constraints on the proterozoic evolution of the aravalli–delhi orogenic belt (nw india) from...

Constraints on the Proterozoic evolution of the Aravalli–Delhi Orogenic belt (NW India)from monazite geochronology and mineral trace element geochemistry

I.S. Buick a,⁎, C. Clark b, D. Rubatto c, J. Hermann c, M. Pandit d, M. Hand e

a Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland 7602, Stellenbosch, South Africab Department of Applied Geology, Curtin University of Technology, Perth, W.A. 6845, Australiac Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australiad Department of Geology, University of Rajasthan, Jaipur, Rajasthan, Indiae School of Earth and Environmental Sciences, University of Adelaide, Adelaide, S.A. 5005, Australia

a b s t r a c ta r t i c l e i n f o

Article history:Received 12 February 2010Accepted 16 September 2010Available online 24 September 2010

Keywords:U–Pb geochronologyMineral REE geochemistryPolymetamorphismRajasthanNW India

The timing and extent of polymetamorphism in the Mangalwar and Sandmata Complexes from the Aravalli–Delhi Orogenic Belt of Rajasthan (NW India) remains contentious, with Archaean, Paleoproterozoic andNeoproterozoic events having previously been postulated. Monazite SHRIMP U–Pb and electron microprobe(EPMA) chemical ages obtained from metasediments in the amphibolite-facies Mangalwar Complex showthat it was metamorphosed at ca. 0.97–93 Ga, with evidence in one sample for an earlier event at ca. 1.82 Ga.Monazite and garnet REE patterns from metapelitic and metapsammitic rocks are characterised by smallnegative Eu anomalies, suggesting that they experienced amphibolite-facies conditions during both events.In contrast, granulite-facies metamorphism in the Sandmata Complex occurred at ca. 1.72 Ga, although themonazite U–Pb system was partially disturbed during a localised high-strain overprint that occurred at ca.1 Ga. A comparison of the REE patterns of porphyroclastic and neoblastic garnets in the Sandmata Complexrocks shows that the second event occurred at amphibolite-facies conditions, consistent with Zr thermometryof rutile in the shear fabric. Garnet REE patterns show that relict granulite-facies garnet porphyroclasts arepresent even in Sandmata samples so sheared and rehydrated that they are now amphibolite-facies schists.REE patterns of isotopically disturbed Sandmata Complex monazite suggest that the age variations generallyreflect partial Pb loss due to solid-state deformation in the later shear zones. New (ca. 1 Ga) monazite growthunder amphibolite-facies conditions occurred only in the most intensely recrystallised and rehydrated rock.In metapelitic rocks the magnitude of Eu anomalies developed in garnet and accessory phases such asmonazite is particularly sensitive as to whether K-feldspar grew at the same time. In the case of the Sandmatagranulites this distinction makes it possible to determine abundance of relict granulite-facies, and newlyformed amphibolite-facies garnet, in extensively sheared, rehydrated and recrystallised granulites, and theextent of new monazite growth during the shearing event.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Understanding the geological history of polymetamorphic high-grade orogenic belts requires the integration of geochronological datawith a number of other techniques, including field-based observa-tions, petrology and geochemistry. A particularly useful approach isprovided by linking U–Pb geochronology to the trace elementcomposition of dated accessory phases and the growth/resorptionhistories in co-existing garnet e.g. Hermann and Rubatto (2003);Buick et al. (2006a); Kelly et al. (2006).One polymetamorphosed terrain where the timing and distribution

of high-grade metamorphism is problematic is the Banded GneissComplex within the Aravalli–Delhi Orogenic Belt (ADOB) of NW India

(Fig. 1). TheBandedGneiss Complex has been generally viewed as amid-to late-Archaean gneiss complex that was variably reworked during thePalaeoproterozoic (Sharma, 1988; Roy and Kröner, 1996;Weidenbeck etal., 1996; Roy et al., 2005).More recently it has been suggested that partsof the Banded Gneiss Complex have a wholly Proterozoic history (Buicket al., 2006b). However, for large parts of the Banded Gneiss Complexrelatively little is known about the timing of metamorphism, the gradesreached, or the significance of the ages obtained in previous studies.In this contribution we present a geochronological (monazite

SHRIMP U–Pb and electron microprobe chemical dates) and LA-ICP-MS mineral trace element study of amphibolite- and (variablyreworked) granulite-facies rocks from the Banded Gneiss Complex.The data confirm the importance of Palaeoproterozoic and earlyNeoproterozoic tectonics in the Aravalli–Delhi Orogenic Belt. Inconjunction with the trace element geochemistry of major silicateminerals, the new geochronological data place constraints on the

Lithos 120 (2010) 511–528

⁎ Corresponding author. Tel.: +27 21 8083128; fax: +27 21 8083129.E-mail address: [email protected] (I.S. Buick).

0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.lithos.2010.09.011

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metamorphic grade of these events, and highlight the behaviour ofaccessory phases during ductile deformation.

2. Geological setting

The Banded Gneiss Complex (BGC; Gupta, 1934; Heron, 1953;Fig. 1a) comprisesmedium- tohigh-grademetasedimentary rocks and arange of mafic and felsic orthogneisses. It is locally unconformablyoverlain by theweak- tomoderately-metamorphosedMesoproterozoicAravalli andNeoproterozoicDelhi Supergroups. TheBGCwasdividedbyGupta (1934) into two regions, BGC-I and BGC-II (Fig. 1). To the south(BGC-I) tonalite–trondhjemite–granodiorite suites and granites haveyielded ca. 3.3–2.5 Ga emplacement ages (Weidenbeck and Goswami,1994; Roy andKröner, 1996). In contrast, although the northerndomain(BGC-II — the present study area) was previously suggested to have asimilar Archaean origin (e.g. Sharma, 1988, 1999; Roy et al., 2005),limited U–Pb zircon provenance data (Buick et al., 2006b) suggests thatsome BGC-II metasediments were deposited in the Palaeoproterozic.The BGC-II can be sub-divided into: a) supracrustal and meta-

igneous granulites of the Sandmata Complex; and b) the MangalwarComplex, a hetereogeneous assemblage of amphibolite-facies meta-sedimentary rocks, meta-granitoids and amphibolites (Guha andBhattacharya, 1995; Fig. 1b). The Sandmata Complex can be furthersub-divided into domains dominated by: a) metapelitic to metap-sammitic granulites (e.g. around Sandmata Hill, Fig. 1b); and b) a suiteof magmatic charnockite–enderbite rocks that have partially beenrecrystallised under amphibolite-facies conditions (e.g. Bhinai;Fig. 1b). In both cases, the granulite-facies rocks are separated byrocks of the Mangalwar Complex by 10 to 100 metre wide high-gradeshear zones. In the Bhinai area (Fig. 1a), felsic orthogneisses of theMangalwar Complex show evidence for localised partial melting inthese shear zones, possibly as the result of structurally channelledfluid infiltration that fluxed melting (Roy et al., 2005).

Granulite-facies rocks from the Sandmata Complex record apolyphase metamorphic history. Early coarse-grained assemblagesrecord P–T conditions of ~7–10 kbar and ~800–900 °C (Guha andBhattacharya, 1995; Dasgupta et al., 1997; Roy et al., 2005; Saha et al.,2008; here termed M1SC). These have been partially recrystallised insteeply-dipping mylonitic high-strain zones (Dasgupta et al., 1997;Roy et al., 2005; Saha et al., 2008; here termed M2SC). These shearzones have been interpreted to be related to the shear zones thatseparate the Sandmata and Mangalwar Complex rocks. It is unclearwhether M1SC and M2SC are part of a single tectonothermal event(Sharma, 1988) or represent unrelated events (Guha and Bhattacharya,1995; Dasgupta et al., 1997).Based on textural criteria, amphibolite-facies rocks from the

Mangalwar Complex were also interpreted to be polymetamorphicby Sharma (1988). He suggested that an early event (here termedM1MC) of probable Archaean age was locally overprinted by an upperamphibolite-facies, a locally anatectic event (here termed M2MC) ofprobable Palaeoproterozoic age. Inferred emplacement ages in therange ca. 1.69–1.64 Ga have been obtained from syn-kinematic felsicintrusions in the Mangalwar Complex (Fareeduddin and Kröner,1998; Roy et al., 2005) using the zircon Pb evaporation technique, andhave been interpreted to date the second of these episodes. However,Buick et al. (2006b) showed that the protolith to one clasticsedimentary unit in the Mangalwar Complex was deposited no earlierthan ca. 1.8–1.7 Ga, and that anatexis both in it, and associated ca.1.72 Ga felsic orthogneisses gneisses, occurred at ca. 0.95 Ga.

3. Analytical methods

Major element analysis of minerals (WDS) was carried out at theUniversity of Melbourne on a Cameca SX50 electron microprobe, andat the Research School of Earth Sciences, Australian NationalUniversity (RSES, ANU) using a Cameca SX100 electron microprobe.

Fig. 1. a) Regional geology of the Aravalli mountains region of NW India; and b) regional geology of the Banded Gneiss Complex, Rajasthan, NW India, showing sample locations(modified after Heron, 1953; Gupta et al., 1997). Abbreviations: ADOB=Aravalli-Delhi Orogenic Belt; CITZ= Central Indian Tectonic Zone; EGB= Eastern Ghats Block; NIB=NorthIndian Block; SIB = South Indian Block.

512 I.S. Buick et al. / Lithos 120 (2010) 511–528

Analytical conditions were 15 kV, 25 nA (University of Melbourne),and 15 kV, 10 nA (ANU). Data were reduced using Cameca PAP(Melbourne) or X-Phi (ANU) matrix corrections. Fe3+ in garnet wasestimated by calculating structural formulae on the basis of 12oxygens and 8 cations per formula unit. Representative analyses aregiven in Electronic Supplementary Table 1, and compositionalvariations are summarized in Table 1.Trace element analyses of the minerals were performed at the

RSES, in thin section or in epoxy mount (two monazite samples), intime-resolved mode using a pulsed 193 nm ArF Excimer laser with120 mJ energy at a repetition rate of 5 Hz coupled to an Agilent 7500quadrupole ICP-MS. Si contents determined by electron microprobewere used as internal standards for silicate minerals. StoichiometricSi, Ce and Ti were used as internal standards for zircon, monazite andrutile, respectively. Ablation spot sizes of 54–86 μm were used forgarnet and plagioclase, 19–32 μm for monazite and 19–24 μm forrutile. NIST-612 glass was used as the external standard for allminerals except for rutile, where NIST-610 was used. Accuracy andreproducibility were monitored through the use of BCR-2 G glass as asecondary standard. Reproducibility for most elements was typically1–5% (1σ) for multiple analyses. Trace element concentrations of the`monazite in the epoxy mount were analysed by positioning ablationspots over SHRIMP analysis spots (see below). Representativeanalyses are given in Electronic Supplementary Table 2.U–Th–Pb analyses of monazite mounted in epoxy from two

samples were obtained using the SHRIMP II ion microprobe at theResearch School of Earth Sciences (ANU). The method used generallyfollowed Williams et al. (1996); in addition, energy filtering (cf.Rubatto et al., 2001) was used to eliminate the interference on 204Pb.The data were collected in sets of six scans throughout the masses.Analysis spots were typically ~20 μm in diameter. The measured206Pb/238U ratios were corrected using reference monazite from theThompson Mine (1766 Ma). The data were corrected for common Pbon the basis of the measured 204Pb, assuming surface derivation andusing the common Pb composition of Broken Hill galena. Datareduction was undertaken using the Squid software of Ludwig(2001). Ages were calculated using Isoplot/Ex software (Ludwig,2000). Isotopic ratios and single spot ages are reported with 1σ error,whereas mean ages are given at the 95% (τ.σ) confidence level (c.l.).Prior to analysis, back-scattered electron (BSE) images were obtainedwith a Cambridge S360 scanning electron microscope using a voltageof 20 kV, current between of ~3 nA and a working distance of 15–20 mm. Isotopic data are given in Table 2.

