ORIGINAL PAPER

U–Cr-rich high Mg-Al granulites from Karimnagar GranuliteBelt, India: implications for Neoarchean-Paleoproterozoicevents in southern India

Chanchal Sarbajna & Sankar Bose & V. Rajagopalan &

Kaushik Das & Anjan Som & A. K. Paul & K. Shivkumar &

K. Umamaheswar & Anjan Chaki

Received: 30 September 2011 /Accepted: 25 October 2012 /Published online: 22 November 2012# Springer-Verlag Wien 2012

Abstract High Mg-Al granulite occurs as enclave withingranite gneisses at Karimnagar, southern India, and it con-tains coarse granoblastic aggregates of orthopyroxene andsapphirine with minor amount of cordierite, spinel andphlogopite. An important chemical characteristic of theseminerals is their extremely high MgO content and thehigh Cr2O3 in sapphirine and spinel. Textural analysisshows sapphirine + orthopyroxene + cordierite as the peak-metamorphic assemblage that possibly evolved though thebreakdown of a spinel-bearing assemblage. Cation exchangegeothermometers involving orthopyroxene, sapphirine andspinel yield temperatures of 600–800 °C with a maximum of860 °C implying an event of high temperature (HT) metamor-phism. Pseudosection analysis in the FeO–MgO–Al2O3–SiO2

chemical system shows the stability of the peak- assemblagebelow 6.2 kbar. Subsequently, the rock underwent hydrationand cooling with the appearance of phlogopite in the

assemblage. Chromium enrichment is possibly inherited fromthe protolith and its presence presumably stabilized sapphirineand spinel below their high-temperature stability field. Therecorded Rb–Sr age of ca. 2,500 Ma in host granite gneissmarks the upper age limit of HT metamorphism. Presence ofpatchy, lobate grains as well as veinlets of uraninite andbrannerite is also a characteristic feature of the rock.Uranium mineralization took place during the post peakmetamorphic stage, sulfide mineralization represented by tinygrains and veinlets of pyrite, millerite and pentlanditecoincided with, and outlasted the uranium mineralization.The U–Th–Pb chemical ages of uraninite grains suggest ca.2,200±12 Ma for the age of uranium mineralization in thegranulite. Based on the field relations, it is surmised that thegranulite metamorphism in the study area is older than ca.2,500Ma and is comparable with an event in the other parts ofEastern Dharwar Craton. It can be conceived as a widespreadevent in southern India.

Introduction

Sapphirine and spinel-bearing high Mg-Al granulites areregarded as important tools to characterize high temperature(HT) to ultrahigh temperature (UHT) crustal metamorphism.Although the interpretation of UHT metamorphism is ratherstraightforward for quartz-bearing high Mg–Al granulite (cf.Harley 2008), interpretation from silica-undersaturated meta-pelites are often dubious. Harley (2008) demonstrated thatpresence of sapphirine + quartz + aluminous orthopyroxene(Al2O3 >7 wt.%) in reduced granulites can be regarded as areliable tool for UHT metamorphism. A number of othernatural occurrences, however, preserve mineral assemblagesdevoid of quartz. Some of these assemblages contain corundum

Editorial handling: K. Stüwe

C. Sarbajna (*) :V. Rajagopalan :A. Som :A. K. Paul :K. Shivkumar :K. UmamaheswarAtomic Minerals Directorate (AMD) for Exploration & Research,Begumpet Hyderabad 500016, Indiae-mail: chanchal_s@hotmail.com

S. BoseDepartment of Geology, Presidency University,Kolkata 700073, India

A. ChakiRaja Ramanna Fellow, Department of Atomic Energy, AMD,Begumpet( Hyderabad, India

K. DasDepartment of Earth & Planetary System Science,Hiroshima University,Hiroshima 739-8526, Japan

Miner Petrol (2013) 107:553–571DOI 10.1007/s00710-012-0242-6

as an essential mineral. Unraveling metamorphic P-T condi-tions in such rocks is problematic, particularly if the ferromag-nesian minerals in such an assemblage contain non-FMAScomponents. Therefore, high Mg–Al granulites containinghigh concentration of Zn, Ni or Cr are proven to be enigmatic.Available natural, and more importantly, experimental datausing these components often furnish intriguing petrologicalcase studies (Harley and Christy 1995). Apart from the P-Tevolutionary history, these case studies help in characterizingthe unusual chemical nature of the protolith and its possiblebearing on the metamorphic process(es).

High Mg–Al granulite occurs as enclaves within gneissesat Kottur, Karimnagar district, Andhra Pradesh, India(Fig. 1). The host gneiss is typically granitic with variable,and at places anomalously high, contents of U (4–1,523 ppm; up to 0.18 wt.% U3O8) and other high fieldstrength elements (Th, Pb, Ce and Zr). We present here anaccount of petrology of high Mg–Al granulite, its unusualchemical characterization and we attempt to interpret themetamorphic evolutionary trend of the rock based onmineral as well as whole rock chemistry and texturalfeatures. Petrological interpretation is also extended toexplain the unusual concentration of uranium in the rock.Geochronological data is utilized to assess the age ofgranulite metamorphism, subsequent granite plutonism andformation of uranium bearing minerals like uraninite andbrannerite.

Background geology

The Karimnagar Granulite Belt (KGB) constitutes a part of theEast Dharwar Craton (EDC) and it occurs along the south-western flank of the NW SE trending Pranhita-Godavari riftbasin (Fig. 1). The belt is predominantly composed of granitegneiss with isolated occurrences of charnockite, enderbite,banded magnetite quartzite and dolerite dykes (Rajesham etal. 1993; Acharyya 1997;Mishra et al. 1999; Acharyya 2000).The granite gneisses and the charnockites contain podiformenclaves of high Mg Al granulite, calc-silicate granulite, ul-tramafic granulite and pyroxene granulite (Rajesham et al.1993; Santosh et al. 2004; Sharma and Prakash 2006).Sharma and Prakash (2006) argued that the peak granulitefacies metamorphism stabilized spinel + sapphirine + ortho-pyroxene at ca. 850 °C and 6 kbar pressure. This was followedby a mid-crustal hydration event that stabilized kornerupine +phlogopite assemblages at 650–600 °C at 4.5–5.5 kbar pres-sures. It is argued that the KGB represents a group of relicArchean supracrustals, which underwent granulite-grademetamorphism older than ca. 2,500 Ma (Rajesham et al.1993). The 2,500 Ma age is the emplacement age of granitegneiss and charnockite based on available Rb–Sr whole-rockdata (Rajesham et al. 1993). The charnockite massif, ultra-mafic intrusives and enderbites yielded Pb–Pb whole rockisochron ages reported in the range of ca. 2,800–2,500 Ga(Yoshida et al. 1996; Santosh et al. 2004) obtained ca. 2,800–

Fig. 1 Geological map of part of Karimnagar District, Andhra Pradesh showing Karimnagar Granulite Belt (KGB) and sample locations (modifiedafter Som et al. 2010). Inset shows the position of the study area within India

554 C. Sarbajna et al.

2,600 Ma emplacement ages of charnockitic gneiss usingElectron Probe Micro Analyzer (EPMA)-based chemical dataof zircon, monazite and uraninite. Recently, Jayananda et al.(2012) documented low-pressure granulite metamorphism at2,635±26 Ma using monazite chemical data from the centralpart of the EDC. The latter is the most convincing evidence infavor of the pre-2.5 Ga tectonothermal events so far.

Uranium minerals are usually rare in high-grade rocks.Yet it is a conspicuous feature of the KGB granulites, whichcontain up to 0.072 wt.% U3O8 as reported by the scientistsof the Atomic Minerals Directorate (AMD) for Explorationand Research (Som et al. 2010). Uraninite and brannerite arethe major uraniferous minerals, although higher concentra-tions of U are also found within monazite and zircon.

Methodology

Representative samples, collected from the high Mg Al gran-ulites and granitoid gneisses from near Kottur area of KGB(Fig. 1), were studied under optical microscope. Mineralchemical composition of silicates, oxides, sulfides and radio-active phases was determined by EPMA. Major, minor andtrace element geochemistry was carried out using wet chem-ical instrumental techniques and Wavelength Dispersive X-ray Fluorescence Spectrometer (WDXRFS). Isotopic analyseswere performed by using a multi- collector Thermal ionizationMass Spectrometer (TIMS, VG-354). These analyses werecarried out at Atomic Minerals Directorate for Explorationand Research (AMD), Department of Atomic Energy, India.

