011Sum 8.000 3.992 3.992 4.000 3.999 4.000 3.999 5.004 5.000 4.999XPrp 0.203XAlm 0.586XSps 0.012XGrs 0.199XMg(Px) 0.555 0.555 0.749 0.726 0.754 0.740yOpx 0.027 0.022XAn 0.602 0.729 0.419

Fe*, all Fe as Fe2+; MG, megacryst; M, matrix; Re, recrystallised.


Fig. 3. A summary diagram showing the microtextural and mineral compositional evolution in felsic granulites and gabbroic norites in relation to early Mesoproterozoicdeformation, metamorphism and mafic magmatism. Black dotted line in garnet and monazite shows the reconstructed outline of these minerals in the Palaeoproterozoic,before partial decomposition in the Mesoproterozoic. Geochronological evidence for Mesoproterozoic evolution, in the form of CHIME monazite chemical agesfrom different microtextural domains in the felsic granulites, is from Bhowmik et al. (2005). See text for details.


Table 4Summary of mineral chemistry in BM2 and BM3 textural domains of the felsic granulite and metanorite

Felsic granulite Rock type/sample no.: BM2 metamorphism Rock type/sample no.: BM3 metamorphism

Grt/Opx ratio Grt relict Opx S(C) Pl S(C) Grt Cor Opx S(R) Pl S(R)

B223A(2) Very high XMg XMg XAn XMg XMg XAn

0.31 0.57–0.58 0.34 0.31 0.58 0.29XGrs yOpx XGrs yOpx

0.056 0.052 0.09 0.0380.009–0.022

B11D Very low XMg XMg XAn

0.29 0.57 0.29XGrs yOpx

0.038 0.083

Metanorite Magmatic composition BM3 metamorphism

Pl (C) Pl (IR) Grt Cor(C) Opx M(C) Opx M(R) Pl (OR) Cpx M(R) Cpx S Cpx Re

B12 XAn XAn XMg XMg XMg XAn XMg XMg XMg

0.60 0.73 0.26 0.55 0.55 0.41 0.75 0.73–0.75 0.74XGrs yOpx yOpx

0.20 0.027 0.022

r(moaX

bccghom0d

3

iraCm0obtraes

3

rsmegp(ttbnacCi(

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ims of coarse orthopyroxene symplectite are depleted in yOpxfrom 0.052 at core to 0.038 at rims). This depletion is evenore pronounced for smaller orthopyroxene symplectites (range

f yOpx = 0.009–0.022). However, irrespective of grain sizend textural location, orthopyroxene symplectites have similarMg.

In sample B11D in contrast, garnet is mostly pseudomorphedy coarse orthopyroxene (Fig. 2d). A smaller volume of plagio-lase is commonly associated with orthopyroxene producing arudely layered corona (Fig. 2d). In contrast to B223A(2), thearnet/orthopyroxene volume ratio is low. The relict garnet coreas a composition of Prp28Alm67Sps01Grs04 (Table 2). Coarserthopyroxene symplectite is magnesian (XMg = 0.57) and alu-inous (yOpx = 0.083). XAn in plagioclase symplectite core is

.29. A comparison of the mineral chemistry in the two texturalomains is presented in Table 4.

.2. Metagabbro

Sample B25 is the most Fe-rich of the gabbroic suite, stud-ed so far (Bhowmik, unpublished data). It is a coarse-grainedock, comprising clinopyroxene, orthopyroxene, plagioclasend minor ilmenite, with accessory apatite and rare monazite.linopyroxene is more abundant than orthopyroxene. All theajor minerals are megacrystic (grain size varying in the range

.5–1.7 mm, with an average of ∼0.85 mm). Despite overprintsf crystal-plastic deformation, relict igneous texture is evidenty the criss-cross orientation of plagioclase laths (Fig. 2e) and by
he presence of (010) albite twinning. Both ortho- and clinopy-oxenes show exsolution textures with clinopyroxene occurrings crystallographically oriented lamellae within the orthopyrox-ne host and vice versa. As a general feature of the gabbroicuite, garnet is absent in the rock.
csat0

.3. Metanorite

It has a mineralogical assemblage of orthopyroxene, clinopy-oxene, plagioclase, ilmenite, garnet and quartz. Despitetrong deformation and metamorphic overprints, the mag-atic texture is still recognizable by the presence of coarse

uhedral outlines of orthopyroxene, clinopyroxene and pla-ioclase and the rare criss-cross orientation of laths oflagioclase. The grain size of these magmatic mineralsfrom 0.15 to 0.5 mm, average ∼0.3 mm) is much finerhan the metagabbro (average ∼0.85 mm). BD3 deforma-ion has produced a strong fabric in the rock, definedy elongate magmatic pyroxenes and plagioclase. Coro-al garnet ± quartz ± clinopyroxene-bearing symplectites occurround elongate and recrystallised grains of orthopyroxene,linopyroxene, plagioclase and also ilmenite (Fig. 2f–h).oronal garnet has an idioblastic habit, commonly protrud-

ng into and truncating deformation twinning in plagioclaseFig. 2h).

Early magmatic composition is recorded in the form ofeverse zoning in plagioclase megacryst (An60 at core to An73 atnner rim) (Tables 3 and 4). Plagioclase outer rim (∼5–10 �m)n contact with coronal garnet is distinctly more sodic (An41).oronal garnet is compositionally homogeneous with uniformlyigh almandine and grossular and relatively lower pyrope con-ents (XGrs = 0.20; XMg = 0.26). Megacrystic orthopyroxene isagnesian (XMg = 0.55) and low in alumina (yOpx in the range

.027–0.022) (Table 3). There is slight rimward decline in yOpx0.027 in core to 0.022 in rim). Megacrystic clinopyroxene inomparison is more aluminous (Altot = 0.06–0.07) and magne-
ian (XMg = 0.75). In general, rims of clinopyroxene megacrystnd recrystallised and symplectite clinopyroxenes, all in con-act with coronal garnet show broadly uniform XMg values of.73–0.75.

A. Basu Sarbadhikari, S.K. Bhowmik / Preca

Table 5aBulk compositions used in pseudosection analysis of felsic granulites

Rock type: felsic granulite

Sample no.: B11D Sample no.: B223A

Mineral proportion 0.91 Opx; 0.05 Grt; 0.55 Grt, 0.28 Pl,0.03 Qtz; 0.01 Pl 0.12 Opx, 0.05 Qtz

Mineral chemistry XMg(Grt) = 0.294; XMg(Grt) = 0.313;XGrs(Grt) = 0.038; XGrs(Grt) = 0.056;XMg(Opx) = 0.573; XMg(Opx) = 0.577;yOpx = 0.083; yOpx = 0.052;XAn = 0.287 XAn = 0.339

Molar % oxides NCFMAS NCFMASSiO2 (S) 48.03 53.35Al2O3 (A) 3.33 12.22MgO (M) 26.62 10.63FeO (F) 21.66 17.77CaO (C) 0.24 3.73Na2O (N) 0.12 2.30X

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Mg: molar Mg/(Mg + Fe).