The EPMA dating of monazite from seven samples was undertakenin polished thin sections using a Cameca SX-51 electronmicroprobe atAdelaide Microscopy, the University of Adelaide. Individual monazitegrains were identified in thin section and imaged by back-scatteredelectron (BSE) to determine internal structure. Operating conditions of20 kV accelerating voltage, a beam current of ~50 nA and a spot size of~3–4 μmwere used for all analyses. Th, U and Pbwere analysed on theTh Mα, U Mβ and Pb Mβ lines respectively using a PET crystal.Background measurement positions were selected to minimise peakoverlapswith other elements. Online and offline numerical correctionswere made for interferences that could not be avoided. An offline Cecorrection, accounting for the addition of apparent Pb due to a secondorder Ce escape peak, reduced Pb concentrations by ~100 ppm. Totalcounting times of 320 s (Pb), 80 s (Th) and 160 s (U) were used toimprove counting statistics and age resolution. A full suite of elements(P, Si, Al, Ce, La, Y, Nd, Sm, Gd, Dy, Pr, Er and Lu) were also analysed toallow PAPmatrix corrections to be applied, and to assess the quality ofthe analysis. Huttonite (ThSiO4), UO2 andNBS824 standardswere usedfor calibration of Th, U and Pb respectively. The MAD monazite U–Pbstandard, was analysed regularly through the analytical sessions.Repeat analyses of MAD yielded an age of 513±14 Ma (2σ; n=27)compared to a TIMS age of 514Ma (Foster et al., 2000). Individual spotages are calculated by substituting the measured Th, U and Pb into theage equation of Montel et al. (1996) and iteratively solving for Pb.Uncertainties for individual spot ages are calculated by propagatingcounting errors through the age equation. Individual population ageswere calculated using the weighted average function in Isoplot/Ex(Ludwig, 2000), using its criterion for elimination of statistical outliers.Analyses are given in Electronic Supplementary Table 3.

4. Mangalwar Complex: petrology and mineral majorelement chemistry

4.1. Raj29 and Raj30

These are samples of upper amphibolite-facies metapelite from theMangalwar Complex near Bhinai (Fig. 1b). Raj29wasdescribedbyBuicket al. (2006b). Both samples are stromaticmigmatites charcterised by1–10mm-wide, discontinuous layers rich in biotite and sillimanite, whichtogether define the fabric, and 2–10 cm-wide leucosomes of grano-blastic quartz and plagioclase and, in the case of Raj30 only, very raremicrocline. Both the layer types locally contain 5–10 mm diameterpoikiloblasts of garnet that contain inclusions of sillimanite, biotite,

Table 1Summary of the composition of major silicate minerals.

Mineral Grt XMg Plg Kfs Bt Ti(pfu)

Qtz Ms Sil Ky Ilm Ru

Alm Py Gr Sps Adr Mol.% XAn XOr XMg

SampleRaj-29 Alm63-72Py20-29Grs5-7Sps1-2 0.28–0.31; 0.34–0.36 (M) 0.48–0.54 (M) 0.25–0.37 * * *

to 0.22 at rimRaj-30 Alm64-72Py19-26Grs6-10Sps1-3 0.29–0.31; 0.35–0.38 (M) 0.90–0.92 (M) 0.46 (M) 0.38–0.41 * * *

to 0.21 at rim 0.44–0.46 (G)Raj-42 Alm82-83Py13-15Grs2-4Sps1 (1) 0.14–0.16 0.21-0.25 (M) 0.37–0.40 0.37–0.38 * *

Alm81-84Py10-13Grs2-4-5Sps1 (2) 0.11–0.14Raj-46 0.30–0.32 (M) * * *Raj-52 Alm70-80Py16-26Gr3Sps1 (1,P) 0.27(C) to 0.17(R) * 0.88 (G) 0.60–0.61 (1, P) 0.16–0.19 * * * * *

Alm75-80Py14-21Gr3-5Sps1 (2) 0.15–0.19 0.86–0.91 (P) 0.53–0.54 (2, R) 0.34–0.37Raj-55 Alm71-72Py22-24Gr0-1 Sps1Adr1-4 (1) 0.34–0.36 (P) 0.88–0.91 (P) 0.60–0.61 (2,R) 0.49–0.51 * * * *

Alm68-73Py18-22Gr4-13Sps0-1Adr0 (2) 0.21–0.24Raj-58 Alm68-71Py25-29Grs3Sps1 (1,P) 0.26–0.30 0.40–0.41 (P) 0.72–0.87 (P, G) 0.68–0.72 (1,G) 0.46–0.59 * * * *

Alm68-77Py19-27Grs3-7Sps1 (2) 0.19–0.27 0.33–0.35 (R) 0.87–0.93 (R) 0.66–0.68 (2,R) 0.42–0.49Raj-59 Alm72-73Py23-24Gr2-4Sps1Adr0-1 0.24–0.25 0.32–0.33(P) 0.76–0.89 (P)

0.21–0.23 (R) 0.90–0.93 (R) 0.63–0.65 (R) 0.45–0.47Raj-66A Alm77-78Py12-14Gro4-6Sps5 (1) 0.13–0.15 0.34–0.37 0.46–0.47 (M) 0.33–0.34 * * * *

Alm76-80Py02-12 Gro6-9Sps5 (2) 0.11–0.15

Abbreviations: (1)=generation 1; (2)=generation 2; M=in matrix; G=included in garnet; P=porphyroclast; *=present; pfu=per formula unit.

513I.S. Buick et al. / Lithos 120 (2010) 511–528

plagioclase and monazite. Monazite also occurs in the external biotitefoliation. Apatite, zircon and ilmenite are additional accessory phases.Garnet grains show little major element compositional zoning(XMg=0.28–0.30; Table 1) except immediately adjacent to matrixbiotite where XMg decreases to values as low as 0.22.

4.2. Raj42

This sample is a fine-grained metapsammitic schist from theMangalwar Complex, sampled ~30 km south of the southern portion ofthe Sandmata Complex (Fig. 1b). It contains the mineral assemblage:quartz–biotite–garnet–plagioclase–ilmenite–monazite–apatite–zircon.More aluminous metapelites at the same outcrop additionally containmuscovite and sillimanite. Garnet in Raj42 occurs as 3–5 mm diameter,anhedral to subhedral porphyroblasts that texturally show evidence fortwo episodes of growth. A core region contains inclusions of quartz,plagioclase, biotite,monazite and ilmenite. These domains have irregularovergrowths that are more inclusion-rich and have faceted rims; theboundary between the two zones is sharp, irregular, and defined by ahigh concentration of ilmenite inclusions (Fig. 2b). Despite the texturalevidence, there is little major element compositional difference betweenthe two garnet types (Table 1).

4.3. Raj46

This is a garnet-free, quartzofeldspathic metasediment that isinterlayered with sillimanite-bearing metapelites. It contains quartz–plagioclase–biotite–muscovite–zircon–monazite and was sampled

~23 km due east of Sandmata Hill (Fig. 1b). In the thin section, biotiteand muscovite define a strong foliation.

5. Sandmata Complex: petrology

5.1. Raj52 and Raj 58 (metapelitic layers)

Both samples are protomylonitic, granulite-facies metapelitesfrom Sandmata Hill (Fig. 1b). Raj 52 contains quartz-sillimanite-kyanite-garnet-biotite-K-feldspar-plagioclase. Ilmenite, rutile, mona-zite, zircon and apatite are accessory phases. Raj 58 shows a similarmineral assemblage, with the exception that it contains much morerutile, and much less ilmenite, in the mylonitic matrix.In both samples garnet occurs as anhedral porphyroclasts (~0.5–

1.5 mm grain diameters; Grt1SC) wrapped by an anastamosing S2SCfoliation containing fine-grained biotite, fibrolitic sillimanite, fine-grainedkyanite, ilmenite and rutile. A texturally distinct secondgenerationoffine-grained, subhedral poikiloblastic garnet (typically ~50–150 μm indiameter; Grt2SC; Fig. 2c) overgrows this foliation and irregularly rimsGrt1SC (Fig. 2d). Kyanite occurs in fine-grained aggregates interpreted tobe pseudomorphs after coarser-grained prismatic sillimanite. Theseaggregates are also aligned within S2SC. Replacement textures betweenkyanite and fine-grained sillimanite in S2SC were not observed.Porphyroclastic Grt1SC contains inclusions of biotite, ilmenite, K-

feldspar and coarse, prismatic sillimanite. Grt1SC is Fe-rich (Table 1),has uniform Ca and Mn contents and shows a decrease in XMg fromcore to rim (Table 1). The fine-grained inclusion-rich Grt2SC is similarin major element composition to Grt1SC rims (Table 1) but formed bydifferent process. Recrystallised plagioclase and K-feldspar aretypically more Na-rich and K-rich, respectively, than porphyroclastic

Table 2SHRIMP monazite data.

Age Age

Spot name % Pb206 U Th Th/U 207Pb/235U 1 σ 206Pb/238U 1 σ ρ 206Pb/238U age 1 σ 207Pb/206Pb 1 σ Conc

comm ppm ppm % % 204corr(Ma)

204corr(Ma)

%

RAJ29-1 0.33 727 22,845 32 1.479 3.61 0.1537 1.38 0.382 921 12 922 69 100RAJ29-2.1 0.26 1403 33,573 25 1.501 2.36 0.1538 1.21 0.510 922 10 951 42 97RAJ29-3.1 0.18 1393 20,236 15 1.537 2.07 0.1588 1.46 0.702 950 13 934 30 102RAJ29-4.1 0.15 1331 29,215 23 1.514 2.28 0.1565 1.19 0.524 937 10 933 40 100RAJ29-4.2 0.24 1275 27,412 22 1.497 2.25 0.1570 1.23 0.546 940 11 903 39 104RAJ29-5.1 0.18 1408 24,045 18 1.502 1.74 0.1545 1.18 0.679 926 10 942 26 98RAJ29-6.1 0.80 13,043 193,034 15 1.769 4.05 0.2038 1.35 0.333 1196 15 707 81 169RAJ29-7.1 0.22 2267 44,536 20 1.620 2.45 0.1645 1.34 0.546 982 12 969 42 101RAJ29-8.1 0.27 1590 31,483 20 1.555 2.64 0.1598 1.35 0.511 955 12 946 46 101RAJ29-8.2 0.00 1536 30,993 21 1.610 1.95 0.1578 1.30 0.667 945 11 1041 29 91RAJ29-9.1 0.12 1241 13,743 11 1.530 1.87 0.1574 1.22 0.652 942 11 942 29 100RAJ29-10.1 0.25 1605 21,849 14 1.492 2.53 0.1570 1.27 0.503 940 11 897 45 105RAJ55-5.1 0.27 470 39,682 87 2.978 9.14 0.2325 1.88 0.206 1348 23 1486 169 91RAJ55-4.1 1.00 470 43,016 95 3.821 4.37 0.2924 1.58 0.362 1653 23 1524 77 108RAJ55-7.1 0.20 458 49,250 111 3.442 2.36 0.2580 1.62 0.688 1480 21 1563 32 95RAJ55-6.1 0.52 426 54,295 132 3.661 2.93 0.2733 1.58 0.540 1558 22 1570 46 99RAJ55-11.1 0.23 769 46,507 63 3.676 1.83 0.2728 1.35 0.741 1555 19 1581 23 98RAJ55-3.1 0.08 674 43,989 67 3.696 2.10 0.2690 1.38 0.655 1536 19 1618 30 95RAJ55-10.1 0.57 641 57,275 92 4.034 2.35 0.2931 1.50 0.639 1657 22 1621 34 102RAJ55-1.1 0.18 626 53,900 89 3.889 2.08 0.2800 1.48 0.713 1591 21 1638 27 97RAJ55-10.2 0.13 745 64,281 89 3.923 2.28 0.2807 1.48 0.647 1595 21 1649 32 97RAJ55-8.2 0.06 3893 73,427 19 3.975 1.20 0.2842 1.08 0.899 1612 15 1651 10 98RAJ55-9.1 0.25 552 50,443 94 4.289 3.17 0.2997 2.39 0.755 1690 36 1693 38 100RAJ55-2.1 0.19 1771 51,785 30 4.426 1.81 0.3076 1.16 0.642 1729 18 1703 26 102RAJ55-8.1 0.00 3288 63,310 20 4.271 1.20 0.2940 1.09 0.911 1661 16 1721 9 97RAJ55-9.2 0.00 612 58,786 99 4.436 2.23 0.3045 1.59 0.715 1713 24 1726 29 99RAJ55-12.1 0.00 1907 66,211 36 4.123 1.22 0.2910 0.92 0.758 1646 13 1675 15 98RAJ55-12.2 0.00 1292 65,285 52 3.939 1.30 0.2836 1.17 0.897 1609 17 1638 11 98RAJ55-12.3 0.00 7115 58,095 8 4.284 0.87 0.2969 0.82 0.942 1676 12 1709 5 98RAJ55-13.1 0.00 4107 67,377 17 4.411 0.89 0.3035 0.84 0.942 1709 13 1722 5 99RAJ55-14.1 0.00 892 58,336 68 3.520 1.31 0.2595 1.10 0.840 1487 15 1594 13 93RAJ55-15.1 0.00 615 70,755 119 4.237 1.41 0.2976 1.20 0.848 1679 18 1683 14 100RAJ55-16.1 0.08 594 49,818 87 3.878 1.46 0.2854 1.20 0.822 1618 17 1597 16 101

