Zoned Monazite and Zircon as Monitors for the Thermal History of Granulite Terranes: an Example from...

Zoned Monazite and Zircon as Monitors for theThermal History of GranuliteTerranes: anExample from the Central IndianTectonic Zone

SANTANU KUMAR BHOWMIK1*, SIMON ALEXANDERWILDE2,ANUBHA BHANDARI1 AND AMIT BASU SARBADHIKARI3

1DEPARTMENT OF GEOLOGY AND GEOPHYSICS, INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR, INDIA2DEPARTMENT OF APPLIED GEOLOGY, CURTIN UNIVERSITY, PERTH, WA 6845, AUSTRALIA3PHYSICAL RESEARCH LABORATORY, AHMEDABAD, INDIA

RECEIVED MAY 17, 2012; ACCEPTED DECEMBER 6, 2013

The growth and dissolution behaviour of detrital, metamorphic and

magmatic monazite and zircon during granulite-facies anatexis in

pelitic and psammo-pelitic granulites and in garnetiferous granite

from the southern margin of the Central Indian Tectonic Zone

(CITZ) have been investigated using reconstructed metamorphic re-

action history, monazite electron microprobe dating and sensitive

high-resolution ion microprobe (SHRIMP) U^Pb zircon geochron-

ology.Whereas the pelitic granulites record medium-pressure granu-

lite-facies metamorphism (BM1 stage), the psammo-pelitic

granulite reached ultrahigh temperatures (TMax48808C at

8·7 kbar). The meta-psammite additionally records two stages of

granulite-facies recrystallization (BM2 and BM3). Irrespective of

variations in the bulk-rock compositions and peak metamorphic con-

ditions, monazite is highly reactive during the BM1 event, producing

complex, chemically zoned crystals. Textural, compositional and

chemical ages of these grains indicate the stability of six compos-

itional domains (CD1 to CD6 in the paragenetic sequence),

of which CD1 represents pre-metamorphic detrital cores of Paleopro-

terozoic age. CD2 and CD3 (combined mean age of

1612�14Ma) mark two stages of recrystallization of detrital

monazite cores during prograde events. Rims of CD4 monazite

(ages between 1615�14 and 1586�14Ma) on partially to com-

pletely equilibrated cores indicate melt crystallization at, or immedi-

ately following, peak BM1P metamorphism. CD5 monazite (age of

1574� 7Ma) is restricted to the psammo-pelitic granulites, and

marks final melt crystallization at the solidus during post-peak cool-

ing (BM1R stage, where R represents retrograde metamorphism).

The metamorphic rim of CD6 monazite (age of 1539� 24Ma)

around partially resorbed CD5 domains is linked to the decompos-

ition of BM1 garnet during the terminal hydration event as part of

a granulite-facies recrystallization event. Compositionally homoge-

neous monazite and rims of chemically zoned monazite grains in

granite together record a magmatic crystallization age of

1604� 9Ma. SHRIMPU^Pb zircon dating of the psammo-pelitic

granulite and garnetiferous granite indicates detrital or inherited

cores of Paleo- to Neoarchean age (3584� 3 to 2530�3Ma),

which have been variously recrystallized and overgrown by new

zircon: (1) at 1658�12Ma; (2) between 1595� 5 and

1590� 6Ma; (3) at 1574� 9Ma.These zircon dates are correlated

with the timing of the following: (1) the protoliths of precursor sedi-

ments of the metasedimentary granulites, deposited between 2530

and 1658Ma; (2) a short-lived high-grade event �65^70Myr

before the culmination of the BM1 granulite-facies event; (3) a

high-Tanatectic event, corresponding to the peak BM1P metamorph-

ism atTMax49008C; (4) final crystallization of anatectic melt at

the solidus (cf. BM1R metamorphic stage).These chronological con-

straints from monazite and zircon, when integrated with the meta-

morphic reaction history and published geochronological data, allow

recognition of three episodes of granulite-facies metamorphism in the

CITZ at 1658Ma (pre-BM1 event), between 1612 and 1574Ma

(BM1 event), and between 1572 and 1539Ma (combined BM2

and BM3 events), as part of a latest Paleoproterozoic to Early Meso-

proterozoic orogenic event. This orogeny is linked to the growth of

the Proto-Greater Indian Landmass.

*Corresponding author. E-mail: [email protected]

� The Author 2014. Published by Oxford University Press. Allrights reserved. For Permissions, please e-mail: [email protected]

JOURNALOFPETROLOGY VOLUME 0 NUMBER 0 PAGES1^37 2014 doi:10.1093/petrology/egt078

Journal of Petrology Advance Access published January 23, 2014

at Indian Institute of Technology K

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KEY WORDS: granulite; metamorphism; monazite; zircon; SHRIMP;

dating

I NTRODUCTIONThe key to interpreting a mineral age in a metamorphiccontext is the ability to place its growth and/or recrystalliza-tion history in a metamorphic reaction pathway and P^T

context. Mineral ages, when properly constrained, revealthe timescales of metamorphism, the knowledge of which iscentral to modeling orogenic processes. Monazite, apartfrom being an excellent mineral chronometer because of itshighThandUconcentrations, high retentivity of radiogenicPb, low common Pb, and resistance to diffusive Pb losseven under granulite-facies conditions (Parrish, 1990;Cherniak et al., 2004), has proved to be a powerful monitorof metamorphic processes (Pyle et al., 2001; Rubatto et al.,2001, 2013; Hermann & Rubatto, 2003; Wing et al., 2003;Kohn & Malloy, 2004; Williams et al., 2007; Kelsey et al.,2008). This is because monazite is extremely reactiveduring most metamorphic and hydrothermal processes,and can grow, dissolve and recrystallize during progradeand retrograde metamorphism at conditions from greens-chist to granulite facies, causing a redistribution of the rareearth elements (REE) between silicates and phosphatesunder changing P^T^X conditions (Foster et al., 2002; Pyle& Spear, 2003; Kelly et al., 2006; McFarlane et al., 2006;Gasser et al., 2012). In addition, monazite-forming mineralreactions have been shown to be linked with the growth ordissolution of key metamorphic minerals, including garnet,K-feldspar and the accessory minerals allanite, xenotime,apatite and zircon (Pyle & Spear, 1999; Ferry, 2000; Pyleet al., 2001; Spear & Pyle, 2002;Yang & Rivers, 2002; Fosteret al., 2004; Gibson et al., 2004; Dahl et al., 2005; Bhowmiket al., 2012). Moreover, interpretation is assisted because theP^T stability limits of metamorphic monazite have beenquantitatively determined for certain rock types and com-positions in several environments (Kelsey et al., 2008; Spear& Pyle, 2010).Like monazite, there are internal structural and trace

element compositional attributes that can be used to distin-guish metamorphic from igneous zircon (Rubatto, 2002;Corfu et al., 2003; Hoskin & Schaltegger, 2003). Its tem-perature of formation can be constrained through zirconthermometry (Watson & Harrison, 2005), its equilibrationwith monazite and other minerals determined throughelement partitioning relationships (Rubatto, 2002;Whitehouse & Platt, 2003; Kelly & Harley, 2005; Harley& Kelly, 2007) and the processes leading to its formationduring metamorphism can be recognized (Roberts &Finger, 1997; Degeling et al., 2001).Despite our understanding of monazite and zircon

chronology, the correct assignment of a monazite orzircon age can still remain complicated, in particularwhere these minerals are chemically zoned and this reflects

age variations. The mineral age can then be variously as-signed to either detrital remnants, prograde or retrogrademetamorphism, or possible polymetamorphism (Braunet al., 1998; Zhu & O’Nions, 1999; Wing et al., 2003; Fosteret al., 2004; Kohn & Malloy, 2004; Rubatto et al., 2006).Nevertheless, the potential of the monazite and zirconchronometers to constrain the time limits of granulite-facies metamorphism has been increasingly realized(Kelsey et al., 2003; Kelly et al., 2006; Bhowmik et al., 2010,2012; Cutts et al., 2011; Ho« gdahl et al., 2012) for the followingreasons: (1) partial melting reactions in pelitic andpsammo-pelitic bulk-rock compositions are petrologicallywell constrained (e.g. Spear et al., 1999; Nair & Chacko,2002). This allows reaction textures and mineral compos-itions of migmatitic metasedimentary rocks to providetight constraints on metamorphic P^T paths (Spear et al.,1999; Saha et al., 2008); (2) there is improved understand-ing of trace element partitioning of monazite^zircon be-tween leucosome and restite phases during partial meltingreactions (Harrison & Watson, 1983; Vavra et al., 1996;Watson, 1996; Bea & Montero, 1999; Rubatto & Hermann,2007; Kelsey et al., 2008; Spear & Pyle, 2010; Kelsey &Powell, 2011); (3) knowledge of Y partitioning betweenmonazite and garnet can be applied to link monazitegrowth to the growth or consumption of peritectic garnet(e.g. Spear & Pyle, 2010); Y-zoned monazite can also beused as an indicator of age zonation (e.g. Foster et al.,2002; Gibson et al., 2004; Bhowmik et al., 2012); (4) the par-titioning behaviour of Th between monazite and allaniteis constrained (Kohn & Malloy, 2004); (5) because mona-zite and zircon are the two dominant accessory phases ingranulites, Th and U zonation in these minerals can beattributed to their relative stabilities, degree of partialmelting, melt composition and melt loss (Bea & Montero,1999); (6) the chemical signature for monazite growth inthe presence of K-feldspar is recognized (Rubatto et al.,2006).Whereas the studies outlined above have provided a

basis for interpreting monazite and zircon ages within theframework of metamorphic P^T paths, there are still chal-lenges to understanding the P^T^X control on the dissol-ution and growth of these minerals in melt-bearingsystems. In this study, we use chemically zoned monaziteand zircon in pelitic and psammo-pelitic granulites fromthe southern margin of the Central Indian Tectonic Zone(CITZ) to study the response of these minerals as a func-tion of the temperature and nature of prograde meltingreactions and melt crystallization during cooling. Thesegranulites, locally referred to as the Bhandara^BalaghatGranulites (BBG), form part of a dismembered sequenceof supracrustal and meta-igneous rocks that occur withina felsic orthogneiss host (Bhowmik et al., 2005; Bhowmik,2006; Bhandari et al., 2011). Previous metamorphic andgeochronological studies indicated a multi-stage

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metamorphic history of the granulite domain, includingevidence for high-temperature reworking (Bhowmik et al.,2005, 2011; Bhowmik, 2006; Roy et al., 2006; BasuSarbadhikari & Bhowmik, 2008; Bhandari et al., 2011).There is, however, a continuing debate on the precise timeconstraints for peak granulite-facies metamorphism(Archean, Early Paleoproterozoic or earliest Mesoprotero-zoic?) and subsequent re-metamorphism, and the timingrelationship between peak metamorphism and felsicplutonism.The present investigation is aimed at resolving these

issues, so as to place a firmer constraint on theT^t historyof the granulite domain. In this study, we analyzed twogranulite samples [for monazite electron microprobe(EMP) dating], one granulite and one granite sample [forlinked monazite EMP dating and sensitive high-resolutionion microprobe (SHRIMP) U^Pb zircon dating], and uti-lized the published monazite results from another granu-lite sample in the BBG. Combining monazite and zircondata, in situY,Th, and U chemical mapping, and results ofmonazite and zircon dating with the metamorphic reac-tion history for three representative granulite samples andone garnetiferous granite sample, we estimate the timingof latest Paleoproterozoic to Early Mesoproterozoic oro-genesis in this segment of the CITZ.

GEOLOGICAL SETT INGThe Bhandara^Balaghat Granulite (BBG) domain is a190 km long by 4^20 km wide granulite^gneiss terranethat is separated from a low- to medium-grade supracrus-tal sequence of Mesoproterozoic age to the north (Centraldomain) and the South Indian Block (SIB) to the south(Fig. 1) (Bhowmik & Roy, 2003; Bhowmik & Spiering,2004; Bhowmik et al., 2011, 2012). The domain consists of asuite of supracrustal and meta-igneous granulites thatrecord four episodes of metamorphism (BM1^BM4) (BasuSarbadhikari & Bhowmik, 2008). BM1 metamorphism,which is the most pervasive event in the terrane, reachedultrahigh-temperature (UHT) metamorphic conditions atseveral locations (BM1P, where P refers to peak meta-morphism at 8^9 kbar,T �930^10008C) (Bhowmik et al.,2005; Bhowmik, 2006; Bhandari et al., 2011). The UHTgranulites show a post-peak, near isobaric cooling P^T

history (BM1R, where R refers to retrograde metamorph-ism, P �9 kbar, T �700^7508C) (Bhowmik et al., 2005;Bhowmik, 2006). The BM1 granulites were subsequentlyre-metamorphosed by two separate granulite-faciesevents; BM2 (P �6·1kbar, T �7258C) and BM3

(P �9·4 kbar, T �7608C) (Basu Sarbadhikari & Bhow-mik, 2008). The BM2 event is correlated with mid-crustalheating, which led to the decomposition of BM1 garnets toproduce orthopyroxeneþplagioclase symplectites in abanded iron formation (BIF) and garnetþorthopyrox-ene-bearing psammo-pelitic granulites, whereas the BM3

metamorphism was interpreted in terms of progradeburial of the BM2 granulites to lower crustal depths (BasuSarbadhikari & Bhowmik, 2008). A terminal, weak greens-chist^amphibolite-facies metamorphic overprint (BM4)has been noted in late shear zones.Based on EMP dating of monazite, the BM1metamorph-

ism has been recently dated at c. 1·6Ga (Bhandari et al.,2011), although there are reports of even older agesof 2·67Ga (garnet Sm^Nd date; Roy et al., 2006) and2·0^2·1Ga (monazite EMP date; Bhowmik et al., 2005).Broadly synchronous with the BM1 event was the emplace-ment of the magmatic protolith of the felsic gneiss host,the timing of which was constrained between 1·60Ga(monazite EMP date) and 1·58Ga (SHRIMP U^Pbzircon date) by Bhowmik et al. (2011). Magmatic zirconand monazite in the granite gneisses also record an over-printing high-temperature metamorphic recrystallizationevent between 1·57 and 1·56Ga (Bhowmik et al., 2011).Monazite from a psammo-pelitic granulite and a granitegneiss sample record a weak metamorphic recrystalliza-tion event between 1·47 and 1·42Ga (Bhandari et al., 2011;Bhowmik et al., 2011). This chronological evidence for twometamorphic recrystallization events post-dating the BM1

event can be tentatively correlated with the BM2^BM3

(between 1·57 and 1·56Ga) and BM4 (between 1·47 and1·42Ga) metamorphism.We selected four samples from the BBG domain for de-

tailed geochronological investigations: two pelitic granu-lite samples (B36H and B27E) for monazite EMP datingand one psammo-pelitic granulite (B42D) and one garne-tiferous granite (B233) sample each for coupled monaziteand SHRIMP U^Pb zircon dating. The massive granitesample, which was collected from a relatively low-straindomain that lacks BM2^BM3 metamorphic overprints,was specifically chosen to obtain its precise magmatic age.We additionally utilized the published monazite textural,compositional and EMP data from psammo-pelitic granu-lite sample B35A [see Bhandari et al. (2011) for details].