. P–T pseudosection modelling

Fig. 2a–c in felsic granulites suggest that both the texturalomains preserve spectacular evidence for operation of BM2nd BM3 mineral reactions in discrete mineralogical and com-ositional domains. These individual microdomains can be usedo constrain the prograde and retrograde segments of the P–Tath of evolution. Using the procedures outlined by Baldwin etl. (2005), an effective bulk composition has been calculatedased on specified mineral proportions and measured mineralhemistry characteristic of the individual domain. Fig. 2a–cave been used for this purpose. The modes of garnet, pla-ioclase, orthopyroxene and quartz, and mineral compositionssed are presented in Table 5a. For the felsic granulites, given:
i) the arrested development of the BM3 garnet + quartz assem-lage by the reacting orthopyroxene + plagioclase symplectite,ii) the relatively small volume of growth and equilibration of
able 5bajor oxide analyses of whole rock compositions for metagabbro and metanorite

used in pseudosection calculation)

ock type Metagabbro(sample no. B25)

Metanorite(sample no. B12)

wt% mol% (NCFMAS) wt% mol% (NCFMAS)

iO2 (S) 49.59 52.66 53.47 58.07iO2 0.58 1.57l2O3 (A) 15.78 9.93 13.69 8.75eOa (F) 11.97 10.69 14.56 12.06nO 0.27 0.21gO (M) 8.71 14.76 5.87 9.80aO (C) 9.67 11.51 8.93 10.18a2O (N) 0.41 0.44 1.07 1.14

2O 0.64 0.14

2O5 0.04 0.209otal 97.66 99.72

Mg 0.58 0.45

a All Fe as FeO; XMg: molar Mg/(Mg + Fe).

hpahetcmstbvcgTXofttsw

mbrian Research 162 (2008) 128–149 137

he BM3 textural domain, and (iii) the compositionally homo-eneous nature of relict garnet in the symplectite domain, coreompositions of garnet and the symplectites have been takeno calculate the effective bulk composition. We have carriedut sensitivity analyses to determine the magnitude of possi-le incorrect estimation of the bulk compositions (presentedn Section 4.3). For the mafic intrusions, bulk rock composi-ions, analysed by the X-ray fluorescence (XRF) spectrometryt the University of Bonn, Germany (see Bhattacharya et al.,998 for the detailed analytical methods) have been used toonstruct P–T pseudosections. Since these rocks record single-ycle metamorphic event (e.g. BM3 metamorphism), whole rocknalysis is preferred over effective bulk composition. Bulk com-ositions have been adjusted to account for FeO in ilmenite andaO in apatite. Noritic gabbro is more magnesian (XMg = 0.56)

han gabbroic norite (XMg = 0.41) (Table 5b), resulting in differ-nces in the positions of calculated garnet-in, orthopyroxene-outnd plagioclase-out curves. P–T pseudosections were calcu-ated in the NCFMAS system using THERMOCALC tc3.25nd the dataset of Holland and Powell (1998; tcds55, November003). The results of pseudosection modelling in felsic gran-lites are presented in Figs. 4–6 and those for the mafic intrusionsn Figs. 7 and 8. Isopleths representing the compositions ofarnet [XMg = Mg/(Mg + Fetot) and XGrs = Ca/(Ca + Fe + Mg)],rthopyroxene [XMg and yOpx], clinopyroxene [XMg and JCpx]nd plagioclase [XAn = Ca/(Ca + Na)] are shown in the figures.

.1. Felsic granulite

.1.1. BM2 metamorphismThe P–T pseudosection for B223A(2) reveals a large

tability field for the assemblage garnet–orthopyroxene–lagioclase–quartz, in which there were successive stages ofarnet breakdown and re-growth of garnet. Except for the XMgsopleth of garnet, which is subhorizontal, the other isoplethsave moderate slopes in P–T space (Fig. 4). The flat XMg iso-leths reveal that increasingly magnesian garnets are stablet high pressures (Fig. 4a). For other isopleths, near isobariceating or near isothermal decompression produce progressivenrichment in yOpx (Fig. 4d) and XAn (Fig. 4e), and deple-ion in XGrs (Fig. 4b) and XMg(Opx) (Fig. 4c) contents. Theompositional parameters of the mineral phases constrain aid-crustal, high-T metamorphism for BM2. Of all the compo-

itional parameters, XMg and XGrs contents in garnet are provedo be robust to constrain this metamorphic condition. This isecause the re-equilibrated garnet, owing to its much largerolume relative to the symplectites, escaped down temperatureompositional resetting in Mg and Fe. Also diffusion of Ca inarnet is extremely sluggish (Chakraborty and Ganguly, 1991).hus, the intersection of the isopleths of XMg(Grt) of 0.31 andGrs of 0.056 provides a tightly constrained BM2 P–T conditionf ∼6 kbar, ∼725 ◦C (Fig. 4f), which is significantly differentrom that estimated by Bhowmik et al. (2005). Significantly,
he compositional parameters of orthopyroxene failed to regis-er the BM2 condition. Because of its relatively smaller grainize, these compositional parameters in orthopyroxene under-ent re-equilibration during post-peak cooling (parameter XMg)


Fig. 4. Calculated P–T pseudosection for felsic granulite [sample B223A(2)], in the system NCFMAS. The pseudosection has been contoured for the isopleths,XMg(Grt) (a); XGrs(Grt) (b); XMg(Opx) (c); yOpx (d); XAn(Pl) (e). “v” indicates the field variance. P–T Path 1, showing post-peak, near isobaric cooling fromultrahigh-temperature metamorphic condition of Palaeoproterozoic age, is taken from Bhowmik et al. (2005) and Bhowmik (2006). P–T Paths 2a, 2b and 2c referto possible prograde segment of the BM2 metamorphic event. (f) Calculation of peak BM2 metamorphic condition using intersection of XMg(Grt) and XGrs(Grt)isopleths, based on the assumption that due to high Grt/Opx volume ratios and slow diffusion rate of Ca in garnet, these compositional parameters escaped significantdown-temperature compositional re-equilibration.


Fig. 5. Calculated P–T pseudosection for felsic granulite [sample B11D], in the system NCFMAS. The contouring of isopleths in the pseudosection is the same asin Fig. 4. Note significant differences in the slope of XMg(Grt) and XGrs(Grt) isopleths in comparison to those in sample B223A(2) (Fig. 4). Both these parametersincrease in value with heating. Path 2d refers to the reconstructed post-peak (with respect to the BM2 metamorphism) cooling arm, based on the compositionalresetting of Fe–Mg in Grt and Opx, octahedral Al in Opx and Ca in Grt and Pl. Estimated peak BM2 condition is taken from sample B223A(2).