Buick et al. (2010) Lithos.


equivalents (Table 1). Fine-grained biotite in S2SC has two differentcompositions; relatively Ti-poor and relatively Mg-rich, and more Ti-rich and relatively Mg-poor (Table 1). Monazite occurs commonly asanhedral grains in the S2SC foliation and also as inclusions in Grt1SC.

5.2. Raj55 and Raj 59 (leucogranitic layers)

Raj55andRaj59are protomylonitic leucogranite interlayers fromtheSandmata Hill (Fig.1b). Raj55 contains an assemblage of quartz–sillimanite–garnet–biotite–K-feldspar–plagioclase, with accessory il-menite, rutile, zircon, monazite and apatite. Raj59 contains a similarassemblage, but has higher modal abundances of Grt1SC, monazite andzircon, lower abundances of late sillimanite and Grt2SC, and addition-ally contains rare fine-grained kyanite pseudomorphs after prismaticsillimanite.In both samples, subhedral Grt1SC grains have major element

compositions that show little zoning and are similar to those preservedonly in cores of garnet from the metapelitic samples (Table 1). Theabundance of Grt2SC is much less than in themetapelites and it is slightlymore Ca-rich than Grt1SC in the same sample (Table 1).Monazite occurs as inclusions in Grt1SC and in the recrystallised

matrix.Monazite in thematrix ofRaj59 showsevidenceof internal plasticdeformation (deformation twinning, and the development of “core andmantle” textures) that is lacking in monazite inclusions in Grt1SC.

5.3. Raj66A (shear zone-hosted schist)

This sample (Fig. 1b) was obtained from a shear zone north of thevillage of Isermand. The shear zone separates supracrustal granulites

from a narrow sliver of Sandmata Complex meta-igneous rocks. Theshear zone trends NE–SW, dips moderately steeply to the SE andaccommodated oblique sinistral movement (Gupta and Choudhuri,2002). It forms part of a conjugate set that on a regional scale allowedthe BGC-II to accommodate late-stage, bulk NW–SE directed short-ening (Gupta and Choudhuri, 2002). Charnockite from the shear zoneis patchily rehydrated. Igneous pyroxenes and K-feldspar are locallystatically partially replaced by aggregates of metamorphic clinopy-roxene+biotite+hornblende. Similar assemblages, accompanied bydynamic recrystallisation of feldspars, occur in mm-wide shear zones.In contrast, metapelitic rocks appear to have more completely

recrystallised. Raj66A is a medium- to fine-grained, K-feldspar-freeschist (Fig. 2g) characterised by alternatingmm-wide layers containingoriented biotite, garnet, fine-grained kyanite and sillimanite, and layerscontaining granoblastic quartz and plagioclase (Table 1). Garnet occursas 1–1.5 mm diameter, anhedral to subhedral grains that are locallyembayed by biotite, sillimanite and ilmenite. Although all the garnetgrains are Fe-rich, based on Ca contents there some evidence for twogarnet generations; low-Ca cores (Grt1SC), and slightly more grossular-rich rims and whole grains (Grt2SC; Table 1). Monazite occurs primarilyin the biotite-rich domains that define the foliation.

6. Mineral trace element chemistry

6.1. Garnet and feldspars (Mangalwar Complex)

Garnet from Raj29 and -30 have similar chondrite-normalised(McDonough and Sun, 1995) REE patterns, characterised by anincrease in relative abundance from La to Lu, and small negative Eu

600 μm

Grt1SC

Grt2SC

Sil+Qtz

Bt

Kfs

Ilm

Raj52

S2SC

2 mm

Grt1

Grt2

Bt

Raj42Grt2

5 mm

Qtz+Kfs+Pl

Grt1SC

Grt1

Grt2SC + Bt + Rt + Sil/Ky

Bt+Sil

5 mm

GrtQtz+Pl±Kfs

Grt

400 μm

Rt

Grt1SC

Grt2SC

Grt2SCIlm

KfsRt

Bt+Ky±Sil

Grt1SC

Grt2SC

1 mm

Raj66ARaj58Raj58

Raj30

Grt1SC

S2SC

S2SC

Raj52

Grt1SCGrt2SC

Bt

Sil

Sil + Qtz

Grt2sC

100 μm

Grt2SC

Grt1SC

S2SC

a)

e) f) g)

b) c) d)

Fig. 2. a) thin section photomicrograph of migmatitic metapelite Raj30 (Mangalwar Complex; plane-polarised light); b) thin section photomicrograph of metapelitic schist Raj42(Mangalwar Complex), showing a sharp truncation surface between two generations of garnet (plane-polarised light); c) BSE image of Raj52 (Sandmata Complex), showing aporphyroclast of early garnet (Grt1SC) wrapped by a fine-grained, mylonitic S2SC sillimanite–biotite (Sil–Bt) fabric overgrown by poikiloblastic Grt2SC; d) BSE image of Raj52, showingthe sharp truncation surface between Grt1SC and rim of pokiloblastic Grt2SC; e) thin section photomicrograph of Raj58 (Sandmata Complex), showing fragments of Grt1SC wrapped byan anastomosing S2SC fabric plane-polarised light); f) BSE image showing two generations of garnet in Raj58; these have different trace element contents and REE patterns (see text);g) thin section photomicrograph of Raj66A (Sandmata Complex), showing schistose fabric. The sample contains two different garnet types; these appear quite similar in texture buthave different REE patterns and origins (see text; plane-polarised light).


anomalies (Eu/Eu*=0.46–0.61 and 0.67–0.98, respectively; Fig. 3a,b;Electronic Supplementary Table 2). From core to rim, garnet in Raj30shows a central plateau and then rimwards decrease in Y, P (Fig. 3c),and HREE abundance, on which are superimposed several excursionsto local maxima.Texturally composite garnet from Raj42 are characterised by REE

profileswith small tomoderately negative Eu anomalies in the cores andovergrowths (Eu/Eu*=0.22–0.30 and Eu/Eu*=0.46–0.62, respective-ly). Compared to REE patterns in the cores, those in the overgrowthsshowamarked depletion of theHREE relative to theMREE (Fig. 3d), andY contents that overlap the lowest values in the cores.Plagioclase is characterised by REE patterns of decreasing relative

abundance from La to Lu, with large positive Eu anomalies (Eu/Eu*=10–87; Fig. 3). The very rare K-feldspar in Raj30 has steeper REEpatterns similar in shape to plagioclase (Fig. 3b), but with loweroverall relative abundances and a steeper relative decrease from theLREE to MREE. As a result, abundances for some of the MREE, inparticular Sm, were below detection limits. Minimum positive Euanomalies of 58–60 were calculated using Sm detection limits as anestimate of maximum concentration. Plagioclase in the differentsamples contains ~10–40 ppm Ti, 380–740 ppm Sr, and ~120–325 ppm P (Supplementary Datafile 2).

6.2. Monazite (Mangalwar Complex)

All monazite grains have REE patterns characterised by relativeenrichment of the L-MREE over the HREE (Fig. 4; Electronic Supple-

mentary Table 2) and small tomoderately negative Eu anomalies (0.23–0.71; mostly N0.4). Monazite from Raj42 shows REE patterns withdifferent steepness, depending on textural setting; the steepest REEpatterns occur for monazite occurring in the matrix.

6.3. Early (Grt1SC) garnet and feldspars (Sandmata Complex)

In contrast to garnet from the Mangalwar Complex, porphyroclasticGrt1SC is characterised by large negative Eu anomalies (Eu/Eu*=0.01–0.08; Fig. 5). REE patterns are generally flat or show a slight decrease inREE abundance from theMREE to theHREE (Fig. 5a, b, c). In contrast, theREE patterns in Raj55 show a variation from relative HREE enrichment(grain cores) to depletion (grain rims) with respect to the MREE(Fig. 5d). Intra-grain variations in the abundance of Y and the HREE areeither very limited (Raj52, Raj58), or showplateau regionswith (Raj55)or without (Raj59) small initial increases in abundance, followed by asteep decrease in concentration within 100–300 μm of the grain rim(Fig. 5e). The relatively low-Ca garnet grains (Grt1SC) from theextensively recrystallised schist Raj66A show similar REE patterns tothose in the granulites (Fig. 5f).The REE patterns for porphyroclastic plagioclase and K-feldspar

are characterised by steeply decreasing relative abundance from theLREE to HREE and either small (plagioclase: Eu/Eu*=1.5–3) or largepositive (K-feldspar: Eu/Eu*=19–45) Eu anomalies (Fig. 5). Com-pared with plagioclase from Manglawar Complex metasediments(Fig. 2), the positive Eu anomalies in plagioclase co-existing with K-feldspar from the Sandmata Complex were much smaller and Ti

Raj29 Mangalwar Complexmetapelitic migmatite

0.01

0.1

1

10

100

1,000

10,000

La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu

Raj30 Mangalwar Complexmetapelitic migmatite

0.01

0.1

1

10

100

1,000

10,000


KfsGrt

Plg

GrtPlg

C

R

0 5 10 15Distance (mm)

0

50

100

150

200

250

P (p

pm)

0

100

200

300

400

500

600

Y (p

pm)

P Y

rim rim

Min

eral

/Cho

ndrit

e

Min

eral

/Cho

ndrit

e

Min

eral

/Cho

ndrit

e

Raj30 Mangalwar Complex

Raj42 Mangalwar Complexgarnet-biotite schist

0.01

0.1

1

10

100

1,000

10,000


PlgGrt overgrowthGrt core

Raj46 Mangalwar Complexmuscovite-biotite schistPlg

c) d)

a) b)

Fig. 3. a) and b) Chondrite-normalised REE patterns of garnet and co-existing feldspars from Raj29 and -30, Mangalwar Complex; c) rim to rim trace element variation in garnet fromRaj30; d) chondrite-normalised REE patterns of garnet and co-existing plagioclase from Raj46, Mangalwar Complex. Note that in Fig. 3b Sm concentrations in Kfs are below detectionlimits, and therefore a full REE pattern could not be determined. Chondrite normalisation factors from McDonough and Sun (1995).


