Early Neoarchaean A-type granitic magmatism by crustal

J. Earth Syst. Sci. (2018) 127:43 c© Indian Academy of Scienceshttps://doi.org/10.1007/s12040-018-0947-y

Early Neoarchaean A-type granitic magmatismby crustal reworking in Singhbhum craton:Evidence from Pala Lahara area, Orissa

Abhishek Topno1, Sukanta Dey1,* , Yongsheng Liu2 and Keqing Zong2

1Department of Applied Geology, Indian Institute of Technology (Indian School of Mines),Dhanbad 826 004, India.2State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences,China University of Geosciences, Wuhan 430 074, China.*Corresponding author. e-mail: [email protected]

MS received 9 May 2017; revised 6 August 2017; accepted 10 August 2017; published online 11 April 2018

Several volumetrically minor ∼2.8 Ga anorogenic granites and rhyolites occur along the marginal part ofthe Singhbhum craton whose origin and role in crustal evolution are poorly constrained. This contributionpresents petrographic, geochemical, zircon U–Pb and trace element, and mineral chemical data on suchgranites exposed in the Pala Lahara area to understand their petrogenesis and tectonic setting. The PalaLahara granites are calc-alkaline, high-silica rocks and define a zircon U–Pb age of 2.79 Ga. These granitesare ferroan, weakly metaluminous, depleted in Al, Ca and Mg and rich in LILE and HFSE. They areclassified as A2-type granites with high Y/Nb ratios. Geochemical characteristics (high SiO2 and K2O,very low MgO, Mg#, Cr, Ni and V, negative Eu anomaly, flat HREE and low Sr/Y) and comparison withmelts reported by published experimental studies suggest an origin through high-temperature, shallowcrustal melting of tonalitic/granodioritic source similar to the ∼3.3 Ga Singhbhum Granite. Intrusion ofthe Pala Lahara granites was coeval with prominent mafic magmatism in the Singhbhum craton (e.g.,the Dhanjori mafic volcanic rocks and NNE–SSW trending mafic dyke swarm). It is suggested that the∼2.8 Ga A-type granites in the Singhbhum craton mark a significant crustal reworking event attendantto mantle-derived mafic magmatism in an extensional tectonic setting.

Keywords. Granite; A-type; geochemistry; Archean; crustal reworking; Singhbhum craton.

1. Introduction

Archaean cratons are archives of episodic juvenilecrust formation and crustal reworking events (Gui-treau et al. 2012; Dey 2013; Hawkesworth et al.2013; Zhai 2014; Dey et al. 2017). It has beennoted that in many cases the two processes are

Supplementary material pertaining to this article is available on the Journal of Earth System Science website (http://www.ias.ac.in/Journals/Journal of Earth System Science).

related. Often an event of juvenile crustal growthis followed shortly by crustal reworking (Hill et al.1992; Rey et al. 2003). Field, geochemical andgeochronological studies provide vital clues inidentifying such events and understanding theprocesses and tectonic setting of crustal growth.Especially, crustal reworking plays an important

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role in stabilization of the cratons and determiningtheir final compositional structure. Crust-derivedA-type granites present a special case of crustalreworking, because they crystallize from magmaswith temperatures significantly higher than thoseof other intracrustal granites (Clemens et al. 1986;Rutter and Wyllie 1988; Creaser et al. 1991;King et al. 2001). These crust-derived A-typegranites originate from a wide range of sources.Understanding their petrogenesis is crucial intracking the evolutionary history of cratons(Eby 1992; Bonin 2007; Frost and Frost 2011;Dey et al. 2014).