Fig. 6. Calculated P–T pseudosection for felsic granulite [sample B223A(2)], in the system NCFMAS, to constrain BM3 metamorphism. P–T Paths 3a–3c refer tothe possible metamorphic pathways, producing coronal garnet + quartz assemblage in the rock. Path 3d is the retrograde segment of the same metamorphism. (f)Calculation of peak BM3 metamorphism using intersection of XGrs(Grt) and XMg(Opx) isopleths, which appear to be least affected during post-peak cooling.

A. Basu Sarbadhikari, S.K. Bhowmik / Preca

Fsc

amtttt

Xnaido(igahctgacutca∼eFmsM

iPw

tidrgdstirBppstngmlmdoPTb(ueS

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ig. 7. Calculated P–T pseudosection for noritic gabbro (sample B25) in theystem, NCFMAS to estimate the possible depths of its emplacement and toonstrain the subsolidus cooling P–T trajectory.

nd also during BM3 metamorphism (parameter yOpx). The esti-ated P–T condition also predicts that plagioclase symplectite at

his condition would have a composition of An36, 2 mol% higherhan that measured (An34) (Fig. 4e). The depletion in the anor-hite content in plagioclase is possibly related to its consumptiono produce grossular-rich BM3 garnet (Fig. 3).

Analysis of the garnet mode isopleths, which is parallel to theGrs isopleths, suggests that the garnet breakdown reaction, gar-et + quartz = orthopyroxene + plagioclase could have followedny of the following three P–T trajectories: (a) near isobaric heat-ng (Path 2a), (b) near isothermal decompression (Path 2b), or (c)ecompression with heating (Path 2c) (Fig. 4a–f). Several linesf evidences appear to suggest model (c) to be valid in this case:1) Firstly, pre-decomposition garnet in the same thin sections distinctly magnesian (XMg = 0.33) relative to the decomposedarnet, which has a reset core composition of XMg = 0.31. Fig. 4and f predict that such magnesian garnets originally stabilized atigher pressures (P ∼9 kbar). (2) Secondly, impoverished yOpxontent (=0.029) in the core of BM1 orthopyroxene, can be takenentatively to infer a lower temperature equilibration of the felsicranulites, pre-dating the BM2 metamorphism (Fig. 4d). Suchn inference is also consistent with recent recognition of deeprustal, post-peak, near isobaric cooling history of this gran-lite domain (Bhowmik et al., 2005; Bhowmik, 2006) (Path 1,his study). Given the relatively slow cooling rate of isobaricallyooled granulites (Harley, 1989), the time gap between the BM1nd BM2 metamorphisms (∼500 Ma for Palaeoproterozoic and1000 Ma for Archaean age of the BM1 metamorphism) is large

nough for the granulites to intersect the steady state geotherm.eatures (1) and (2) when combined with the mid-crustal peaketamorphism, suggests prograde heating accompanying pres-

ure fall of relatively cooler, deep crust at the onset of theesoproterozoic event (BM2 metamorphism) (Path 2c, Fig. 4f).
Compared to sample B223A(2), XMg and XGrs isopleths
n garnet of sample B11D have different orientations in the–T space (Fig. 5a and b). The former has steep slopes,ith XMg in garnet increasing during heating (Fig. 5a). Con-

grlb

mbrian Research 162 (2008) 128–149 141

rary to the previous example (Fig. 4b) grossular contentsn garnet (Fig. 5b) increase with heating. This suggestsifferent reaction mechanism for the formation of orthopy-oxene symplectite in this rock. Compared to the reactionarnet + quartz = orthopyroxene + plagioclase, which inducedecomposition of substantial grossular component in garnet inample B223A(2), garnet in this sample broke down, essen-ially by the reaction, pyrope = Mg-Tschermak + enstatite. Thiss evident from the high yOpx (=0.083) content in the orthopy-oxene symplectite (Table 2). The estimated P–T condition ofM2 metamorphism, deduced from sample B223A(2) whenlotted in Fig. 5f, suggests that none of the compositionalarameters in garnet, pyroxene and plagioclase has been pre-erved. Because of low garnet/orthopyroxene volume ratios,here was significant down temperature re-equilibration of gar-et and orthopyroxene. At this metamorphic condition XMg inarnet would have a value of ∼0.36 (∼7 mol% higher than thateasured). Similarly, estimated yOpx (∼0.10) is about ∼2 mol%

ower from that measured. Because of the extremely weaketamorphic overprint of BM3 metamorphism in this textural

omain, we have used these compositional criteria and the slopef XMg(Grt) isopleth to deduce a schematic post-peak cooling–T trajectory for the BM2 metamorphism (Path 2d, Fig. 5f).he falling pressure during cooling is additionally supportedy the inferred shallow emplacement depth of the metanoritesee below). Based on contrasting garnet/orthopyroxene vol-me ratios in the symplectite domains of the two samples, thentire P–T path of the BM2 metamorphism has been deduced inection 5.

.1.2. BM3 metamorphismGrossular-rich coronal garnet, replacing the pyroxene-

lagioclase symplectites, in sample B223A(2) can be producedy (a) near isobaric cooling (Path 3a), (b) near isothermal loadingPath 3b), or (c) pressure increase, accompanying rising temper-ture (Path 3c) (Fig. 6). The compositional features of 9 mol%rossular contents in coronal garnet (Fig. 6b), and the sodicomposition of the outer rim of symplectite plagioclase (An29)Fig. 6e), provide strong compositional support for a phase ofressure increase. Using intersection of grossular and XMg(Opx)sopleths, P–T condition of ∼9 kb, 725 ◦C has been estimated forhe peak BM3 metamorphism (Fig. 6f), which is higher than thatalculated previously (P ∼8 kbar, T ∼675 ◦C, Bhowmik et al.,005). Since the felsic granulites underwent significant coolingT ≤ 500 ◦C), following the peak BM2 metamorphism, as estab-ished earlier, we infer Path 3c as the prograde arm for the BM3etamorphism (Fig. 6f). Depletion in yOpx contents in the rim of

ymplectite orthopyroxene2, with the degree of depletion beingfunction of the grain size of orthopyroxenes (yOpx = 0.01 in

maller orthopyroxene vs. yOpx = 0.038–0.022 in coarser ones),oupled with increase in XMg(Opx) from 0.56 to 0.58, when eval-ated in Fig. 6c, d and f, suggest a phase of cooling followingeak BM3 metamorphism. Based on the assumption that no new
arnet was produced subsequent to this metamorphic phase, theetrograde cooling arm, in all likelihood, followed the grossu-ar isopleth of garnet (Path 3d, Fig. 6). This is further evaluatedelow.