SAMPLE DESCR IPT IONSMetapelitic granulite samples B36H andB27EThese samples were collected from locations 20m apartnear Pipariya village (Fig. 2), where an interlayered sequenceof metapelitic granulite and metagabbro-norite (now two-pyroxene granulite) is exposed. Previous studies in this areareported in situ biotite dehydration melting in aluminousmetasediment, producing stromatic migmatite bandingwith abundant large peritectic garnet (Bhowmik, 2006).Sample B36H consists of garnetþ cordieriteþ silliman-

iteþK-feldsparþquartzþplagioclaseþbiotiteþmusco-viteþ rutile, with accessory ilmenite, monazite and zircon(monazite:zircon 41). Migmatite banding is demarcated

BHOWMIK et al. THERMAL HISTORYOF GRANULITE TERRANES

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Fig. 1. Location of the Bhandara^Balaghat granulite (BBG) domain at the southern margin of the Central IndianTectonic Zone (CITZ). Theblack square marks the study area, shown in detail in Fig. 2. The position of the Central Indian Shear/Suture (CIS) in this figure and Fig. 2 isafter Bhowmik et al. (2012). RKG refers to Ramakona-Katangi Granulite domain. Inset shows the location of the CITZ in the tectonic frame-work of the Archean cratonic blocks [North Indian Block (NIB) and South Indian Block (SIB)] and Proterozoic mobile belts [Aravalli^DelhiMobile Belt (ADMB) in the west; the CITZ in the center; the Chhotanagpur Gneissic Complex (CGC) in the east; the Shillong PlateauGneissic Complex (SPGC) in the NE; the Eastern Ghats Mobile Belt (EGMB) in the SE] of India. The NIB and SIB contain the Archeannuclei of Bundelkhand (BKN) and Singhbhum (SN)^Bastar (BN)^Karnataka (KN), respectively.


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by alternating layers of garnetþ cordieriteþ sillimanite andK-feldspar (perthitic)þ plagioclaseþquartz (Fig. 3a).Garnet porphyroblasts (�20 modal %) have inclusions ofprograde biotite (biotite1), fibrolitic sillimanite (sillimanite1)and quartz (Fig. 3b). Sillimanite in the matrix is coarse-grained and wraps around garnet (Fig. 3c) and cordierite.The latter, like garnet, also occurs as porphyroblasts(Fig. 3d). These porphyroblastic phases show equilibrium,straight grain boundary relationships with each other.There are also rare rutile porphyroblasts in the matrix.Garnet, cordierite and K-feldspar are separated from eachother by a retrograde biotite (biotite2)þ sillimanite (silli-manite2)þ quartz assemblage (Fig. 3d and e). K-feldspar isalso replaced by muscoviteþquartz symplectite (Fig. 3e).Sample B27E is mineralogically and texturally similar to

B36H, except that it lacks cordierite and rutile. Layers richin K-feldspar (perthitic) and quartz alternate with gar-netþ sillimanite-bearing layers. As in sample B36H, garnetand K-feldspar are separated by a symplectite consisting ofbiotite2þ sillimanite2þ quartz. K-feldspar is also exten-sively replaced by retrograde white mica and quartz.

Psammo-pelitic granulite sample B42DThis is a garnetþorthopyroxeneþplagioclase (antiper-thitic)þ quartzþK-feldsparþbiotiteþ anthophylliteþ il-meniteþmonaziteþ zircon psammo-pelitic granulite

from the Larsara area (Figs 2 and 4a). The rock is inter-leaved with sapphirineþ spinelþ sillimaniteþ garnetþcorundumþmesoperthite-bearing, silica-undersaturatedaluminous granulite, granulite-facies BIF and metamor-phosed gabbro-norite (two-pyroxene granulite), inter-preted to record UHT metamorphic conditions. Themineralogical evolution of this rock was previouslydescribed by Bhowmik et al. (2005) and Basu Sarbadkikari& Bhowmik (2008). The studied sample was taken from adiatexite migmatite outcrop where clusters of porphyro-blastic garnet, with or without orthopyroxene, occur in acoarse-grained mosaic of plagioclase (antiperthitic) andquartz. Despite overprinting by later recrystallization,local delicate intergrowths of plagioclase and quartz(Fig. 4a) indicate crystallization from anatectic melt.Porphyroblastic garnet (�20 modal %), locally with K-feldspar inclusions, is armoured by coronal orthopyrox-eneþplagioclase symplectite (Fig. 4b^d). Early stabilizedporphyroblastic orthopyroxene (orthopyroxene1) and leu-cosome plagioclase (plagioclase1) are distinguished fromcoronal orthopyroxene (orthopyroxene2) and plagioclase(plagioclase2). A coronal variety of garnetþquartz�K-feldspar symplectite occurs as 100^150 mmwide idioblas-tic overgrowths around porphyroblastic garnet againstorthopyroxene2 and plagioclase2. This variety (garnet2)can be distinguished from texturally early porphyroblastic

Fig. 2. Simplified geological map of the BBG domain, showing the locations of samples used in this study and previously published geochrono-logical sites. Geochronology data sources for metamorphic and magmatic events are superscripted as follows: 1, Bhandari et al. (2011); 2,Bhowmik et al. (2011).


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Fig. 3. (a^e) Back-scattered electron (BSE) images and photomicrographs of pelitic granulite sample B36H, showing key textural features.Mineral abbreviations used in this and other figures are after Kretz (1983). (a) BSE image showing the stability of the peak metamorphic assem-blage of porphyroblastic Grtþporphyroblastic CrdþKfsþ SilþQtz. These minerals are segregated in different layers of melanosome andleucosome, producing a granulite-facies migmatite banding. Also identified are locations of detailed textural features shown in Fig. 3b^e (rect-angles with bold black outlines) and monazite (Mnz) grains A, E, H and G (white or black circles). (b) Porphyroblastic Grt contains inclusionsof Qtz, Bt and Sil. (c) Grt is wrapped by a coarse foliation defined by Sil. (d) Porphyroblastic Grt and Crd are separated by retrogradeBtþSilþQtz assemblage. (e) Kfs is partially rimmed by MsþQtz symplectite and Bt.


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Fig. 4. (a) X-ray element and BSE images showing textural features of psammo-pelitic granulite sample B42D. (a) Map of the intensity of AlX-rays showing the occurrence of porphyroblastic Grt (shown within black and white dotted lines) and Opx as clusters within a mosaic of inter-grown Pl and Qtz. In the central part of the image, aggregates of coarse Pl contain rafts of Grt. In contrast, intergrowths of coarse Pl andQtz, resembling crystallized melt, occur in the lower left part of the image (shown in a white rectangle). Also shown are the locations ofFig. 4b^f (white rectangles) and monazite grains A, B and C (white open circles). It should be noted that there are minor distortions of Al X-ray intensity at the edges of the map. (b^d) Mg, Fe and Ca X-ray intensity images of a textural domain showing the occurrence of Opx2þPl2assemblage as a corona around porphyroblastic Grt (Grt1). A second generation Grt (Grt2) with Qtz intergrowths is Ca-rich and forms a meta-morphic overgrowth around Grt1 at its contact with Pl2 and Opx2. (e) BSE image showing the development of Grt2þQtz intergrowtharound Grt1by replacing coronal Pl2 and Opx2. Grt1/2 and Opx2 are partially and completely replaced by coronal Bt. Also shown is the locationof Fig. 4f. (f) Fracture-controlled replacement of Grt1 by Bt, which also affected monazite grain A.


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garnet (garnet1). K-feldspar associated with garnet1 andgarnet2 is designated as K-feldspar1 and K-feldspar2, re-spectively. Both generations of garnet and orthopyroxeneare locally replaced by retrograde biotite (Fig. 4e and f),with anthophyllite also replacing orthopyroxene1 andorthopyroxene2.

Mineral compositions in the granulitesSelected mineral compositions are presented in Table 1.Analyses were carried out using a CAMECA SX-100 elec-tron microprobe fitted with four spectrometers at theDepartment of Geology and Geophysics, Indian Instituteof Technology (IIT), Kharagpur. The analysis conditionsincluded 15 kV accelerating voltage, 15 nA beam currentand 1 mm beam size; full details of the analytical protocolhave been presented by Bhandari et al. (2011). In metapeli-tic granulite sample B27E, the cores of large porphyro-blastic garnet are magnesian (Prp29Alm67Grs03Sps01),whereas the rims against biotite2 are ferroan(Prp13Alm82Grs03Sps02) (Table 1). In psammo-peliticgranulite sample B42D, garnet1 is characteristicallypoorer in Ca (Prp31Alm62Grs06Sps01) than garnet2 (Prp31^28Alm64^61Grs12^10Sps01) (Table 1). The cores of coarseorthopyroxene1 are aluminous [Altot¼ 0·17 a.p.f.u. on a6(O) basis] and have XMg values of 0·57^0·54. Inclusionsof orthopyroxene1 and orthopyroxene2 within garnet1 andgarnet2, respectively, are less aluminous (Altot¼ 0·06^0·12) and more magnesian (XMg¼ 0·61^0·65). Relativelycoarser orthopyroxene2 grains are compositionally homo-geneous with intermediate Al (Altot¼ 0·08) and XMg

(¼ 0·57) contents. Irrespective of textural type, plagioclaseis compositionally homogeneous with XAn¼ 0·35^0·36.Biotite inclusions within garnet1 are more magnesian(XMg¼ 0·71) relative to coronal biotite (XMg¼ 0·67).

Granite sample B233This is a garnetiferous leucogranite from the Hurkitolaarea (Fig. 2), where metapelitic granulites occur as dis-rupted rafts and lenses within the granite. The analysedsample was taken from a coarse diatextite band, wherethe granite is host to a green spinel-bearing garnet^cor-dierite granulite enclave, �2m long and 1m wide.Sampled at a distance of 3m from the enclave, the graniteis composed of K-feldsparþquartzþplagioclaseþbio-titeþ garnet, with accessory monazite and zircon, and sec-ondary muscovite. The rock is undeformed, showing ahypidiomorphic granular texture, with sparse ovoid crys-tals of garnet in a quartzo-feldspathic groundmass. Thegarnets may represent incongruent solids formed from anin situ biotite melting reaction in the granulite enclave,derived from the break-up of the enclave at the site of em-placement, or they may represent xenocrysts entrainedfrom either the magmatic source region or the wall-rockduring emplacement.

METAMORPH IC REACT IONH ISTORY AND P^T CONDIT IONSTextural and compositional features allow us to reconstructthe metamorphic reaction history and also to comment onthe possible P^T paths recorded in these rocks.

Metapelitic granulite samples B36H andB27ETextural features indicate the stability of garnetþK-feld-sparþ sillimanite in B27E and garnetþ cordieriteþK-feldsparþ sillimaniteþ rutile in sample B36H at peakmetamorphic conditions (BM1P), following the two modelbiotite melting reactions:

biotiteþ plagioclaseþ sillimaniteþ quartz

! garnetþK-feldsparss þmeltð1Þ

and

biotiteþ plagioclaseþ sillimaniteþ quartz

! garnetþ cordieriteþK-feldsparss� ilmeniteþ rutileþmelt:

ð2Þ

Reaction (1), which produces peritectic garnet, requiresthe more ferroan bulk-rock composition of sample B27E,whereas the relatively more magnesian bulk compositionof sample B36H allows the formation of peritectic cordier-ite [reaction (2)]. Given the low rutile and ilmenite con-tents of sample B36H, the progress of reaction (2) and theP^T stability of garnetþ cordieriteþK-feldsparþ silli-maniteþmelt can be evaluated with a simplifiedNKFMASH petrogenetic grid (Spear et al., 1999). Thegrid predicts medium-pressure (P� 4^8 kbar) granulite-facies metamorphism with TMax4700^8408C. The grid,which is isoplethed with XFe [¼ Fe/(FeþMg)] contoursin garnet, also predicts a minimum temperature of 750^7708C at 6^9 kbar during the BM1P metamorphism forthe stability of the magnesian garnet cores (XFe¼ 0·70) inpelitic granulite sample B27E.Both these samples record the development of retrograde

biotiteþ sillimaniteþquartz and muscoviteþquartz sym-plectites, for which the following reactions are suggested:

garnetþ cordieriteþK-feldsparþmelt

! biotiteþ sillimaniteþ quartzð3Þ

garnetþK-feldsparþmelt! biotite

þ sillimaniteþ quartzð4Þ

and

K-feldsparþ sillimaniteþmelt! muscoviteþ quartz:

ð5Þ


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Table 1: Representative mineral chemical analyses in psammo-pelitic granulite (sample B42D) and pelitic granulite

(sample B27E)

Sample no.: B42D B42D B42D B42D B42D B42D B42D B42D B42D B42D B42D B42D B42D B42D B42D

Mineral: Grt1 Grt1 Grt2 Grt2 Grt2 Opx1 Opx1 Opx1 Opx2 Opx2 Opx2 Bt1 Bt2 Pl1 Pl1

Spot no.: 29-2 30-2 19-1 20-1 42-2 3-4 8-4 8-1 40-2 22-2 22-6 3-2 8-2 28-3 29-3

Textural setting: P(C) P(C) Ov(C) Ov(C) Ov4Opx1 P(C) P(C) I^Grt2 I^Grt2 COR(R) COR(R) I^Grt1 COR Meg(C) Meg(C)

SiO2 38·75 38·68 38·51 38·65 38·42 50·37 49·76 50·97 52·59 51·11 51·04 37·77 38·32 59·38 59·48

TiO2 b.d.l. b.d.l. b.d.l. b.d.l. 0·04 0·01 0·06 b.d.l. b.d.l. 0·08 0·07 2·84 2·8 0·01 b.d.l.

Al2O3 21·57 21·67 21·52 21·89 21·35 3·88 3·84 2·67 1·35 1·95 1·87 15·82 15·48 25·22 25·32

Cr2O3 0·09 0·2 b.d.l. 0·01 0·16 0·1 0·05 0·12 0·01 0·13 0·04 0·19 0·1 0·07 0·04

FeO* 28·59 29·46 28·56 29·41 30·48 26·19 28·39 24·74 23·56 26·71 26·99 12·33 13·37 b.d.l. 0·05

MnO 0·49 0·64 0·51 0·57 0·47 b.d.l. 0·13 0·16 0·1 b.d.l. 0·14 0·07 0·06 b.d.l. b.d.l.

MgO 8·11 8·21 7·12 6·79 6·5 19·21 18·53 21·43 22·77 20·05 19·83 17·22 15·52 b.d.l. b.d.l.

CaO 2·12 2·16 4·19 4·28 3·85 0·25 0·23 0·12 0·23 0·21 0·16 b.d.l. b.d.l. 7·61 7·49

Na2O b.d.l. b.d.l. b.d.l. b.d.l. 0·02 b.d.l. b.d.l. 0·02 0·03 b.d.l. b.d.l. 0·06 0·13 7·56 7·65

K2O 0·01 0·01 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0·04 0·02 0·02 b.d.l. 9·07 9·03 0·13 0·14

Total 99·73 101·03 100·41 101·6 101·29 100·01 100·99 100·27 100·66 100·26 100·14 95·37 94·81 99·97 100·16

(O) 12 12 12 12 12 6 6 6 6 6 6 11 11 8 8

Si 3·011 2·971 2·982 2·966 2·97 1·904 1·876 1·903 1·944 1·928 1·931 2·758 2·837 2·654 2·653

Ti – – – – 0·002 – 0·002 – – 0·002 0·002 0·156 0·156 – –

Al 1·976 1·963 1·965 1·98 1·946 0·173 0·171 0·117 0·059 0·087 0·083 1·361 1·351 1·329 1·331

Cr 0·006 0·012 – 0·001 0·01 0·003 0·002 0·003 0·056 0·004 0·001 0·011 0·006 0·002 0·001

Fe 1·858 1·893 1·85 1·887 1·971 0·827 0·896 0·773 0·672 0·843 0·854 0·753 0·828 – 0·002

Mn 0·032 0·042 0·034 0·037 0·031 – 0·004 0·005 0·003 – 0·005 0·004 0·004 – –

Mg 0·939 0·94 0·822 0·777 0·749 1·082 1·041 1·192 1·254 1·127 1·118 1·873 1·713 – –

Ca 0·177 0·178 0·348 0·352 0·319 0·01 0·009 0·005 0·009 0·009 0·007 – – 0·365 0·358

Na – – – – 0·003 – – – 0·002 – – 0·009 0·018 0·655 0·662

K 0·001 – – – – – – 0·002 0·001 0·001 – 0·845 0·853 0·008 0·008

Sum 7·999 8 8 8 8 4 4 4 4 4 4 7·771 7·766 5·013 5·015

Sample no.: B42D B42D B42D B42D B42D B42D B42D B27E B27E

Mineral name: Pl1 Pl1 Pl1 Pl2 Pl2 Kfs Kfs Grt Grt

Spot no.: 30-3 22-2 14-2 1-200 1- 191 38-3 41-2 23 17

Textural setting: Meg(R) I^Grt1 I^Grt1 COR(C) COR(R) I^Grt1 I^Grt2 P(C) P(R)

SiO2 59·39 58·78 58·83 59·01 59·03 64·22 63·76 38·54 37·34

TiO2 0 0·02 0 0 0 0 0 0 0·03

Al2O3 25·45 25·23 25·53 25·45 25·57 18·14 18·79 22·48 21·63

Cr2O3 b.d.l. b.d.l. 0·06 b.d.l. 0·05 b.d.l. b.d.l. 0·11 b.d.l.