Mineral chemical analyses were performed using aCAMECA SX-50 EPMA, equipped with a wavelength dis-persive spectrometry (WDS) using TAP, LIF and PET crys-tals with an accelerating voltage 15 kV, a beam current of40 nA, a beam diameter of 2–5 μm and counting times of10–60s. Both natural and synthetic mineral standards wereused for the elemental peak calibrations. The raw data wascorrected by using the PAP correction programme. All thenecessary peak overlap corrections were considered in orderto minimize interferences between the various elements. Kα-lines were used for Na, Mg, Al, Si, P, K, Ca, Ti, Cr, Mn, Fe,Co, Ni, Cu, Zn,; Lα lines for Mo, Ag, Cd, Sb, Ba, Y, La, Ce,Yb and Lu; and Lβ-1ines for Pr, Nd, Sm, Gd, Tb, Dy, Hoand Er; Mα : lines for Th, Pb; and the Mβ line for U. Afterrepeated analysis of respective standards, it was observedthat the error on major and minor element concentration wasless than 2 wt.% (relative) at 2σ (95 %) confidence. Thecalibration was checked routinely by using an andraditestandard prepared by MAC, UK. Other details are describedin detail in Rajagopalan and Paul (2011).

Analytical data of uraninite was compared with a numberof international mineral standards like uraninite (UO2), ThO2,MnTiO3, REE-fluorides, Yand silicates etc. Counting time on

analytical lines as well as half of this time for backgroundcounts on both sides of the peak were 10 s for U, 20 s for Thand 40 s for Pb following the method of Kemp (2003). TheEPMA-based chemical ages were determined on the assump-tion that the chemical concentration of U, Th and total Pbremained in a closed system and all the Pb is radiogenic.While calibrating, each element was iterated on the basis ofminimum three acquisitions. The error (2σ-level) for chemicalage computation is <1 %, especially when Pb content isgreater than 20 %. Using the measured values of U, Th andPb, chemical ages were calculated for each point (average ofthree) applying the empirical formula after Kato et al. (1999).Calculated errors in the chemical age are mainly related touncertainties of Pb counts (c.f. Bowels 1990).

For whole-rock analyses (major, minor, trace elements in-cluding REE) samples were first cleaned using ultrasoniccleaner, then dried and powdered with the help of crushers, aball mill and a pulverizer to −100 mesh. Finally, the powderedsamples were grinded to −250 to −300 mesh in an agate mortarand after coning and quartering, representative samples wereanalysed using different analytical techniques. Silicon, Ti and Pwere analyzed by Ultraviolet (UV)-visible Spectrophotometer(Analytic JENA AG, SPECORD 200BU) while Na and Kwere measured by Flame Photometer. Al, Fe (total), Ca, Mg,Mn and Li were measured by Atomic Absorption Spectrometer(AAS) with a GBC-make Avanta-P Spectrometer. U was mea-sured by fluorimetry (JARREL ASH, USA). Loss-on-ignitionwas determined gravimetrically by heating powdered samplesfor 2 h at 950 °C. REE and Th were analyzed by InductivelyCoupled Plasma–Optical Emission Spectrometer (ICP- OES;ULTIMA-2). The values of the international CertifiedReference Material (CRM), BR (USGS) analysed along withthe samples are in good agreement. Trace elements like Cr, Ni,Zn, Ga, Rb, Ba, Sr, Y, Zr, and Nb were analyzed using asequential WDXRFS (Panalytical, MagiX PRO; PW-2440).Certified Reference Materials (CRM; United States GeologicalSurvey, Geological Survey of Japan and Centre National De LaRecherche Scientifique, France) like BR, BEN, BIR-1, DRN,PCC1, PM-S ST-2, UBN, DTS-1, WSE, JGb-1, JGb-2, JB-3were used for analysis of trace elements byWDXRFS technique(Govindraju 1994). The calibration of theWDXRFS instrumentwas checked routinely using the certified standard referencematerial BR (USGS). Analytical Uncertainties are ±1 % forSiO2, Al2O3, Fe2O3, CaO and MgO; ±2 % for MnO, K2O andP2O5; ±3 % for TiO2, FeO, P2O5 and ±5–10 % for otherelements. For trace elements (by WDXRFS) and REE (byICP-OES), the errors are less than 5 % for concentrations<30 ppm and 10 % for concentrations <5 ppm.

Isotopic analyses for Rb and Sr from the granitoid gneisssamples were performed using a multi-collector TIMS (VG-354). Whole-rock samples were first dissolved in 1 M HClfollowed by decomposition in 4-ml HF and 2-ml HNO3. Srand Rb contents were determined by isotope dilution mass

U–Cr-rich high Mg-Al granulites from Karimnagar Granulite Belt 555

spectrometry (Pandey et al. 1997). Ion exchange techniqueswere used to separate elements for isotope analyses. Rb andSr were separated using BiO-Rad AG-50*12 cation ex-change resin. The Sr isotopic ratio was normalized to86Sr/88Sr 0 0.1194. The total blanks were Rb <3 ng and Sr<5 ng. The analytical uncertainties at 2σ were 2 % for Rb,87Rb/86Sr and 0.05 % for 87Sr/86Sr.

Petrography

High Mg–Al granulites are a melanocratic (dark brown),hard, compact medium-grained rocks, comprising sapphi-rine (Spr), orthopyroxene (Opx) and cordierite (Crd) asmajor minerals, while spinel (Spl), rutile (Rt) and phlogopite(Phl) are present as disseminated minor phases in variableamounts. Pyrite, pentlandite, millerite, uraninite, brannerite,monazite and zircon occur as accessory minerals. Localabundance of phlogopite and ferruginous encrustations arealso noticeable. Microscopically the rock exhibits subequi-granular granoblastic texture. Feldspar and quartz grains areconspicuously absent in the rock. Opx is colorless, non-pleochroic and shows a mosaic texture with triple pointjunctions reflecting static crystallization (Fig. 2a). Spr +Opx (Fig. 2b) as well as Spr + Opx + Crd intergrowths formthe rock matrix. Large Phl blades occur as replacementwithin the relatively smaller grain-sized matrix of Opx +Spr + Crd (Fig. 2c). Green Spl is clouded with exsolvedmagnetite and mostly included within Spr (Fig. 2d), whereasskeletal intergrowth of Spl + Crd surrounded by coarser Opx+ Spr intergrowths are noticed. Coarse to fine-sized rutilealso occur as intergrowths with Crd (Fig. 2e). Phl grains arecoarse and show pale brown to orange pleochroism. Thesegrains include Spr (Fig. 2f) and partially pseudomorph theSpr + Opx intergrowth. Ilmenite occurs along the crystallo-graphic planes of Phl as exsolution of Ti-bearing compo-nents from an early Ti-rich biotite (Fig. 2g). Dusty pyriteand tiny specks of other sulphide minerals rimming Phl arealso present (Fig. 2h); the latter also includes pyrite and/oruraninite patches and veinlets (Fig. 2i).

The presence of U-bearing minerals is a characteristicfeature of the rock. Uraninite is the major U-bearing mineraland it occurs mainly as patches in Phl and/or along the marginof Opx and Spr grains that are in contact with Phl (Fig. 2i). Phlhas grown at least in two stages as shown by cleavage andoptical continuities across the boundary line that separates theinner and outer stages of the grains growth (Fig. 2k). Trails ofsubequant grains of uraninite cluster mostly occur along theboundary line dividing the inner and outer growth stages ofPhl grains (Fig. 2k). In Phl-free microdomains, patches ofuraninite straddle at the grain boundaries of Spr and Opx(Fig. 2i, k). Veins containing discrete uraninite crystals (50–100 μm long) are also discernible in some grain boundaries as

well as through the grain interiors. Occurrence of major ra-dioactive phase, like uraninite, is also confirmed by the pres-ence of dense α-tracks on cellulose nitrate (CN) films(Fig. 2l). Brannerite is tiny and intimately associated withpatchy uraninite.