0.1

1

10

100

1,000

10,000

100,000

1,000,000


Monazite near Grt rimMonazite in grain mount

Raj29 (Mangalwar Complex) Monazite

Raj30 (Mangalwar Complex) Monazite

Min

eral

/Cho

ndrit

e

Min

eral

/Cho

ndrit

e

0.1

1

10

100

1,000

10,000

100,000

1,000,000


Monazite in matrixMonazite in Grt

0.1

1

10

100

1,000

10,000

100,000

1,000,000


Monazite

Raj46 (Mangalwar Complex)

0.1

1

10

100

1,000

10,000

100,000

1,000,000

La Ce Pr NdPmSm Eu Gd Tb Dy Ho Er Tm Yb Lu

Monazite in matrix

Raj42 (Mangalwar Complex)

Monazite included in early Grt

Fig. 4. Chondrite-normalised REE patterns for monazite from the Mangalwar Complex.

0.01

0.1

1

10

100

Grt1SCKfs

Raj52 Sandmata Complex

1000

10000


La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu

Min

eral

/Cho

ndrit

eM

iner

al/C

hond

rite

Inferred granulite-facies garnet (Grt1SC)Plg

Raj66a : reworkedSandmata Complex

0.01

0.1

1

10

100

1,000

.01

.1

1

10

100

1,000

10,000

La Ce Pr NdPmSmEu Gd Tb Dy Ho Er TmYb Lu

Raj58 Sandmata Complex

KfsGrt1SC Grt1SC

0.01

0.1

1

10

100

1,000

10,000

La Ce Pr NdPmSmEu Gd Tb Dy Ho Er TmYb Lu

Raj59 Sandmata Complex Grt Leucogranite Layer

KfsPlg

10,000

0.01

0.1

1

10

100

1,000

10,000Raj 55 Sandmata Complex

KfsPlg

Grt1SC

0.0 0.5 1.0Distance (mm)

0

100

200

300

400

500

600

Y (p

pm)

Rim Rim

Raj55 Grt1SC (Sandmata Complex)

a) b) c)

d) e) f)

Fig. 5. a) and b) Chondrite-normalised REE patterns for Grt1SC and porphyroclastic feldspars in: Raj 52 and Raj 55, respectively; c) rim to rim compositional variation in Y in a Grt1SCporphyroclast, Raj55; d), e) and f) chondrite-normalised REE patterns for Grt1SC garnet and feldspars in Raj 58, Raj 59 and Raj66A, respectively. All samples are from the SandmataComplex.


contents were much higher (~50–360 ppm; Supplementary Table 2).The exception is plagioclase from the K-feldspar-free schist Raj66A,which has both the largest positive Eu anomaly (Eu/Eu*=90–161;Fig. 5f) and had amongst the lowest Ti contents (10–12 ppm).

6.4. Monazite (Sandmata Complex)

Monazite is characterised by large negative Eu anomalies (Eu/Eu*=0.01–0.05), similar to Grt1SC in the same rocks, and by highlyvariable relativedepletion in theHREE (Fig. 6; Electronic SupplementaryTable 2). The exception came from three analyses from Raj66A, whichhad Eu/Eu*=0.14–0.59. The remainder of monazite from this samplehad large negative Eu/Eu*, similar to the other Sandmata samples.

6.5. Grt2 garnet (Sandmata Complex)

Poikiloblastic Grt2SC in Raj 52 and -58 has REE patterns (Fig. 7a)that show an increase in relative abundance from the LREE to theMREE, a marked decrease from the MREE to the HREE, and variablydeveloped small negative to small positive Eu anomalies (Eu/Eu*=0.22–1.67, mostly N0.40). Overall, Grt2SC has much lower Y(Fig. 7b), Li, Ti, Sc, Zr, and MREE–HREE abundances than Grt1SC(Electronic Supplementary Table 2). Grt2SC was too fine-grained toanalyse in Raj59, and in Raj55 only two analyses, slightly contami-nated by biotite, were obtained. Their REE patterns are similar to thoseobtained from Raj52 and 58 (Eu/Eu*=0.28 and 0.40; Fig. 7a).In Raj66A, Grt2SC is also characterised by modest negative to small

positive Eu anomalies (Eu/Eu*=0.31–1.33; Fig. 7c). Compared withGrt1SC in the same rock it has much lower Ti contents (Fig. 7d), andcompared with Grt2SC from the granulite-facies rocks shows relativelyless depletion of the HREE with respect to the MREE.

6.6. Zircon and rutile (Sandmata Complex)

Trace element concentrations were determined from low Th/U,anatectic zircon from Raj55, in an epoxy grainmount previously dated

by Buick et al. (2006b). This zircon has Ti contents in the range 5.6–20 ppm (Electronic Supplementary Table 2). REE patterns arecharacterised by relatively flat HREE to MREE, a decrease in relativeabundance from the MREE to the LREE, large negative Eu anomalies(Eu/Eu*=0.03 to 0.13; Fig. 8) and small positive Ce anomalies. Theflat zircon MREE–HREE patterns are consistent with the presence ofgarnet in the rock (e.g. Hermann and Rubatto, 2003).Rutile from the M2SC mylonitic fabric has variable but generally

high Cr, V and Nb contents, and Zr in the range 71–448 pm (ElectronicSupplementary Table 2). There appears to be no clear correlationbetween the Zr content and textural setting.

7. SHRIMP monazite U–Pb geochronology

7.1. Raj29migmatitic amphibolite-faciesmetapelite (Mangalwar Complex)

BSE images of monazite grains from migmatitic metapelite Raj29show only weak euhedral zoning, or grains are unzoned (Fig. 9a).Monazite grains analysed from the grain mount by SHRIMP have Th, Uand Th/U in the ranges 1.4–4.5 wt.%, 0.07–0.22 wt.% and 11–32,respectively (Table 2). The isotopic compositions of eleven out oftwelve analyses from ten grains define a statistically homogeneouspopulation and yields a concordia age of 941±9 Ma (Fig 9b). This ageis equal within error of the imprecise 938±32 Ma LA-ICP-MS age(Buick et al., 2006b) obtained from rare, narrow low Th/U metamor-phic zircon from the same sample.

7.2. Raj55 granulite-facies leucogranite (Sandmata Complex)

BSE images from monazite in the grain mount show a range ofzoning types (Fig. 8c); the majority of the anhedral, 80–200 μmdiameter grains show weak patchy zoning and irregular, discontin-uous, and very narrow (b10 μmwide) bright rims. The patchy zoninglocally transects domains that show weak euhedral zoning (Fig. 9c).The latter were too narrow to be analysed by SHRIMP. Fourteenanalyses were obtained from 11 grains in the grain mount.


Raj55 Sandmata Complexgarnet leucogranite

Mon

azite

/Cho

ndrit

eM

onaz

ite/C

hond

rite

0.1

1

10

100

1,000

10,000

100,000

1,000,000


Raj52 Sandmata Complexgranulite-facies metapelite


Raj58 Sandmata Complexgranulite-facies metapelite

10,000

100,000

1,000,000

0.1

1

10

100

1,000


Raj59 Sandmata Complexgarnet leucogranite


Raj66A Sandmata Complexamphibolite-facies metapelitic schist

Fig. 6. Chondrite-normalised REE patterns for monazite in metapelites and leucogranitic interlayers from the Sandmata Complex.


The analyses show a wide range of U (0.04–0.39 wt.%), Th (4.0–7.3 wt.%) and Th/U (20–130; Table 2). The isotopic data are discordantand have apparent 207Pb–206Pb spot ages between 1726±29 Ma and1486±169 Ma. In two grains where multiple spots were analysed,

significant age differences occurred between spots analysed withinthe same BSE zone (one of these is shown in Fig. 9c). The SHRIMPmonazite data lie on adiscordia segmentwith upper and low intercepts of1680±50Ma and 1087±220Ma (MSWD=1.8), respectively (Fig. 9d).The seven oldest analyses have the same 206Pb/207Pb within uncertainty,are all within 3% of concordance and yield a weighted mean 207Pb–206Pbage of 1714±10Ma (MSWD=1.5). This is taken to be the best estimatefor the age of metamorphism, and is within error of the weighted mean207Pb–206Pb age of a concordant population of the low Th/U zircon fromthe same sample (1723±10Ma; Buick et al., 2006b). The remainder ofthe low Th/U zircon analyses were also discordant as a result of (poorlyconstrained) late Mesoproterozoic/early Neoproterozoic Pb loss (Buicket al., 2006b).

8. Monazite EPMA chemical ages

8.1. Raj30migmatitic amphibolite-faciesmetapelite (Mangalwar Complex)

Monazite grains fromRaj30 are similar in shape, size and appearancein BSE to those from Raj29, dated by SHRIMP (above). Seventy-twoEPMA analyses were made from monazite grains in thin section fromRaj30, both in garnet and in the rock matrix. Spot ages are in the rangeca. 1.18–0.61 Ga (Electronic Supplementary Table 3), and the relativeprobability distribution is slightly asymmetric and skewed towardslower ages (Fig. 10a). After rejection of two analyses on statistical

low-Caearly Grt1SC

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Higher-Ca Grt2SC rimsand whole grains

0.01

0.1

1

10

100

1,000

10,000

Min

eral

/Cho

ndrit

e

Raj66A: amphibolite-facies schist

1.0 1.5 2.0 2.5CaO (Wt %)

0

100

200

300

400

500

600

700

Ti (p

pm)

Eu/Eu* < 0.15(Grt1SC)Eu/Eu* > 0.2 (Grt2SC)

mixedanalyses?

Raj66A: amphibolite-facies schist

a) b)

d)c)

Raj55 Grt1SC

0.01

0.1

1

10

100

1,000

10,000


Raj52Raj55Raj58

0 100 200 300 400 500 600 700 800Y (ppm)

0.0

0.5

1.0

1.5

2.0

Eu/

Eu*

Porphyroclastic Grt1SCRaj52Raj55Raj58

Late Grt2SCRaj52Raj55Raj58

Grt2SC in shear fabricM

iner

al/C

hond

rite

Fig. 7. a) Chondrite-normalised REE patterns for fine-grained, poikiloblastic Grt2SC in granulites Raj52, Raj55 and Raj58 from the Sandmata Complex. The shaded field shows the REEpatterns for Grt1SC from Raj55 for reference; b) plot of Eu/Eu* vs. Y (ppm) for Grt1SC and Grt2SC from the Sandmata Complex granulites; c) chondrite-normalised REE patterns fortexturally late garnet (overgrowths and whole grains), Raj66A; d) discrimination between early and late-formed garnet in terms of Ti (ppm) vs. CaO (wt.%), Raj 66A. Analyses withlarge negative Eu anomalies and low Ti contents are assumed to be physical mixtures of the two garnet types.

0.01

0.1

1

10

100

1,000

10,000


Raj55:~1.72 Ga metamorphic zircon

Zirc

on/C

hond

rite

Eu/Eu* = 0.03-0.05

Th/U = 0.02-0.52

Fig. 8. Chondrite-normalised REE patterns for low Th/U anatectic zircon from Raj55(Sandmata Complex), analysed fromanepoxy grainmount studied byBuick et al. (2006b).


grounds, a weighted mean age of 933±29 Ma (MSWD=6.4) wascalculated for this sample, with the higher than expected MSWDindicating residual scatter in the data beyond analytical uncertainty. Inorder to seewhethermonazite includedwithin garnet might have beenprotected and hence yield a better constrained age, a weighted meanage was also calculated using only those analyses obtained frommonazite inclusions in garnet. The calculated weighted mean ageindeed shows somewhat less scatter than that obtained from entiredataset, but is identical to it within error (972±35Ma, MSWD=4.2).