FeO* 0·17 0·19 0·23 0·08 0·27 0·65 1·03 30·72 36·04

MnO b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0·63 1·11

MgO b.d.l. b.d.l. 0·03 b.d.l. b.d.l. 0·02 b.d.l. 7·35 3·11

CaO 7·54 7·37 7·43 7·47 7·49 b.d.l. 0·41 1·2 1·19

Na2O 7·69 7·56 7·5 7·41 7·27 1·08 1·51 0·01 0·02

K2O 0·08 0·1 0·05 0·08 0·1 14·95 14 0·01 0·04

Total 100·31 99·25 99·65 99·5 99·78 99·06 99·49 101·05 100·51

(O) 8 8 8 8 8 8 8 12 12

Si 2·646 2·646 2·637 2·647 2·641 2·988 2·951 2·975 2·985

Ti – – –. – – – – – –

Al 1·337 1·339 1·349 1·346 1·349 0·995 1·025 2·045 2·04

Cr – – 0·002 – 0·002 0·025 0·04 0·005 –

Fe 0·006 0·007 0·009 0·003 0·01 – – 1·985 2·41

Mn – – – – – – – 0·04 0·075

Mg – – 0·002 – – 0·001 – 0·845 0·37

Ca 0·36 0·356 0·357 0·359 0·359 – 0·02 0·1 0·1

Na 0·664 0·66 0·651 0·644 0·631 0·098 0·136 – 0·005

K 0·005 0·006 0·003 0·005 0·005 0·887 0·826 – 0·005

Sum 5·018 5·015 5·01 5·004 4·997 4·994 4·998 7·995 7·99

*Total Fe given as FeO.Mineral abbreviations in this and other tables are after Kretz (1983). P(C)/P(R), porphyroblastic core/rim; COR, corona; Meg(C)/Meg(R),megacrystic core/rim; ^, against; b.d.l., below detection limit (see text for Grt1 and Grt2 nomenclature).


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These reactions, when evaluated using the petrogeneticgrid, indicate rehydration of the granulite-facies anhydrousminerals as a consequence of H2O released from the crys-tallizing anatectic melt (Spear et al., 1999) during a phaseof post-peak cooling (cf. BM1R). The relatively high modalabundance of porphyroblastic garnet (�20 modal %) alsosuggests incomplete hydration of the peak granulite-faciesassemblage as a consequence of the loss of melt during orafter peak granulite-facies metamorphism (e.g. White &Powell, 2002; Guernina & Sawyer, 2003). The appearanceof muscovite via reaction (5) took place after the granuliteterrane had cooled toT �7008C.

Psammo-pelitic granulite sample B42DTextural and compositional features indicate a more com-plex metamorphic evolution for sample B42D, involvingtwo stages of garnet growth, an intervening phase ofgarnet decomposition, and a terminal hydration event.The peak metamorphic assemblage of garnet1þorthopyr-oxene1 could be produced following a biotite dehydrationmelting reaction of the type:

biotite1 þ plagioclase1 þ quartz! garnet1þ orthopyroxene1 �K-feldsparþmelt:

ð6Þ

This reaction occurs in intermediate to ferroan bulk-rock compositions at lower crustal depths and atT� 850^9008C (Stevens et al., 1997; Nair & Chacko, 2002; Johnsonet al., 2008). Such a highTof biotite dehydration melting isalso inferred from the relatively aluminous composition oforthopyroxene1 and the general absence of K-feldspar inthe leucosome. Whereas the K-feldspar-free nature of theleucosome can be attributed to a low potassium content,the rare presence of K-feldspar within garnet1 suggeststhat the former, stabilized as reaction (6), was intersectedatT� 8008C; but reacted later at an elevated temperature,leading to its complete dissolution in the melt. Such an in-terpretation finds support from recent studies involvingphase diagram calculations (P^T pseudosections) for avariety of pelitic and greywacke bulk-rock compositions(e.g. Johnson et al., 2008). The results of these studies showthat the absence of K-feldspar in the leucosome could berelated to aTMax that exceeded that of the K-feldspar-outcurve, which for these protolith compositions varies be-tween 850 and 9008C.Reaction (6) was followed by the decomposition of

garnet1 to an intergrowth of orthopyroxene2þ plagio-clase2, which suggests the model reaction:

garnet1 þ quartz! orthopyroxene2 þ plagioclase2: ð7Þ

Regrowth of garnet (garnet2), intergrown with quartz,with or without K-feldspar, and replacing orthopyroxene2and plagioclase2, suggests the operation of the followingtwo reactions:

orthopyroxene2 þ plagioclase2! garnet2 þ quartz ð8aÞ

and

biotiteþ plagioclase2 þ quartz! garnet2þK-feldsparþH2O=melt:

ð8bÞ

The formation of K-feldspar with garnet2 demands bio-tite to be a reactant [reaction (8b)], which could have sta-bilized as one or more of the following: (1) remnants ofprograde biotite in reaction (6); (2) a product of hydrationduring melt crystallization along a post-peak coolingpath; (3) a replacement product of orthopyroxene2.Because of the multi-stage metamorphic history of sampleB42D it is not possible to discriminate between these. Aterminal hydration event affected both varieties of garnetand orthopyroxene, producing late retrograde biotite andanthophyllite.Following Basu Sarbadhikari & Bhowmik (2008), we

correlate the growth of peritectic garnet1 and orthopyrox-ene1 to peak BM1P, orthopyroxene2þ plagioclase2 to BM2,garnet2þ quartz�K-feldspar2 to peak BM3P, and late bio-tite and anthopyllite to retrograde BM3R in the BBGdomain.Mineral compositions in sample B42D allow estimation

of P^T conditions for BM1 and BM3 metamorphismusing geothermobarometry on the assemblage garnetôrthopyroxene^plagioclase^quartz (GOPS). Because thecomposition of garnet in equilibrium with the orthopyrox-ene2þ plagioclase2 symplectite is unlikely to have survivedoverprinting by BM3 metamorphism, we have not calcu-lated the P^T conditions for BM2 metamorphism. Thecombination of magnesian cores in garnet1, aluminouscores in orthopyroxene1 and plagioclase appears to repre-sent an equilibrium assemblage for the peak BM1 meta-morphism. The assemblage of grossular-rich garnet2 andthe rim compositions of contact coronal orthopyroxene2and plagioclase2 is used to calculate the peak P^T condi-tion for the BM3 metamorphism.Temperatures were estimated using garnetôrthopyrox-

ene Fe^Mg exchange thermometry (Ganguly et al., 1996)and orthopyroxene Al-thermometry (Harley & Green,1982; Aranovich & Berman, 1997; Pattison et al., 2003). Forthe exchange and the first two orthopyroxene Al-therm-ometersTwas calculated at a reference pressure, the valueof which was obtained by applying the calibration ofPattison et al. (2003). This is an Fe^MgÂl thermobarom-eter that uses a convergence technique to account for lateFe^Mg exchange, and allows simultaneous estimation ofboth P andT. For this, we have used model 2 in the cali-bration of Pattison et al. (2003) for estimating the mole frac-tion of Al(VI) in orthopyroxene [XAl (Opx)¼ (Al/2)/2].Pressures were calculated using the Fe (GAFS: Moecheret al., 1988; Essene, 1989) and Mg (GAES: Newton &Perkins, 1982; Essene, 1989; Eckert et al., 1991) end-memberGOPS barometric formulations.The results of these calcula-tions are presented inTable 2.


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Fe^Mg exchange thermometry yields temperatures inthe range 860^9358C for BM1metamorphism.The calibra-tion from Pattison et al. (2003) gives a consistent tempera-ture of �8858C, which is nearly identical to that obtainedusing the method of Harley & Green (1982). The calibra-tion of Aranovich & Berman (1997) yielded higher T inthe range of 980^10208C. Because the calibrations ofPattison et al. (2003) and Harley & Green (1982) have con-sidered adjustments for natural garnet and orthopyroxenecompositions by adopting activity^composition relation-ships, our preferred peak BM1 temperature estimate isbased on these two thermometers. The estimated pressuresfor the same metamorphism at a reference temperature of�9008C range from 7·2 to 9·6 kbar.This spread in pressureestimates is dependent on the chosen calibration as Feend-member barometers (PE(Fe), PM and PP) give higherpressures (8·4^8·8 bar) than Mg end-member barometers(PNP and PEc) (7·2^8·1kbar), a feature also observed inprevious studies (e.g. Zulbati & Harley, 2007). The excep-tion is, however, the pressure estimate from the Mg end-member calibration of Essene (1989), which records thehighest pressures in the range 9·2^9·6 kbar. Given theferroan compositions of the garnet^orthopyroxene assem-blages in this rock and the greater degree of sensitivity ofthe Mg end-member GOPS barometer to mineral compos-ition adjustments, pressure estimates from Fe end-membercalibrations are preferred here. Because the barometric for-mulation of Pattison et al. (2003) has also incorporatedcompositional adjustments to remove the effects of lateFe^Mg exchange, the results from this calibration are con-sidered reasonable. Given these caveats, the average P^T

condition for the peak metamorphism (BM1P) is placed at8·7�0·6 kbar, 880�358C (error 2s). When comparedwith previous temperature estimations from the sameLarsara Hill area (Bhowmik et al., 2005; Bhowmik, 2006),

this result for psammo-pelitic granulite sample B42D islower by 70^1208C, and should be treated as a minimumtemperature estimate for BM1P metamorphism. Using thesame formulations, the peak P^T conditions for the BM3

(BM3P) metamorphism are constrained at 9� 0·8 kbar,740�508C. A summary of the metamorphic conditions re-corded in these rocks is presented inTable 3.

ANALYT ICAL METHODSMonazite EMP datingMonazite grains from samples B36H, B27E, B42D andB233 were analyzed in situ by electron probe microanalysis(EPMA) using a Cameca SX100 system and a JEOL JSM6490 scanning electron microscope (SEM) at theDepartment of Geology and Geophysics, Indian Instituteof Technology, Kharagpur. Based on the results of back-scattered electron (BSE) imaging with the SEM andX-ray element imaging by EPMA, monazite grains wereselected for spot chemical analyses. Analytical proceduresare described below; further details have been presentedby Pant et al. (2009) and Bhowmik et al. (2010).For chemical mapping of monazite, we generated X-ray

element maps for Y, Th and U at 20 kV and 100 nA, with50ms dwell times and a step size of 0·1 mm at a resolutionof 512�512 pixels (for grains D, E, G, H and O of sampleB36H and grains K, M and N of sample B27E). For grainA of sample B36H, the step size was 0·2 mm and the reso-lution was 600� 600 pixels, whereas for sample B42D, thestep size and resolution for grains A, B and C were 0·1 mmand 300�300 pixels, respectively. A high-resolutionX-ray element map for Pb was additionally generated forgrain B in sample B42D at 20 kV and 150 nA, with 70msdwell time and a step size of 0·1 mm at a resolution of460�500 pixels.

Table 2: Results of geothermobarometry in psammo-pelitic granulite sample, B42D

Grt–Opx thermometry Grt–Opx–Pl–Qtz barometry

TG TP THG TAB PRef. PNP PEc PE(Mg) PE(Fe) PM PP TRef.

862 882 866 983 8 7·8 7·3 9·2 8·8 8·8 8·8 900

936 883 877 1019 8 8·1 7·6 9·6 8·5 8·6 8·4 900

852 884 866 981 8 7·7 7·2 9·2 8·8 8·8 8·7 900

925 885 878 1015 8 8·0 7·5 9·6 8·5 8·6 8·5 900

820 741 734 889 8 9·0 7·7 10·7 9·1 9·0 8·9 750

783 757 733 879 8 8·9 7·4 10·4 9·1 9·0 9·1 750

Abbreviations used for T (8C) and P (kbar) estimate: PRef./TRef., reference P and T; TG, THG and TAB,T calculated using the calibrations of Ganguly et al. (1996), Harley & Green (1982) and Aranovich &Berman (1997), respectively; PNP, PEc, PM and PE, P calculated using calibrations of Newton &Perkins (1982), Eckert et al. (1991), Moecher et al. (1988) and Essene (1989), respectively; Tp andPp, temperature and pressure estimates following Pattison et al. (2003).


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Spot analysis of monazite was conducted at an accelerat-ing voltage of 20 kV, a beam current of 200 nA, and abeam size of 1 mm. X-ray data were calibrated againstwell-characterized natural and synthetic standards. Thespecific standards used for monazite analysis were: galenafor Pb; UO2 for U; ThO2 for Th; synthetic silica^alumin-ium glass containing 4% REE for La, Ce, Nd, Pr, Sm,Ho, Dy and Gd; apatite for P and Ca; yttrium aluminiumgarnet (YAG) for Y; corundum for Al; hematite for Fe;Th-glass for Si. Whereas the analyses of Pb, U and Thwere used to calculate U^Th^Pb ages, the remainingelements allowed a more accurate X-PHI matrix reduction

of the data.This larger array of elements also allowed com-prehensive grain compositional characterization, which isuseful for interpreting the growth and dissolution behav-iour of monazite and in separating complex monazite agepopulations.X-ray lines selected for age analysis were Y La, Pb Ma,

U Mb and Th Ma. Pb Ma was measured on LPET; thepeak counting time was 300 s and the background wasmeasured on both sides for 150 s. For uranium, the U Mbline was used to avoid interference with the Th Mb line;the counting time was 200 s on the peak and 100 s on thebackground. The thorium Ma peak was also counted for

Table 3: Summary of metamorphic conditions of analysed rock types, monazite textural location and its compositional

features

Metamorphic/magmatic mineral assemblage Mnz grain Textural location Compositional domains Thermal history

Pipariya, pelitic granulite, sample B36H

GrtþCrdþSilþKfsþQtz (BM1P) H Melanosome CD1 ! CD2 ! CD3 ! CD4 BM1: medium-P granulite-

BtþSilþQtz (BM1R) O Leucosome CD1 ! CD2 ! CD3 ! CD4 facies metamorphism

MsþQtz (BM1R) D Leucosome CD1 ! CD2 ! CD3 ! CD4

A Leucosome CD2 ! CD3 ! CD4

E Melanosome CD2 ! CD3 ! CD4

G Inclusion in BM1 Grt CD2 ! CD4

Pipariya, pelitic granulite, sample B27E

GrtþSilþKfsþQtz (BM1P) K Melanosome CD2 ! CD3 ! CD4 BM1: medium-P granulite-facies

BtþSilþQtz (BM1R) M Leucosome CD2 ! CD3 ! CD4 metamorphism

MsþQtz (BM1R) N Leucosome CD4

Larsara Hills, psammo-pelitic granulite, sample B42D

Grt1þOpx1�Kfs1þPlþQtz (BM1P) B Inclusion in BM1 Grt CD1 ! CD4 BM1P: 8·7 kbar, 8808C

Opx2þPl2�Bt (BM2) C Inclusion in BM1 Grt CD1 ! CD2 ! CD4 BM3P: 9 kbar, 7408C

Grt2þQtzþKfs2 (BM3P) A Inclusion in BM1 Grt CD5 ! CD6

Bt�Ath (BM3R)

Dongargaon, psammo-pelitic granulite, sample B35A*

GrtþCrdþOpxþPlþQtzþ IlmþRt (BM1P) B Inclusion in BM1 Crd CD4 ! CD5 BM1P: 8·1 kbar, 9308C

L Leucosome CD2 ! CD4 ! CD5

J Leucosome CD4 ! CD5

Hurkitola, granite, sample B233

KfsþQtzþPlþBtþGrtþ IlmþMs(secondary) 1 Interstitial to Kfs–Pl–Qtz CD4 Associated GrtþCrdþSpl

4 Included in Kfs CD4 granulite recorded medium-P

5 Interstitial to Pl–Kfs CD4 granulite-facies metamorphism

6 Interstitial to Bt–Pl–Qtz CD4

7 Included in Ilm CD4

*Textural and P–T data for B35A are from Bhandari et al. (2011).Mineral abbreviations after Kretz (1983). BM1–2–3(P/R), stages of metamorphism at peak (P) and retrograde (R) conditions;CD1–CD6, compositional domains of Mnz (see text and Table 5 for further details); arrows with compositional domainsindicate paragenetic sequence of Mnz formation. Interpretation of Mnz compositional domains and their sequences isfrom this study.