1 Preca

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.2. Mafic intrusions

.2.1. Metagabbro
The diagnostic feature of garnet absence in the metagab-
ros suggests that the subsolidus cooling of the mafic intrusionsook place at a pressure less than that of the garnet-inurve (Fig. 7). Both experimental and natural rock data

aNtt

ig. 8. P–T pseudosection calculated in the system, NCFMAS for the metagabbroic no

Grs(Grt) (a); XMg(Grt) (b); XMg(Opx) (c); yOpx (d); XAn(Pl) (e); XMg(Cpx) (f). (g)etamorphism. Also shown are inferred depth of emplacement and subsolidus coolin


uggest that the net transfer reaction producing the gar-et + clinopyroxene + quartz assemblage from the subsolidusrthopyroxene + plagioclase one, becomes extremely sluggish
t temperatures below 650–700 ◦C, owing to slow rate ofaSi–CaAl diffusion in plagioclase at this temperature condi-
ion (Grove et al., 1984). Sample B25, being the most ferroan ofhe gabbroic suite analysed so far, the pseudosection provides

rite bulk (sample B12). The pseudosection has been contoured for the isopleths,Reconstruction of prograde (Path 4a) and retrograde arms (Path 4b) of BM3

g history of the norites.


( Cont

am

4

crsm((icviNwstrcoTrDsom(fs

crt(fbc(uniwmpalicrtr((n

4

Fig. 8.

n upper pressure limit of P ≤ 6 kbar for the emplacement ofetagabbro in the BBG domain (Fig. 7).

.2.2. MetanoriteBecause of ferroan bulk rock composition, the garnet-in

urve in the metanorite (Fig. 8) occurs at a much lower-Pelative to that in the metagabbro (Fig. 7). The P–T pseudo-ections predict the following compositions of the subsolidusinerals at this low-P condition: XMg(Opx) ∼0.41–0.42

Fig. 8c), yOpx ∼0.062–0.03 (Fig. 8d), XMg(Cpx) = 0.54–0.62Fig. 8f) and XAn = 0.77 (Fig. 8e). The mineral compositionsn the cores of megacrystic pyroxenes reveal that subsolidusompositions in these minerals are totally reset. Partial preser-ation of subsolidus composition is, however, noted in thenner rim of reverse zoned plagioclase megacryst (An73; Analo. B-6; Table 3). It is inferred that plagioclase outer rimas even more anorthitic, commensurate with the predicted

ubsolidus composition. However, due to the operation ofhe anorthite-consuming, garnet-forming reactions, orthopy-oxene + plagioclase = garnet + clinopyroxene + quartz andlinopyroxene + plagioclase = garnet + quartz, this componentf anorthitic plagioclase is likely to have been consumed.extural features further reveal post-kinematic growth (withespect to BS3 foliation) of garnet in this rock (Fig. 2f–h).rawing analogy with the metagabbros that near isobaric

ubsolidus cooling was not responsible for the appearancef garnet in the metabasites, the upper limit of the emplace-
ent depth of the norite is constrained at ≤4.5–5 kbar
Fig. 8g). The estimate is 1.0–1.5 kbar shallower than thator the metagabbro, consistent with its relatively finer grainize.

ci

inued ).

The rim compositions of megacrystic and recrystallisedlinopyroxenes (XMg = 0.73–0.74), the composition of clinopy-oxene symplectite (XMg = 0.73–0.75), the outer rim composi-ion of plagioclase (An41), all in contact with coronal garnetXGrs = 0.20 at core) appear to mark equilibrium compositionsor the formation of garnet + clinopyroxene + quartz assem-lage. The intersection of these isopleths provides a robust P–Tonstraint of ∼9.4 kbar, 760 ◦C for the BM3 metamorphismFig. 8g), which is close to that estimated from the felsic gran-lites (Fig. 6f). As shown before in the felsic granulites, theorites also record compositional evidence for post-peak cool-ng. The analysed garnet core has a composition of XMg = 0.26,hich is ∼5 mol% lower than that predicted for the peak BM3etamorphism (Fig. 8b and g). The slope of the XMg(Grt) iso-

leth suggests that such a reduction of XMg values could bechieved by cooling, accompanying pressure fall (Fig. 8b). Sucharge-scale resetting of XMg composition of garnet during cool-ng was due to low garnet/pyroxene volume ratios. With theomposition of the plagioclase outer rim in contact with garnet,emaining fixed at An41, the retrograde cooling arm is inferredo follow the XAn isopleth (Fig. 8e). The prograde (Path 4a) andetrograde (Path 4b) arms of BM3 metamorphism deduced hereFig. 8g) resemble with those derived from the felsic granulitesPaths 3c and 3d, respectively, Fig. 6f), implying regional-scaleature of the BM3 metamorphic event.

.3. Limitations of method

Having gained significant insight into the metamorphic pro-esses through P–T pseudosection modelling, it also becomesmportant to analyse the limitations of the approach, and the

1 Preca

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ffects of possible errors on the interpretations made above.e explore here the possible effects of three sources of error

n the reconstructed topology: (a) calculated activities of min-ral end members and the thermodynamic dataset used, (b) thehoice of the chemical systems and (c) the estimated mineralodes.For the first one, the uncertainties associated with the

ositions of boundaries separating fields of different mineralssemblages, using the internally consistent thermodynamicataset of Holland and Powell (1998) are in the range of ±10 ◦CZeh and Holness, 2003). The usefulness of our chosen activityodels is also reflected in good agreement between calculated

nd measured mineral assemblage and mineral composition.For all the pseudosections calculated in this work, the model

ystem NCFMAS is chosen. In the felsic granulites, ignoringn, Ti and K seems to be justified for the following reasons: (1)

pessartine occurs in low concentration in garnet (∼1 mol%), (2)iotite, ilmenite and perthites are extremely low in abundance.
he effects of possible presence of ferric cation are difficult tossess, owing to the uncertainties in the calculated ferric in garnetnd orthopyroxene (Pattison et al., 2003). In an important work,elsey et al. (2003) have demonstrated the possible effects of
msta

ig. 9. Sensitivity analyses in the topology of P–T pseudosection, in felsic granulite [saf plagioclase mode, (b) 10% underestimation of plagioclase mode, (c) 10% overestimode. The shift in P–T locus of XMg(Grt), XGrs(Grt) and XMg(Opx) isopleths due to er

). The direction of shift of the isopleths and calculated P–T conditions of BM2 and B


ncertainties in Fe3+ calculations in the location of yOpx iso-leths in P–T space. An underestimation of the estimated Fe3+

ontent in orthopyroxene will cause the P–T locus of the yOpxsopleth to shift towards higher temperatures, resulting in overes-imation of calculated temperatures (and pressures). The reverses true for Fe3+ overestimation. Estimated Fe3+ in the analysedrthopyroxenes of felsic granulites (through stoichiometry andharge balance) is low (in the range of 0.00–0.04 per formulanit). This coupled with the general absence of magnetite inhese rocks suggest that Fe3+

, even if present, is unlikely toause a significant shifting in the P–T position of yOpx isopleths.owever, in the mafic intrusions, the haematite component in

olid solution with ilmenite, the latter being an important con-tituent in these rocks, is likely to have influence on the stabilityf the garnetiferous assemblage. Relatively oxidising conditionsould enhance the stability of ilmenite-bearing assemblage and

ause the garnet-in curve to shift to relatively higher pressuren the TiNCFMASO system. However, the general absence of

agnetite in these rocks and low calculated ferric in ilmenite inample B12 (our unpublished data) suggests that the calculatedopology in the NCFMAS system is unlikely to be significantlyltered.

mple B223A(2)], for errors in estimated mineral modes: (a) 10% overestimationation of orthopyroxene mode, and (d) 10% underestimation of orthopyroxene

rors in calculated mineral modes are compared with those of the original (CaseM3 metamorphism are shown by white and black arrows, respectively.