Several pulses of orogenic granitoid magmatismoccurred in the Singhbhum craton (figure 1)between 3.45 and 3.3 Ga (Saha 1994; Mishraet al. 1999; Nelson et al. 2014; Upadhyay et al.2014; Dey et al. 2017). These granitoids are inter-leaved with >3.3 Ga supracrustal belts. Results ofrecent geochronological investigations (Tait et al.2011; Nelson et al. 2014; Upadhyay et al. 2014;Dey et al. 2017) suggest that this craton was stabi-lized at ∼3.25 Ga followed by intrusion of the 3.1Ga A-type Mayurbhanj Granite. However, severalvolumetrically minor but geochemically distinct∼2.8 Ga anorogenic granites and rhyolites occuralong the marginal part of the Singhbhum craton(Mishra et al. 2000; Bandyopadhyay et al. 2001;Chattopadhyay et al. 2015). The petrogenesis ofthese rocks and their role in crustal evolution arepoorly constrained owing to the inadequate geo-chemical and geochronological data. In this work,we present petrographic, whole-rock geochemical,zircon U–Pb and trace element, and mineral chem-ical data on such granites exposed in the PalaLahara area, south-western margin of the Singhb-hum craton (figure 2). We deliberate on the natureof the source of these granitoids and the source-rock melting conditions. Finally, the tectonic set-ting of emplacement of the granitoids and their rolein crustal evolution are discussed.

2. Regional geology

The Singhbhum craton is bounded to the southby the Eastern Ghats Mobile Belt (EGMB), tothe west by the Bastar craton, to the north bythe North Singhbhum Mobile Belt (NSMB) andto the east by recent alluvium (figure 1). Thecentral part of the Singhbhum craton is occu-pied by polyphase 3.45–3.3 Ga granitoids (Mishraet al. 1999; Acharyya et al. 2010; Tait et al.2011; Nelson et al. 2014; Upadhyay et al. 2014;

Dey et al. 2017). This granitic terrain is encircledby various supracrustal assemblages ranging inage from Palaeoarchaean to Mesoproterozoic (Saha1994). Traditionally the oldest among them isbelieved to be the >3.3 Ga Older MetamorphicGroup or OMG comprising strongly-deformed,amphibolite facies pelitic schists, calc-gneisses,para-amphibolites and ortho-amphibolites (Saha1994; Sharma et al. 1994; Hofmann and Mazum-dar 2015). The OMG is tectonically interleavedwith a group of moderately to strongly deformed3.45–3.33 Ga TTGs (tonalites, trondhjemites andgranodiorites) and granites collectively termed asOlder Metamorphic Tonalite Gneiss (OMTG) (Nel-son et al. 2014; Upadhyay et al. 2014; Hofmann andMazumdar 2015).

Voluminous granitoid magmatism, ranging incomposition from trondhjemite, granodiorite togranite, occurred in the Singhbhum craton between3.35 and 3.29 Ga (Mishra et al. 1999; Tait et al.2011; Nelson et al. 2014; Upadhyay et al. 2014;Dey et al. 2017). These granitoids (SinghbhumGranite) are bordered by the >3.3 Ga green-schist facies Iron Ore Group (IOG) sediments dis-tributed in three prominent basins (Jamda-Koira,Tomka-Daitari and Gorumahisani-Badampahar)(Saha 1994; Mazumder et al. 2012). Along theeastern margin of the Singhbhum craton, the A-type Mayurbhanj granite was emplaced at 3.1Ga (Nelson et al. 2014). Subsequently, duringNeoarchaean to Palaeoproterozoic, several shallowvolcano-sedimentary basins (Dhanjori, Simlipal,Malangtoli and Jagannathpur) developed over theMesoarchaean to Neoarchaean basement of the cra-ton (Bose 2009). To the north, the Singhbhumcraton is bordered by the Neoarchaean to Mesopro-terozoic metasedimentary and metavolcanic rocksof the Singhbhum Mobile Belt (Singhbhum Group)(Mazumder et al. 2015).