8.2. Raj42 amphibolite-facies metapelite (Mangalwar Complex)

Monazite in this sampleoccurs as small, equant toelongate (20–50 μmdiameter) grains that show little zonation in BSE images. A total ofseventy-seven EPMA analyses were obtained from this sample frommonazite grains included in the cores of the texturally two-phase garnetsand in the external foliation. The data show a clear difference in agedepending on textural setting (Fig. 10b; Electronic SupplementaryTable 3). All analyses obtained from monazite included in garnetcores yield spot ages in the range ca. 2.02–1.66 Ga (weighted meanage=1824±43Ma, MSWD=1.5, n=23), whereas those in the matrixyield spot ages in the range ca. 1.14–0.82 Ga (weightedmeanage=969±18Ma, MSWD=1.06, n=54; Fig. 10b). The ca. 1.82 Ga population ischaracterised by a wide range of Y (0.11–2.2 wt.%; Fig. 10c), Th (0.26–4.5 wt.%) andU (0.12–1.2 wt.%) contents, and variable but low Th/U (1.0–9.3; mostly b5). The ca. 0.97 Ga population is characterised by a muchmore restricted and lower range of Y contents (0.10–0.37 wt.%; Fig. 10c),moreuniformTh (2.7–5.9 wt.%) andU(0.56–1.1 wt.%) contents, andTh/Uin the range 3.6–8.3 (mostly 4–5; Electronic Supplementary Table 3).

8.3. Raj46 amphibolite-facies metapsammite (Mangalwar Complex)

In this sample monazite occurs as ~50–80 mm diameter, roundedequant to slightly elongate grains. It shows very weak, broad

compositional zoning in BSE. Fifty electron microprobe analyseswere obtained from monazite in this sample. Spot ages are in therange 1.20–0.79 Ga (Electronic Supplementary Table 3), and on arelative probability diagram they define a single population, with aweighted mean age of 967±24 Ma (MSWD=0.92; Fig. 10d).

8.4. Raj52 granulite-facies metapelite (Sandmata Complex)

Monazite typically occurs as 40–80 μm diameter grains in themylonitic matrix and, less commonly, in porphyroclastic Grt1SC. BSEzoning patterns in this, and other Sandmata samples are similar tothose for Raj55 (dated by SHRIMP, above). The very narrow,discontinuous bright rims were too narrow to analyse. As aconsequence, ages were obtained only from within the BSE-dark,weakly zoned interior of monazite grains in the sheared rock matrix.Ninety-eight EPMA spots were analysed for this sample and yield arange of spot ages from ca. 1.84 to 1.26 Ga (Fig. 11a; ElectronicSupplementary Table 3). The relative probability diagram suggeststhat the data define a single population that is slightly asymmetric,tailing towards younger ages, with a peak at ca. 1.6 Ga. The weightedmean of ninety-four of the ninety-eight analyses is 1587±17 Ma(MSWD=2.8), with the higher than expectedMSWD indicating someexcess scatter. This age is younger than the inferred timing of peakmetamorphism in the Sandmata Complex, and the reasons for thisdiscrepancy are discussed later.

8.5. Raj58 granulite-facies metapelite (Sandmata Complex)

Monazite grains are rounded, 20–80 μm in diameter. They occurboth in Grt1SC and in the protomylonitic matrix. The monazite locallyshows discontinuous, 1–5 μm wide, BSE-bright rims, as described forRaj55. Otherwise it is characterised by weak, irregular and broadzoning in BSE images. The majority of analyses were obtained frommonazite inclusions within porphyroclastic Grt1SC. One-hundred and

200 μm

942±11

955±12

945±11

937±10

940±11 1732±20

1597±42

1726±29

1721±9

1651±10

1649±32

1621±34

840

880

920

960

1000

1040

1080

1120

0.13

0.15

0.17

0.19

0.21

0.23

1.2 1.4 1.6 1.8 2.0

Concordia Age = 941 ± 9 Ma(95% confidence)MSWD (of concordance) = 0.52,Probability (of concordance) = 0.47

included

not included

0.18

0.22

0.26

0.30

0.34

1 2 3 4 5

1800

1600

1400

1200

1000

207Pb/235U 207Pb/235U

206 P

b/23

8 U

206 P

b/23

8 U

Raj29 Mangalwar Complex Raj55: Sandmata Complex

Oldest concordant grains:Weighted mean 207Pb-206Pb age =1714 ± 10 Ma (MSWD = 1.5; n = 7)

Intercepts at 1680 ± 50 Ma and1087 ± 220 Ma (MSWD = 1.8)

data ellipes are 2σ data ellipes are 2σ

a)

b) d)

c)

Fig. 9. a)Representativemonazite BSE image fromsampleRaj29 (MangalwarComplex)with SHRIMPspot 206Pb–238U ages (±1σ); b) concordia diagram forU–Pb isotopic data fromRaj29;c) representative monazite BSE images from sample Raj55 (Sandmata Complex) with SHRIMP spot 207Pb–206Pb ages (±1σ); d) concordia diagram for U–Pb isotopic data from Raj55.


five EPMA analyses were obtained from Raj58. The spot ages rangefrom ca. 1.91 Ga to ca. 1.28 Ga (Electronic Supplementary Table 2),and the resulting relative probability distribution (Fig. 11b) ismarkedly asymmetric, tailing towards younger ages. A statisticallymeaningful weighted mean age could not be calculated for allanalyses because of excess scatter in the data. However, analysesfrom monazite included in Grt1SC yielded a weighted mean age of1737±15 Ma (MSWD=2.5, 85 of 91 analyses) still with a smallamount of residual scatter. This age is identical within uncertainty tothat inferred by Buick et al. (2006b) for the peak of granulite-faciesmetamorphism and timing of emplacement of charnockite–enderbitegranitoids in the Sandmata Complex.

8.6. Raj59 granulite-facies leucogranite (Sandmata Complex)

Monazite occurs as ~100–200 μm diameter, rounded to anhedralinclusions in Grt1SC, along Grt1SC grain boundaries and as grains withscalloped grain boundaries in the recrystallised mylonitic matrix.Taken as a whole, there appears to be multiple age peaks in theprobability distribution in the ca. 1.2–1.7 Ga range (Fig. 11c), and astatistically meaningful age could not be obtained from the completedataset, or frommonazite occurring only in themylonitic matrix. Agesobtained from the latter are mostly b1.6 Ga. In contrast, 38 of 43analyses of monazite included in Grt1SC yield an age of 1696±20 Ma(MSWD=1.1), within error of that obtained from Raj58, and theSHRIMP age obtained from monazite in Raj55.

8.7. Raj66A amphibolite-facies metapelite (Sandmata Complex)

Monazite is uncommon and relatively small (30–80 μm). It occurspredominantly in the external biotite foliation. Eighty-nine EPMAanalyses were obtained from 10 grains. The analyses show a widerange of Y (0.05–8.5 wt.%) and Th (0.05–8.4 wt.%) contents, and Th/U(0.38–56), and yield spot ages ranging from ca. 1.83 Ga to ca. 0.71 Ga(Fig. 11d; Electronic Supplementary Table 3). The ages show abimodal distribution. The great majority falls within the ca. 1.4–1.8 Garange, and was obtained from grains in both the matrix and in garnet.Together, they do not yield a statistically meaningful age populationdue to excess scatter. However, 9 out of 10 analyses obtained withinGrt1SC yielded aweightedmean age of 1733±33 Ma (MSWD=1.1). Asubordinate population consists of 12 spot ages that scatter around1 Ga. This minor age group has Y, Th and Th/U that fall towards thelower end of the range measured for the monazite as a whole.

9. Accessory phase thermometry (Sandmata Complex)

Thermobarometry of rocks from the Sandmata and MangalwarComplexes has been the subject of several other studies (e.g. Sharma,1988; Roy et al., 2005; Saha et al., 2008) and is not the primary focus ofthis contribution. Moreover, the XMg of porphyroclastic Grt1SCconverges with that of the low-XMg neoblastic Grt2SC garnet in themost amenable rocks for P–T estimates; the strongly rehydrated andreworked metapelitic rocks. This suggests that the major element

Num

ber

Relative probability

RAJ 30 (Mangalwar Complex)

Mean = 933 ± 29 Ma,95% conf.MSWD = 6.4 (n = 70 of 72),

400 600 800 1000 1200 1400 16000

5

10

15

20

400 800 1200 1600 2000 2400 2800 3200

Age (Ma)

Num

ber


Monazite inclusions in garnetMean = 1824 ± 43, 95% conf.

MSWD = 1.5, probability = 0.054

Monazite in matrixMean = 969 ± 18, 95% conf.Wtd by data-pt errs only, 0 of 54 rej.MSWD = 1.06, probability = 0.36

RAJ 42 (Mangalwar Complex)

0

2

4

6

8

10

12

14

16

18

500 700 900 1100 1300 1500 1700Age (Ma)

Num

ber


RAJ46 (Mangalwar Complex)

Mean = 967 ± 24 (95% conf.)MSWD = 0.92, probability = 0.63

14

12

10

8

6

4

2

0

d)c)

a) b)

Fig. 10. a) and b) relative probability distributions and histograms of EPMA ages obtained from Raj30 and Raj42 (both Mangalwar Complex), respectively; c) Y (ppm) vs. spot age formonazite from Raj 42; d) relative probability distributions and histograms of EPMA ages obtained from Raj46.


composition of the porphyroclastic garnetwas largely reset during thelater shearing event.Nevertheless, granulite-facies rocks from the Sandmata Complex

contain two accessory phases, zircon and rutile, whose trace elementcontents can be used for estimating the temperature of the M1SC andM2SC events, respectively. Ti-in zircon thermometry using thecalibration of Ferry and Watson (2007) was performed on previouslydated low Th/U, 1.72 Ga anatectic zircon from leucogranitic layerRaj55 from the Sandmata Complex. Zircon from the Raj55 zircon grainmount of Buick et al. (2006b) contains 5.6–20 ppm Ti (ElectronicSupplementary Table 3). Because the peak granulite-facies assem-blages in Raj55 and -59 contain ilmenite, rather than rutile, the Ti-in-zircon thermometer needs to be corrected for the reduced activity ofTiO2. For most felsic magmatic and metamorphic rocks aTiO2 is in therange 0.6–0.9 for rutile-absent parageneses (e.g. Hayden andWatson,2007). Using these constraints the Ti-in-zircon thermometer yieldstemperatures between 797±43 °C (1σ; aTiO2=0.6) and 757±49 °C(1σ, aTiO2=0.9) for Raj55.For the M2SC assemblage, temperatures in samples Raj55, Raj58

and Raj59 were estimated using the Zr content of rutile co-existingwith zircon and quartz, using the calibration of Ferry and Watson(2007). Si and Rb contents were used to monitor the possibility ofcontamination of the LA-ICP-MS by garnet, sillimanite or biotite, withwhich the rutile is intergrown. All three samples produced a similarrange of temperatures: Raj55 (541–667 °C; average=614±48 °C,1σ; n=12); Raj58 (547–675 °C; average=639±36 °C, 1σ; n=24);Raj59 (553–667 °C; average=639±42 °C, 1σ; n=16). There was nosystematic difference in the temperatures obtained from rutile withinthe external S2SC fabric and rutile included in garnet (Grt2SC) that

overgrows this fabric. The data suggest that the rutile in this fabricgrew under mid-amphibolite-facies conditions.