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200 s, with 100 s measured on the background. Both U andTh were measured on PET crystals. The REE measuredand the X-ray lines used were La (La), Ce (La), Nd (Lb),Pr (Lb), Sm (La), Ho (Lb), Dy (La) and Gd (Lb).Background positions were selected using WDS spectra,taking care to avoid major interferences. The backgroundcount rate was interpolated using a linear equation con-structed with the two background positions and the re-spective count rates. Counting time for REE at the peakvaried between 40 and 60 s. TheY La line was used for yt-trium and was counted for 40 s on the peak. Interferenceof Y Ll on Pb Ma, Th Mz on Pb Ma and of Th Mg onU Mb were corrected after measuring the interferinglines during calibration and thereafter applying the over-lap correction. Detection limits at 200 nA were 155 ppmfor Y, 145 ppm for Pb, 380 ppm for Th and 300 ppm for U.Further details of the monazite analytical protocol are pre-sented inTable 4.Prior to and during analytical sessions, analyses of exter-

nal monazite age standards were carried out periodicallyto monitor short-term systematic error and to check foroverall accuracy and reproducibility of results betweenthe sessions. The monazite age standard (monazite30625A) used in this study is from a collection of Moacyrmonazite from Brazil. Monazite 30625A yielded a207Pb/235U date of 483�1Ma and a 206Pb/238U date of487Ma [isotope dilution thermal ionization mass

spectrometry (ID-TIMS) method; Crowley et al., 2005].However, there are reports of older dates from other separ-ates of Moacyr monazite [e.g. aTIMS 207Pb/206Pb date of509·3�0·5Ma and 206Pb/238U date of 517Ma, quoted bySpear et al. (2009) and an ID-TIMS 208Pb/232Th date of506�1·0Ma, 207Pb/235U date of 506·7�0·8Ma and206Pb/238U date of 515·2�0·6Ma, quoted by Dumondet al. (2008)]. These age differences appear to suggest thatmore than one monazite age population may be presentin Moacyr monazite (Spear et al., 2009). Our analyses ofthis monazite age standard yielded a long-term weightedmean age of 506·6�5·5Ma (2s error, MSWD¼ 0·28,n¼ 39), which, although it deviates from some of the pub-lished ages, is still within the precision limit of Moacyrmonazite ages (e.g. EMP age of 497�10Ma and isotopicage of 509·3�0·5Ma; Spear et al., 2009) reported in the lit-erature.The close agreement of the EMP ages of our stand-ard (507Ma) and that determined at the laboratory ofRensselaer Polytechnic Institute, USA (‘Moacyr Middle’monazite 497�10Ma) is also reflected in broadly thesame range of measured Pb by EPMA in these laboratories(IIT-KGP: mean 1592�62 ppm, 2s error, n¼ 39, thisstudy; Rensselaer Polytechnic Institute: range 1600�58to 1622�58 ppm, Table 6 of Spear et al., 2009). Thecalculated mean age of the monazite standard(mean¼ 513�16Ma, MSWD¼ 0·067, n¼ 4) in two ana-lytical sessions of this study is within 1·3% of its long-term

Table 4: Operating conditions for electron microprobe analysis with CAMECA SX100 machine at the IIT Kharagpur

laboratory

Element X-ray line Crystal Standard Spectral

position

Peak

time (s)

Bg time (s) Bg Off1 Bg Off2 Intensity Detection

limit (ppm)

Al Ka TAP Corundum 32458 20 10 600 1785·5 90

Si Ka TAP Th glass 27736 20 10 600 870·4 75

P Ka LPET Apatite 70355 20 10 500 168·1 115

Ca Ka PET Apatite 38384 20 10 500 439·6 130

Fe Ka LIF Hematite 48084 10 5 500 189·9 615

Y La TAP YAG 25108 40 20 600 485·4 155

La La LPET Si–Al glassþ 4% REE 30466 40 20 500 59·1 130

Ce La LPET Si–Al glassþ 4% REE 29278 40 20 500 61·1 135

Pr Lb LIF Si–Al glassþ 4% REE 56084 40 20 6230 2·1 595

Nd Lb LIF Si–Al glassþ 4% REE 53807 40 20 1700 2 865

Sm La LIF Si–Al glassþ 4% REE 54621 40 20 500 3·5 465

Gd Lb LIF Si–Al glassþ 4% REE 45861 40 20 2000 2·2 755

Dy La LIF Si–Al glassþ 4% REE 47394 60 30 400 3·5 450

Ho Lb LIF Si–Al glassþ 4% REE 40926 60 30 1170 1·9 865

Pb Ma LPET Galena 60406 300 150 –1000 2000 264·3 145

Th Ma PET ThO2 47265 200 100 600 88·9 380

U Mb PET UO2 42439 200 100 –996 774 142·7 300

Bg, Background.


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average. Summarizing, EPMA-determined monazite datafrom the Kharagpur laboratory can be used for age deter-minations. Relative probability plots and weighted meanages of monazite were computed with the Isoplot program(Ludwig, 2003).

SHRIMP U^Pb zircon geochronologyThe zircon separation and processing techniques are iden-tical to those described by Bhowmik et al. (2011). U^Pbanalyses were conducted using the WA ConsortiumSHRIMP IIA ion microprobe housed at CurtinUniversity. Detailed analytical procedures have beendescribed by Nelson (1997) and Williams (1998). Isotopicratios were monitored by reference to Sri Lankan gemzircon standard (CZ3) with a 206Pb/238U ratio of 0·0914that is equivalent to an age of 564Ma. Pb/U ratios in theunknown samples were corrected using the ln(Pb/U)/ln(UO/U) relationship as measured on CZ3. All ageshave been calculated from the U and Th decay constantsrecommended by Steiger & Ja« ger (1977) and were cor-rected using the measured 204Pb. The analytical data werereduced, calculated and plotted using the Squid (v. 1.0)and Isoplot (v. 3.0) programs (Ludwig, 2003), although thedata tables were constructed using the Krill 007 programof Peter Kinny at Curtin University. Reported ages(41000Ma) are quoted using the 207Pb/206Pb data(Table 3). Single analyses in the data table and concordiaplots are presented with 1s errors, whereas uncertaintiesin weighted mean ages are quoted at the 95% confidencelevel (2s), unless otherwise indicated.

CHEMICAL COMPOSIT IONALAND GEOCHRONOLOGICALRESULTSMonazite EMP datingA total of 18 monazite grains [nine from two pelitic granu-lite samples B36H (six grains) and B27E (three grains),three from psammo-pelitic granulite sample B42D and sixfrom granite sample B233] were selected and analyzed.The textural settings and compositional features of theseanalyzed grains, together with those from the publisheddataset of a psammo-pelitic granulite (three grains fromsample B35A) are summarized in Tables 3 and 5.Compositional features and age information for severalrepresentative monazite grains (grains O, D and A fromsample B36H; grains B and A from sample B42D; grainK from sample B27E; grain B from sample B35A; grains1, 4, 5, 6 and 7 from sample B233) are shown in Figs 5and 6, and those of the remaining monazite grainsare presented as supplementary data (Figs A1 and A2)(supplementary data are available for downloading athttp://www.petrology.oxfordjournals.org) The complete

monazite compositional dataset is presented inSupplementary DataTable A1.

Pelitic and psammo-pelitic granulitesCombined BSE and X-ray element (Y,Th and U) imaging(Figs 5 and 6) shows that the majority of monazite grainsare texturally and compositionally zoned; most having acore^rim relationship. In this study, the core^rim termin-ology of zoned monazite (and also of zircon; see below) isused in a broad sense, with ‘cores’ being taken as texturallyolder domains that are mostly, but not solely, located atthe center of grains. As will be shown below, the cores arecommonly complex, with the presence of several domains.In contrast, texturally younger domains that surroundand/or truncate the cores are called rims. Based on the tex-tural and compositional features of the cores and rims of15 monazite grains from four granulite samples, six com-positional domains (CD) have been recognized, whichform a paragenetic sequence from CD1 to CD6 (Table 5).

Compositional domain CD1

This domain, which is present in five of the 15 monazitegrains examined in detail from the granulites, is depletedin Th (Fig. 5c, h and m) but enriched in Y (Fig. 5b, gand l) and U (Fig. 5d, i and o). Spot monazite ages arecharacteristically older than c. 1·6Ga, with a range from1·69 to 1·89Ga (Fig. 5e, j and p).


In three of the five monazite grains where CD1 is present,an intermediate compositional domain, which forms dis-continuous seams on CD1, occurs as part of a compositecore (grain O, Fig. 5e; grain D, Fig. 5j). It is enriched inY(Fig. 5b and g) similar to CD1, but differs from the latterin having elevated Th concentrations (Fig. 5c and h; alsograin H in Supplementary Data Fig. A1b and c). In six ofthe remaining 10 monazite grains, the innermost coreregion is composed entirely of compositional domain CD2(e.g. grain A, Fig. 6a^e; grain K, Fig. 6f^j; see Table 5 andSupplementary Data Fig. A1j^x for further examples). Thiscompositional domain yielded younger monazite spotages than CD1 in the range 1·57^1·67Ga (Fig. 6e and j,and Supplementary DataTable A1).


In seven monazite grains, CD3 constitutes an outer coreregion, 2^10 mm in thickness, which shows progressive de-pletion in Y towards the rim (Figs 5b, g and 6b).Monazite spot ages in the range of 1·59^1·64Ga(Supplementary DataTable A1) were obtained from CD3,and are similar to those of CD2.


In 11 of the 15 monazite grains from the granulites, thecores are surrounded by a rim that is uniformly low in Y


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Table5:

Classificationofmonazitecompositionaldomains

ingranulites

Compositional

domains,

sample

Domainmorphologyan

dtexturalzonation

Compositional

attributes

Interpretation

I.CD1:

B36H

Highly

variab

lein

size,shap

ean

dinternal

organ

ization;gen

erally

assm

allrelics

withscalloped

toirregularoutlines

intheinteriorofovalto

rectan

gular-

shap

edco

resofco

mpositionally

zoned

Mnzgrains(grainsH,O,D).

The

domainis

either

surrounded

bysymmetricorasym

metricdistributionofra-

dially

outw

ardco

ncentric

layers

ofCD2an

dCD3domainsoris

sharply

abutted

against

domainCD4(grainsH,O,D)

Low

Th(ThO2¼1–4wt%);

highY

(Y2O3¼1–3);low–h

ighU

(UO2¼0·2–

0·8);Th/U¼1–11;HREE-enrich

ed[(Dy/

Sm) N¼0·08–0·11]

Pre-m

etam

orphic

(w.r.t.BM

1even

t)

detritalco

re

B42D

Single

(grain

B)to

multiple

isolatedrelics(grain

C)in

theoval-shap

edco

resof

compositionally

zoned

Mnzgrains

II.CD2:

B36H,B27E,

B35A

andB42D

Rem

nan

tpatch

eswithresorbed

grain

outlines

atthecenters

ofoval-shap

ed

cores(A

,Ein

B36H;K,M

inB27E;Lin

B35A),

asthin

layers,radially

outw

ardto

CD1(H

,O,D

inB36H),

asoval-shap

edgrainswith(C

inB42D)

orwithout(G

inB36H)relicsofCD1.

CD2is

surrounded

byCD3orCD4.

HighTh(5–8);highY(1–2);U

variab

lefrom

0·3to

1·3;

Th/U

(11–34);

mod–h

igh

HREEen

rich

men

t[(Dy/Sm) N¼0·04–

0·11]

Recry

nlinkedto

1ststag

eofdissol-

ution–rep

ptn

ofdetritalco

re(pro-

BM

1even

t)

III.CD3:

B36H

andB27E

Outerm

ost

layerin

ovalto

rectan

gular-shap

edco

resofco

mpositionally

zoned

Mnzgrainseither

maintainingtheco

reoutline(O

,D,A,Ein

B36H;M

in

B27E)oritsouterboundaryag

ainst

CD4is

serrated

(Hin

B36H)

HighTh(5–8);

low

Y(0·7–b

.d.l.);low

U

(0·3–0·4);

Th/U¼18–73;

HREE-dep

leted

[(Dy/Sm) N50·02]

2ndstag

eofdissolution–rep

ptn

ofCD1–

CD2co

reduring1s

tap

pearance

of

subsolidusorperitecticGrt(pro-BM

1)

IV.CD4:

B36H,B27Ean

d

B35A

Rim

aroundvariouslyresorbed

CD1–CD2–CD3(H

,O,D

inB36H),CD2–CD3(A

,

Ein

B36H;M

inB27E)an

dCD2(K

inB27E;Lin

B35A)co

res;

asco

mbined

veinsan

drims(G

inB36H);

rarely

ashomogen

eousgrains(N

inB27E;Jin

B35A)

HighTh(3–11);low

Y(b.d.l.);low

U(0·1–

0·5);Th/U¼11–140;HREE-dep

leted

[(Dy/Sm) N50·02]

Form

edas

peritecticmineral

atBM

1Por

duringmeltcrystallizationat

orim

-

med

iately

followingBM

1P

B42D

Asouterlayerofoval-shap

edco

rearoundCD1(grain

B)

V.CD5:

B42D

andB35A

Coarse

Mnzco

mpositional

domainwithfaintoscillatory

zoningan

dpartially

preserved

euhed

ralcrystaloutlineeither

attheco

reofco

mpositionally

zoned

Mnz(A

inB42D)oras

single

grainswithrelicsofCD4(B

inB35A);rarely

as

thin

rim

aroundco

mposite

CD2–CD4(L

inB35A)orCD4(J

inB35)co

m-

positional

domains

ExtremelyhighTh(11–22);

low

Y(0·3–

b.d.l.);low–m

odU

(0·1–0·6);

Th/U

(36–

160);[(Dy/Sm) N¼0·04–0·11]

Growth

duringfinal

freezingofmelt

duringBM

1Rat

T�8008C

VI.CD6:

B42D

Asmetam

orphic

overgrowth

aroundCD5

Higher

Yan

dHREEthan

CD5

Growth

linkedto

decompositionofGrt

duringBM

3Rhyd

ration

Mod,moderate;

Recry

n,recrystallization;pro-,

prograde;

ppt,

precipitation;repptn,reprecipitation.