Mineral chemistry

Representative chemical analyses of the major minerals arepresented in Table 1. Spl is magnesian (XMg 00.67–0.69) withsubstantial amount of Cr (Cr2O3 0 13.1–15.4 wt.%).Negligible amounts of ZnO and NiO (<0.5 wt.%) are foundin all the analyzed grains. Stoichiometric recalculation showssmall, but variable amounts of ferric iron (up to 2.6 wt.%Fe2O3), chiefly as magnetite component. Spr is extremelymagnesian (XMg 0 0.93–0.94) and contains high amounts ofCr (Cr2O3 0 2.4–2.9 wt.%). All the compositions are peralu-minous as evident from their proximity to the 7:9:3 composi-tions (Fig. 3). Minor amounts of TiO2 (<0.2 wt.%) and ZnO(<0.1 wt.%) are present. Recalculated compositions show anappreciable amount of ferric iron (Fe2O3 0 2.5–2.9 wt.%,Table 1). Opx is broadly homogeneous in terms of Fe-Mgdistribution (XMg 0 0.89–0.90), but shows a variation of thealumina contents (Al2O3 0 5.1–6.3 wt.%). The recalculationbased on stoichiometric balancing shows 2.3–3.1 wt.%Fe2O3. Such a moderate amount of ferric iron and alumin-ium implies presence of Tschermak components (Table 1).A minor amount of Cr is present in all the analyzedgrains (Cr2O3 0 0.2–0.3 wt.%). Crd is remarkably homo-geneous and almost pure Mg cordierite (XMg 0 0.97). Thelow oxide totals (97.6–98.5 wt.%), indicate the presenceH2O and/or CO2 in the channel structure of cordierite. Phl(XMg 0 0.91) shows compositional homogeneity and allthe analyzed grains contain small amounts of Ti (TiO2 0

1.2–1.6 wt.%).Tables 2 and 3 lists representative analyses of sulfide

minerals and uraninite. Uraninite is essentially a mixtureof uranous (UO2) and uranic (UO3) oxides, whose relativeproportions largely depend on aging and the degree ofautooxidation. U occurs mostly in U4+ state in this mineral.Analyses of seven uraninite grains are presented in Table 3;each set represents the average of three EPMA analyses.The EPMA data reveal that uraninite is characterized byhigh UO2 (65.24 to 69.20 wt.%), PbO (22.62–25.99 wt.%), low ThO2 (0.56–1.14 wt.%) and CaO (0.62–1.08 wt.%). The total RE2O3 contents in these uraninitegrains vary in the range 1.06–1.48 wt.%. The amounts ofU, Th, Zr, Ti and REE in uraninite are known to vary withcrystal structure (Heinrich 1958) and conditions of forma-tion. The low concentrations of Th, REE and Ca of theanalyzed grains imply low temperatures of formation(Fritsche and Dahlkamp 1997).

556 C. Sarbajna et al.

Whole rock chemistry

Major, minor and trace element data of seven samples ofhigh Mg-Al granulites are listed in Table 4. The MgOcontent in all samples is very high and is comparable tothe sapphirine-bearing granulites from the Vestfold Hills ofEast Antarctica (Harley 1993), In-Ouzzal, Algeria (Bernard-

Griffiths et al. 1996) and Kakanuru area of Eastern GhatsBelt, India (Kamineni and Rao 1988) (see Table 4). Thewhole-rock chemistry is totally different from that of theenclosing granitoid gneiss (Table 4). All the samples aremafic to ultramafic with very high Mg numbers (Mg 86–91), MgO (19.1–31.91 wt.%) and high alumina (Al2O3 0

14.00–18.50 wt.%) content. The bulk chemistry is

Fig. 2 Photomicrographs from the studied high Mg-Al granulite (a)Orthopyroxene (Opx) showing mosaic structure. (b) Coarse inter-growth of orthopyroxene (Opx) and sapphirine (Spr). Phlogopite(Phl) replaces Opx. (c) Intergrown sapphirine (Spr) and orthopyroxene(Opx) are enclosed within poikiloblastic cordierite (Crd). Phlogopite(Phl) is also present. Large phlogopite blade replacing relativelysmaller grain-sized Opx + Spr + Cdr matrix in the left lower corner.(d) Tiny spinel (Spl) inclusion within sapphirine (Spr) in associationwith orthopyroxene (Opx). Phlogopite (Phl) flakes randomly cut sap-phirine grains. (e) Granular intergrowth of rutile (Rt) and cordierite(Crd). Adjacent orthopyroxene (Opx) grains are replaced by phlogopite(Phl). (f) Tiny sapphirine (Spr) inclusions within coarse flakes ofphlogopite (Phl). Associated cordierite (Crd) grains partially

pseudomorph the sapphirine–orthopyroxene (Opx) intergrowth. (g)Oriented ilmenite (Ilm) needles within phlogopite (Phl). Note thepresence of tiny zircon (Zrn) grains within phlogopite. (h) Clouds oftiny pyrite (Py) grains replacing phlogopite (Phl) in association oforthopyroxene (Opx) and cordierite (Crd). (i) Discrete grains of urani-nite (Urn) and pyrite (Py) within phlogopite (Phl) observed underreflected light. Sapphirine (Spr) is also present as inclusion. (j) Trailsof tiny millerite and pyrite (Mil + Py) within cordierite (Crd) observedunder reflected light. Sapphirine (Spr) occurs as inclusion withincordierite. (k) Patches and grains of uraninite (Urn) along veins withinphlogopite (Phl) crystal. Note the presence of uraninite at the microfracture in sapphirine (Spr). (l) Dense α-tracks of uraninite on cellulosenitrate (CN) film as observed under reflected light

U–Cr-rich high Mg-Al granulites from Karimnagar Granulite Belt 557

compatible with theconstituentminerals, namely Spr,Mg–Opx,Cdr, Spl and Phl. The very low concentrations of Na2O (0.01–0.73 wt.%) and CaO (0.30–2.12 wt.%) are attributed to theabsence of feldspar in the rock. The low to moderate K2O(0.36–1.87 wt.%) content explains the minor presence of Phl.Ni content is very high in all the samples (474–1,738 ppm),which is compatible to the presence of millerite and pentlanditeas accessory minerals (Table 2). Interestingly, Cr content is quitehigh in all the samples (0.24–0.68 wt.% Cr2O3), what corrobo-rates the high Cr-contents in sapphirine and spinel (Table 1).

It is also evident that the samples are more enriched in LargeIon Lithophile Elements (LILE) compared to the high fieldstrength elements (HFSE) with distinct enrichment of U (153–611 ppm) and Pb (47–235 ppm) in some samples, which containuraninite and brannerite as accessory minerals. The primordialmantle-normalized spider diagram (Wood et al. 1979) showsenrichment of LILE like Rb, Ba, K, Nb, La, Ce, Nd, Zr, Sm and

Yand depletion of Sr and Ti (Fig. 4). The LILE contents of thesesamples are significantly higher compared to the Prydz Baysample (Table 4). The K/Rb ratio varies in the range 148–300(except one sample with 36), which, however, is lower than thePrydz Bay sample values, indicating moderate to high Rb-enrichment in the present case. The total REE contents in foursamples vary in the range 99–887 ppm with (La/Yb)N 0 5.32–38.50, (Gd/Yb)N 0 2.90–4.00 and Eu/Eu* 0 0.11–0.20. Thechondrite-normalized (Boynton 1984) REE plot shows twocharacteristic enrichment patterns (Fig. 5). Sample KOTR-23shows the highest concentration of total REE (887 ppm) with asteep LREE [(La/Yb)N 0 38.50] and relatively shallow HREE[(Gd/Yb)N 0 4.00] pattern and with a distinct negative anomalyfor Eu Eu/Eu* 0 0.11. This may be attributed to the presence ofLREE-enriched minerals like monazite and zircon, and theabsence of feldspar in the sample. This enrichment pattern(Fig. 5) is similar to the samples of the Eastern Ghats Belt and

Fig. 2 (continued)

558 C. Sarbajna et al.

Table 1 Representative EPMA analyses of major minerals of the studied high Mg-Al granulites

Phase Opx Spr

Spot No. 3 5 6 7 12 1 5 6 7 8 10

SiO2 52.42 51.85 51.84 52.50 52.27 13.19 13.13 12.97 13.17 13.30 13.01

TiO2 0.10 0.10 0.12 0.08 0.11 0.07 0.10 0.09 0.05 0.10 0.05

Al2O3 6.08 5.52 5.58 5.14 6.25 58.92 58.79 58.73 59.56 58.67 59.25

Cr2O3 0.27 0.25 0.26 0.22 0.20 2.59 2.74 2.89 2.36 2.38 2.54

Fe2O3 2.32 3.10 2.48 2.28 2.34 2.65 2.72 2.92 2.63 2.47 2.79

FeO 6.45 6.14 6.90 6.90 6.60 2.20 2.39 1.99 2.13 2.46 2.12

MnO 0.10 0.11 0.09 0.13 0.12 0.00 0.00 0.00 0.00 0.00 0.03

MgO 31.45 31.23 30.80 31.15 31.28 18.73 18.62 18.67 18.81 18.64 18.66

CaO 0.05 0.06 0.08 0.06 0.00

NiO 0.03 0.10 0.09 0.10 0.05 0.18 0.11 0.20 0.16 0.19 0.14

Total 99.29 98.45 98.23 98.58 99.22 98.53 98.59 98.46 98.87 98.20 98.58

Based on 6 O Based on 10 O

Si 1.84 1.84 1.84 1.86 1.83 0.79 0.79 0.78 0.79 0.80 0.78

Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Al 0.25 0.23 0.23 0.21 0.26 4.17 4.16 4.16 4.19 4.16 4.19