10. Discussion

10.1. Mineral REE patterns as indicator of metamorphic grade

The trace element abundances, and in particular, the chondrite-normalised REE patterns of major silicate, and dated accessoryminerals can potentially place important constraints on sub-solidusand anatectic metamorphic processes. A number of studies haveshown that the REE patterns of garnet are a sensitive monitor ofmetamorphic processes and conditions (e.g. Bea, 1996; Hermann andRubatto, 2003; Buick et al., 2006a, Kelly et al., 2006; Gregory et al.,2009; Kotková and Harley, 2010).In this study the REE patterns for garnet, monazite and zircon from

the Sandmata Complex, and garnet andmonazite from theMangalwarComplex show systematic differences in the magnitude of the Euanomaly. Sandmata Complex M1SC garnet, monazite and zircon showlarge negative Eu anomalies, M2SC garnets show small negative tomoderate positive Eu anomalies, and Magalwar Complex amphibo-lite-facies garnet and monazite show small negative Eu anomalies(Figs 5, 6, 7, and 12a). Because Eu may occur as either Eu+2 or Eu+3,variations in Eu/Eu* may reflect variations in oxidation state. Mineralssuch as zircon additionally contain Ce, which also occurs with variablevalence (Ce3+ and Ce4+). If oxygen fugacity (fO2) is the only controlon the valence state of Eu then positive Ce anomalies should be not beaccompanied by Eu anomalies in zircon. However, this is generally notobserved (Hoskin and Schaltegger, 2003), suggesting that other

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RAJ 66A ReworkedSandmata Complex

MSWD = 2.8 (all Mnz in matrix)

MSWD = 3.6 (all Mnz data)

MSWD = 2.5 (Mnz incs in Grt)

MSWD = 4.3 ( all Mnz)

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MSWD = 5.3 ( Mnz in matrix)

MSWD = 2.4

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a) b)

c) d)

Fig. 11. Relative probability distributions and histograms of EPMA ages obtained from Sandmata Complex samples: a) Raj52; b) Raj58; c) Raj59; and d) Raj66A.


factors control the size of mineral Eu anomalies. Because feldsparsstrongly concentrate Eu, processes affecting feldspar stability andfractionation have commonly been invoked to explain the size andmagnitude of Eu anomalies in zircon. Zircon and co-genetic mineralstypically have similar magnitude Eu anomalies (e.g. Bea, 1996;Hermann and Rubatto, 2003; Gregory et al., 2009; Kotková andHarley, 2010), in turn suggesting that feldspar stability controls thesign and magnitude of the Eu anomaly of these minerals. Therefore,we view the feldspar growth or consumption as playing thecontrolling role in understanding the differences in the magnitudeand sign of Eu anomalies of the different minerals in this study.In incompletely recrystallised low- to medium-grade metamor-

phic rocks (e.g. those with coarse-grained igneous protoliths), sub-solidus garnet formation is typically coronal and strongly spatiallyassociated with proximity to plagioclase feldspar, which typicallyprovides the Ca and Al for garnet formation. Sub-solidus garnetformed from very small-scale, diffusion-controlled reactions thatconsume plagioclase typically acquire the positive Eu/Eu* of thereactant feldspar (e.g. Hermann and Rubatto, 2003; Thöni et al., 2008;Gregory et al., 2009), and are characterised by positive Eu anomalies.In contrast, in metapelites where metamorphic equilibration occursmore completely and on a larger length-scale under lower greenschistto amphibolite-facies conditions, garnet in textural and chemicalequilibrium with plagioclase typically shows negligible or smallnegative Eu anomalies e.g. Harris et al. (1992), Bea (1996), Pyle et al.(2001), Corrie and Kohn (2008; calculated from tabulated data intheir online data supplement).At uppermost amphibolite to granulite grades, K-feldspar is stable

in metapelitic bulk compositions due to dehydration melting ofmuscovite and/or biotite. The production of peritectic garnet+K-feldspar, in particular, typically occurs at temperatures in excess ofN750–800 °C through dehydration melting reactions that consumebiotite. In contrast to lower grades, garnet formed with K-feldsparthrough dehydration melting in the granulite-facies typically shows amuch larger negative Eu anomaly (Watt and Harley, 1993; Bea, 1996;Hermann and Rubatto, 2003; Kotková and Harley, 2010). Comparablylarge negative Eu anomalies occur in these garnets, regardless ofwhether they reside in melanosomes or leucosomes, or as a functionof bulk rock Eu/Eu* (e.g. leucosomes that are predominatly crystal-lised liquids or cumulate in character). Moreover, the magnitude ofthe negative Eu anomaly in garnet from metapelitic granulites issimilar to that in magmatic garnet from peraluminous K-feldspar-bearing granites or pegmatites (e.g. Thöni et al., 2008; Villaros et al.,2009). This suggests that the co-formation with K-feldspar is a

primary control on the magnitude of the negative Eu anomaly in co-existing garnet. Which feldspars (plagioclase or plagioclase+K-feldspar) occur in a rock, and whether they grew or were consumedwith respect to accessory phase growth appears to control themagnitude of the Eu anomaly in the REE patterns of minerals such aszircon, monazite, apatite and allanite e.g. Hermann and Rubatto(2003) Kelly et al. (2006), Buick et al. (2006a), Rubatto et al. (2006),Gregory et al. (2009).These general observations allow the following conclusions to be

reached regarding the metamorphic evolution of metapelitic rocksfrom the Mangalwar and Sandmata Complexes.

10.2. Metamorphic evolution of the Sandmata Complex

The granulite-facies metapelites and interlayered leucogranitelayers from the Sandmata Complex contain coarse-grained porphyr-oclastic garnet (Grt1SC), andmonazite with comparably large negativeEu anomalies (Eu/Eu*=0.01–0.08; Figs 5, 6, and 12a). The garnet REEpatterns are consistent with growth in the presence of K-feldspar,which occurs as inclusions in the Grt1SC. Mineral inclusion relation-ships in the Sandmata Complex granulites suggest that Grt1SC wasproduced by a dehydration melting reaction of the general form:

Qtz + Bt + Sil + Pl = Grt + Kfs + Liq + Ilm ð1Þ

This is consistent with Ti-in-zircon saturation thermometryperformed on anatectic zircons in Raj55, which yielded temperatureswell in excess of the wet solidus. Since new zircon growth in anatecticrocks typically occurs during cooling and melt crystallisation after themetamorphic peak (e.g. Kelsey et al., 2008), the Ti-in-zircontemperatures are likely to represent minimum estimates for thepeak of M1SC.While porphyroclastic K-feldspar in the Sandmata Complex

granulites shows a large positive Eu anomaly, plagioclase from thesame rocks has a much smaller Eu anomaly (Fig. 5). Although somestudies in the literature show large positive Eu anomalies for co-existing feldspars in anatectic or granitic magmatic rocks (Bea, 1996;Bea and Montero, 1999; Acosta-Vigil et al., 2010), others show thesame relationship as seen in Fig. 5 (Bea, 1996; Gregory et al., 2009).The reason for this difference is unclear. Plagioclase is a reactant inreaction (1), above), whereas K-feldspar is a peritectic product of thesame reaction. Both plagioclase and K-feldspar could also potentiallyhave crystallised from the melt on cooling. Reactant plagioclaseshould show large positive Eu anomaly, similar to that developed in

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Fig. 12. Distinction betweenmineral growth under amphibolite- and granulite-facies conditions in terms of a) DyN/LuN vs. Eu/Eu* (garnet, monazite and zircon), and b) Eu/Eu* vs. Ti(plagioclase and K-feldspar).


plagioclase from amphibolite-facies Mangalwar Complex metasedi-ments (Fig. 3). Therefore, the plagioclase in the Sandmata complexgranulites is unlikely to have a peak metamorphic origin. Onealternate possibility is that the Grt1SC+K-feldspar show complemen-tary large negative and positive Eu anomalies because they formedtogether as products of the melting reaction, and that the plagioclasecrystallised from the melt during cooling. Sequestration of Eu intoearlier formed peritectic Kfs could result in a suppressed Eu anomalyin late crystallising plagioclase. If this is the case then the magnitudeof the positive Eu anomaly in plagioclase could be used to determinewhether it was a reactant duringmelting or a late-stage crystallisationproduct.The second generation of fine-grained garnet (Grt2SC) that occurs

within, and overprinting zones of dynamic recrystallisation showsmarkedly different REE patterns to that of the coarse-grained Grt1SCporphyroclasts. Grt2SC REE patterns are characterised by moderatenegative to positive Eu anomalies (Eu/Eu*=0.22–1.67; Fig. 12a),overall much lower Ti, Zr, Y, REE abundances and marked relativeenrichment in the MREE with respect to either the LREE or HREE(Figs 7, 12a). Given the small size of these grains, and the similarity ofmajor element composition between the Grt1SC and Grt2SC it ispossible that some of the analyses of Grt2SC that show the largestnegative Eu anomalies result from analysing physical mixtures ofGrt2SC and relict Grt1SC. Nevertheless, the positive Eu anomalies insome Grt2SC analyses suggest that they formed under sub-solidusconditions as a result of short-range (diffusion-controlled) reactionsthat involved feldspar breakdown (cf. Hermann and Rubatto, 2003).Most of the biotite in the metapelitc granulites is texturally late andoccurs in the S2SC foliation; some of this may formed by reactionbetween granulite-facies minerals and crystallising partial melt at theend of M1SC, however textural relationships are obscured by the lattershearing. S2MC contains biotite with a range of TiO2 contents on a thinsection scale; the more Ti-rich biotite is similar to rare inclusions ingarnet and may have formed during high-temperature melt crystal-lisation. The low-Ti biotite probably formed as part of S2SC via areaction that also produced rutile and Grt2SC ie:

Kfs + Grt1SC + Pl + Ilm + H2O=melt = Sil= Ky + Bt + Grt2SC + Rt:

ð2Þ

The extremely low Y and HREE contents of Grt2SC, compared withGrt1SC suggests that the former grew in a Y- and HREE-depletedenvironment due to the stability of zircon and only limitedbreakdown of Grt1SC, which retained most of the bulk rock HREE.This is consistent with the occurrence of abundant relict Grt1SC in all ofthe samples.The occurrence of rutile within the S2SC biotite-sillimanite±

kyanite shear fabric and as inclusions in Grt2SC, places a furtherconstraint on this reaction. Zr-in-rutile thermometry from severalsamples yields a range of temperatures (541–675 °C) at, or below thewet pelite solidus at 5–10 kbar. This suggests that the narrow (mm- tocm-wide) shear zones most likely formed synchronously with limitedwater infiltration at sub-solidus conditions, rather than due to meltcrystallisation.One sample (Raj66A), from a major shear zone separating

supracrustal and igneous units on the Sandmata Complex, is a schist.Basedonmajor and trace element chemistry it contains two generationsof relatively coarse-grained garnet. The first is characterised by REEpatternswith large negative Eu anomalies (Eu/Eu*=0.03–0.11), similarto that of Grt1SC from granulite-facies metapelites in the SandmataComplex. The second generation (Grt2SC; Fig. 7c and d) is slightly moreCa-rich and shows moderate negative to positive Eu anomalies (Eu/Eu*=0.31–1.33). Although the sample now lacks K-feldspar, the Grt1SCREE pattern strongly suggests that K-feldspar was part of the earliestassemblage preserved in the rock, and that this assemblage therefore

originally reached granulite-facies conditions, similar to Raj52 and -58.The magnitude of the Eu anomaly in Grt2SC is consistent with theoccurrence of plagioclase,without K-feldspar, asobserved in thepresentmineral assemblage. InRaj66A, the plagioclase lacks the high Ti contentsand small positive Eu anomaly found in plagioclase that co-exists withK-feldspar in the Sandmata metasedimentary granulites (Fig. 12b).Instead, it shows low-Ti contents and a very large positive Eu anomaly,similar to that found in K-feldspar-free amphibolite-facies metasedi-ments from the Mangalwar Complex (Fig. 12b). This suggests that theplagioclase in this rock completely recrystallised during the rehydrationof an originally granulite-facies assemblage that consumed K-feldspar(Eu source) and produced biotite (Ti sink).In summary, the mineral REE patterns in Raj66A suggest that this

rock originally contained co-existing K-feldspar and garnet, but thatthe K-feldspar was completely replaced by biotite during hydration inthe shear zone, via a reaction similar to that inferred to have occurredon a much more limited scale in the granulites. Extensive breakdownof Grt1SC is also consistent with the higher MREE–HREE concentra-tions, and relative enrichment of the HREE over the MREE in Grt2SCfrom this sample compared to others where there was limited Grt2SCdevelopment.