The SW part of the Singhbhum craton, thefocus of the present study, exposes the Malayagirisupracrustal belt and associated granite gneissesdesignated as the Pala Lahara gneisses (figure 1).Recent zircon U–Pb dates suggest them to be∼2.8 Ga (Nelson et al. 2014; Chattopadhyay et al.2015), much younger than the Singhbhum Gran-ite and IOG. The stratigraphic status of theserocks is not well constrained. Some workers (e.g.,Sarkar et al. 1990; Saha 1994; Mohanty et al. 2008;Chattopadhyay et al. 2015) consider this terraneas the deformed, marginal part of the Singhbhumcraton. Others include the Pala Lahara gneissesand associated supracrustal rocks in the Rengali

J. Earth Syst. Sci. (2018) 127:43 Page 3 of 22 43

Figure 1. Geological map of the Singhbhum craton showing the location of the study area (after Saha 1994). Inset shows thedisposition of Eastern Ghats Province, Rengali Province, Singhbhum Mobile Belt and Chhotanagpur Granite Gneiss withrespect to the Singhbhum craton (after Crowe et al. 2003). The location of ∼2.8 Ga A-type granites and Dhanjori maficvolcanic rocks are marked within the inset as follows. 1: Bhuban, 2: Rengali, 3: Golabhand, 4: Pala Lahara, 5: Temperkola,6: Pitamahali, and 7: Dhanjori.

Province, a wedge-shaped transitional domainsandwiched between the Singhbhum craton and theEastern Ghats Province of the EGMB (Crowe et al.2003; Ghosh et al. 2016). The Rengali Province isa domain with distinct lithological and structuralcharacteristics compared to the adjacent part of theEastern Ghats Mobile belt and Singhbhum craton(Nash et al. 1996; Crowe et al. 2003). The domainexposes medium- to high-grade gneisses includinggneissic charnockites and mafic granulites andinterleaved metamorphosed volcano-sedimentaryassemblages of Mesoarchaean to Palaeoproterozoicage (Mahalik 1994; Nash et al. 1996; Crowe et al.

2003; Mahapatro et al. 2011; Bose et al. 2015;Ghosh et al. 2016). Texture-controlled U–Th–Pbchemical ages of monazites from the supracrustalrocks suggested polymetamorphic events at 3.06,2.78, 2.42, 0.98–0.94 and 0.57–0.54 Ga (Mahapa-tro et al. 2011; Chattopadhyay et al. 2015; Ghoshet al. 2016). It has been suggested that the RengaliProvince recorded evidences of multiple orogeniesincluding Grenville-age (0.98–0.94 Ga) dockingof the Eastern Ghats Province with proto-India(Chattopadhyay et al. 2015). Bose et al. (2016), onthe basis of SHRIMP zircon U–Pb dating, proposedthat the main orogenic events within the Rengali


Province occurred during the Neoarchaean time(c. 2800–2500 Ma) as a result of southward growthof the Singhbhum craton.

3. Geology of the Pala Lahara areaand samples

In the Pala Lahara area, supracrustal rocks ofthe Malayagiri Group (or Pala Lahara Group) aresurrounded by granitic gneisses (figure 2). TheMalayagiri Group mainly consists of greenschist toamphibolite facies quartzites (metasandstones orpsammites), schistose muscovite quartzites (psam-mopelites), metapelites, amphibolites/metabasalts,and banded iron formations (BIFs) forming asynclinorial complex (Prasad Rao et al. 1964;Sarkar et al. 1990). Nelson et al. (2014) reporteda weighted mean 207Pb/206Pb SHRIMP zirconage of 2806±6 Ma from a dacitic tuff of theMalayagiri Group from Golabhand area. Theadjacent Pala Lahara granitic gneisses are grey,mesocratic, distinctly banded, foliated and, often,migmatitic (figure 3a). Foliations within thegneisses trend NE–SW, E–W to NW–SE. Min-eral lineations, defined by parallelism of elongatedhornblende and magnetite grains, plunge SW, ESEor NE. Mafic minerals like biotite, hornblende andmagnetite are segregated into darker bands whichare intercalated with lighter coloured quartzo-feldspathic bands (figure 3b). Parallel alignmentof biotite-rich streaks commonly defines a strongfoliation (figure 3c).