Cr 0.01 0.01 0.01 0.01 0.01 0.12 0.13 0.14 0.11 0.11 0.12

Fe3+ 0.06 0.08 0.07 0.06 0.06 0.12 0.12 0.13 0.12 0.11 0.13

Fe2+ 0.19 0.18 0.21 0.21 0.20 0.11 0.12 0.10 0.11 0.12 0.11

Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Mg 1.64 1.65 1.63 1.64 1.64 1.68 1.67 1.67 1.67 1.67 1.67

Ca 0.00 0.00 0.00 0.00 0.00

Ni 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.01

Sum 4.00 4.00 4.00 4.00 4.00 7.00 7.00 7.00 7.00 7.00 7.00

XMg 0.90 0.90 0.89 0.89 0.89 0.94 0.93 0.94 0.94 0.93 0.94

Phase Spl Crd

Spot no 1 2 3 4 5 6 7 8 1 2 3

SiO2 0.22 0.20 0.18 0.18 0.21 0.20 0.20 0.19 50.35 50.95 50.34

TiO2 0.02 0.01 0.01 0.01 0.01 0.03 0.02 0.02 0.01 0.00 0.00

Al2O3 50.53 50.94 50.76 50.86 50.87 50.95 50.96 50.05 33.94 33.95 33.89

Cr2O3 15.41 15.35 13.77 13.30 13.80 13.13 13.65 13.26

Fe2O3 1.58 1.68 2.60 1.03 1.70 1.27 1.55 1.83

FeO 13.96 12.78 15.00 15.94 15.14 15.38 15.38 15.63 0.76 0.63 0.67

MnO 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.00 0.00

MgO 17.50 17.87 16.70 17.70 17.31 17.52 17.36 17.97 12.85 12.90 12.69

CaO 0.09 0.02 0.01 0.01 0.06 0.03 0.06 0.01 0.02 0.00 0.01

NiO 0.33 0.28 0.27 0.16 0.28 0.35 0.27 0.38

ZnO 0.26 0.36 0.22 0.01 0.32 0.01 0.13 0.26

Na2O 0.06 0.07 0.00

K2O 0.01 0.01 0.01

Total 99.91 99.50 99.53 99.21 99.71 98.88 99.59 99.61 98.03 98.51 97.61

Based on 4 O Based on 18 O

Si 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 5.02 5.04 5.03

Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Al 1.60 1.62 1.63 1.62 1.62 1.62 1.62 1.59 3.98 3.96 3.99

Cr 0.33 0.33 0.30 0.28 0.29 0.28 0.29 0.28

Fe3+ 0.06 0.05 0.07 0.09 0.08 0.08 0.08 0.12 0.06 0.05 0.06

Fe2+ 0.29 0.28 0.33 0.29 0.30 0.29 0.30 0.27 0.00 0.00 0.00

Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.91 1.90 1.89

U–Cr-rich high Mg-Al granulites from Karimnagar Granulite Belt 559

the In-Ouzzal. In contract the other three samples show flatterREE patterns with a weaker negative Eu anomaly. They aresimilar to the In-Ouzzal sample, except that they contain moreREE andU. The overall chemistry of the studied rock suggests aprotolith of mafic to ultramafic character.

Geothermobarometry

Spr, Opx, Cdr, Spl and Phl are the major constituents in thestudied high Mg–Al granulites and which are devoid of feld-spars and quartz. Therefore, application of well-calibratedgeothermometers is severely limited in the present study. Fe2

Fig. 3 Compositional variation of sapphirine plotted along the Tscher-mak substitution line. All the compositions are peraluminous and plotclose to 7:9:3

Mg 0.70 0.72 0.68 0.71 0.70 0.71 0.70 0.72 0.00 0.00 0.00

Ca 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Ni 0.01 0.01 0.00 0.00 0.01 0.01 0.01 0.01

Zn 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.01

Na 0.01 0.01 0.00

K 0.00 0.00 0.00

Total 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 10.98 10.97 10.97

XMg 0.71 0.72 0.68 0.71 0.70 0.71 0.70 0.73 0.97 0.97 0.97

Phase Phl

Spot no 1 2 3 4 5 6 7 8 9 10 12

SiO2 40.92 40.09 40.27 40.61 40.32 40.54 40.28 40.22 40.59 40.70 40.17

TiO2 1.47 1.31 1.34 1.31 1.12 1.25 1.22 1.28 1.25 1.31 1.60

Al2O3 17.68 17.86 17.73 17.73 17.57 17.78 17.08 17.62 17.19 17.08 17.81

FeO 4.03 3.79 3.99 4.11 4.00 4.11 4.05 4.12 3.96 3.97 4.07

MnO 0.00 0.00 0.04 0.01 0.04 0.00 0.02 0.00 0.00 0.00 0.02

MgO 22.32 22.49 21.96 22.24 22.95 23.09 22.89 23.41 22.96 23.22 23.24

CaO 0.03 0.08 0.08 0.02 0.07 0.00 0.02 0.03 0.05 0.04 0.10

Na2O 0.73 0.76 0.81 0.75 0.84 0.68 0.95 0.84 0.92 0.98 0.81

K2O 7.73 7.54 7.74 7.86 7.32 7.33 7.78 7.68 7.66 7.76 7.59

H2O 4.09 4.10 4.04 4.07 4.09 4.07 4.07 4.12 4.10 4.11 4.08

Total 99.00 98.02 98.00 98.71 98.32 98.85 98.36 99.32 98.68 99.17 99.49

Unit formula based on 20 O + 4 (OH)

Si 5.26 5.20 5.23 5.24 5.21 5.21 5.23 5.16 5.24 5.23 5.15

AlIV 2.67 2.73 2.71 2.69 2.68 2.69 2.61 2.66 2.61 2.59 2.69

AlVI 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Ti 0.14 0.13 0.13 0.13 0.11 0.12 0.12 0.12 0.12 0.13 0.15

Fe2+ 0.43 0.41 0.43 0.44 0.43 0.44 0.44 0.44 0.43 0.43 0.44

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Mg 4.27 4.35 4.25 4.28 4.42 4.42 4.43 4.48 4.42 4.45 4.44

Ca 0.00 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.01 0.01 0.01

Na 0.18 0.19 0.20 0.19 0.21 0.17 0.24 0.21 0.23 0.24 0.20

K 1.27 1.25 1.28 1.29 1.21 1.20 1.29 1.26 1.26 1.27 1.24

OH 3.51 3.55 3.50 3.51 3.53 3.49 3.52 3.53 3.53 3.53 3.49

Cations 14.23 14.26 14.27 14.27 14.28 14.26 14.35 14.34 14.31 14.34 14.32

XMg 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91

Table 1 (Continued)

560 C. Sarbajna et al.

Tab

le2

RepresentativeEPMA

data

ofsulfideminerals(m

illerite,py

rite

andpentland

ite)from

Kotturhigh

Mg–

Algranulite

Phase

Mil

Py

Pn

Point

No.