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Fig. 5. BSE and Y, Th and U X-ray element images of representative monazite grains from metapelite sample B36H (grains O and D) andfrom psammo-pelite granulite sample B42D (grain B), showing core and rim domains. The cores of ellipsoidal to ovoid shape are marked by


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(Figs 5b, g and 6b, g) and U (Figs 5d, i and 6d, i), but en-riched inTh (Figs 5c and 6c) relative to the core. The rimis generally oval-shaped (grains O, D and B, Fig. 5a^p;grains A and K, Fig. 6a^j) and shows rare partially de-veloped crystal faces (e.g. grain H; upper right corner ofSupplementary Data Fig. A1b^d).Boundaries between the rim and core domains are

highly variable; being ovoid (Figs 5e, j, p and 6e), amoeb-oid, or curvilinear (Fig. 6j). Because of these properties,the monazite rim is inferred to mark a distinct event thatis younger than CD3. Monazite spot ages show a largespread from 1·70 to 1·52 (Figs 5e, j, p and 6e, j; andSupplementary DataTable A1).Two monazite grains in psammo-pelitic granulite

sample B42D show CD4 directly around CD1 (grain B,Fig. 5k^p; grain C, Supplementary Data Fig. A1f^i). Onecharacteristic feature of CD4 in grain B is its compos-itional heterogeneity in Pb (PbO¼ 0·78^0·96wt % in thelower part vs 0·57^0·70wt % in the upper part of Fig. 5n)and U (U2O3¼0·43^0·59wt % in the lower part vs 0·12^0·30wt % in the upper part in Fig. 5o). Lower values ofPb and U in the CD4 rim of grain B are comparable withthose from the same compositional domain in samplesB36H and B27E (Supplementary Data Table A1). Thiscompositional domain with elevated Pb and U concentra-tions records Paleoproterozoic spot ages (range 1·68^2·14Ga) (Fig. 5n and p), which are older than the latestPaleoproterozoic to Early Mesoproterozoic ages obtainedfrom the relatively lower Pb- and U-bearing parts of thesame grain (Fig. 5n and p). As mentioned above, the latestPaleoproterozoic to Early Mesoproterozoic ages are char-acteristic of CD4 in samples B36H and B27E. We discussthe possible reasons for this below.


Four of the 15 monazite grains, all from psammo-peliticgranulite samples B35A and B42D, show the presence ofan extremely thorian (ThO2¼14·1^22·1wt %) componenteither as a thin rim around CD4 (grains L and J,Supplementary Data Fig. A2a^h) or as coarse crystals(grain B, Fig. 6k^n) with or without (grain A, Fig. 6o^s)relics of CD4 monazite. Because this type of monazite istexturally younger than CD4, it is named CD5. Spotmonazite ages in this domain range from 1·54 to 1·61Ga(Supplementary DataTable A1).


This domain is seen only in monazite grain A from sampleB42D and occurs as an idioblastic rim around CD5(Fig. 6o). It has lowerTh and U, but elevated Yconcentra-tions relative to CD5 (Fig. 6p^r). Spot ages from thisdomain range from 1·56 to 1·50Ga and are thus youngerthan those from CD5 (Fig. 6s).

Granite sample B233Monazite grains from the granite are texturally simplewith a general absence of core^rim relationships (grains,1, 5, 6 and 7, Fig. 7a^d), except in grain 4, where a faintcore^rim relationship is observed in the BSE image(Fig. 7e). The grains occur either as inclusions or at the in-terstices of primary magmatic minerals and show a varietyof crystal habits that range from equant or rectangular toelliptical. All the six grains analyzed are compositionallyhomogeneous, with high Th (8·1^10·1%) and low Y(below detection limit, b.d.l.) and U (0·02^0·3%) concen-trations. Monazite spot ages show a range from latestPaleoproterozoic to Early Mesoproterozoic. Despite thesimilarity of the spot ages, grain-scale (intra- and intergra-nular) age heterogeneity is recognized (Fig. 7a^e). In atleast three monazite grains (1, 5 and 6), spot ages areyounger than 1628Ma, with a cluster between 1600 and1590Ma. In the two other monazite grains, the cores arealways older than 1580Ma (Fig. 7d and e), with the oldestspot ages clustering between 1646 and 1654Ma. Rims ofthe same grains are as young as 1585Ma.We further evalu-ate this age distribution in the granite monazite below.

Monazite EMP dating: a summarySummarizing the key compositional features of these com-positional domains (Fig. 8a^h), several first-order observa-tions can be made. CD1 is distinguished from all otherdomains by lowTh, but high U (Fig. 8a), high Y, but lowTh/U ratio (510) (Fig. 8b) and enrichment in heavy rareearth elements (HREE) (Fig. 8c and d). CD1 monazite isdepleted in huttonite and cheralite contents relative toCD2 (Fig. 8e and f) and also records the oldest ages(Fig. 8h) of all samples analyzed. CD2 is distinguishedfrom CD1 and CD4 by a combination of higher Yconcen-tration and low to moderate Th/U ratios (10^35) (Fig. 8b).In (Dy/Sm)N vs Y and XHREE vs Y plots, CD2 shows alarge array (Fig. 8c and d). The field of CD2 is, however,more restricted in the pelitic granulite samples and

Fig. 5 Continueddashed lines in BSE images (a, f, k). Despite modifications of these cores through successive phases of dissolution^reprecipitation, the innermostcore regions of all these grains show the preservation of detrital remnants. Based on compositional zonation of the cores and rims, a numberof domains (referred to as CD1, CD2, CD3, etc.) are distinguished. Spot values on the X-ray images in this figure and Fig. 6 represent relevantoxide wt %. For each monazite grain in this and other samples, apart from the three X-ray element images, an image showing the reconstructedcompositional domains that are preserved in each grain, together with monazite spot ages, is also presented. Pb X-ray image is shown for mon-azite grain B in B42D with spot ages (in bracket) being added with Pb oxide data (n). The domain boundaries are marked with dotted linesin the Y, Th and U images. The uncertainties (2s) of monazite spot ages for different domains in this figure are in the range �80^140Ma forCD1, �70Ma for CD2, �60^70Ma for CD3, and �55^75Ma For CD4. bdl, below detection limit.


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Fig. 6. (a^j) BSE andY,Th and UX-ray compositional maps and images showing reconstructed compositional domains (CD2, CD3 and CD4)of monazite cores and rims from two representative grains from pelitic granulite samples B36H and B27E. It should be noted that monazitecores in these grains lack compositional domain CD1. For details, see caption to Fig. 5. The uncertainties (2s) of monazite spot ages for the do-mains in this figure are in the range �50^70Ma for CD2, �55^75Ma for CD3, and �50^80Ma for CD4. (k^s) Sketches of compositionallyzoned monazite grain B from sample B35A (after Bhandari et al., 2011) (k^n) and X-ray element (Y, Th and U) images of monazite grain Afrom sample B42D (o^s). These images are used to fix the timing of growth of extremely thorian (ThO2 �14^22wt %) monazite (cf. compos-itional domain CD5) in the overall chronological framework of the evolution of the various compositional domains. The occurrence of theyoungest monazite (CD6) as a rim around CD5 monazite should be noted (o^s). The uncertainties (2s) of monazite spot ages for CD5 andCD6 domains are �30^70Ma and �50^65Ma, respectively.


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Fig. 7. Back-scattered electron images (a^e) of five selected monazite grains from garnetiferous granite sample B233.Y,Th and U oxide wt %,as well as monazite spot ages, are shown in that order from top to bottom in the data for each spot (white circles). The white dotted lines in(d) and (e) separate core and rim domains of zoned monazite grains. The uncertainties (2s) of monazite spot ages in this sample are �35^55Ma.


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Fig. 8. Monazite compositional plots for pelitic (B36H, B27E) and psammo-pelitic (B42D, B35A) granulites and granite (B233) samples.(a) Th (ppm) vs U (ppm), (b) Y (c.p.f.u., where c.p.f.u. is cation per formula unit) vs Th/U, (c) chondrite-normalized (Dy/Sm)N vs Y, and(d) XHREE vsY plots for CD1, CD2, CD3 and CD4 monazite grains in granulites and those from the granite. It should be noted that althoughmonazite grains from the granite and the CD4 domain are compositionally similar, those from granite show extreme fractionation of Th overU [see inset in (b)]. (e) Si (c.p.f.u.) vs ThþU (c.p.f.u.) and (f) Ca (p.f.u.) vs ThþU (p.f.u.) plots showing the distinctions between CD1 andCD2 compositional domains in huttonite and cheralite contents. (g) (Dy/Sm)N vs (Sm/Nd)N plot for compositional domains CD5 and CD6.See text for details. (h) Th/U vs monazite spot age plot for granulite and granite samples.


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overlaps with CD1 (Fig. 8c and d).This implies that high-Yand HREE-bearing CD2 is genetically linked to CD1.Although the field of CD3 overlaps with CD2 in Th/U vsYand XHREE vs Y plots (Fig. 8b and d), the (Dy/Sm)N vsY plot discriminates CD3 from CD2 (Fig. 8c). CD4 is dis-tinguished from CD2 and CD1 by the strong fractionationof Th over U (Th/U ratio¼15^140) and extremely low Yand HREE concentrations (Fig. 8b^d). Extremely highTh (Fig. 6l and q) and Th/U ratios characterize CD5,whereas CD6 is enriched in middle REE (MREE) andHREE compared with CD5 (Fig. 8g).Monazite from the garnetiferous leucogranite shows ex-

treme fractionation of Th over U (Th/U ratio up to 700,inset of Fig. 8b) and has lowYand HREE concentrations,similar to CD4 (Fig. 8a^d).The probability distribution diagrams for monazite from

the six compositional domains of the individual granulitesamples and the garnetiferous granite are shown in Fig.9a^l. For both pelitic (sample B36H) and psammo-pelitic(sample B42D) granulites, CD1 is the oldest of all the com-positional domains. Figure 9a and f shows that, whereasthe main age population of the CD1 domain is between1774 and 1776Ma, there are also dates that are older than1·77Ga. Given the similarity of spot ages for the CD2 andCD3 domains in the two pelitic granulite samples, theprobability distribution diagram is calculated for the com-bined domains. This shows a unimodal age population forboth the samples (Fig. 9b and d), and with mean ages of1612�14Ma (MSWD¼ 0·38, n¼ 20) for sample B36Hand 1611�16Ma (MSWD¼ 0·43, n¼16) for sample B27E.There is intra-sample variation of CD4 ages (Fig. 9c, eand g). For the pelitic granulites, the peak age is1586�14Ma (MSWD¼1·8, n¼ 34) in sample B36H(Fig. 9c) and 1615�14Ma (MSWD¼1·2, n¼ 26) insample B27E (Fig. 9e); there are also some older andyounger dates. In the psammo-pelitic granulite (sampleB42D), there are several age peaks with the youngest oneat �1608Ma (Fig. 9g), which is broadly similar to that ob-tained from the pelitic granulites. We discuss the possiblereasons behind the age variations below. The unimodalage population for CD5 monazite in sample B35A recordsa mean age of 1579� 9Ma (MSWD¼ 0·22, n¼18)(Fig. 9j). There is a larger spread of ages for CD5 monazitein sample B42D, but the mean age of 1580�25Ma(MSWD¼ 2·7, n¼11) (Fig. 9h) is the same as in sampleB35A. The mean age of CD6 monazite, which is recordedonly in sample B42D (Fig. 9i), is estimated at1539�24Ma (MSWD¼ 0·77, n¼ 5). As mentioned above,monazite grains from the leucogranite show age zonation.Zoned monazite cores show a broadly unimodal age distri-bution (Fig. 9k) with a mean age of 1637�16Ma(MSWD¼ 0·89, n¼ 9). In contrast, rims of zoned mona-zite and homogeneous monazite crystals collectivelydefine a younger population (Fig. 9l) with a mean age of

1604�9Ma (MSWD¼1·01, n¼ 32).We discuss the signifi-cance of this age zonation after presentation of U^Pbzircon dates from the same rock.

SHRIMP U^Pb zircon datingPsammo-pelitic granulite sample B42D

A total of 37 spots on 25 zircon grains were analyzed in twosessions, along with 19 analyses of the CZ3 standard thatrecorded a 1s variation in Pb/U isotopic ratios of 1·66%and 1·27% in analytical sessions 1 and 2, respectively: theresults are presented in Table 6. The zircon grains aresmall (diameter �50^125 mm), rounded and reddish incolour. Cathodoluminescence (CL) imaging reveals thatthe majority of the analyzed grains consist of cores andrims (Fig. 10a^g). For clarity of description, the zirconshave been classified into three types: (1) Group I zircons,which have core^rim structures with the cores showing os-cillatory zonation; (2) Group II zircons, which also havecore^rim relationships, but the cores are generally struc-tureless; (3) Group III zircons, which lack core^rimstructures.In Group I zircon cores, there are considerable vari-

ations in the internal structures, the intensity of lumines-cence and the Th/U ratios. Thus, in grains 2, 15, 24 and 4,the cores display oscillatory zonation and have eithermultifaceted or rounded exteriors (Fig. 10a^c). The Th/Uratio is moderate (0·35^0·48) except in grain 2, where theratio is low (0·08). In grain 20, the oscillatory zoned corewith relatively high aspect ratio shows well-developedcrystal faces with pyramidal terminations (Fig. 10d). TheTh/U ratio is high at �1·05. These features of Group Izircon cores are consistent with crystallization from melt.In Group II zircons, the cores are generally composed ofstructureless ovoid to ellipsoidal domains (diameter �50^120 mm) or show weakly developed internal banding(Fig. 10e^h). The cores of grains 1, 3, 10, 12 and 26(Fig. 10e, f and h) are moderately enriched in U (range822^1180 ppm) but depleted inTh (range 38^91ppm) andhave characteristic low Th/U ratios in the range 0·04^0·09, perhaps the result of metamorphism (Vavra et al.,1996). The remaining cores (grains 5, 25, 24, 21, 19, 16and 7) (e.g. Fig. 10g and Table 6) have relatively higherTh/U ratios (0·21^0·70) and wide variations in U (562^3138 ppm) and Th contents (125^2117 ppm). Although therelatively higher Th/U ratios may imply a magmaticorigin, the structureless nature of these zircons suggestslater recrystallization (see Grant et al., 2009).The cores of both Group I and II zircons are mantled by

multiple generations of zircon rims of variable thickness(width �10^50 mm). In some grains, these rims truncate in-ternal compositional banding in the cores and have cross-cutting relationships between themselves. Their formationinvolved both partial dissolution and recrystallization ofthe cores, and new zircon growth. The innermost rim


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around zircon cores (rim 1) is a low-luminescent, darkzircon layer of lobate shape (Fig. 10a, b and g) with a lowTh/U ratio of �0·07. Immediately outward of rim 1occurs a more highly luminescent, low-U domain (Fig. 10aand g), which is referred to as rim 2. In several zircongrains, rim 2 directly embays the core (Fig. 10d and h). Athird, volumetrically minor domain (rim 3) is developedaround rim 2 (Fig. 10a and d) and is a low-luminescentoutermost layer with relatively high U (498 ppm) and Th(156 ppm) contents, with aTh/U ratio of 0·32.The Group III zircons can be classified into the follow-

ing textural types: type a, elongate zircon with weakly de-veloped oscillatory zonation, showing rounding ofpyramidal faces (grain 6) (Fig. 10i); type b, sector-zoned,tabular-shaped, moderately luminescent zircon withroundish terminations showing traces of faint oscillatoryzonation (grain 17) (Fig. 10j); type c, soccer-ball-shaped,

sector-twinned zircons having multi-faceted exteriors(grain 13) (Fig. 10k) or roundish terminations (grain 14)(Fig. 10l). In the last type, a highly luminescent layer,resembling rim 2, mantles a core of low luminescence.TheTh/U ratios in zircons of these textural types are char-acteristically high (0·84^0·97). Textural types a and b andthe high Th/U ratios of these zircons are consistent withtheir growth from high-temperature anatectic melt (Wanet al., 2011), in equilibrium with a monazite-free residualbulk composition (Stepanov et al., 2012), followed by subso-lidus recrystallization at relatively high temperatures.Type c textures suggest zircon growth from high-T, low-water content melts, analogous to those described byVavra et al. (1996) and Harley et al. (2007).Concordant analyses of cores in Group I zircons record

three ages: 3584�3Ma (grain 4), 3398�15Ma (grain 24)and 2556� 8Ma (grain 20) (Table 6, Fig. 10c and d).