Petrographic study on the collected Pala Laharagranite gneiss samples have shown them to bemedium- to fine-grained, inequigranular rockscomposed mainly of quartz, K-feldspar (bothmicrocline and orthoclase) and plagioclase withsubordinate biotite and hornblende (figure 3d).Accessory minerals include zircon, apatite, epidote,allanite, titanite and magnetite. K-feldspar dom-inates in abundance over plagioclase. The grainsof K-feldspar are anhedral mostly occurring withinthe interstitial places between larger subhedral toanhedral plagioclase and anhedral quartz grains.Some of the larger K-feldspar grains enclose smallergrains of quartz, biotite and hornblende. Biotitegrains generally occur as individual thin flakes orare segregated into narrow linear bands. Perthitesare common. Hornblende grains are small, anhedralto subhedral in nature. Sometimes biotite, horn-blende and titanite form clusters. The rock dis-plays strong deformation. Dimensional orientation

of quartz grains and subparallel alignment ofthe biotite-rich streaks define a distinct foliationwithin the rock. At places, folding of this foliationresulted in formation of crenulation cleavages sug-gesting polyphase deformation (figure 3e). WingedK-feldspar porphyroclasts with recrystallized mar-gin are common. Frequently, the quartz grainsare granulated resulting in formation of subgrainsand polygonization indicating dynamic recrystal-lization. The sample PLL40, collected from theNW part of the study area (figure 2), is signifi-cantly different. The major minerals in this sam-ple are quartz, plagioclase, microcline and biotitewith accessory muscovite, epidote, titanite, apatite,magnetite and zircon. The rock shows inequigran-ular texture. Compared to other Pala Lahara gran-ite samples, it is distinctly richer in plagioclaseand biotite whereas magnetite contents are muchless.

Leucocratic quartzo-felspathic veins and patchesoften occur within the banded gneiss. Sarkar et al.(1990) identified three phases of folding withinrocks of the Malayagiri Group – the first twophases being NNW–SSE trending followed by athird one trending E–W, the folds being definedby bedding planes and axial plane schistosity. Therelationship between the Malayagiri Group andthe granitic gneisses is controversial. According toSarkar et al. (1990), the gneisses form the base-ment of the Malayagiri supracrustal rocks. Theirargument was based on the higher grade of meta-morphism of the gneisses and absence of graniticveins within the Malayagiri Group. On the otherhand, Mohanty et al. (2008) suggested that thegranitic gneisses and the Malayagiri supracrustalrock were affected by synchronous folding and sim-ilar grade of metamorphism. According to them,the gneisses are younger as they contain raftsand enclaves of amphibolites and metasedimentaryrocks presumably of the Malayagiri Group.

The area north of the Pala Lahara graniticgneisses is occupied by weakly metamorphosedquartzites and conglomerates of the E–W trendingMankarchua basin (Chakrabarti et al. 2011). Thecontact between rocks of the Mankarchua basinand the Pala Lahara gneisses is faulted and theirrelation is not clear.

4. Analytical results

Details of whole-rock major and trace elementdeterminations (fusion ICP-OES and ICP-MS),


Figure 2. Geological map of Pala Lahara area (modified after Mohanty et al. 2008).

zircon U–Pb dating, trace element and Hf isotopeanalyses (LA-ICPMS and LA-MC-ICPMS), andelectron microprobe mineral analyses are given inAppendix 1 (supplementary file).

4.1 Whole-rock major and trace elements

The gneisses of the Pala Lahara area show nar-row range of compositions with high SiO2 (71.5–76.0 wt%) and K2O (4.3–5.4 wt%) contents(table 1). All the samples are weakly metalumi-nous and classified as granites in the normativeAb–An–Or diagram (figure 4a). These granitesare typically ferroan with high Fe2O3 (3.7–5.8wt%) and very low MgO (0.04–0.28 wt%) contents

and Mg numbers (Mg# = 2–11) (figure 4b). TheAl2O3, CaO and Na2O contents are also low. TheK-rich nature is reflected in high K2O/Na2O ratios(1.5–2.1) and high-K calc-alkaline to shoshonitecharacter (figure 4c). The TiO2/MgO ratios arealso high (2.0–7.9).