12

34

56

78

910

111

21

2

Fe

1.00

2.95

3.52

1.32

1.29

0.73

0.86

0.53

3.41

0.96

1.38

45.60

44.96

24.10

18.95

S35

.15

36.86

34.78

35.56

34.96

35.49

35.05

35.85

34.58

35.96

34.84

53.69

53.77

34.49

40.00

Ni

62.76

60.03

61.06

62.28

62.11

62.54

62.56

62.01

60.49

61.47

62.71

0.84

0.42

37.63

36.38

Co

0.66

0.35

0.48

0.58

0.22

1.14

0.46

0.95

0.81

0.54

0.12

0.39

0.94

0.64

1.38

Cu

––

0.01

––

–0.06

––

–0.06

0.06

0.04

0.05

0.23

Zn

–0.08

0.07

––

––

–0.19

0.16

–0.09

–0.04

–

As

––

––

0.02

––

–0.04

–0.06

0.09

0.04

0.12

Mo

––

––

––

––

––

––

––

–

Ag

0.04

–0.02

0.06

0.01

0.13

––

0.01

0.10

–0.01

0.04

0.01

0.06

Cd

–0.02

–0.03

–0.02

––

0.03

0.02

–0.06

0.01

0.04

0.02

Sb

––

–0.01

––

0.05

0.02

––

–0.03

––

–

Pb

––

––

––

––

––

––

––

–

Total

99.61

100.29

99.94

99.84

98.61

100.05

99.04

99.36

99.56

99.21

99.11

100.83

100.27

97.04

97.14

Unitform

ula

Fe

0.01

80.05

30.06

30.02

40.02

30.01

30.01

50.00

90.06

10.01

70.02

50.81

60.80

50.43

20.33

9

S1.09

61.14

91.08

51.10

91.09

01.10

71.09

31.118

1.07

81.12

11.08

61.67

41.67

71.07

51.24

7

Ni

1.06

91.02

31.04

01.06

11.05

81.06

61.06

61.05

71.03

11.04

71.06

80.01

40.00

70.64

10.62

0

Co

0.011

0.00

60.00

80.01

00.00

40.01

90.00

80.01

60.01

40.00

90.00

20.00

70.01

60.011

0.02

3

Cu

––

––

––

0.00

10.00

0–

–0.00

10.00

10.00

10.00

10.00

4

Zn

–0.00

10.00

1–

––

––

0.00

30.00

2–

0.00

10.00

1–

As

––

––

––

––

0.00

1–

–0.00

10.00

10.00

10.00

2

Mo

––

––

––

––

––

––

––

–

Ag

––

–0.00

1–

0.00

1–

––

0.00

1–

––

–0.00

1

Cd

––

––

––

––

––

–0.00

1–

––

Sb

––

––

––

––

––

––

––

–

Pb

––

––

––

––

––

––

––

–

U–Cr-rich high Mg-Al granulites from Karimnagar Granulite Belt 561

Table 3 Representative EPMA data of uraninite from Kottur high Mg–Al granulite

Phase Uraninite

Point no 2 3 5 6 7 9 10

UO2 68.54 68.87 67.92 69.20 68.78 68.11 65.24

ThO2 0.90 0.91 0.93 0.82 0.56 0.66 1.14

PbO 25.99 24.12 24.40 25.41 22.62 23.99 25.58

Al2O3 0.00 0.00 0.00 0.00 0.01 0.02 0.01

SiO2 0.00 0.21 0.37 0.27 0.14 0.22 0.19

P2O5 0.00 0.02 0.00 0.01 0.02 0.01 0.03

CaO 0.62 0.89 0.97 1.00 0.77 1.08 0.65

TiO2 0.03 0.07 0.07 0.08 0.07 0.08 0.05

MnO 0.00 0.03 0.05 0.03 0.01 0.01 0.01

FeO 0.69 0.20 0.40 0.20 0.10 0.16 0.19

Y2O3 0.19 1.29 1.00 1.00 1.78 0.93 0.19

La2O3 0.00 0.02 0.02 0.08 0.01 0.01 0.01

Ce2O3 0.04 0.24 0.17 0.03 0.25 0.1 0.01

Pr2O3 0.11 0.00 0.06 0.00 0.01 0.11 0.19

Nd2O3 0.00 0.15 0.05 0.15 0.06 0.05 0.16

Sm2O3 0.20 0.00 0.00 0.00 0.01 0.09 0.04

Gd2O3 0.09 0.23 0.00 0.22 0.26 0.01 0.24

Tb2O3 0.00 0.00 0.16 0.09 0.01 0.01 0.08

Dy2O3 0.10 0.22 0.00 0.14 0.51 0.01 0.08

Ho2O3 0.16 0.00 0.67 0.00 0.09 0.15 0.2

Er2O3 0.30 0.24 0.00 0.37 0.01 0.21 0.01

Tm2O3 0.00 0.11 0.00 0.00 0.01 0.01 0.03

Yb2O3 0.06 0.00 0.17 0.03 0.24 0.01 0.01

Lu2O3 0.00 0.00 0.08 0.16 0.01 0.3 0.01

Total 98.02 97.82 97.49 99.29 96.34 96.34 94.35

Unit formula based on 4 oxygen

U 1.530 1.537 1.516 1.545 1.535 1.520 1.456

Th 0.021 0.021 0.021 0.019 0.013 0.015 0.026

Pb 0.702 0.651 0.659 0.686 0.611 0.648 0.691

Al 0.000 0.000 0.000 0.000 0.001 0.002 0.001

Si 0.000 0.021 0.037 0.027 0.014 0.022 0.019

P 0.000 0.002 0.000 0.001 0.002 0.001 0.003

Ca 0.022 0.032 0.035 0.036 0.028 0.039 0.023

Ti 0.002 0.005 0.005 0.006 0.005 0.006 0.004

Mn 0.000 0.001 0.002 0.001 0.000 0.000 0.000

Fe 0.039 0.011 0.022 0.011 0.006 0.009 0.011

Y 0.010 0.069 0.053 0.053 0.095 0.050 0.010

La 0.000 0.001 0.001 0.003 0.000 0.000 0.000

Ce 0.001 0.009 0.006 0.001 0.009 0.004 0.000

Pr 0.004 0.000 0.002 0.000 0.000 0.004 0.007

Nd 0.000 0.005 0.002 0.005 0.002 0.002 0.006

Sm 0.007 0.000 0.000 0.000 0.000 0.003 0.001

Gd 0.003 0.008 0.000 0.007 0.009 0.000 0.008

Tb 0.000 0.000 0.005 0.003 0.000 0.000 0.003

Dy 0.003 0.007 0.000 0.005 0.016 0.000 0.003

Ho 0.005 0.000 0.021 0.000 0.003 0.005 0.006

Er 0.009 0.008 0.000 0.012 0.000 0.007 0.000

562 C. Sarbajna et al.

+–Mg exchange thermometers using suitable mineral pairse.g. spinel-cordierite, sapphirine-orthopyroxene and spinel-sapphirine were applied in this study. While calibrations byVielzeuf (1983), Das et al. (2005, 2006) and Kawasaki andSato (2002) are based on experimental data, the calibration byOwen and Greenough (1991) is retrieved from natural mineralcompositional data. Results are summarised in Table 5. It isworthwhile to mention that possible errors in such estimatesare to some extent sourced from errors in Fe3+ recalculationscarried out on EPMA analyses. Moreover, the effect of elementslike Cr is not taken into account in the activitymodels used in theafore-mentioned geothermometric calibrations.Moreover, differ-ential down-temperature resetting for coexisting mineral pairsmay also have contributed to the wide scatter of estimatedtemperature values. Geobarometric estimates are difficult forthe present mineral assemblages. An average pressure of 8 kbarhas been considered for temperature calculations. The Spl-Cdrthermometer of Vielzeuf (1983) for this assemblages gives T0460–540 °C, where as the model of Das et al. (2005) gives T0540–610 °C using the same set of chemical data. The Spr-OpxFe–Mg exchange thermometer (Kawasaki and Sato 2002) gives520–710 °C, with one result showing a high estimate of 830 °C.Kawasaki and Sato (2002), however, used their experimentallyconstrained thermometer to very low-Cr Spr-Spl (XCr

Spr <0.016and XCr

Spl <0.047) pairs from different natural rocks. The Spl-Spr thermometer of Owen and Greenough (1991) gives T0810–950 °C, whereas the model of Das et al. (2006) gives T0860–1,020 °C using the same phase chemical data. Temperatures ofthe Spr-Spl thermometer are comparatively high, whereas thepeak assemblage of Spr + Opx delimits much lower temper-atures (<720 °C,mostly). To our understanding, the anomalouslyhigh temperature results are computational artifacts as none ofthe used models accounts for the excess Cr in Spr and coexistingSpl. Recent experimental data from the system MgO–Al2O3–SiO2–Cr2O3 show that increasing Cr content enlarges the stabil-ity field of Spr towards lower temperatures (Brigida et al. 2007).Therefore, the apparently estimated “ultrahigh temperature” con-ditions cannot be reconciled from the circumstantial evidence.