10.3. Relating monazite growth to polymetamorphism in the SandmataComplex

Monazite from the Sandmata Complex granulite sample Raj55yielded a discordant array on the concordia diagram (Fig. 9). Theseven oldest grains gave a weighted mean 207Pb–206Pb age of 1714±10 Ma, and the analyses as a whole have an upper intercept at ca.1.68 Ga, and a poorly defined late Mesozoic/early Neoproterozoiclower age intercept (Fig. 9). The 1714±10 Ma age is within error ofthat obtained by Buick et al. (2006b) from low Th/U zircon from thesame sample, and the emplacement age of granitic bodies elsewherein the complex (Sarkar et al., 1989; Buick et al., 2006b). Monazite fromother samples from the Sandmata Complex yielded PalaeoproterozoicEPMA spot ages with asymmetric age distributions skewed towardsthe younger ages. The oldest EPMA ages (Raj 58, Raj59 and Raj66A)were obtained from monazite included in Grt1SC. Their weightedmean ages (1737±15 Ma, 1696±20 Ma and 1733±33 Ma; respec-tively) are similar to the more precise SHRIMP monazite age fromRaj55.LA-ICP-MS trace element geochemistry allows a constraint to be

placed on the behaviour of monazite in these recrystallised, intenselydeformed rocks. Regardless of the range of monazite EPMA ages, allmonazite analysed for trace elements by LA-ICP-MS from the samplesRaj55, -58 and -59 is characterised by very large negative Euanomalies (Eu/Eu*=0.01–0.06; Fig. 6), the magnitudes of which aresimilar to those of Grt1SC in the same rocks (Fig. 5). Co-existing garnetand monazite typically show Eu anomalies of similar magnitude andsign (e.g. Hermann and Rubatto, 2003; Buick et al., 2006b; Kelly et al.,2006; Rubatto et al., 2006). Significantly, although the secondgeneration garnet that formed in the mylonitic fabric shows eithersmall negative or positive Eu anomalies, with the exception of Raj66A(discussed separately below) this was not observed in any of themonazite grains on the 20–30 μm scale of the LA-ICP-MS analyses.This suggests that little or none of the monazite dated in thesesamples formed through the reactions that formed the new, sub-solidus garnet (Grt2SC). Therefore, we suggest that all the monazite inthese samples formed in the presence of K-feldspar, regardless of theirrange of spot ages. Moreover, the steepness of the REE pattern fromthe MREE to HREE suggests that the monazite formed in the presenceof garnet (e.g. Hermann and Rubatto, 2003; Buick et al., 2006b; Kellyet al., 2006; Rubatto et al., 2006; Raj29 or Raj30 vs. Raj46 in thisstudy).The weighted mean EPMA ages range from 1719±18 Ma (Raj58)

to 1587±17 Ma (Raj52), which might be interpreted to indicate


several episodes of Palaeoproterozoic to early Mesoproterozoicmetamorphism. However, on textural and trace element grounds allsamples contain only a single early garnet generation that predatesmylonitisation. This suggests that the range of monazite EPMA agesdoes not reflect multiple episodes of granulite-facies metamorphism,each (based on REE patterns) producing garnet+K-feldspar throughdehydration melting reactions, and each therefore requiring succes-sively higher temperatures as the assemblage becamemore refractorydue to melt loss. Instead, all monazite appears to have formed duringthe first (granulite-grade)metamorphic event with themonazite ageshaving been subsequently variably reset.The range of spot ages obtained frommonazite via SHRIMP or EPMA

could have resulted from a number of processes, including Pb volumediffusion, fluid-mediated recrystallisation, Pb loss due to internal latticestrain, or new monazite growth. As discussed above, the last of thesepossibilities is unlikely (except for Raj66A, below), at least during thesecondmetamorphic event to have affected the rocks (M2SC).Moreover,for even the smallest monazite grains observed in the granulites andleucogranitic layers (30–50 μmradius), the Pbdiffusiondata of Cherniaket al. (2004) imply closure temperatures to Pb volume diffusion inexcess of 900 °C for typical regional metamorphic cooling rates, wellabove the temperatures reached duringM2SC. Thus Pb volume diffusioncannot explain the range of ages.EPMA data show that the oldest chemical ages occur for monazite

included in Grt1SC, suggesting that this garnet has shielded somemonazite from resetting compared to monazite in the matrix, wherethe effects of M2SC–D2SC shearing were greater. It is known that the U–Pb system in monazite may potentially be reset by fluid-mediateddissolution-precipitation reactions (e.g. Townsend et al., 2000). Giventhat high-strain zones are commonly associated with fluid influx andthat there is preferential resetting of EPMA ages in monazite in themylonitic matrix, this may be a viable mechanism for monazite ageresetting in the Sandmata Complex. However, monazite affected bydissolution/reprecipitation is commonly porous (e.g. Townsend et al.,2000), which was not observed in this study and, moreover, themonazite REE patterns argue against such sub-solidus reactions, asnoted above. This mechanism therefore also seems unlikely.In the biotite-poor leucogranitic interlayers, in particular, mona-

zite shows microstructural evidence for crystal-plastic deformationduring M2SC. Such textures include undulose extinction, deformationtwinning and the formation of sub-grains (Fig. 13). None of thesefeatures was observed in monazite included in Grt1SC. Recent studiesof deformed zircon have demonstrated a link between grain-scaleplastic deformation and enhanced mobility of the REE, U and Th(Reddy et al., 2006), raising the possibility of deformation-induced

resetting of the radiogenic U–Pb system in zircon. Therefore, at thisstage we tentatively suggest that the age variations in M1SC monazitemay reflect partial and variable Pb loss controlled by plasticdeformation during M2SC.The exception to this model comes from Raj66A, where the EPMA

age probability distribution diagram shows a weak clustering of agesat ca. 1 Ga (Fig. 11d), suggesting new growth, rather than anasymmetric tail towards younger ages seen for other Sandmatasamples (Fig. 11a, b, c). REE analyses of Raj66A monazite show eitherlarge negative, or negligible Eu anomalies (Fig. 6e), similar to garnetfrom the same sample. Since the EPMA and LA-ICP-MS data werecollected from different grains this suggests, but does not conclusivelydemonstrate, that Raj66A contains a minor ca. 1 Ga monazitepopulation that grew under amphibolite-facies conditions.The age of the lower intercept in the discordant SHRIMP data from

Raj55monazite, theminor EPMAmonazite age population in themostsheared and recrystallised Sandmata Complex metasediment(Raj66A), and discordance in zircon data from the Sandmata Complex(Sarkar et al., 1989; Buick et al., 2006b) suggest that S2MC may haveoccurred at approximately 1 Ga, broadly coeval with pervasivemetamorphism in the Mangalwar Complex.

10.4. The metamorphic evolution of the Mangalwar Complex

The 941.1±8.6 Ma SHRIMP U–Pb concordia age obtained frommonazite in migmatitic metapelite Raj29 near Bhinai is within error ofthe less precise LA-ICP-MS U–Pb age of rare, low Th/U zirconovergrowths in the same sample (Buick et al., 2006b) and of theEPMA monazite age of Raj30, and with LA-ICP-MS U–Pb ages of lowTh/U zircon grains and overgrowths found in a migmatitic felsicorthogneiss at the sheared contact between the Mangalwar andSandmata Complexes at Bhinai (949±11 Ma; Buick et al., 2006b). Themineral assemblages in Raj29 and Raj30 showno textural evidence forpolyphase metamorphism, and in Raj30 ca. 0.97–0.93 Ga EPMA ageswere obtained for monazite included in garnet rims and cores. Y- andP-zoning (Fig. 3c) in the garnet from this rock are broadly consistentwith co-crystallisation of monazite (Pyle et al., 2001; Yang and Rivers,2002). All of these lines of evidence suggest that these rocksexperienced a single cycle of metamorphism at ca. 940–950 Ma.In contrast to samples from the Sandmata Complex, both monazite

and garnet in Raj29 and Raj30 show small negative Eu anomalies,even though Raj30 contains rare K-feldspar. At face value, theoccurrence of garnet-bearing leucosomes in muscovite-absent, bio-tite+sillimanite migmatites such as these might suggest that thegarnet formed through biotite+sillimanite dehydration melting, and

150 μm

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Fig. 13. a) Photomicrograph (cross polarised light) showing partially recrystallised monazite from the quartzofeldspathic matrix of Raj59 (Sandmata Complex). The monazite grainhas a core that shows deformation twinning, and a mantle of unstrained sub-grains; b) chondrite-normalised REE patterns from the core and mantle sub-grains in a).


hence that the grade of metamorphism during the ca. 940 Ma eventwas in the granulite-facies at this locality (at least above 800–850 °Cat 5–10 kbar based on partial melting experimental studies; e.g.Stevens et al., 1997). However, the lack of a large negative Eu anomalyin the garnet suggests that it did not form through a reaction thatproduced K-feldspar, and therefore that the rare K-feldspar in Raj30formed later than the garnet (and ~940 Mamonazite, which also lacksan appreciable Eu anomaly). Early formation of garnet, before K-feldspar is consistent with the lack of K-feldspar inclusions in thegarnet, in contrast to case of the Sandmata granulites, where K-feldspar inclusions are common in Grt1. The single grain of K-feldsparobserved in Raj-30 occurs in a leucosome segregation, not in thebiotite–sillimanite-rich domains where garnet occurs. There is notextural evidence for biotite pseudomorphs that might indicate thereplacement of K-feldspar+ferro-magnesian minerals, as commonlyoccurs in granulite-facies metapelites during cooling from peakmetamorphic P–T conditions. We therefore suggest that the K-feldspar crystallised from the melt, during cooling, rather than withgarnet as a peritectic product of partial melting. The garnet in this rockmay have formed during upper amphibolite-facies anatexis, or underprograde sub-solidus conditions.In the south of the field area, metapsammite Raj46 yielded a

weighted mean EPMA monazite age of 967±24Ma, identical withinerror to the SHRIMPU–Pbmonazite age fromRaj29, the EPMA age fromRaj30, and the LA-ICP-MS age obtained from a migmatitic felsicorthogneiss at Bhinai by Buick et al. (2006b). Sample Raj42 alsorecorded a weighted mean EPMA age of 969±18Ma. Although datedfrom only a few localities, ca. 0.97–0.93 Ga metamorphism apparentlyoccurs along at least ~130 km of the strike length of BGC-II. Raj42 alsorecorded an older population at 1824±43 Ma that was confined to thecores of texturally composite garnets. The ~1.82 Ga date is older thanthe age of granulite-grade metamorphism in the Sandmata Complex,and the inferred depositional age of Raj29 (b1.7 Ga; Buick et al., 2006b).Garnet and monazite REE patterns from the Mangalwar Complex

samples lack the large negative Eu anomalies that characterisemonazite and early garnet (Grt1) from the Sandmata Complexgranulites that formed in equilibrium with K-feldspar. This suggeststhat, unlike the major shear zone-hosted amphibolite-facies schistRaj66A in the Sandmata Complex, none of these Mangalwar Complexrocks had a previous granulite-facies history.