The Pala Lahara granite gneisses (henceforthcalled as Pala Lahara granites) are distinctly richerin high field strength elements (HFSE) (Zr, Hf, Nband Ta), U, Th, total REE, Zn and Y compared toSinghbhum Granite (table 1 and figure 5a). Largeion lithophile elements (LILE) (Rb, Ba and Pb)concentrations are moderate to high. The Sr con-tents (50–66 ppm) and Sr/Y ratios (0.5–0.8) aremarkedly low. Chondrite-normalized REE patterns


Figure 3. (a) Field feature of the Pala Lahara granite showing distinctly foliated, banded nature; 0.75 km NNW of Shriram-pur. (b) Prominent gneissic banding within the Pala Lahara granite; 0.4 km NNW of Shrirampur. (c) Parallel alignmentof biotite-rich streaks defines a prominent foliation within the Pala Lahara granite; 1.9 km east of Shrirampur. (d) Pho-tomicrograph of Pal Lahara granite gneiss. Stretching of quartz and subparallel arrangement of biotite grains has imparteda strong foliation within the rock, XN. (e) Same as (d) under plane polarised light. Note folding of foliations (marked byyellow lines) resulting in formation of crenulation cleavages. Bt: biotite, Hbl: hornblende, Kfs: K-feldspar, Pl: plagioclase,Qz: quartz, Ttn: titanite.

show well fractionated LREE, flat HREE andprominent negative Eu anomaly (Eu/Eu* ∼0.4)(figure 5b). Primitive mantle (PM) normalizedmultielement variation diagrams show enrichmentof LILE, Th, U and LREE and prominent troughsat Sr, P and Ti (figure 5a). The samples also showmoderate PM-normalized troughs at Ba, Nb, Taand Y, which are mainly due to the high con-centration of the neighbouring LILE or LREE.Very low concentrations of Cr, Ni, V and Sc aredistinctive.

The sample PLL40 is different. Compared toother Pala Lahara granite samples, it containsmore Al2O3, MgO, CaO, Na2O and Sr, and isdepleted in Fe2O3, K2O, LILE, HFSE, U and Th(table 1 and figure 5a). Consequently, this sam-ple is characterized by lower K2O/Na2O (0.62)and higher Mg# (31) plotting in the magnesianand medium-K field (figure 4b, c). The absoluteREE contents are much less. Chondrite normalizedpattern shows moderately fractionated REE with-out Eu anomaly (figure 5b). Geochemically this


Table 1. Major (Wt%) and trace (ppm) element compositions of Pala Lahara granitoids