Fig. 9. Probability density diagrams with histograms of spot monazite ages from the various compositional domains of pelitic (samples B36H,a^c; B27E, d, e) and psammo-pelitic (samples B42D, f^i; B35A, j) granulites and in granite sample B233 (k, l). Probability plots with unimodalage populations are shown with their mean ages.


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Table6:

U,T

h,PbSH

RIM

Pdataforzirconsfrom

theBBGdomainintheCentralIndianTectonicZone

Spot

no.

U (ppm)

Th

(ppm)

Th/U

Pb

(ppm)

204Pb/

206Pb

f206%

207Pb*/

206Pb*

208Pb*/

206Pb*

206Pb*/

238U

207Pb*/

235U

208Pb*/

232Th

%Con.

206Pb/2

38U

age(M

a)

207Pb/2

35U

age(M

a)

207Pb/2

06Pb

age(M

a)

208Pb/2

32Th

age(M

a)

Tex.

type

Psa

mmo-peliticgranulite,sa

mple

B42D

GroupI

1-4

458

221

0·48

433

0·0000

0·02

0·3230�0·0007

0·1256�0·0006

0·7578�0·0123

33·75�0·56

0·1970�0·0034

101

3636�45

3603�16

3584�3

3635�58

C

1-15

414

162

0·39

313

0·0000

0·00

0·2784�0·0006

0·1062�0·0006

0·6328�0·0103

24·29�0·41

0·1715�0·0030

943161�41

3280�16

3354�3

3199�52

C

2-15

568

137

0·25

220

0·0000

0 ·01

0·2279�0·0018

0·0829�0·0034

0·4509�0·0025

14·17�0·31

0·1498�0·0042

792399�50

2761�29

3037�29

2822�119

C

2-24

631

216

0·35

373

0·0000

0·04

0·2866�0·0010

0·0910�0·0021

0·6883�0·0016

27·17�0·18

0·1755�0·0026

993376�41

3390�18

3398�15

3268�85

C

2-2

499

370·08

214

0·0000

0·05

0·2360�0·0019

0·0195�0·0012

0·4996�0·0015

16·23�0·24

0·1190�0·0026

852612�32

2891�23

3091�30

2273�58

C

2-8

374

170

0·47

170

0·0001

0·14

0·1938�0·0026

0·1240�0·0014

0·5282�0·0016

14·02�0·31

0·1358�0·0022

992734�35

2751�29

2764�43

2575�56

C

2-20

595

606

1·05

249

0·0000

0·02

0·1700�0·0005

0·2915�0·0016

0·4865�0·0016

11·39�0·17

0·1345�0·0023

100

2556�35

2556�16

2556�8

2550�58

C

GroupII

1-1

1180

600·05

486

0·0001

0·22

0·1495�0·0004

0·0150�0·0005

0·4079�0·0066

8·41�0·14

0·1198�0·0044

942205�30

2276�15

2340�4

2286�79

C

1-3

822

750·09

282

0·0000

0·04

0·1375�0·0005

0·0259�0·0004

0·3424�0·0055

6·49�0·11

0·0979�0·0023

861898�27

2044�15

2195�6

1888�42

C

1-10

1125

910·08

531

0·0000

0·04

0·1672�0·0003

0·0228�0·0003

0·4606�0·0074

10·62�0·17

0·1301�0·0026

972442�33

2490�15

2530�3

2472�47

C

2-12

1029

380·04

314

0·0000

0·03

0·1306�0·0015

0·0103�0·0016

0·3549�0·0015

6·38�0·21

0·0902�0·0035

931958�25

2030�19

2103�27

1746�60

C

2-26

923

670·08

351

0·0000

0·02

0·1505�0·0021

0·0191�0·0015

0·4430�0·0015

9·18�0·26

0·1097�0·0025

101

2364�30

2356�24

2350�37

2104�52

C

1-5

591

125

0·21

336

0·0000

0·00

0·1841�0·0006

0·0564�0·0005

0·5329�0·0086

13·53�0·23

0·1422�0·0027

102

2754�36

2717�16

2691�5

2687�48

C

1-16

571

272

0·48

349

0·0000

0·02

0·1827�0·0005

0·1314�0·0007

0·5397�0·0087

13·59�0·23

0·1487�0·0026

104

2782�37

2722�16

2677�5

2802�45

C

2-16

1559

474

0·31

708

0·0000

0·05

0·1870�0·0005

0·0840�0·0005

0·5284�0·0015

13·59�0·16

0·1391�0·0016

101

2735�33

2722�15

2712�8

2633�42

C

2-25

933

435

0·48

408

0·0000

0·06

0·1816�0·0007

0·1297�0·0005

0·5082�0·0015

12·69�0·17

0·1355�0·0016

992649�33

2657�16

2663�12

2568�41

C

2-21

562

169

0·31

251

0·0001

0·08

0·1888�0·0008

0·0922�0·0011

0·5188�0·0015

13·45�0·17

0·1508�0·0019

992694�34

2712�16

2725�13

2838�55

C

2-19

3138

2117

0·70

1280

0·0000

0·01

0·1621�0·0003

0·1948�0·0004

0·4749�0·0015

10·61�0·15

0·1326�0·0015

101

2505�30

2490�14

2477�5

2516�38

C

1-7

505

267

0·53

370

0·0000

0·06

0·2749�0·0007

0·1424�0·0007

0·5990�0·0097

22·70�0·38

0·1616�0·0028

913026�39

3214�16

3334�4

3028�49

C

GroupsI–IIrims

2-2M

840

580·07

210

0·0001

0·14

0·1031�0·0006

0·0234�0·0012

0·2911�0·0015

4·09�0·16

0·0822�0·0043

991647�22

1652�13

1658�12

1596�68

Rim

1

2-2R

498

156

0 ·32

115

0·0000

0·07

0·0980�0·0004

0·0935�0·0008

0·2678�0·0015

3·60�0·16

0·0762�0·0018

971530�21

1548�13

1574�9

1483�27

Rim

3

2-4

7025

0·37

250·0006

0·82

0·2070�0·0040

0·0904�0·0020

0·4110�0·0019

11·34�0·47

0·0789�0·0048

782220�36

2552�43

2828�69

1535�73

Rim

2

2-8

111

480·45

270·0024

3·93

0·1273�0·0007

0·1888�0·0013

0·2693�0·0018

3·47�0·60

0·0633�0·0114

103

1537�25

1520�48

1496�109

1240�142

Rim

2

1-12

7358

0·79

240·0006

0·92

0·0990�0·0024

0·2318�0·0057

0·2801�0·0052

3·82�0·12

0·0821�0·0026

991592�26

1598�26

1606�45

1595�49

Rim

2

1-11

645

580·09

217

0·0000

0·05

0·1268�0·0005

0·0240�0·0005

0·3387�0·0055

5·92�0·10

0·0908�0·0024

921880�26

1965�15

2055�6

1757�45

Rim

2

1-18

9861

0·62

300·0002

0·29

0·0992�0·0015

0·1732�0·0033

0·2776�0·0050

3·80�0·09

0·0771�0·0021

981579�25

1592�20

1609�27

1502�39

Rim

2

2-19

4929

0·60

110·0015

2·43

0·1286�0·0030

0·2264�0·0017

0·2550�0·0022

3·80�0·67

0·0749�0·0085

831464�29

1593�54

1769�116

1459�124

Rim

2

2-20

5937

0·64

140·0007

1·14

0·1023�0·0013

0·1970�0·0018

0·2672�0·0022

3·41�0·48

0·0722�0·0060

103

1526�30

1506�38

1477�80

1409�85

Rim

2

2-23

5335

0·68

120·0001

0·21

0·0998�0·0014

0·2024�0·0018

0·2649�0·0022

3·58�0·35

0·0769�0·0041

951515�30

1545�28

1588�51

1498�62

Rim

2

2-24

6238

0·63

160·0003

0·42

0·1477�0·0040

0·1661�0·0017

0·3063�0·0028

6·09�0·51

0·0766�0·0040

761722�42

1989�44

2279�73

1491�60

Rim

2

2-25

152

220·15

350·0018

2·95

0·1164�0·0008

0·0887�0·0039

0·2628�0·0019

3·30�0·56

0·0420�0·0460

104

1504�25

1480�43

1446�100

832�383

Rim

2

2-26

289

910·32

740·0002

0·37

0·0983�0·0006

0·1070�0·0011

0·2968�0·0016

3·89�0·23

0·0905�0·0043

110

1675�23

1612�19

1529�32

1750�76

Rim

2

1-9

945

820·09

270

0·0002

0·27

0·0973�0·0004

0·0243�0·0007

0·2932�0·0047

3·93�0·07

0·0816�0·0028

105

1658�23

1620�14

1572�8

1586�53

Rim

2

(continued

)


23



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Table6:

Continued

Spot

no.

U (ppm)

Th

(ppm)

Th/U

Pb

(ppm)

204Pb/

206Pb

f206%

207Pb*/

206Pb*

208Pb*/

206Pb*

206Pb*/

238U

207Pb*/

235U

208Pb*/

232Th

%Con.

206Pb/2

38U

age(M

a)

207Pb/2

35U

age(M

a)

207Pb/2

06Pb

age(M

a)

208Pb/2

32Th

age(M

a).

GroupIII

1-6

226

189

0·84

710·0000

0·00

0·0997�0·0006

0·2440�0·0017

0·2734�0·0046

3·76�0·07

0·0797�0·0015

961558�23

1584�15

1618�12

1549�28

1-17

7169

0·97

240·0001

0·16

0·0998�0·0018

0·2812�0·0046

0·2786�0·0052

3·83�0·11

0·0810�0·0021

981584�26

1600�22

1621�34

1575�39

1-13

8174

0·91

270·0002

0·27

0·0963�0·0019

0·2617�0·0047

0·2776�0·0051

3·68�0·11

0·0796�0·0021

102

1579�26

1568�23

1553�37

1549�40

1-14

213

196

0·92

700·0001

0·09

0·0980�0·0010

0·2622�0·0025

0·2800�0·0047

3·78�0·08

0·0800�0·0016

100

1591�24

1589�17

1587�19

1556�30

Garnetiferousgranite,sa

mple

B233

1353

126

0·36

102

0·0001

0·21

0·0974�0·0009

0·1005�0·0018

0·2778�0·0039

3·73�0·07

0·0782�0·0018

100

1580�20

1578�14

1576�17

1521�34

2344

204

0·59

106

0·0000

0·04

0·0988�0·0008

0·1820�0·0018

0·2789�0·0040

3·80�0·07

0·0857�0·0016

991586�20

1593�14

1602�15

1662�29

3269

730·27

760·0000

0·00

0·0996�0·0008

0·0800�0·0011

0·2791�0·0041

3·83�0 ·07

0·0819�0·0017

981587�21

1600�14

1617�15

1591�32

4539

504

0·93

184

0·0000

0·01

0·1001�0·0007

0·2636�0·0016

0·2902�0·0040

4·01�0·06

0·0819�0·0013

101

1643�20

1636�13

1627�13

1590�24

5409

146

0·36

119

0·0001

0·17

0·0972�0·0007

0·0984�0·0014

0·2815�0·0038

3·77�0·06

0·0775�0·0016

102

1599�19

1587�13

1570�14

1509�30

6552

684

1·24

193

0·0000

0·04

0·0984�0·0006

0·3623�0·0018

0·2778�0·0038

3·77�0·06

0·0811�0·0012

991580�19

1586�12

1595�11

1576�23

7283

790·28

800·0001

0·13

0·0981�0·0008

0·0809�0·0015

0·2778�0·0040

3·76�0·07

0·0802�0·0020

100

1580�20

1583�14

1588�16

1560�37

8792

226

0·29

340

0·0000

0·04

0·1417�0·0004

0·0878�0·0006

0·4049�0·0053

7·91�0·11

0·1246�0·0019

972191�24

2221�12

2248�5

2373�34

9287

110

0·38

166

0·0002

0·29

0·1719�0·0009

0·1148�0·0015

0·5179�0·0073

12·28�0·19

0·1553�0·0032

104

2690�31

2626�15

2577�9

2917�55

10218

113

0·52

109

0·0001

0·09

0·1602�0·0010

0·1449�0 ·0016

0·4436�0·0065

9·80�0·16

0·1244�0·0024

962367�29

2416�15

2457�10

2370�44

*Correctedusingmeasured

204Pb.

Forhyp

hen

ated

spotnumbersthefirstpart(1-or2-)indicates

SHRIM

Psession(first

orseco

nd)an

dthefinal

partisthegrain

number.2-2M

/R,Man

tle/Rim

;C,co

re;Re,

recrystallized;f206%¼(common

206Pb/total206Pb)�

100;

%Con.isper

centco

nco

rdan

cedefined

as[(206Pb/2

38U

age)/(

207Pb/2

06Pbag

e)]�

100.

Tex.type,

texturaltype.


24



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The core of Group II zircon grain 10, with a low Th/Uratio, yields a concordant date of 2530�3Ma (Fig. 10f).The remaining Group I and II cores show small degreesof reverse discordance (range 102^104% in Group II) anda greater range of normal discordance (79^94% in GroupI and 86^97% in Group II) (Table 6) and record a largespread in 207Pb/206Pb ages from 3334� 4 to 2103�27Ma(Table 6, Fig. 11a). Relatively coarse rim 1 and rim 3 do-mains in grain 2 yield concordant dates of 1658�12Maand 1574�9Ma, respectively (Fig. 10a). Analyses of rim 2in eight grains, directly adjacent to Group I and Group II

cores, display a range of discordance (from 78 to 116%)and a spread in 207Pb/206Pb ages from 2828�69 to1446�100Ma with a dominant cluster at c. 1·6Ga(Table 6). In the concordia diagram, the discordant ana-lyses from cores of Groups I and II and rim 2 plot alongtwo poorly defined discordia lines with upper and lowerintercept ages at 3430�37Ma and 1590� 49Ma (Fig.11b) and 2564� 80Ma and 1568�63Ma (Fig. 11c), re-spectively. Despite the relatively large errors, the upperintercept ages fall within the range of concordant dates ob-tained from zircon grains 24 (3398�15Ma), 20

Fig. 10. Cathodoluminescence images of representative dated zircon crystals in psammo-pelitic granulite sample B42D. Larger and smallerellipses mark locations of SHRIMP spots during first and second analytical sessions, respectively. Dotted boundaries separate the textural do-mains (see text for details). SHRIMP 207Pb^206P ages are reported in Ma�1s. (a) Faint, oscillatory zoned core, surrounded by three gener-ations of metamorphic rims, which are radially distributed away from the core, as follows. The innermost rim (rim 1) of curvilinear shape is alow-luminescent, U-rich zone that embays the core. The central rim 2 is a high-luminescent, U-poor zone that surrounds rim 1. The outermostrim 3 is another low-luminescent zone, which truncates rims 1 and 2. (b) Dark oscillatory zoned core with multifaceted exterior, rimmed bythin zones of rims 1 and 2. (c) Dark, oscillatory zoned core, embayed by a bright luminescent zone. (d) Elongate, oscillatory zoned, euhedralzircon core with pyramidal terminations, surrounded by rim 2 and rim 3. (e, f) Ovoid, structureless, dark, U-rich core, discontinuously sur-rounded by rim 2 in (e). (g) Dumb-bell-shaped zircon with an elongate inner core. This is surrounded by successive thin rims (rim 1 and rim2). (h) Relic of a detrital core within rim 2, the latter containing a thin, laterally continuous, dark band, which merges with the outline of thecore.This feature appears to indicate that the dark band marks the outer limit of the detrital core. (i) Elongate oscillatory zoned zircon, showingpartially preserved euhedral crystal outline with pyramidal terminations. (j) Luminescent, sector-twinned zircon, showing traces of fine, oscil-latory zonation (upper left). (k) Bright luminescent, soccer-ball-shaped zircon. (l) Oval zircon showing a darker recrystallized central zone sur-rounded by rim 2.