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The third source of error stems from the possible errors instimates of the effective bulk compositions, resulting fromrrors in the calculations of mineral modes (Zeh, 2001). Weave evaluated this aspect in sample B223A(2). Ten percentageverestimation of plagioclase in the calculated bulk shifts esti-ated P–T conditions of BM2 and BM3 metamorphism to higher

nd lower P–T conditions, respectively (Fig. 9a). The reverse isrue for plagioclase underestimation (Fig. 9b). Thus, 10% under-stimation in plagioclase mode would give an unrealisticallyigh BM3 pressure of over 12 kbar (�P = 3 kbar) (Fig. 9b). Forrthopyroxene, both overestimation (Fig. 9c) and underestima-ion (Fig. 9d) do not cause major changes in the estimated BM3etamorphic condition. In the former case, the P–T condition

f BM2 metamorphism is shifted to lower P (�P = ∼0.5 kbar)nd T (�T = ∼50 ◦C) (Fig. 9c). For a low estimated orthopy-oxene mode, an unreasonably high P–T condition for BM2etamorphism is predicted (Fig. 9d). Although, error analy-

is of estimated mineral modes gives indications of significanthanges in the P–T locus of these isopleths (Fig. 9a–d), caus-ng over- or underestimations of calculated P–T conditions ofetamorphism, the slopes of the compositional isopleths in gar-

et, orthopyroxene and plagioclase remain broadly unchanged.herefore, conclusions concerning the slopes of the P–T vectors

or these metamorphic events still remain robust. Moreover, ourstimated P–T conditions of BM2 and BM3 metamorphism arelso consistent with that calculated from the metagabbro andetanorites. This rules out large-scale error in calculated effec-

ive bulk compositions in the felsic granulites. The metamorphicndings drawn through P–T pseudosection modelling, therefore,emain intact and can be utilized for tectonic modelling.

. Discussion

One of the key problems concerning the metamorphic evo-ution of polycyclic granulites is to constrain how these rocksesponded to later reactivations during their incorporation intoyounger mobile belt. This is because, apart from partial com-ositional resetting of minerals, the high-temperature mineralssemblages belonging to the earlier events are unlikely to yieldhe conditions of the superimposed net transfer reactions inesponse to the younger events, except in the case of rocks withigh Mg–Al bulk compositions. Even in the latter situation, therograde segment of the P–T path of the younger event is rarelyocumented, making it difficult to interpret the tectonic settingf the younger event. We have addressed this aspect in this studyy using rocks from the polymetamorphic BBG domain in theausar Mobile Belt and demonstrate that the older granulitesesponded in multiple stages to younger orogenic events. Despitehe high variance of the assemblages involved in these metamor-hic overprints, P–T pseudosection modelling provides criticalew information on the early Mesoproterozoic crustal evolutionn the CITZ.

In a previous study, Bhowmik et al. (2005) established
wo phases of Mesoproterozoic metamorphism (BM2 andM3) in the BBG domain, which reworked Palaeoproterozoic
Archaean, after Roy et al., 2006) ultrahigh-temperature gran-lites (Fig. 10a). In the absence of a prograde segment of the P–T

pztr

mbrian Research 162 (2008) 128–149 145

ath, these were considered to be part of the same Mesoprotero-oic metamorphic event (Fig. 17b of Bhowmik et al., 2005).he present study reveals that instead of a single metamorphicvent, there were two separate metamorphic pulses in the earlyesoproterozoic, having contrasting metamorphic P–T paths.BM2 has a clockwise metamorphic P–T path, involving an

mportant phase of heating (≥250 ◦C) during decompressionrior to cooling during continued decompression (Fig. 10b).eating during decompression is also a feature of crust thickenedy thrusting. However, the extent of heating (≥250 ◦C) duringnitial decompression, documented here, is much larger than

odelled for tectonically thickened crust, but consistent withagmatically active extensional zones. Based on this evidence

nd also the syn-metamorphic mafic intrusions, we present anlternate interpretation in terms of mid-crustal extension of nearormal crustal thickness (Harley, 1989; Sandiford and Powell,986) for the BM2 metamorphism.

Subsequent to this metamorphic event, the granulites andhe mafic intrusions were at mid-crustal levels, and were re-

etamorphosed, in response to a regional-scale tectonothermalvent (BD3/BM3). BM3 exhibits a counterclockwise P–T path,nvolving a prograde burial to ∼9.4 kbar, 760 ◦C, followedy post-peak cooling to ∼6.5 kbar, 500 ◦C (Fig. 10c). Tec-onic thickening of ∼17–19 km of crust is also supported bytructural findings, which demonstrate a thrust-sense of move-ent during the BD3 deformation (Bhowmik et al., 2005;asu Sarbadhikari, 2006). Crustal thickening established here

s unlike that result from continent–continent collision, wherehickening takes place in cool crust with temperature increas-ng following thickening (England and Thompson, 1984). Inhis case, increasing temperature during loading, being followedy cooling indicates crustal thickening of a hot crust (Spearnd Markussen, 1997). The thermally perturbed nature of theid-crust, before thickening was possibly related to renewedagmatic accretion in the form of metanorite intrusions. Asresult, cooling commenced immediately after the heat sourceas removed or if the overlying crustal slab was relatively cooler.Published geochronological data provide some constraints

n the timing of these two metamorphic events. Based on elec-ron microprobe dating of monazites from different texturalomains in sample B223A(2), Bhowmik et al. (2005) retrievedonazite chemical ages in the range ∼1.53 Ga to ∼1.45 Ga.he older of these two age domains was estimated from theM2 symplectites around porphyroblastic garnet and also in theatrix, away from the effects of penetrative BD3 deformation

Fig. 3, this study). In both these textural settings, Mesopro-erozoic monazites appeared as metamorphic overgrowths onalaeoproterozoic cores. In contrast, monazites dated from theS3 mylonite foliation correspond to the younger age domainf ∼1.45 Ga (Fig. 3, this study). If indeed the age variation, asorrelated with the textural settings, is real and does not reflectnalytical uncertainties of the dating method, the ∼1.53 ageata appears to indicate the minimum age of the BM2 metamor-
hism. Bhowmik et al. (2006) reported 1.59 Ga SHRIMP U–Pbircon date from a new felsic-granulite sample, lying close tohe dated monazite sample, B223A(2) (Fig. 1b). The formerecords the same BM2 and BM3 textural features, studied here.