10.5. Implications for regional tectonics

While it has been suggested that both the Mangalwar andSandmata Complexes are polymetamorphic, there is no consensusas to when these events occurred and to what extent the twocomplexes share a metamorphic history. Some workers havesuggested that the Sandmata Complex experienced a polyphaseArchaean and Palaeoproterozoic history. In contrast, in both this studyand that of Buick et al. (2006b), no evidence of an Archaean historyhas been found in granulite-grade metasediments or interlayeredleucogranites. The M1SC history, involving granulite-facies metamor-phism and anatexis, was synchronous at ca. 1720 Ma with theemplacement of a voluminous charno-enderbite suite. The subse-quent reworking of the granulites under high-pressure amphibolite-facies conditions (M2SC) in general did not result in new growth ofaccessory phases, but rather variable Pb loss in both monazite (thisstudy) and zircon (Sarkar et al., 1989; Buick et al., 2006b). The best ageestimate for reworking comes from a minor population of (probablynewly formed) monazite in Raj66A, the most thoroughly recrystal-lised of the metasediments, at ca. 1 Ga.The Mangalwar Complex was also suggested to have had an

Archaean–Paleoproterozoic metamorphic history (e.g. Sharma, 1988;1999). However, SHRIMP U–Pb monazite, U–Pb zircon LA-ICP-MS(Buick et al., 2006b) and EPMA monazite age constraints all suggestthat mid-to upper amphibolite-facies metamorphism and local

anatexis occurred predominantly in the interval ca. 0.97–0.93 Ga. Insome samples (Raj29, -30, -46) early Neoproterozoic metamorphismappears to be the only metamorphic event to have affected theserocks. However, in one sample (Raj42) there is evidence for a previousPalaeoproerozoic event, albeit different in age to M1SC in theSandmata Complex. As was the case in the Sandmata Complex,there is no evidence for Archaean metamorphism preserved inmonazite from this terrain, which would be expected if the rockswere generally polymetamorphic at amphibolite-facies grades giventhe high closure temperature of monazite to Pb volume diffusion(Cherniak et al., 2004).The timing of tectonic juxtaposition of the Sandmata and

Mangalwar complexes has typically been viewed as Palaeoproterozic,at ca. 1.7 Ga (e.g. Sharma, 1988; Roy et al., 2005). However, it isunclear whether all of the sedimentary protoliths to ManglawarComplex metasediments were deposited at this time (e.g. Raj29;Buick et al., 2006b). Based on the current dataset, and that of Buick etal. (2006b), it appears more likely that the two complexes werejuxtaposed at ca. 1 Ga. The timing of regional metamorphism in theManglawar Complex and tectonic juxtaposition with the SandmataComplex partly overlaps the timing of 987–968 Ma Andean-typeconvergent margin extrusive and intrusive felsic magmatism in theadjacent Delhi Supergroup (Deb et al., 2001; Pandit et al., 2003). Thegeochronological data from this study reinforces the suggestion ofBuick et al. (2006b) that the tectonic history of the BGC-II is distinctfrom that of BGC-I; the former is dominated by Palaeoproterozoic andNeoproterozoic intrusive and metamorphic events, and appears tolack an Archaean history, whereas the orogenic history of the latterappears to be wholly Archaean. The two complexes should betherefore formally separated in the future.In a recent petrological study of the Banded Gneiss Complex, Saha

et al. (2008) proposed a three-stage metamorphic evolution inmetasedimentary granulites from the Sandmata Complex in a domain~7–9 km NE of the Sandmata Hill area. They recognised an earliestgranulite-facies assemblage identical to the M1SC assemblage in thisstudy; this was reworked in high-strain zones containing new garnet,kyanite, biotite and partial melt (their M2). In the Sandmatagranulites, Saha et al. (2008) estimated peak P–T conditions of 12–15 kbar, 815 °C for theirM2. PeakM2 high-pressuremetamorphism inthe Sandmata Complex was followed by a clockwise retrogradeevolution to amphibolite-facies conditions (M2R; ~6 kbar, 625 °C).Saha et al. (2008) also estimated high-pressure (~10 kbar) amphib-olite-facies conditions for kyanite-bearing migmatites in the Mangal-war Complex and correlated this event with M2 in the SandmataComplex. M2 was followed by sillimanite-grade shearing event (M3).The textural relationships described by Saha et al. (2008) between

new garnet and biotite–kyanite fabrics in their M2 are very similar tothose attributed to M2SC in this study except that we did not observeany partial melt associated with this garnet generation. Moreover, Zr-in-rutile thermometry for rutile in S2SC and included in Grt2, and theREE patterns of Grt2 in our study suggest that they formed at muchlower (amphibolite-facies) conditions than those described in theirstudy. It is possible that the difference in apparent grade reflectsregional variations in the M2 overprint. Moreover, Saha et al. (2008)related their M2 event, which juxtaposed the Mangalwar andSandmata Complexes, to Paleoproterozoic tectonics, and suggestedminor M3 shearing at ca. 1 Ga. In contrast, we suggest that M1SCoccurred at ca. 1.7 Ga and M2Sc at ca. 1 Ga, and that the Sandmata andMangalwar complexes were juxtaposed at ca. 1 Ga. This interpreta-tion seems to be in better agreement with a very recent contributionfrom Bhowmik et al. (2010), who independently datedmonazite fromthe Sandmata and Mangalwar Complexes using the EPMA (chemicaldating) method. The ages obtained largely overlap with thoseobtained in this contribution, and were obtained from differentsample localities. Monazite EPMA ages in the range ca. 1.73 to ca.0.89 Ga were obtained from the Sandmata Complex metapelitic


granulites, with the oldest of these inferred to date peakM1 (ourM1SCmetamorphism), and the youngest to date the high-P M2 event (ourM2SC). Monazite ages in the range ca. 0.98 to ca. 0.89 Ga wereobtained from Mangalwar Complex metasediments and inferred todate moncyclic metamorphism in theMangalwar complex, coincidentwith M2SC in the Sandmata Complex. The Bhowmik et al. (2010) fromthe Mangalwar Complex reinforce the importance of early Neopro-teroic tectonics in the Aravalli–Delhi Orogenic belt, as shown in thisstudy. However, our older 1.82 Ga monazite age suggests that part oftheMangalwar Complex has an earlier metamorphic history. Togetherwith the detrital zircon evidence of Buick et al. (2006b) for post 1.7 Gadeposition of sedimentary precursors of one Manglwar Complexmetasediment this age suggests that one of the challenges for futurework will to be to determine basement-cover relationships within theMangalwar Complex.In the broader context of the Paleoprotoerozoic evolution of the

BGC-II, it is worth noting that both Palaeoproterozoic zircon (Sarkar etal., 1989; Buick et al., 2006b) and monazite (this study) in this terrainshow evidence for significant isotopic discordance. Part of theevidence for the fine-scale sub-division of themetamorphic/structuralevolution of the Banded Gneiss Complex (BGC-I and-II) is based oneither early 207Pb–206Pb SIMS (Weidenbeck & Goswami, 1994) orsingle grain Pb evaporation techniques (e.g. Roy & Kröner, 1996;Fareeduddin & Kröner, 1998; Roy et al., 2005) in which it is notpossible to assess isotopic discordance. For example, Roy et al. (2005)obtained Pb evaporation zircons ages in the range ca. 1.69–1.62 Gafrom partially melted felsic orthogneisses from the shear zoneseparating the Sandmata Complex and Mangalwar Complex nearBhinai. They suggested that this age range bracketed the history ofthis shear zone, from fluid-fluxed melting of the felsic orthogneissesto subsequent reworking at lower temperatures below the wetsolidus, under amphibolite-facies conditions. However, igneousPalaeoproterozoic zircon in these migmatitic rocks is variablydiscordant and contains minor low Th/U rims formed at 949±11 Ma (Buick et al., 2006b). Because of the extent of the ca. 0.97–0.94 Ga overprint, and the occurrence of zircon discordance in theBGC it may be necessary to re-visit some of the key samples andlocalities using SIMS or LA-ICP-MS techniques where concordance canbe assessed.

11. Conclusions

In this study, integration of monazite geochronologywithmineral-scale trace element geochemistry has been used to place constraintson the evolution of polymetamorphic high-grade rocks in the BGC-II,and the mechanism of monazite intra-grain age variation. Metasedi-ments from the Mangalwar Complex record geographically wide-spread evidence for amphibolite-facies metamorphism at ca. 950 Ma.One sample additionally records evidence for a Palaeoproterozoicevent (ca. 1.82 Ga) that also reached amphibolite-facies grades. In theSandmata Complex metapelitic rocks reached granulite-facies condi-tions at ca. 1.72 Ga and were reworked under sub-solidus conditionsat 1 Ga. The preservation of monazite REE patterns developed duringthe granulite-facies M1 event in the Sandmata Complex suggests thatlittle new monazite growth occurred during the second event, andthat age dispersion resulted from deformation-assisted Pb loss duringca. 1 Ga shearing. The extent of metamorphism during the Neopro-terozoic has implications for the interpretation of the range ofPalaeoprotoerozoic ages previously obtained from the BGC.Granulite-facies metapelitic rocks commonly have polyphase

histories, which may or may not be temporally related. Severalaspects of garnet trace element chemistry can be used to unravelthese histories. Compositional zoning of Y and P can be used todetermine whether garnet co-crystallised with phosphate minerals(in particular, monazite; Pyle et al., 2001; Yang and Rivers, 2002) thatcan be dated. In addition, because of their robustness to resetting, REE

patterns in garnet and co-existing minerals such as monazite providean important memory of co-existingmineral assemblages. Garnet andmonazite (and other accessory phases such as apatite, zircon andallanite) that formed part of the same paragenesis typically showsimilar Eu anomalies. Large negative Eu anomalies (Fig. 12a) provide aqualitative indicator of whether or not the garnet and accessoryminerals grew with K-feldspar (and hence at granulite grades), evenwhere the K-feldspar may have been completely replaced duringsubsequent melt crystallisation or later rehydration. Similarly, theextent of depletion in the HREE compared to the MREE in secondgeneration garnet provides a qualitative indication of the how muchof the REE budget has been sequestered in relict, earlier formedgarnet, with implications for Sm/Nd dating of the second event. Theclear link between the magnitude of Eu anomalies developed ingarnet, feldspars and accessory phases (monazite and zircon in thisstudy, but also allanite and apatite, see Gregory et al., 2009) providesan important link between ages extracted from polymetamorphicrocks and the grades at which these overprints developed.Supplementarymaterials related to this article can be found online

at doi:10.1016/j.lithos.2010.09.011.

Acknowledgements

ISB acknowledges a Professorial Fellowship and Discovery GrantDP0342473 from the Australian Research Council (ARC) and a BlueSkies Grant from the National Foundation for Research (South Africa).DR acknowledges an ARC QEII Fellowship, and DR and JH were fundedby Discovery Grant DP055670. Angus Netting (Adelaide University),Alexander Primyak (University of Melbourne) and Ashley Norris(ANU) helped in the operation of electron microprobes. CharlotteAllen and Charles Magee are thanked for help with the LA-ICP-MS(ANU). The ANU Electron Microscopy Unit provided access andtechnical support for imaging and EDS analysis. Constructive reviewsby Toby Rivers and an anonymous reviewer were greatly appreciated.

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