Serial no. 1 2 3 4 5 6 7

Sample PLL 35 PLL 36 PLL 37 PLL 38 PLL 39 PLL 43 PLL 40

SiO2 75.02 76.01 74.13 74.06 71.51 73.89 72.48

TiO2 0.315 0.296 0.305 0.454 0.492 0.333 0.292

Al2O3 11.13 11.46 11.52 11.76 11.76 11.77 14.22

Fe2O3(T ) 3.93 3.76 4.38 4.70 5.77 3.74 2.51

MnO 0.061 0.052 0.07 0.078 0.085 0.062 0.041

MgO 0.04 0.07 0.06 0.28 0.13 0.17 0.56

CaO 0.98 0.90 1.18 1.60 2.12 1.20 2.32

Na2O 2.90 2.65 2.81 2.91 2.79 2.62 4.33

K2O 4.72 5.34 4.92 4.82 4.31 5.37 2.69

P2O5 0.01 0.02 0.04 0.09 0.06 0.07 0.09

LOI 0.17 0.13 0.15 0.12 0.03 0.37 0.88

Total 99.28 100.69 99.57 100.87 99.06 99.60 100.41

K2O/Na2O 1.63 2.02 1.75 1.66 1.54 2.05 0.62

A/CNK 0.96 0.98 0.96 0.93 0.91 0.97 1.02

Mg# 2 4 3 11 4 8 31

Be 5 4 3 6 5 4 2

Rb 198 192 145 208 177 177 91

Cs 3.6 2.0 <0.5 2.5 1.3 1.3 4.5

Sr 50 52 56 60 66 48 243

Ba 842 887 1072 928 796 933 323

Tl 0.7 0.8 0.6 1.0 0.7 0.8 0.5

Pb 44 53 34 47 35 32 4

Th 34.6 37.1 33.2 39.4 32.8 27.3 5.0

U 6.8 6.8 4.7 6.6 4.3 3.5 0.6

Y 81 87 103 98 84 92 13

Zr 681 692 791 735 745 529 150

Hf 15.9 16.2 19.1 18.0 17.5 9.9 2.6

Nb 47 47 49 56 46 28 8

Sn 9 7 8 11 7 4 2

Ta 3.2 4.9 3.3 5.2 4.6 1.7 1.1

Mo <2 2 <2 4 2 2 <2

V 5 6 5 14 6 7 20

Cr <20 <20 <20 <20 <20 <20 <20

Sc 4 4 4 6 8 5 3

Ni <20 <20 <20 <20 <20 <20 <20

Ag 3.3 3.2 4.3 3.7 3.9 5.8 1.7

Zn 100 110 110 210 110 90 60

Ga 17 16 19 20 18 18 18

Ge <1 <1 1 1 1 2 1

La 125 144 133 149 112 141 27.9

Ce 232 272 252 268 211 238 49.9

Pr 24.9 28.6 27.5 26.9 21.9 27.0 5.4

Nd 83.0 94.2 94.7 90.7 74.9 96.1 19.5

Sm 16.4 18.5 19.6 18.6 14.7 17.3 3.4

Eu 2.09 2.25 2.55 2.54 2.08 2.15 0.90

Gd 14.3 16.3 17.4 17.1 14.1 14.2 2.7

Tb 2.5 2.7 3.0 3.0 2.4 2.4 0.4

Dy 14.9 15.8 17.4 17.6 14.3 15.0 2.3

Ho 3.1 3.3 3.6 3.6 3.0 3.0 0.4

Er 9.2 9.7 10.6 11.2 9.2 9.0 1.2

Tm 1.40 1.47 1.60 1.74 1.45 1.45 0.19


Table 1. (Continued.)

Serial no. 1 2 3 4 5 6 7

Sample PLL 35 PLL 36 PLL 37 PLL 38 PLL 39 PLL 43 PLL 40

Yb 9.1 9.8 10.7 11.9 9.9 9.3 1.2

Lu 1.45 1.60 1.71 1.95 1.65 1.30 0.17

LaN/YbN 9.0 9.6 8.1 8.2 7.4 9.9 15.2

GdN/YbN 1.2 1.3 1.3 1.1 1.1 1.2 1.8

Eu/Eu* 0.42 0.40 0.42 0.44 0.44 0.42 0.91

Sr/Y 0.62 0.60 0.54 0.61 0.79 0.52 18.69

Ga/Al 2.89 2.64 3.12 3.21 2.89 2.89 2.39

Y/Nb 1.72 1.85 2.10 1.75 1.83 3.29 1.63

ZST 927 933 940 918 907 895 766

Longitude 85◦09′37′′E 85◦09′13′′E 85◦08′11′′E 85◦09′01′′E 85◦09′23′′E 85◦11′15′′E 85◦05′17′′ELatitude 21◦25′16′′N 21◦24′47′′N 21◦24′17′′N 21◦23′59′′N 21◦23′49′′N 21◦23′16′′N 21◦27′14′′N

A/CNK = molecular Al2O3/(CaO–1.67P2O5 + Na2O + K2O), Mg# = molecular MgO/(MgO+FeOTotal). LaN/YbN and

GdN/YbN are ratio of chondrite-normalized elemental concentrations. Eu*= (GdN*SmN)1/2. ZST: zircon saturation tem-

perature (◦C).

sample is comparable to the ∼3.3 Ga SinghbhumGranite samples reported from the central part ofthe Singhbhum craton (figure 5a, b).