Fig. 10. Cartoons showing the multistage amalgamation processes in the Central Indian Tectonic Zone as modelled from the Sausar Mobile Belt at its southernmargin. (a) Two possible models for the ultrahigh-temperature granulite metamorphism, along a counter clockwise (CCW) P–T path in the Bhandara–Balaghatgranulite (BBG) domain (Bhowmik et al., 2005, Bhowmik, 2006): (i) an Archaean event in the South Indian Block (SIB) of unspecified tectonic setting or (ii) aproduct of a Palaeoproterozoic event, in a possible island arc setting. (b–c) Development of an early Mesoproterozoic hot orogen. The tectonic switching from earlylithospheric extension to contractions, resulting from successive stages of slab retreat and flat subduction led to the suturing of arcs, back arcs and continents inthe CITZ. This produced two short-lived metamorphic events (BM2 and BM3) with contrasting P–T paths (this study). (d) Further slab retreat in between early tolate Mesoproterozoic caused another phase of lithospheric extension of the thickened crust. (e) Grenville-aged (?) continent–continent collision, leading to the finalamalgamation in the CITZ. Metamorphic P–T paths from the northern Ramakona-Katangi granulite (RKG) domain, in the orogenic core have classical clockwiseP–T loops (Bhowmik and Roy, 2003; Bhowmik and Spiering, 2004). The model predicts accretion of the early Mesoproterozoic hot orogen in the foreland side ofthe Grenville-aged orogen. CIS—Central Indian Shear.

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ased on this SHRIMP data, we constrain the timing of the BM2etamorphism at ∼1.59 Ga.Roy et al. (2006) recently calculated Sm–Nd mineral-whole

ock isochron age of ∼1.40–1.42 Ga from garnetiferous (theirample no. 2Px/7) and non-garnetiferous (their sample nos.

G/3, 2Px/1 and 2Px/3) metagabbros, which is close to the1.45 Ga monazite chemical ages. These Sm–Nd dates were

nterpreted as cooling ages, based on their assumption that gar-et (of coronal variety) in sample 2Px/7 appeared in response toooling. Although, no geological map, showing sample loca-ions is given by the authors, the latitude–longitude data ofhese samples presented in Table 1 of Roy et al. (2006) indi-ate that these samples were collected from three locations,amely Larsara (samples, 2Px/7 and 2Px/3), Dongariya (sam-le 2Px/1) and Sahapur (MG/3). Petrographic descriptions ofhe mafic intrusions given in Roy et al. (2006) show that gar-etiferous (sample 2Px/7) and non-garnetferous (sample 2Px/3)afic intrusions of Larsara area have physical resemblance withetanorite and metagabbro of our study. If true, the Sm–Nd

ates would likely to bracket the upper limit of the BM3 meta-orphism. There is an increasing body of evidence that similaresoproterozoic granulite-facies metamorphism is more com-on than originally believed (e.g. East Antarctica, Kelly et

l., 2002; Eastern Ghats Belt, India, Mezger and Cosca, 1999;ovach et al., 2001; Simmat, 2003 quoted in Dobmeier andaith, 2003). This study, however, provides much tighter con-

traints on the P–T path of evolution of the Mesoproterozoicranulite-facies event(s).

. Suggested geodynamic model

.1. Archaean/Palaeoproterozoic (?) evolution

Although of relatively restricted geographic distribution, theranulites of the BBG domain, at the craton–mobile belt inter-ace record a complex tectonic history. This is shown by a seriesf cartoons in Fig. 10a–e. The metamorphic findings presentedere, when collated with those derived from other domains ofhe Sausar Mobile Belt (Bhowmik and Roy, 2003; Bhowmiknd Spiering, 2004; Roy et al., 2006) provide an opportunityo establish a coherent geodynamic model of evolution of thisegment of the CITZ. These findings suggest multistage crustalvolutionary processes were operative in the Central Indian Tec-onic Zone, that are inconsistent with simple monocyclic modelsresented previously. Those models considered that Palaeopro-erozoic subduction of the North Indian Block below the Southndian Blocks led to the stitching of these crustal blocks alonghe Central Indian Suture Zone (Yedekar et al., 1990; Jain etl., 1991, 1995; Mishra et al., 2000). Such a model wouldequire a clockwise metamorphic P–T path from the rocks ofhe BBG domain. However, the counterclockwise P–T trajec-ory deduced by Bhowmik et al. (2005) and Bhowmik (2006)Fig. 10a), contradicts this simple collisional model. Instead,
HT metamorphism along a counterclockwise P–T path isenerally explained by model of lower crustal magmatic accre-ion of mantle-derived mafic magmas (Bohlen, 1987; Harley,989). Such ultrahigh temperatures and CCW P–T trajectories
acm

mbrian Research 162 (2008) 128–149 147

re indicative of continental rifts or island arc tectonic settingsHarley, 1989). Because of the allochthonous character of theBG domain and insufficient geochemical and geochronologi-al characterization, it is not possible at this stage to discriminateetween the two possibilities. Nevertheless, the metamorphicvent led to the cratonisation (through isobaric cooling) of theBG crust. The age of this cratonisation is, however, debatable.hile a Palaeoproterozoic age was indicated by the monazite

hemical ages (Bhowmik et al., 2005), an Archaean age wasuggested recently based on Sm–Nd garnet mineral ages (Royt al., 2006). A robust isotopic geochronological tool, such ashe SHRIMP U–Pb zircon dating method needs to be applied toesolve this controversy.

.2. Early Mesoproterozoic evolution

With respect to the Mesoproterozoic, the granulites of theBG domain record an early extension (BM2 metamorphism)

Fig. 10b) and a later compression (BM3 metamorphism)Fig. 10c). These metamorphisms, occurring within a shortime span, can be best explained by tectonic switching inccretionary orogens, where lithospheric extension was imme-iately followed by contractions, consistent with a thermallyoftened back-arc region (Thompson et al., 2001; Collins,002). If true, this would suggest the closure of a back arc,ue to collision with a possible arc component in the northFig. 10c). The model predicts the presence of an early Meso-roterozoic hot and narrow orogen at the southern margin ofhe CITZ. Subsequent to this phase of amalgamation in theITZ, leading to the suturing of arcs and back arcs, a furtherhase of slab roll back caused another phase of lithosphericxtension (Fig. 10d). This is indicated by the intrusion ofD4 olivine gabbro dykes and the development of BM4 low-orthopyroxene–clinopyroxene–plagioclase assemblage in theetagabbros (Table 1).