Peak metamorphic conditions were further constrainedfrom pseudosection analysis in the system FeO–MgO–Al2O3–SiO2 (FMAS) using the program PERPLEX(Connolly and Petrini 2002). Bulk chemical data, presentedin Table 4, was used for this purpose. (Sample KOTR 21) has

been used to calculate the FMAS pseudosection. The absenceof feldspar allows us to eliminate K2O, Na2O and CaO asmajor components. Phlogopite is not considered as a part ofthe peak assemblage and rutile is present only as trace amount.The database file (hpver02.dat) was used after Holland andPowell (1998), while the activity-composition models (filesolut07ver.dat) of Opx, Spr and Spl were taken fromHolland and Powell (2003). The resultant P-T pseudosection(Fig. 6) shows the stability of the peak assemblage Spr + Opx+ Crd below 6 kbar; at higher pressures assemblages like Spr+ Opx + Crd + Sil and Spr + Opx + Crd + Qtz are stabile.Since the degree of freedom in this system is high (at leastdivariant), it is important to further constrain the position ofpeak metamorphic stage. Compositional isopleths of coexist-ing minerals (XMg of Spr and Opx) have been used for thispurpose, but the measured XMg values do not intersect withinthe stability field as shown in the Fig. 6. This possibly occursdue to the late stage Fe2+–Mg exchange among the ferromag-nesian minerals. On the other hand, experimental, theoreticaland natural data show that the Al content in orthopyroxene ismore robust in preserving the peak metamorphic temperature(Harley 2008). After plotting the Al-isopleths of orthopyrox-ene (corresponding to 5.5–6.3 wt.% Al2O3) in the pseudosec-tion, a small temperature window of 800–880 °C below 6 kbarpressure has been considered for establishing the stability of theassemblage Spr +Opx +Crd. This is very similar to the peakP-T values of 6.2 kbar, 850 °C estimated from other areas of KGB(Sharma and Prakash 2006). The constructed FMAS phasediagram looks very similar to the theoretical phase diagram inthe system MgO–Al2O3–SiO2 (MAS) of Gasparik (1994),presumably due to the Mg-rich nature of the bulk rock. Oneinteresting observation to be made in the diagram is the totaldisappearance of Spl, suggesting that Spl was never stabilizedwith Spr and Opx. Theoretical analysis in the MAS systemshows that Spl appears only below 4 kbar (Gasparik 1994),which also may explain the absence of Spl in the pseudosection. However, presence of Cr and Ni (and/or Ti) in thesystem can enlarge the stability of Spl and Spr at the expense ofCrd (Brigida et al. 2007). The effect of these elements on theFMAS system could not be quantified since thermodynamicparameters of Cr-bearing Spl and Spr are not available.

The temperature condition of the retrograde stage was con-strained using Phl. Experimental data of Henry et al. (2005)

Table 3 (continued)

Phase Uraninite

Point no 2 3 5 6 7 9 10

Tm 0.000 0.003 0.000 0.000 0.000 0.000 0.001

Yb 0.002 0.000 0.005 0.001 0.007 0.000 0.000

Lu 0.000 0.000 0.002 0.005 0.000 0.009 0.000

Chemical age (Ma) 2,190±11 2,058±14 2,100±12 2,136±12 1,960±15 2,073±11 2,243±11

U–Cr-rich high Mg-Al granulites from Karimnagar Granulite Belt 563

Table 4 Major, trace and REE data of seven samles of the studied highMg-Al granulite (KOTR samples). Samples from Prydz bay(No.65316), Eastern Ghats belt(No.S-27) and In Ouzzal (No. INZ-

98) are presented for comparison. Compositional ranges for majorand selected elements of the host granite gneiss are also included inthe Table (No.GG)

Sample No KOTR21 KOTR22 KOTR23 KOTR-24 KOTR-25 KOTR-132 KOTR-134 S-27a 65316b INZ-98c GGd

SiO2 46.18 43.22 42.39 50.8 50.32 48.00 46.00 41.9 39.41 44.3 66.00–77.28

TiO2 0.46 0.29 0.45 0.53 0.32 0.40 0.26 1.12 0.11 0.63 *

Al2O3 15.03 14.39 14.5 14.9 16.29 14.00 18.50 20.15 28.05 18.4 7.85–14.26

Fe2O3(t) 7.76 8.00 5.61 6.15 5.99 8.20 4.72 6.41 2.42 19.00 *

MnO 0.09 0.04 0.04 0.07 0.07 0.07 0.04 0.03 0.02 0.12 0.82–7.81

MgO 25.43 31.91 28.78 21.22 19.1 26.10 23.60 26.91 29.98 20.4 0.01–0.12

CaO 1.16 1.33 2.12 0.51 0.66 0.70 0.30 0.02 0.08 0.31 0.09–2.7

Na2O 0.01 0.01 0.01 0.73 0.11 0.49 0.70 0.05 0.17 0.2 0.43–3.14

K2O 1.87 0.85 1.04 1.63 0.36 0.52 3.04 2.25 0.28 1.1 *

P2O5 0.31 0.26 1.26 0.28 0.05 0.44 0.30 0.16 0.02 0.03 0.01–3.91

Cr2O3 0.41 0.59 0.68 0.28 0.24 0.26 0.46 * 0.01 0.08 1.64–7.48

NiO 0.07 0.15 0.13 0.07 0.06 0.10 0.22 * 0.01 0.04 <0.01–0.20

Total 98.78 101.04 97.00 97.18 93.57 99.28 98.14 99 100.56 104.61

Trace and REE (in ppm)

Ba 313 169 58 256 715 80 328 * 47 569 *

Rb 105 40 42 51 82 20 84 * 3.6 67 56–1011

Sr 119 107 108 13 5 16 24 * 1.6 31 45–3693

Zr 55 41 55 102 315 245 265 * 72 66 5–43

Nb 53 44 40 50 35 32 46 * 3 14 *

Co 15 34 18 20 19 16 18 * 23 40.7 *

Zn 104 109 70 99 88 75 89 * 31 *

Y 48 26 84 45 85 48 54 * 10 18 *

Ga 42 86 65 23 35 29 32 * * * 19–1623

Pb 235 121 47 60 65 85 94 * 5 4–1523

U 12 8 5 430 153 399 611 * * 0.48 <10–802

Th 14 11 12 30 184 45 60 * * 7.35 *

La 17 19 231 16.5 * * * 173.4 * 9.53 53–160

Ce 33 35 383 30 * * * 330.6 * 16.48 *

Pr 4.1 4.1 37 4.0 * * * * * *

Nd 17 18 147 17.2 * * * 142.8 * 6.82 *

Sm 6 6 30 6 * * * 26.5 * 1.67 *

Eu 1.5 1.6 3.1 1.4 * * * 2.68 * 0.28 *

Gd 9 10 24 9 * * * 18.36 * 1.8 *

Dy 8 9 17 8 * * * * 2.01 *

Er 4 4 9 3.5 * * * * 1.4 *

Yb 3 3 6 3.1 * * * 15.7 * 1.64

REE(t) 102 110 887 99 * * * 710 * 42

K/Rb 147.85 176.41 205.56 265.32 36.45 215.84 300.44 * 645.67 113.14

K/Ba 49.60 41.75 148.85 52.86 4.18 53.96 76.94 * 49.46 13.32

Rb/Sr 0.88 0.37 0.39 3.92 16.40 1.25 3.50 * 2.25 2.16

Ba/Sr 2.63 1.58 0.54 19.69 143.00 5.00 13.67 * 29.38 18.35

Mg# 87 89 91 87 86 86 91 96 89

Eu/Eu* 0.20 0.19 0.11 0.19 * * * * 0.12 0.16

(La/Yb)N 5.48 5.59 38.50 5.32 * * * * 11.04 5.81

(Lan/Sm)N 2.98 3.28 7.70 2.75 * * * * 6.54 5.71

(Gd/Yb)N 2.90 2.94 4.00 2.90 * * * * 1.17 1.10

U3O8% 0.001 0.001 0.001 0.051 0.018 0.047 0.072 * * 0.000057

* Not analyseda Saphirine granulites, Eatern Ghats Belt, India (Kamineni and Rao 1988)b Saphirine granulite, Vestfold Hills, East Antartica (Harley 1993)c Al-Mg granulites, In-Ouzzal, Hoggar, Algeria (Bernard-Griffiths et al. 1996)d Range of composition of the host granite gneiss samples

564 C. Sarbajna et al.

show that Ti-partitioning in Phl is a function of temperature formid- to deep-crustal rocks (4–6 kbars). All the Phl compositionsshowTi values ranging from 0.12 to 0.15 a.f.u (20 oxygen basis)that plot close to the 650 °C isotherm for the given XMg values(Table 5). \The occurrence of sulfide minerals in the retro-grade assemblage also provides a temperature estimate forthis stage. Experimental data in the Fe–Ni–S system suggestthat appearance of pentlandite (with 21–40 at.% Ni which issimilar to the present compositions) from the monosulfidesolid solution implies cooling below 610 °C (Naldrett et al.1967; Raghavan 2004). However, the stable coexistence ofpentlandite with millerite and pyrite implies even muchlower temperature below 230 °C (Graterol and Naldrett 1971;Misra and Fleet 1973).