4.2 Zircon U–Pb dating, trace element and Hfisotope study

The sample PLL35, a typical representativesample of the Pala Lahara granite, was chosen forgeochronological work. It was collected from 3.5 kmSW of Pala Lahara (figure 2). The LA-ICPMSzircon U–Pb isotope and trace elemental data onthis sample are given in tables 2 and 3, respec-tively. The zircon grains recovered from this sampleare pink and mostly euhedral, equant to prismaticwith oscillatory zoning in CL images (figure 6a).These zircons are generally small ranging in lengthfrom 30 to 100 μm and in width from 20 to 50μm.The zircon grains show moderate U and Th con-tents with high Th/U ratios of 0.47–0.73 (table 2).On the concordia diagram, the analyses are concor-dant to moderately discordant and define an upperintercept age of 2792±75 Ma (MSWD = 5.5) (fig-ure 6b). Seven least discordant analyses (maximumdiscordance 5%) define a similar weighted average206Pb/207Pb age of 2788±59 Ma (MSWD = 4.6),which is interpreted as the time of crystallizationof the rock.

Most of the zircon analyses exhibit steep HREEpattern, positive Ce anomaly and negative Euanomaly in chondrite-normalized diagram (fig-ure 6c), suggesting their igneous origin. Two zirconanalyses (#1 and 6) are characterized by LREEand U enrichments compared to other analyses(table 3). Higher U contents probably induced

damage within the zircon structure (Hoskin 2005),which was responsible for lead loss and consequentdiscordance (5% and 9% respectively).

Only two spots (#1 and 2) could be analysedfor Hf isotope due to the small size of the zircons.These spots yielded ε Hf2.79Ga values of –2.5 and+0.1 and two-stage model ages of (TDM2) of 3.36and 3.19 Ga, respectively (table 4).

4.3 Amphibole and biotite chemistry

4.3.1 Amphibole

The structural formula for the amphibole analysesfrom Pala Lahara granite is determined on ananhydrous basis, assuming 23 oxygen atoms performula unit (a.p.f.u.) (table 5). The Fe3+/Fe2+

ratio was calculated following the charge balanc-ing method of Schumacher (1997). The amphibolegrains display narrow range of compositions. Theyare rich in total FeO (∼28 wt%) with low MgO(∼2.5 wt%) and Mg# (∼14) making all of themferroan. They are classified as calcic amphibolesaccording to the nomenclature of Leake et al.(1997) and Hawthhorne et al. (2012) and plotmostly in the ferro-pargasite field (figure 7a).All the amphibole analyses display greater than10 wt% Al2O3, sufficient to balance the Si defi-ciency in the tetrahedral sites. The Na and Kcontents are low.

4.3.2 Biotite

Structural formula of biotite is calculated on thebasis of 22 oxygen (table 5). Biotite grains of


Figure 4. Plots of Pala Lahara granite gneisses. (a) Nor-mative Ab–An–Or classification. Fields after Barker (1979).(b) FeO* vs. SiO2 plot. FeO∗ = FeOTotal/(FeOTotal+MgO).Fields of ferroan and magnesian from Frost and Frost (2008).(c) K2O vs. SiO2 plot. Fields after Peccerillo and Taylor(1976).

the Pala Lahara granite are classified as annitewith VIAl < 0.5 a.f.p.u. (Speer 1984; Tischen-dorf et al. 2007) (figure 7b). For all the grainstetrahedral sites are completely filled with Si andAl. The biotite grains are rich in FeO (∼31wt%) and TiO2 (2.7–3.3 wt%) and depleted inMgO (3.2–3.6 wt%) with low Mg# (∼0.16). On aAl(tot) vs. Mg diagram the composition of biotitecan be linked with the type of parental magma(Nachit et al. 1985). Biotites from the Pala Lahara

granite mostly plot in the subalkaline field inconformity with its ferroan character (figure 7c). Inthe FeO*–MgO–Al2O3 biotite discrimination dia-gram (Abdel-Rahman 1994), the biotite analysesfrom the Palal Lahara granite plot in the fieldof biotites from anorogenic alkaline igneous rocks(figure 7d).