.3. Late Mesoproterozoic evolution

Nevertheless, the granulites were still at depth, at the endf this early Mesoproterozoic event. Their exhumation and tec-onic juxtaposition with Sausar supracrustal rocks would requiren even younger tectonothermal event. Recent studies havestablished the existence of a regional high-P metamorphic beltith a clockwise metamorphic P–T path (Fig. 10e) from theamakona-Katangi granulite domain, at the northern margin of

he Sausar Mobile Belt (Bhowmik and Roy, 2003; Bhowmiknd Spiering, 2004). Based on available geochronological data,howmik and Roy (2003) speculated a Grenvillian age for thisigh-P metamorphic event, also supported by recent geochrono-ogical findings (Roy et al., 2006). If correct, this could be thevent that led to the final closure of the Sausar basin (Fig. 10e),nd also the final amalgamation in the Central Indian Tectonicone (Bhowmik and Roy, 2003).

In summary, the present study demonstrates thatllochthonous polycyclic tectonic sheets, occurring at theratonic foreland of younger orogen, record significant infor-ation of older tectonothermal histories, the signatures of

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hich are almost absent from the main part of the orogen. Inarticular, an early Mesoproterozoic hot orogen, lying at thenterface of the craton and a Grenville-aged younger orogen,s deduced in this study, can act as potential markers fornter-orogen correlations in adjoining Proterozoic mobile beltsf Peninsular India.

cknowledgements

We thank Prof. S. Dasgupta, Prof. M.M. Raith and Dr. S.arkar for inviting us to write this paper. ABS acknowledges

he doctoral (Indian Institute of Technology, Kharagpur) andost-doctoral research fellowships (Council of Scientific andndustrial Research, Sanction No. 9181(579)/06-EMR-1). SKBhanks the Department of Science and Technology for partialunding of the research (Grant No. ESS/23/VES/129/2000).nalytical data for monazite age dating and bulk chemical com-ositions were generated in the University of Bonn, Germany,or which SKB thanks the Indian National Science Academy-eutsche Forschungsgemeinschaft research fellowship, M.M.aith, B. Spiering and R. Hoffbauer. The paper was critically

eviewed by Prof. Toby Rivers and an anonymous reviewer.heir comments have helped to improve the manuscript. It ispleasure to acknowledge stimulating discussion with Prof. A.hattacharya concerning many aspects of orogenic evolution.

eferences

aldwin, J.A., Powell, R., Brown, M., Moraes, R., Fuck, R.A., 2005. Modellingof mineral equilibria in ultrahigh-temperature metamorphic rocks from theAnapolis–Itaucu complex, central Brazil. J. Metamorph. Geol. 23, 511–531.

asu Sarbadhikari, A., 2006. Polymetamorphic history of the Central Indian Tec-tonic Zone: constraints from reaction texture, mineral chemistry and electronmicroprobe dating of monazites. Unpublished Ph.D. Thesis. Indian Instituteof Technology, Kharagpur, India.

hattacharya, A., Raith, M., Hoernes, S., Banerjee, D., 1998. Geochemical evo-lution of the massif-type anorthosite complex at Bolangir in the EasternGhats Belt of India. J. Petrol. 39, 1169–1195.

howmik, S.K., 2006. Ultra high temperature-metamorphism and its signifi-cance in the Central Indian Tectonic Zone. In: Sajeev, K., Santosh, M. (GuestEds.), Extreme crustal metamorphism and related crust-mantle processes.Lithos Spec. Issue 92, 484–505.

howmik, S.K., Roy, A., 2003. Garnetiferous metabasites from the SausarMobile Belt: petrology, P–T path and implications for the tectonothermalevolution of the Central Indian Tectonic Zone. J. Petrol. 44, 387–420.

howmik, S.K., Spiering, B., 2004. Constraining the prograde and retrogradeP–T paths of granulites using decomposition of initially zoned garnets: anexample from the Central Indian Tectonic Zone. Contrib. Miner. Petrol. 147,581–603.

howmik, S.K., Sarbadhikari, A.B., Spiering, B., Raith, M.M., 2005. Mesopro-terozoic reworking of Palaeoproterozoic ultrahigh-temperature granulitesin the Central Indian Tectonic Zone and its implications. J. Petrol. 46,1085–1119.

howmik, S.K., Basu Sarbadhikari, A., Wilde, S., 2006. 1. 6 Ga metamorphicre-heating and subsequent compression of UHT granulites from the CentralIndian Tectonic Zone. In: Brown, M., Piccoli, P.M. (Eds.), InternationalConference with Program and Abstracts on the Granulites and Granulites.
Brasilia, Brazil, p. 16.
ohlen, S.R., 1987. Pressure–temperature–time paths and a tectonic model forthe evolution of granulites. J. Geol. 95, 617–632.

hakraborty, S., Ganguly, G., 1991. Compositional zoning and cation diffusionin aluminosilicate garnets. In: Ganguly, J. (Ed.), Diffusion, Atomic Ordering

M


and Mass Transport. Advances in Physical Geochemistry, vol. 8. Springer,Berlin, Heidelberg, New York, Tokyo, pp. 120–175.

ollins, W.J., 2002. Hot orogens, tectonic switching, and creation of continentalcrust. Geology 30, 535–538.

obmeier, C., Raith, M.M., 2003. Crustal architecture and evolution of the East-ern Ghat Belt and adjacent regions of India. In: Yoshida, M., Windley, B.W.,Dasgupta, S. (Eds.), Proterozoic East Gondwana: Supercontinent Assemblyand Breakup, Geological Society of London, Special Publications, vol. 206,pp. 145–168.

ngland, P.C., Thompson, A.B., 1984. Pressure–temperature–time paths ofregional metamorphism I. Heat transfer during the evolution of regions ofthickened continental crust. J. Petrol. 25, 894–928.

oncalves, P., Nicollet, C., Montel, J.M., 2004. Petrology and in-situ U–Th–Pbmonazite geochronology of ultrahigh-temperature metamorphism from theAndriamena mafic unit, north-central Madagascar. Significance of a petro-graphical P–T path in a polymetamorphic context. J. Petrol. 45, 1923–1957.

rove, T.L., Baker, M.B., Kinzler, R.J., 1984. Coupled CaAl–NaSi diffusion inplagioclase feldspar: experiments and applications to cooling rate speedom-etry. Geochim. Cosmochim. Acta 48, 2113–2121.

and, M., Dirks, P.H.G.M., Powell, R., Buick, I.S., 1992. How well established isisobaric cooling in Proterozoic orogenic belts? An example from the Aruntainlier. Geology 20, 649–652.

arley, S.L., 1989. The origins of granulites: a metamorphic perspective. Geol.Mag. 126, 215–247.

arley, S.L., Christy, A.G., 1994. Titanium-bearing sapphirine in a partiallymelted aluminous granulite xenolith, Vestfold Hills, Antarctica: geologicaland mineralogical implications. Eur. J. Mineral. 7, 637–653.

arley, S.L., Fitzsimons, I.C.W., 1995. High-grade metamorphism and defor-mation in the Prydz Bay region, East Antarctica: terranes, events andregional correlations. Memoir of Geological Society of India, vol. 34,pp. 73–100.