Geochronology

The Rb–Sr isotope data for nine samples and the Pb–Pb isotopedata for eight samples of the granitoid gneiss are given inTable 6. Of these, six samples (excluding three samples:

GCS-3537, 3538, 3539) hosting the high Mg–Al granuliteyield an Rb–Sr errorchron age of 2,530±220Ma with an initial87Sr/86Sr ratio of 0.7036±0.0027 and MSWD of 20, (Table 6,Fig. 7). Pb–Pb isotope ages for the seven studied samples(Table 6), excluding GC-3538 yield an errorchron age of2,530±190 Ma (Fig. 8). The MSWD of 20 of the Rb–Sr ageindicates minor geological disturbances, with incomplete ho-mogenization of the Rb–Sr system or some post crystallizationeffects (Fig. 7). Hence, the obtained age has a high uncertainty.The high initial 87Sr/86Sr ratio indicates involvement of radio-genic crustal material in formation of the granitoids.

Based on EPMA chemical dating of uraninite, an appar-ent age of ca. 2,200±12 Ma (1,960±15 to 2,243±11 Ma,Table 3) is inferred. However, the spread of the individualages is quite high so that it is difficult a reliable age for agroup of analyses with a reasonable level of certainty. Thelow analytical totals in some uraninite analyses may becaused by high concentrations of vacancies, presence ofelements not included in analysis, and standardization inac-curacies as described by Kemp (2003).

Discussion

Metamorphic evolution

The high Mg–Al granulites of Kottur, KGB are unusual inmany aspects. Overall textural features, as documented earlierin the Petrography section, reflect that the Spl + Opx + Crdassemblage in this melanocratic rocks were stable in the peakmetamorphic stage in the absence of quartz and feldspar andthat U mineralization took place during the retrograde meta-morphic stage when hydration stabilized Phl in the system.

Occurrence of Spl inclusion within Spr possibly implies thatSpl was part of the prograde assemblage. The evolution of themineral assemblage can be conceived following the reaction.

Splþ Crd ¼> Spr þ Opx ð1Þ

Fig. 4 Primordial mantle-normalised trace element plotsof the studied high Mg–Algranulites. Additional data areplotted for comparison. Sample65316 is from Prydz Bay(Harley 1993), S-27 fromEastern Ghats Belt (Kamineniand Rao 1988) and INZ-98 fromIn Ouzzal (Bernard-Griffiths etal. 1996)

Fig. 5 Chondrite normalized REE patterns of the studied high Mg–Algranulites. Samples S-27 and INZ-98 are also plotted for comparison

U–Cr-rich high Mg-Al granulites from Karimnagar Granulite Belt 565

Occurrence of Crd in the matrix with stable contact withSpl and Opx implies overstepping of thepreceding reaction(Eq. 1 due to non-availability of Spl in the reaction site.

This reaction occurs at high temperatures in the mod-el FMAS system in silica-undersaturated bulk composi-tions. In the theoretical FMAS(O) petrogenetic grid, thisreaction actually represents the (Qtz, Grt, Sil) divariantassemblage stabilized at temperature >900 °C (Hensen1986). However, presence of non-FMAS components couldshift the stability fields of Spl and Spr to lower temperatures.

Geothermobarometric calculations and pseudosection analy-sis in the FMAS system suggest a temperature window of800–880 °C at a pressure below 6 kbar for the peak assem-blage (Fig. 6). Corundum is also absent in the assemblagealthough similar rocks with corundum are reported from ad-jacent localities in the KGB by Sharma and Prakash (2006),who also reported kornerupine as secondary mineral in therocks due to hydration of the peak assemblage of Opx + Spl.Localized hydration is evidenced by the occurrence of latecoarse Phl in the assemblage. Geothermometric calculationsgave temperatures around 650 °C for this stage thereby indi-cating cooling of the granulites. A further drop in temperatureis suggested by the stability of the accessory sulphides pyritemillerite pentlandite, which also coincides with the formationof uranium bearing phases.

The appearance of Phl at a later stage, in the absence ofalkali feldspar, could be ascribed to the influx of hydrousand potassic fluid into the system that gave rise to thefollowing reaction.

Opxþ Spr þ H2Oþ Kþ ¼ Phlþ Qtz ð2Þ

Although presence of quartz is predicted from stoichio-metric balancing, it is conspicuously absent in the sample.Presence of small amounts of Ti in biotite may warrantpresence of rutile as a reactant, but it can also come fromdecomposition of Spr and Opx.

Nature of protolith

The unusual bulk chemical and mineralogical composition ofthe studied granulite samples allow us to speculate about thenature of the protolith. Although Spr + Opx or Spr + Splassemblages are reported from KGB granulites (cf.Rajesham et al. 1993), none of these occurrences show anom-alous Cr contents. It is known that Cr substitutes for AlIV inSpr imparting a stabilizing effect (Friend 1982). AlthoughCr2O3 content in the studied Spr grains ranges up to about3 wt.%, Spr from other natural occurrences show as high as7.5 wt.% Cr2O3 (Friend and Hughes 1977; Friend 1982). Arecent experimental study could show that incorporation of Cr

Table 5 Range of temperatures calculated using different geothermometers

Mineral pairs Pressure Range of calculated ln KD Estimated range oftemperature(T)

Models employed

Sapphirine-spinel (Fe2+–Mg) 8 kbar (assumed) 1.28251–1.85129 810–950 °C Owen and Greenough 1991

860–1,020 °C Das et al. 2006

Spinel-cordierite (Fe2+–Mg) 8 kbar (assumed) −2.7500–2.5485 460–540 °C Vielzeuf 1983

540–610 °C Das et al. 2005

Sapphirine-orthopyroxene (Fe2+–Mg) 0.5973–0.9714 520–710 °C Kawasaki and Sato 2002

Biotite Ti saturation 4–6 kbar ~650 °C Henry et al. 2005

Fig. 6 P-T pseudosection in the system FeO–MgO–Al2O3–SiO2

(FMAS) constructed with the help of the PERPLEX program. Bulkcomposition used for the construction is FeO:MgO:Al2O3:SiO2 06.98:25.43:15.03:46.18 which is the bulk chemical composition of thesample KOTR21 (Table 4). Fields of different divariant and trivariantassemblages are shown in the diagram. The peak metamorphic assem-blage Spr + Opx + Crd forms a trivariant field, which is bound uppressure by the assemblages Spr + Opx + Sil + Qtz and Spr + Opx +Sil + Crd at higher and lower temperatures respectively. Isopleths of Al(in p.f.u.) in orthopyroxene are also shown. The shaded area bounded bystippled line demarcates the P-T window for the peak assemblage withmeasured Al-contents in orthopyroxene. See text for details

566 C. Sarbajna et al.

in the MAS system enlarges the stability of Spr to lowertemperatures (Brigida et al. 2007). In this experimental study,Spr coexisting with Spl is found to contain 10–30 wt.%Cr2O3, which is far beyond what is observed in natural rocks.A thorough literature research, however, reveals many suchcase studies. Cr, Ni and Co enriched high Mg–Al granulitesoccasionally can show such unusual chemistry for which threeprocesses have been discussed: (1) pre-metamorphic process-es, (2) syn-metamorphic processes and (3) their combinations.Pre-metamorphic processes include (a) mixed volcanic-sedimentary or evaporitic source rocks (Warren 1979; Moineet al. 1981; Friend 1982; Harley 1993), (b) weathering(Donohue and Essene 2005), (c) segregation during the sedi-mentation process, and (d) diagenesis. Syn-metamorphic pro-cesses include (e) metasomatism (Kamineni and Rao 1988)and (f) partial melting and melt extraction (Grant 1968;Vielzeuf et al. 1990). Although all these possibilities do exist,most of the Cr-enriched high Mg–Al granulites and schistsrepresent chemically anomalous protoliths, what, in somecases, has been proven with isotopic data (Harley 1993). Ithas been argued that such high concentrations of Cr, Ni etc. in

high Mg–Al rocks are caused by deposition of immatureshale-type sediments (komatiitic early crust) during earlyArchean time (Bernard-Griffiths et al. 1996). Metasomatismof an ultramafic rock is an alternative possibility (Kamineniand Rao 1988), but for this model one needs to have mafic-ultramafic rocks in the vicinity, what is not the case in theKGB. Another plausible model is the advanced partialmelting-cum-melt loss leaving “restitic” anhydrous residue.This model would require unequivocal evidence for partialmelting and melt-extraction from the “restitic” aluminousgranulite (Vielzeuf et al. 1990). In the present situation, neithermetasomatism of ultramafic rock nor melting of high Mg–Alschists seems therefore probable. The voluminous granitoidgneiss hosting the high Mg–Al granulites of KGB are muchyounger in age (~2,500 Ma, Rajesham et al. 1993; presentstudy) compared to the age of granulite metamorphism(>2,500 Ma). The wide variation in Cr values in differentsamples, on the other hand, implies inheritance from mixedsedimentary protolith as envisaged by many workers (cf.Harley 1993).