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Fig. 11. U^Pb concordia diagrams for zircons from psammo-pelitic granulite sample B42D (a^d) and garnetiferous granite sample B233 (e, f).(a) Plot of all zircon analyses in the concordia diagram. The cluster of zircon analyses at �1·6Ga is highlighted in (d). (b, c) Selected zirconanalyses plot along two discordia lines with upper and lower intercept ages at 3430�37Ma and 1590� 49Ma (b) and 2564� 80Ma and1568�63Ma (c). Also identified are textural types and spot numbers of zircon analyses. C/R, core/rim; Zrn, zircon. (d, f) U^Pb concordia dia-grams for samples B42D (d) and B233 (f). (See text for details.)


26



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(2556� 8Ma) and 10 (2530�3Ma). The lower interceptages for both discordia converge at c. 1·57^1·59Ga (Fig.11b and c), which is within error of the metamorphic ageof the monazite as established above, and indicate possibledisturbance of the zircon at this time (see below for tightertime constraints on this metamorphic event).The four ana-lyses of Group III zircons, together with four analyses ofzircons from rim 2 domains, all showing low degrees ofnormal and reverse discordance, but with low errors(Table 6), record a concordia age of 1590� 6Ma (MSWDof concordance¼ 0·65) (Fig. 11d).

Granite sample B233

A total of 10 analyses were made on 10 zircon grains, alongwith nine analyses of the CZ3 standard that recorded a1svariation in Pb/U isotopic ratios of 1·38% over the ana-lytical session. SHRIMPU^Pb zircon dating of this garne-tiferous granite sample (Table 6) yields 207Pb/206Pb datesof 2577�9, 2457�10 and 2248�5Ma, and a cluster be-tween 1627�13 and 1570�14Ma. On the concordia dia-gram, the zircon analyses plot along a poorly defineddiscordia with upper and lower intercept ages of2476�76Ma and 1599�54Ma (MSWD¼ 2·4), respect-ively (Fig. 11e). The youngest age cluster marks the crystal-lization age of the granite, with seven zircon analysesyielding a concordia age of 1595�5Ma (MSWD of con-cordance¼ 0·13) (Fig. 11f). The three older analyses, whichform part of a 2·48Ga zircon age population and resemblezircon dates from Group I and Group II cores in sampleB42D, record the age of inherited zircons from either thesource or the wall-rock during emplacement.

DISCUSSIONLinking the growth and dissolution ofmonazite with the metamorphic reactionhistory of the BBG granulitesThe monazite chemical and age information presentedabove, when combined with the metamorphic information,allows us to reconstruct the mechanisms of monazitegrowth and dissolution (CD1 to CD6) in the BBG granu-lites. Considering the variability of the metamorphic con-ditions recorded by the four samples studied, and thepossible effects on monazite stability, it is important tofirst briefly summarize the key metamorphic findings. Allthe samples record evidence for copious biotite dehydrationmelting at T� 8008C, but in different bulk-rock compos-itions, producing an array of peritectic mineral assem-blages, both among the samples and between differentcompositional layers within a single sample: garnet(B27E, B35A), garnetþ cordierite (B36H, B35A), and gar-netþ aluminous orthopyroxene (B42D, B35A). Mineralassemblages in the pelitic granulite samples from Pipariya(B36H and B27E) indicate a medium-pressure granulite-facies metamorphism. The psammo-pelitic granulite

samples B42D and B35A from Larsara and Dongariyarecord lower crustal granulite-facies metamorphism, ap-proaching UHT conditions (Bhowmik et al., 2005;Bhowmik, 2006; Bhandari et al., 2011; this study). The peli-tic granulites record post-peak cooling to temperatures�7008C. The post-peak metamorphic reaction history insample B42D is analogous to that described by BasuSarbadhikari & Bhowmik (2008) and is interpreted interms of two stages of metamorphic reworking (BM2 andBM3) of the UHTgranulite: (1) mid-crustal prograde heat-ing (BM2); (2) reburial of the BM2 granulites to lower crus-tal depths (BM3P) and their rehydration (BM3R). Giventhis background information concerning the metamorphicevolution, we attempt to model the compositional domainsCD1 ! CD6. The evolutionary history of the compos-itional domains in selected monazite grains is explainedwith the help of a series of schematic diagrams (Fig. 12a^c).

Monazite recrystallization (CD1^CD2^CD3)

The domains CD1 to CD3 constitute the cores of compos-ite monazite grains. Despite modifications during the for-mation of monazite rims, ovoid to rectangular cores stillremain in a number of grains. CD1 (age older than1·77Ga) pre-dated the formation of the CD2 and CD3 do-mains (combined mean age¼1612�14Ma) by4160Myr,and can be interpreted as detrital monazite remnants thatsurvived dissolution at the onset of partial melting.There are two lines of evidence that suggest monazite

CD2 originated by a mechanism of dissolution and repreci-pitation, but without wholesale dissolution [terminologyafter Putnis (2002)]. These are: (1) textural evidence forpartial to complete pseudomorphing of CD1 by CD2 (par-tial in grains O, D, sample B36H, Fig. 5a^j; complete ingrain A, sample B36H, Fig. 6a^e; grain K, sample B27E,Fig. 6f^j); (2) where CD1 monazite is preserved, there isalso a sharp compositional contrast (e.g. inTh) across thedomain boundary, which cannot be explained by solid-state diffusion given the low diffusivity of this element atthe estimatedT condition of CD2 (T58008C; see below)(Cherniak & Pyle, 2008). Because this variety of monazitewas produced by replacement along reaction fronts withinthe crystal itself (e.g. Harlov et al., 2011; Kelly et al., 2012)(Fig. 12a and b), we relate it to monazite recystallizationand differentiate it from monazite that forms throughnew growth. The high Y, HREE and Th contents of CD2suggest that dissolution^reprecipitation of CD1 was drivenby a metamorphic fluid (e.g. alkali or halogen-rich,Harlov et al., 2011) or melt (e.g. produced by muscovite de-hydration melting) that was not in equilibrium withgarnet.In several monazite grains, there are compositional and

textural attributes which suggest that the cores, composedof CD2 alone or with composite CD2^CD1 components,underwent further dissolution and reprecipitation, produ-cing CD3 (Fig. 12a and b). The progressive depletion in Y


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and HREE in CD3, with Th contents either showing aminor decrease or increase (Fig. 8a^d), indicates a newfluid or melt composition in equilibrium with the first ap-pearance of subsolidus or peritectic garnet.

Anatexis at granulite facies (CD4)

The next stage of monazite evolution, extensively recordedin samples B36H, B27E and B35A, involved large-scaledissolution of monazite cores (composed of CD1^CD3) as

Fig. 12. Schematic diagrams showing the spectrum of multistage monazite growth and dissolution in the metasedimentary rocks of the CentralIndianTectonic Zone during prograde, peak and retrograde granulite-facies metamorphism.The signatures of the complete monazite evolution-ary history are differentially preserved in different compositional domains in individual monazite grains.The dashed line at each stage of mona-zite evolution marks the limit of its dissolution in the next stage. The dotted lines in (a) and (b) mark the extent of later monazite rims,producing compositional domain CD4. Oz, oscillatory zoned.


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revealed by their serrated to amoeboid outlines. In mona-zite grains O and D in sample B36H, the dissolution af-fected even the innermost part of the core domains (seeCD1) (Fig. 5e and j), and may in part be responsible forthe asymmetrical distribution of the compositional do-mains CD1^CD2 and CD3 in the preserved cores of thesegrains. In the majority of cases, the variably resorbedcores are rimmed by compositionally homogeneous mona-zite that characterizes CD4 (Fig. 12a and b), locally witheuhedral crystal faces (e.g. grain H, sample B36H,Supplementary Data Fig. A1aê).This newly grown mona-zite is impoverished inYand HREE, and taken as evidencethat CD4 was in equilibrium with peritectic garnet underpeak granulite-facies conditions (Spear & Pyle, 2010). Weinfer that this monazite rim crystallized from melt eitheratTMax or immediately followingTMax.As mentioned above, CD4 monazite from psammo-

pelitic granulite (sample B42D) shows age heterogeneity(e.g. grain B, Figs 5p, 9g and 13a). A 1600Ma age popula-tion is recorded in Pb-poor parts of the rim (Fig. 5nand p), whereas high Pb content CD4 monazite yieldedolder Paleoproterozoic dates (Figs 5n, p and 13a). One ofthe following two interpretations is suggested. (1) Themonazite rim of grain B was of Paleoproterozoic age, andthus constituted a detrital component in the psammo-peli-tic protolith of the rock. Incomplete metamorphic recrys-tallization of the rim at c. 1·6Ga led to differential Pb loss.In the segment of the rim where there was little or no Pbloss, the original Paleoproterozoic age is retained. In con-trast, complete removal of radiogenic Pb occurred in partof the rim that yielded c. 1·6Ga ages. (2) Alternatively, theolder date from the rim was possibly due to localized inher-itance of common Pb during its growth at 1·6Ga. Sharpcompositional contrast in Y, Th and Pb across the bound-ary between CD1 and CD4 appears to rule out option (1).However, for option (2) to be valid, the presence ofcommon Pb in the monazite rim needs to be established.One of the underlying principles of electron microprobe

dating of monazite is that common Pb is either absent ornegligible at the time of monazite growth (Parrish, 1990).Although generally true for monazite growth, numerousmeasurements of 204Pb, which is an indicator of commonPb, show its variable concentration in natural monazitegrains (e.g. Williams et al., 1983; Copeland et al., 1988;Corfu, 1988; DeWolf et al., 1993; Zhu et al., 1997; Stern &Berman, 2000), raising some concerns as to the assump-tions of the behaviour of common Pb in monazite.Possible crustal inheritance of Pb in monazite has beensuggested from a number of studies (e.g. Chatterjee &Bhattacharji, 2004; Seydoux-Guillaume et al., 2012).Although the amount of 204Pb and total common Pb is

not directly measured for the CD4 domain in monazitegrain B of sample B42D, we evaluated this through twochemical plots (Fig. 13b and c). CD4 areas with higher

spot monazite ages show consistently higher Pb/(ThþU)ratios relative to those of c. 1·6Ga areas (Fig. 13b).Measured Pb for the former is also higher than calculatedradiogenic Pb (at a reference time of 1·6Ga) for the samedomain (Fig. 13c). We interpret these high Pb contentCD4 areas as possible evidence for the presence ofcommon Pb. However, this needs to be tested by detailedisotopic studies. If true, the uncorrected Paleoproterozoicdates in CD4 rims in grains B and C from sample B42Dare artefacts, and cannot be used to calculate the forma-tion age.

Growth of high-Th monazite (CD5)

With the formation of CD4 rims, monazite evolution waseffectively frozen in the metapelite granulite samplesB36H and B27E. However, monazite continued to form inmeta-psammo-pelitic samples B42D and B35 (Fig. 12c).CD5 has locally preserved euhedral crystal faces and hasa consistent lowYcomposition, features that collectively in-dicate crystallization from melt, still in equilibrium withperitectic garnet. However, it also has high Th contentsand Th/U ratios (up to 160). The origin of high-thorianCD5 monazite is complicated. Watt & Harley (1993) re-ported extremely thorian monazite (ThO2¼21·4wt %)in granulite-facies metapelites from Brattstrand Bluffs,East Antarctica. They correlated its growth with crystal-lization of anatectic melt produced by biotite dehydrationmelting reactions. In contrast, Kelly et al. (2012) reportedhigh-Th rims (�22wt % ThO2) in monazite from highMgÂl granulites from the Oygarden Islands, EastAntarctica. These rims are in textural equilibrium withsapphirineôrthopyroxene symplectites that formed as aresult of garnet breakdown. They interpreted the high-Thrim domains to have formed through recrystallization andnot by new growth.We suggest that CD5 crystallized from an extremely

high-T anatectic melt in which Th was fractionated overU. Melt compositions of this type may have originatedthrough a combination of processes, including biotite dehy-dration melting, loss of accumulated melt during progradeheating, remelting of former restite (formed during BM1

metamorphism) but at an elevated temperature (TMax of�9008C) and fractional crystallization of melt. Becauseof the very high partition coefficient of Th in mona-zite (more than 1800% higher than that for U; Stepanovet al., 2012), remnant monazite in the restite will be ex-tremely enriched in Th and will have a high Th/U ratio.Dissolution of such refractory monazite at high tempera-tures is likely to produce localized, highly thorian mona-zite (as for CD5 monazite). The restricted occurrenceof this monazite type to the UHT metamorphosedpsammo-pelitic granulites is also consistent with its high-temperature origin.


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A second metamorphic cycle (CD6)

The evolution of monazite in the BBG granulites termi-nated with the formation of CD6 in psammo-pelitic granu-lite sample B42D. Compositional and textural featuressuggest that development of CD6 as an idioblastic rimaround partially resorbed CD5 (Fig. 6o^s) is the result ofmetamorphic overgrowth (Fig. 12c). The occurrence ofcomposite monazite grain B within BM1 garnet, and inclose spatial association with retrograde biotite (Fig. 4f),and higher Y and MREE^HREE concentrations in theCD6 rim relative to those in CD5 (Figs 6p and 8g) collect-ively connect the growth of CD6 to the decomposition ofgarnet during the terminal hydration event as part of theBM3 metamorphic event.

Interpretation of monazite and zircon agesEarly monazite and zircon: Archean to Paleoproterozoicprovenance for the BBG granulites

The results of monazite and zircon geochronology can nowbe utilized to identify and constrain the detrital remnants(pre-BM1) of both minerals. Zircon Lu^Hf isotopic

analyses indicate that the protoliths of the BBG granuliteshave an Archean heritage (Bhowmik et al., 2011). TheSHRIMP U^Pb zircon ages from two samples in thisstudy (psammo-pelite B42D and garnetiferous graniteB233) exhibit two broad age populations: Paleo- toNeoarchean (3584^2530Ma) and Late Paleoproterozoicto Early Mesoproterozoic (1658^1574Ma). The Group Iand Group II zircon cores in sample B42D dominate thePaleo- to Neoarchean zircon population. A combinationof features such as general rounded shapes, CL patternsthat vary with different zircon grains and different zirconages (3584�3, 3398�15, 2556� 8 and 2530�3Ma) col-lectively suggest a detrital origin for these zircon cores.Similarly, pre-Late Paleoproterozoic zircon cores alsooccur in the garnetiferous granite sample B233 and recordthe age of inherited zircons from either the source or thewall-rock during emplacement. The U^Pb detrital zircondate of 3584�3Ma is close to the oldest dated zircon(3582·6�4Ma) from the Indian continental lithosphere(e.g. Bastar granite, Rajesh et al., 2009) suggesting that theprovenance of supracrustal granulites of the BBG domain

Fig. 13. (a) Reconstructed age heterogeneity map of CD4 monazite rim in grain B from psammo-pelitic granulite sample B42D. (b, c)Measured Pb vs measured ThþU (b) and measured Pb vs calculated radiogeneic Pb (calculated at 1·6Ga) (c) plots in CD4 rim. The part ofCD4 rim that shows spot ages older than c. 1·6Ga, the average age of this domain in the granulites, has high measured Pb (see text for details).


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consisted of the oldest Indian continental crust or itsreworked equivalent. Based on the zircon dates, the deposi-tional age of the precursor sediments of the psammo-pelitic granulite is bracketed between �2530Ma (the ageof the youngest concordant detrital zircon) and �1658Ma(the age of the oldest metamorphic rim, rim 1; see discus-sion below).