ensen, B.J., Zhou, B., Thost, D.E., 1995. Are reaction textures reliable guides tometamorphic histories? Timing constraints from garnet Sm–Nd chronologyfor ‘decompression’ textures in granulites from Sostrene Island, Prydz Bay,Antarctica. Geol. J. 30, 261–271.

olland, T.J.B., Powell, R., 1998. An internally consistent thermodynamicdataset for phases of petrological interest. J. Metamorph. Geol. 16, 309–343.

ain, S.C., Yedekar, D.B., Nair, K.K.K., 1991. Central Indian shear zone: a majorPre-Cambrian crustal boundary. J. Geol. Soc. Ind. 37, 521–531.

ain, S.C., Nair, K.K.K., Yedekar, D.B., 1995. Geology of theSon–Narmada–Tapti lineament zone in Central India. In: Geoscien-tific studies of the Son–Narmada–Tapti Lineament Zone, Geological Surveyof India, Special Publications, vol. 10, pp. 1–154.

elly, N.M., Clarke, G.L., Fanning, C.M., 2002. A two-stage evolution of theNeoproterozoic Rayner structural episode: new U–Pb sensitive high resolu-tion ion microprobe constraints from the Oygarden group, Kemp Land, EastAntarctica. Precambrian Res. 116, 307–330.

elsey, D.E., White, R.W., Powell, R., 2003. Orthopyroxene–sillimanite–quartzassemblages: distribution, petrology, quantitative P–T–X constraints andP–T paths. J. Metamorph. Geol. 21, 439–453.

ovach, V.P., Simmat, R., Rickers, K., Berezhnaya, N.G., Salnikova, E.B., Dob-meier, C., Raith, M.M., Yakovleva, S.Z., Kotov, A.B., 2001. The WesternChamockite Zone of the Eastern Ghats Belt, India—an independent crustalprovince of late Archaean (2.8 Ga) and Palaeoproterozoic (1.7–l.6 Ga) ter-rains. In: International Symposium and Field Workshop on the Assemblyand Breakup of Rodinia and Gondwana, Osaka, Gondwana Res. 4, October26–30, 2001, pp. 666–667.

retz, R., 1983. Symbols for rock-forming minerals. Am. Mineral. 68, 277–279.uehner, S.M., 1992. Experimental determination of the emplacement pressure

of Proterozoic mafic dykes within portions of the East Antarctic Shield.Lithos 28, 21–42.

uehner, S.M., Green, D.H., 1991. Uplift history of the East Antarctic Shield:constraints imposed by high-pressure experimental studies of Proterozoic
mafic dykes. In: Thomson, M.R.A., Crame, J.A., Thomson, J.W. (Eds.), Geo-logical Evolution of Antarctica. Cambridge University Press, Cambridge, pp.1–6.
ezger, K., Cosca, M.A., 1999. The thermal history of the Eastern Ghats Belt(India) as revealed by U–Pb and 40Ar/39Ar dating of metamorphic and mag-

Preca

M

P

P

R

R

R

R

S

S

S

T

V

Y

Z


matic minerals: implications for the SWEAT correlation. Precambrian Res.94, 251–271.

ishra, D.C., Singh, B., Tiwari, V.M., Gupta, S.B., Rao, M.B.S.V., 2000. Twocases of continental collisions and related tectonics during the Proterozoicperiod in India—insights from gravity modelling constrained by seismic andmagnetotelluric studies. Precambrian Res. 99, 149–169.

asschier, C.W., Bekendam, R.F., Hoek, J.D., Dirks, P.G.H.M., Debcorder,H., 1991. Proterozoic geological evolution of the northern Vestfold Hills,Antarctica. Geol. Mag. 128, 307–318.

attison, D.R.M., Chacko, T., Farquhar, J., McFarlane, C.R.M., 2003. Tempera-tures of granulite facies metamorphism: constraints from experimental phaseequilibria and thermobarometry corrected for retrograde exchange. J. Petrol.44, 867–900.

amachandra, H.M., 1999. Petrology of Bhandara and Balaghat granulite beltin parts of Maharashtra and Madhya Pradesh. Unpublished Report of Geo-logical Survey of India. 55 pp.

amachandra, H.M., Roy, A., 2001. Evolution of the Bhandara–Balaghat gran-ulite belt along the southern margin of the Sausar Mobile Belt of CentralIndia. Proc. Indian Acad. Sci. (Earth Planet. Sci.) 110/4, 351–368.

ivers, T., Martignole, J., Gower, C.F., Davidson, A., 1989. New tectonic divi-
sions of the Grenville Province, southeast Canadian shield. Tectonics 8,63–84.
oy, A., Kagami, H., Yoshida, M., Roy, A., Bandyopadhyay, B.K., Chat-topadhyay, A., Khan, A.S., Huin, A.K., Pal, T., 2006. Rb–Sr and Sm–Nddating of different metamorphic events from the Sausar Mobile Belt, central

Z

mbrian Research 162 (2008) 128–149 149

India: implications for Proterozoic crustal evolution. J. Asian Earth Sci. 26,61–76.

andiford, M., Powell, R., 1986. Deep crustal metamorphism during continen-tal extension: modern and ancient examples. Earth Planet. Sci. Lett. 79,151–158.

immat, R., 2003. Identifizierung hochgradig metamorpher Provinzen desEastern Ghats Belt in Indien anhand einer EMS-Studie von Monazit-Altersmustern. Ph.D. Thesis. University of Bonn, Germany.

pear, F.S., Markussen, J.C., 1997. Mineral zoning, P–T–X–M phase relations,and metamorphic evolution of some Adirondack granulites, New York. J.Petrol. 38, 757–783.

hompson, A.B., Schulmann, K., Jezek, J., Tolar, V., 2001. Thermally soft-ened continental extensional zones (arcs and rifts) as precursors to thickenedorogenic belts. Tectonophysics 332, 115–141.

ernon, R.H., 1996. Problems with inferring P–T–t paths in low-P granulitefacies rocks. J. Metamorph. Geol. 14, 143–154.

edekar, D.B., Jain, S.C., Nair, K.K.K., Dutta, K.K., 1990. The central Indiancollision suture. In: Precambrian of Central India, Geological Survey ofIndia, Special Publishing, vol. 28, pp. 1–37.

eh, A., 2001. Inference of a detailed P–T path from P–T pseudosections using
metapelitic rocks of variable composition from a single outcrop, ShackletonRange, Antarctica. J. Metamorph. Geol. 19, 329–350.
eh, A., Holness, M.B., 2003. The effect of reaction overstep on garnet micro-textures in metapelitic rocks of the Ilesha Schist Belt, SW Nigeria. J. Petrol.44, 967–991.

Constraining the metamorphic evolution of a cryptic hot Mesoproterozoic orogen in the Central Indian...

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