Table 6 Rb–Sr and Pb–Pb isotopic data of granite gneisses in the study area

Sample no Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr 206Pb/204Pb 207 Pb/204Pb 208 Pb/204Pb

GC-3535 126 160 2.303 0.79184 27.955 17.899 42.421

GC-3537 192 164 3.421 0.81742 20.125 16.656 37.999

GC-3541 153 193 2.315 0.78953 20.094 16.544 37.131

GC-3534 165 411 1.167 0.74346 19.277 16.347 44.238

GC-3542 131 427 0.899 0.73699

GC-3536 124 402 0.8977 0.73527 19.223 16.398 40.906

GC-3540 38 507 0.219 0.71208 16.228 15.981 36.722

GC-3538 41.4 226 0.5306 0.73557 23.823 16.981 136.73

GC-3539 58.7 542 0.3142 0.72569 18.586 16.29 37.673

%errors (2σ) 2.00 1.00 2.00 0.05 0.20 0.20 0.20

Fig. 7 Rb-Sr isochron whole rock plot of host granite gneisses Fig. 8 207Pb-206Pb isochron plot of host granite gneisses

U–Cr-rich high Mg-Al granulites from Karimnagar Granulite Belt 567

Uranium mineralization

Uranium mineralization is a conspicuous feature of thestudied part of KGB, where almost all the rocks (includingthe high Mg–Al granulite) have varying amounts of U.Apart from Kottur, similar U enrichments are also reportedfrom the enclaves of Domalkunta (up to 0.083 % U3O8),Pattipak (up to 0.035 % U3O8), Peddur (up to 1.96 % U3O8)and Nallagonda (up to 0.022 % U3O8) areas (Fig. 9). It iswell known that the concentration of U in high-grade meta-morphic rocks is very low (Taylor and McLennan 1985;Rollinson 1993). During progressive regional metamor-phism, U behaves as an incompatible element and is nor-mally expelled from the system during partial melting andsubsequent melt extraction. However, data on few naturaloccurrences from high-grade metamorphic rocks reportedanomalous concentration of U in the form of uraninite grains

(Andreoli et al. 2006 from Western Namaqualand Belt,South Africa). Uraninite is conspicuous, although monaziteis much more widespread in such assemblages (Andreoliand Hart 1990; Andreoli et al. 2006). A suitable petrologicalprocess should be called for to interpret such anomalousenrichment of uranium, which can also provide enoughclues to identify its possible source. Andreoli et al. (2006)further showed that the mafic/ultramafic granulites enrichedin U are actually widespread over a very wide area withinGondwana stretching from South Africa to India andAntarctica, and perhaps beyond, for which they coined theterm “Erlank Anomaly”. Actually this is a broad region ofmid-to-lower crust rocks, where U increases with metamor-phic grade. Clearly, in the area of the ErlankAnomaly thewell-established views about U partitioning between amphibolite-facies gneisses and granulites do not seem to be valid (cf.Rollinson 1993).

Fig. 9 U-values from the granite gneisses and aluminous granulite in the study area. Measured U values are shown in terms of wt. % U3O8

Table 7 Summary of important thermal events within the KGB

Event Age Method Reference

Emplacement of granite gneiss and charnockite 2,500 Ma Rb–Sr whole rock isochron Rajesham et al. 1993

Age of charnockite, ultramafic intrusive and enderbite 2,800–2,500 Ma Pb–Pb whole rock isochron Yoshida et al. 1996

Emplacement of charnockitic gneiss 2,800–2,600 Ma Zircon EPMA chemical Santosh et al. 2004

Age of Hyderabad Karimnagar granite 2,490 Ma Rb–Sr whole rock isochron Crawford 1969

Chinnatumbalam/Gilhesugur granite 2,435±70 Ma Rb–Sr whole rock isochron Pandey et al. 1988

Age of potassic granite, Dharmawaram 2,237±46 Ma Rb–Sr whole rock isochron Singh et al. 2004

Age of granite gneiss 2,530±220 Ma Rb–Sr errorchron Present study

2,530±90 Ma Pb–Pb errorchron Present study

568 C. Sarbajna et al.

The anomalous concentration of U-bearing minerals (ura-ninite and brannerite) is a significant feature of the studiedhigh Mg–Al granulites. Trails and clusters of sub-equanturaninite grains, mostly occurring along the boundary of innerand outer growth zones of Phl (Fig. 2c), imply uraninitecrystallization at a later stage of Phl growth. Petrographicobservation also shows that Phl has grown at the post-peakmetamorphic stage. Tiny specks of sulphide minerals with orwithout uraninite, rimming and veining Phl blades indicatethat the sulphide mineralization could have coincided with theuranium mineralization, and most likely outlasted the latemetamorphic stage. Chemical ages on uraninite grains suggestca. 2,200 Ma for this event, which is younger than granulitemetamorphism and the emplacement of the granite. Assumingthat U-mineralization is related to granite emplacement, thepresently determined age of ca. 2.2 Ga may imply partialreopening of the U–Pb system in the uraninite grains; howev-er, there is the alternative possibility that granitic magmatismcontinued in this terrain. The younger age for the uraninitemay be correlated with the time of emplacement of youngerK-rich granites (ca. 2,237 Ma age; Singh et al. 2004). The lowRE2O3 and ThO2 contents of the uraninite grains furtherindicate low temperatures of formation (Fritsche andDahlkamp 1997). This is in sharp contrast to the situation inWestern Namaqualand, where pre-metamorphic U and Thenrichment caused thermal perturbations in the crust thateventually lead to UHT metamorphism (Andreoli et al.2006). A similar radioactivity-driven thermal model is putforward to explain granulite metamorphism in the NorthernMarginal Zone of the Limpopo Belt (Kramers et al. 2001). Inthese cases, high-grade metamorphism is controlled by U-Th-REE enriched, H2O-poor fluids and melts (Taylor andMcLennan 1985; Rollinson 1993). The tectonic model ofAndreoli et al. (2006) envisages collision of two U-enrichedterranes resulting in tectonic imbrication that would transportthe radioelement-bearing sediments and granites to lowercrustal depths. In absence of a working tectonic model forKGB, this could not be tested in the present case.

Timing of tectonothermal events within KGB and correlation

Available geochronological data from KGB suggest a ther-mal high-grade event at ca. 2,800–2,600 Ma related withcharnockite magmatism (zircon U–Pb EPMA dating,Santosh et al. 2004). Inherited cores of some zircon grainspreserved even much older components (ca. 3,100–2,900 Ma). However, the actual age of high-temperaturemetamorphism (such as studied here) in the KGB is un-known. Yoshida et al. (1996), based on Pb–Pb whole rockisochron age data, argued that the timing of high-grademetamorphism is ca. 2,900–2,800 Ma. But these ages werenot reproduced in later studies (Santosh et al. 2004). A high-precision geochronological study is warranted to resolve

this issue. It is important to note that the KGB is a part ofthe East Dharwar Craton (EDC) and that there are unambig-uous documentations of high-grade metamorphism in theother parts of EDC. The recent finding of ca. 2,625 Ma low-pressure granulite metamorphism in the central part of the EastDharwar Craton bears a testimony to this (Jayananda et al.2012). Although we could not estimate the age of granulitemetamorphism in this study, the estimated age data of ca.2,530 Ma for the emplacement of granitoid magma shouldbe regarded as the upper age limit for the high temperaturemetamorphism in the KGB. This is comparable to the reportedemplacement ages of granitoids from Dharmavaram (Singh etal. 2004), Hyderabad (Crawford 1969), Chinnatumbalam andGilhesugur (Pandey et al. 1988) areas as summarized inTable 7. The textural characteristics of uraninite and therecorded chemical age (~2,200 Ma) implies the formation ofuranium bearing phases at a much later stage after peakmetamorphism. The tectonothermal events in KGB, spanning700Ma (ca. 2.90–2.20 Ga), could result from important crust-building events in the EDC.

Acknowledgments The authors are thankful to Director, AMD, forgiving permission to publish this research article. SB and KD acknowl-edge the DST FIST-sponsored research laboratory facilities at the Presi-dency University, Kolkata and Bengal Engineering and ScienceUniversity, Shibpur, respectively. AC thankfully acknowledges Depart-ment of Atomic Energy for Raja Ramanna Fellowship grant. We thankDr. Subrata Karmakar for his help and suggestions, particularly thoserelated to the geochronological calculations. SB thanks Saptarshi Mallickfor his assistance. J.G. Raith, Editor in Chief, and two anonymousreviewers are thanked for their thoughtful comments on the manuscripts.These comments immensely helped to improve the quality of this paper.

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