Timing and duration of metamorphism of the BBGgranulites

The younger monazite and zircon ages allow us to con-strain the timing and duration of the granulite-facies eventin the BBG domain. Monazite EMP ages from CD2 andCD3 (1612�14Ma) mark the timing of prograde meta-morphism in the metapelites. A monazite age of CD4 (be-tween 1615 and 1586Ma) constrains the timing of meltcrystallization at or immediately following the BM1P meta-morphism in the pelitic granulites. Because of the relativelylower resolution of the monazite chemical ages and thesimilarity of these ages from the three domains, we do notattach any significance to the possible timescale of progradeheating as implied by the metamorphic context of theirgrowth history. However, we stress that this range of agesis indistinguishable from the 1·6Ga monazite age from theUHT metamorphosed garnet^cordierite^orthopyroxenegranulite sample B35A reported by Bhandari et al. (2011).The earliest recognizable metamorphic zircon growth

(rim 1) around detrital zircon in sample B42D yields aconcordant 207Pb/206Pb age of 1658�12Ma. The lowTh/U ratio (¼ 0·07) of rim 1 is consistent with a meta-morphic origin, whereas the rounded shape also appearsto indicate modification of its original habit owing to dis-solution in melt.We therefore relate the growth of rim 1 tohigh-grade metamorphism. There are two possible explan-ations for the �1658Ma metamorphic age: (1) it marksthe timing of the prograde segment of the UHTgranulite-facies metamorphism in the BBG domain; or (2) the datemay indicate a thermal pulse unrelated to the BM1 meta-morphic event. The age of rim 1 zircon pre-dates the pro-grade metamorphism in the granulites by �46Myr, muchlonger than the errors on single zircon and monazitedates. Given the short-lived nature of the heating pulses(e.g. 10^20Myr) both in modern and ancient orogens, weargue that the 1658Ma metamorphic pulse, which is re-corded only in sample B42D, marks the timing of a short-lived high-grade(?) metamorphic event, nearly 65^70Myrbefore the culmination of the main granulite-facies eventin the BBG domain (see below).SHRIMP U^Pb zircon dates from the psammo-pelitic

granulite record two younger age populations at 1590� 6and 1574�9Ma. There is compelling evidence to interpretthe 1590Ma zircon date as the timing of high-T anatexis.Group III zircons in the felsic granulites with partiallypreserved oscillatory zonation and high Th/U ratios(0·84^0·97) appear to have crystallized from anatectic

melts at this time. Nearly identical zircon ages(1595�5Ma) have also been obtained from the garnetifer-ous granite (sample B233), where oscillatory zoned zirconrecords uniformly high Th/U ratios. The close spatialassociation of the granites with the partially melted,garnet^cordierite^spinel granulite enclaves provides fur-ther support for a pervasive crustal melting event at 1595^1590Ma. The monazite age population at 1604�9Mafrom the granite is within the range of its U^Pb zirconage (1595Ma) and thus marks the timing of mineralgrowth. In contrast, monazite from the older age popula-tion (mean age of 1637�16Ma) possibly represents resetinherited components from the source rocks. Consideringthat the TMax of BM1P metamorphism reached 950^10008C, it is possible that the zircon U^Pb ages inpsammo-pelitic granulites and granites are closure agesrather than growth or crystallization ages. Pb diffusiondata for zircon (Cherniak & Watson, 2000) suggest thatfor a representative 100 mm zircon grain (as in this study),the complete erasure of Pb isotopic memory correspondingto TMax condition would require timescales at such ex-treme crustal metamorphic conditions of up to 100Myr,which is much longer than the duration of typical meta-morphic events at TMax. This would suggest that the1590Ma zircon ages obtained in this study are growthages rather than closure ages. We therefore interpret the1595^1590Ma U^Pb zircon dates as marking the timingof peak UHT metamorphism (BM1P event) in the BBGdomain. Seen in this context, the discordant behaviour ofsome of the Archean detrital zircon cores (Fig. 11b and c)can be related to isotopic disturbance in response to oneor more processes, namely fluid alteration and meta-morphic recrystallization. These findings also negate ear-lier suggestions for a Paleoproterozoic (Bhowmik et al.,2005) or Archean (Roy et al., 2006) age for UHT meta-morphism in the BBG domain.Further time constraints for the isobaric cooling history

of the BM1 granulites come from rim 3 zircon in sampleB42D. This variety has moderate Th/U ratios (¼ 0·32)and yields a concordant 207Pb^206Pb date of 1574�9Ma,which is indistinguishable from the monazite EMP age of1575�9Ma from the CD5 compositional domain.We cor-relate the 1574Ma age with the final freezing of the anatec-tic melt along the isobaric cooling P^T path (BM1R

metamorphism). Considering that the solidus temperatureof biotite melting in a psammo-pelitic bulk-rock compos-ition (as in sample B42D) is �8008C (Johnson et al.,2008), we consider that the anatectic melt froze at aboutthis temperature, effectively constraining the growth tem-perature of rim 3 zircon. We use these chronological con-straints from zircon and monazite to estimate theduration of cooling of the UHT granulites (from 1000 to8008C) to be in the range of 21^16Ma and a time inte-grated cooling rate of 10^138C Ma^1.


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There is both petrological and geochronological evi-dence in support of younger high-grade metamorphicevents that reworked the HT^UHTgranulites of the BBGterrane (Basu Sarbadhikari & Bhowmik, 2008; Bhowmiket al., 2011; this study). Bhowmik et al. (2011) constrainedthe timing of a high-grade recrystallization event between1572�7 and 1555�17Ma, immediately following the BM1

thermal pulse. This metamorphism caused recrystalliza-tion and compositional resetting of 1603^1584Ma mag-matic zircon and monazite in granites (Bhowmik et al.,2011). The monazite compositional domain CD6, whichwas formed during the waning phase of the BM3 meta-morphic event, yielded an even younger age of1539�24Ma. Based on these ages, we constrain thetiming of the BM2 and BM3 high-grade events between1572 and 1539Ma.

Latest Paleoproterozoic to EarlyMesoproterozoic orogenesis in the CITZand its tectonic implicationsWe now use the metamorphic and geochronological find-ings to deduce the T^t history of the BBG granulites(Fig. 14). The BBG crust witnessed three separate high-grade events: (1) at 1658Ma; (2) between 1612 and1574Ma; (3) between 1572 and 1539Ma. These form partof a single latest Paleoproterozoic to Early Mesoprotero-zoic orogenic event. Whereas the metamorphic conditionsfor the first event are unknown, the other two events arecorrelated with BM1 and the combined BM2^BM3 meta-morphic events, respectively, which are well established inthe BBG domain from previous studies. The recognitionof three high-T metamorphic events, the intermediate oneof which locally reached UHT metamorphic conditions,provides strong support for the episodic nature of highheat flow beneath the northern margin of the SouthIndian Block. Monazite and zircon ages indicate thatgranulite-facies metamorphic conditions (T� 8008C), cor-responding to the last two thermal pulses, lasted for about25Myr (Fig.14).We relate these pulses to mantle-scale ther-mal perturbations, which play a critical role in producingextreme thermal conditions, conducive to the generationof UHT metamorphic belts (for general reviews seeHarley, 1998, 2008; Brown, 2007; Kelsey, 2008). One of theimportant consequences of these mantle perturbations ismantle melting and the generation of basaltic magma.Intraplating and/or overplating of voluminous basalticmagma, with or without crustal extension, is said to pro-duce granulite terranes with counter-clockwise P^T paths(Harley,1989; Bohlen, 1991). Clark et al. (2011) recently pro-posed a combination of factors in a collisional orogen togenerate UHT metamorphic conditions at middle tolower crustal depths: (1) the presence of a thick upper crus-tal layer with elevated concentrations of radiogenic heat-producing elements; (2) crustal thickening producing a

wide mountain plateau; (3) low erosion rates maintainingthe plateau for a long duration (of the order of 120Myr).In the BBG domain, the short-lived nature of the UHTthermal pulse (520Myr; this study) and its counter-clockwise P^T path are not consistent with the model ofClark et al. (2011) to produce anomalously hot BBG lowercrust during continental collision. The close spatial associ-ation of mafic igneous rocks of different types (e.g.gabbro-norite, norite, orthopyroxenite and olivine gabbro)and supracrustal granulites, broadly in 1:1 igneous to supra-crustal proportion in exposed outcrops of the BBGdomain (Fig. 2; see also Bhowmik et al., 2005; Bhowmik,2006), provides evidence for episodic mantle melting andconsequently periodic thermal pulses to induce granulite-facies metamorphism in this terrane. Orogenesis of thistype is fundamentally different from classical continent^continent collisional orogeny. A hot orogenesis model hasbeen previously suggested for the tectonothermal evolutionof the BBG domain (Basu Sarbadhikari & Bhowmik,2008; Bhandari et al., 2011). Bhowmik et al. (2011) recentlyproposed an active continental margin setting for thesouthern margin of the CITZ at 1·6Ga, owing to south-ward subduction of oceanic lithosphere beneath the SouthIndian Block (SIB). This produced a magmatic arc(Tirodi biotite gneiss unit in the central and northern do-mains; see Fig. 1) and a coeval back-arc basin (BBGdomain) along the northern margin of the SIB. These twotectonic units with contrasting isotopic compositions(Bhowmik et al., 2011) were sutured along the CentralIndian Shear (CIS) by an arc^continent collision between1·57 and 1·54Ga (Bhowmik et al., 2012; this study) to pro-duce a Proto-Greater Indian landmass (Bhandari et al.,2011; Bhowmik et al., 2011).

Fig. 14. Schematic diagram showing the T^t history of the latestPaleoproterozoic to Early Mesoproterozoic orogenesis in the CITZ.BM1 to BM3 are metamorphic stages; subscripts P and R with BM1,BM2 and BM3 indicate peak and retrograde metamorphic stages(see text for details).


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Table7:

Sum

maryofthermalhistoryofLatePaleoproterozoictoEarlyMesoproterozoicgranuliteterranes

Granuliteterran

esNumber

ofthermal

pulses

andtheir

timings

Metam

orphism

andmag

matism

Durationof

cooling

P–T

path

Tim

ingof

cratonization

Referen

ces

BBG,CITZ,India

Multiple,at

1·66

Ga,

between1·64

and

1·57

Ga,

1·57–1·56an

d1·54

Ga

HTto

UHTgranulitemetam

orphism,synch

ronous

graniteplutonism;UHT(8–9

kbar,910–10008C

)at

1·59–1·6Ga;

final

meltXnat

solidusat

1·74

Ga

16–21Myr

CCW

Between1·66

and1·54

Ga

This

study

Ongole

domain,EGB,India

Multiple,between1·76

and1·72

Ga,

and1·63

and1·61

Ga

Early,dry,HTmag

matism

at1·76–1·72Ga;

UHTmeta-

morphism

(48kb

ar,410008C

)at

1·63

Ga;

final

melt

Xnat

solidusat

1·61

Ga

�20

Myr

CCW

atc.

1·6Ga

S99,D09,B12

SPGC,India

CGC,India

Single

at1·6Ga


and1·62

Ga,

at

1·55

Ga

HTgranulitemetam

orphism

(7·5kb

ar,8508C)

n.k.

CCW

at1·6Ga

C07

Early

granulitemetam

orphism

at1·65–1·66Ga;

syn-

chronousdry

felsic

mag

matism;an

orthosite

pluto-

nism

at1·55

Ga

n.k.

n.k.

at1·55

Ga(?)

C08,R11,S12

SMC,ADMB,India

Single

at1·73–1·72Ga

Peakgranulitemetam

orphism

(8kb

ar,8208C)an

dsyn-

chronousdry,HT,felsic

plutonism

at1·73–1·72Ga

n.k.

n.k.

at1·72

Ga

DA97,S89,

BU06,BH10

Western

Gaw

ler

Craton,Australia


and

1·69

Ga(Kim

ban

orogen

y),an

d1·69

and1·66

Ga

PeakHT–U

HTmetam

orphism

(5–7

kbar,870–9508C)at

1·69

Ga

�30

Myr

CCW

at1·66

Ga

CU13

NorthernGaw

ler

Craton,Australia

Single,between1·6an

d

1·58

Ga

PeakHT–U

HTmetam

orphism

(6·5–9

kbar,850–9258Cat

1·6Ga

�20

Myr

CW

at1·58

Ga

CU11

Oyg

arden

Group,EastAntarctica

Single,between1 ·65

and1·6Ga

Thermal

even

tat

1·65–1·6Ga

n.k.

n.k.

at1·6Ga

KL02

n.k.,notkn

own;Xn,crystallization;CCW/C

W,co

unter-clockwise/clockwise;

BBG,Bhan

dara–Balag

hat

granulitedomain;CITZ,Cen

tral

IndianTectonic

Zone;

EGB,

Eastern

GhatsBelt;

SPGC,ShillongPlateau

Gneissic

Complex;

CGC,Chhotanag

purGneissic

Complex;

SMC,San

dmataComplex;

ADMB,Aravalli–D

elhiMobile

Belt.Referen

ces:

S99,Sen

gupta

etal.(1999);D09,Upad

hyayet

al.(2009);B12,Bose

etal.(2012);C07,Chatterjee

etal.(2007);C08,Chatterjee

etal.(2008);R11,

Rekhaet

al.(2011);S12,San

yal&

Sen

gupta

(2012);DA97,Dasgupta

etal.(1997);S89,Sarkaret

al.(1989);BU06,Buicket

al.(2006);BH10,Bhowmik

etal.

(2010);CU13,Cuttset

al.(2013);CU11,Cuttset

al.(2011);KL02,Kelly

etal.(2002).


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When the thermal history of the Central Indiangranulites is compared with that of well-studied LatePaleoproterozoic to Early Mesoproterozoic granulitesfrom other segments of the Indian shield, and also fromadjoining Gondwana continents (Table 7), several key fea-tures are evident: (1) Late Paleoproterozoic to EarlyMesoproproterozoic orogenesis appears to be an integralfeature of vast stretches of the former Gondwanan frag-ments and suggests that orogenesis in this time period wasmore widespread than previously recognized; (2) severalshort-lived thermal pulses (�20^30Myr) are a general fea-ture of this orogenic episode, locally reaching UHT meta-morphic conditions and spanning over 60^100Myr; (3)high apparent thermal gradients at peak metamorphism,and with counter-clockwise metamorphic P^T paths, arecommon features of these granulite domains; (4) granu-lite-facies metamorphism and subsequent cratonizationstarted at c. 1·72Ga and was completed by 1·62^1·54Ga inmost of these granulite belts.In the context of the continuing debate on the nature of

Proterozoic tectonics between the two supercontinent as-sembly events in the Paleoproterozoic (Columbia) and inthe Late Mesoproterozoic and Early Neoproterozoic(Rodinia), the data summarized above provide strongsupport for the development of a Late Paleoproterozoicto Early Mesoproterozoic landmass, akin to a proto-supercontinent involving the Indian Shield.

ACKNOWLEDGEMENTSS.K.B. acknowledges The Institute for GeoscienceResearch (TIGeR) Visiting Senior Research Fellowship inthe Department of Applied Geology, Curtin University,for zircon geochronology work. S.A.W. acknowledges theJohn de Laeter Centre for Isotopic Research, Perth, foraccess to the SHRIMP facility. We also acknowledge thehelp of Aloka Dey, Honey Das and Prabhakar Naragaduring electron microprobe analyses. Suggestions by K.Mezger, M. Brown and M. Kohn on some aspects ofmonazite growth, and by Prabhakar and N. C. Pant onmonazite analytical protocol have helped us to improvethe paper.We thank Toby Rivers, Nigel Kelly and two an-onymous reviewers for detailed reviews, and Geoff Clarkefor competent editorial handling that led to significant im-provement of the paper.

FUNDINGS.K.B. and A.B. would like to thank the Department ofScience and Technology (DST) for partial funding of theresearch [grant no. IR/S4/ESF-8(9)/2005].

SUPPLEMENTARY DATASupplementary data for this paper are available at Journalof Petrology online.

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