5. Temperature and pressure

5.1 Temperature

Three zircon analyses (#7, 9 and 10) displayanomalously high Ti contents (354–411 ppm) sug-gesting presence of micro-inclusions (e.g., rutile).The other zircon analyses have Ti contents varyingfrom 9.8 to 43.3 ppm. The apparent crystalliza-tion temperatures of these five zircons, determinedusing the Ti-in-zircon thermometer (Watson et al.2006), range from 740 to 889◦C with an averageof 803 ± 46◦C (two standard deviations) (table 3).This average is distinctly higher than the averagevalue of 653 ± 123◦C Ti-in-zircon temperature fora variety of felsic to intermediate rocks obtainedby Fu et al. (2008). The zircon saturation temper-atures, calculated from whole-rock compositionsof the Pala Lahara granite (Watson and Har-rison 1983; Miller et al. 2003), range from 895to 939◦C with an average of 920 ± 16◦C (twostandard deviations) (table 1). These results areconsistent with the commonly observed fact thatTi-in-zircon temperatures are generally lower thanzircon saturation temperatures for granitic rocks(Fu et al. 2008). The zircon saturation temper-ature of the 2.81 Ga Golabhand rhyolite sample(originally described as dacite) from the adjacentMalayagiri Group (Nelson et al. 2014) is 886◦C.The zircon saturation temperature of the sodicsample PLL40 is much lower (766◦C), which issimilar to those of the Singhbhum Granite (range757–823, average 782 ± 25◦C; data from Nel-son et al. 2015). The comparison suggests hotnature of the parental magma of the Pala Laharagranites and the coeval rhyolite of the Malyagiribelt.

5.2 Pressure

The pressure of crystallisation of hornblendegrains of the Pala Lahara granite gneiss is obtai-ned using Al-in-hornblende barometer fromequations provided by different workers such as


0.1

1

10

100

1000

Cs Rb Ba Th U K Nb Ta La Ce Sr Nd P Zr Hf Sm Ti Tb Y

Rock

/Prim

i�ve

man

tle

PLL 35 PLL 36PLL 37 PLL 38PLL 39 PLL 43PLL 40 Singhbhum granite

1

10

100

1000

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

Rock

/Cho

ndrit

e

(b)

(a)

Figure 5. (a) Primitive mantle (Sun and McDonough 1989) normalized multi-element variation diagram and (b) Chondrite(Boynton 1984) normalized REE plots of Pala Lahara granites. Also plotted, for comparison, the Singhbhum Granites fromthe central part of the craton and rhyolite sample from the Malayagiri belt (Nelson et al. 2014). Note that the sample PLL40from the Pala Lahara area and the Singhbhum Granites are similar and have much lower LILE, REE, HFSE, U and Thcontents.

Hammarstrom and Zen (1986), Hollister et al.(1987), Schmidt (1992) and Anderson and Smith(1995). The calculated average pressures arr-ived at using different equations are P1 = 4.7Kbar, P2 = 7.2 Kbar, P3 = 7.7 Kbar and P4 =7.5 Kbar, respectively. These values primarilycorrespond to the depth of upper to middlecrust.

6. Discussions

6.1 A-type granite affinity

Important geochemical features of the Pala Laharagranitic gneisses include their ferroan character,elevated K2O, K2O/Na2O and TiO2/MgO, andenrichment in HFSE (Hf, Zr, Ti, Nb and Ta),


Table

2.U–Pb–Thisotope

analysesandage

ofzirconsfrom

thePala

Lahara

granitesample

PLL35,Singhbhum

craton.

Pb

Th

UT

h/U

207P

b/206P

b207P

b/235U

206P

b/238U

Sopt

no.

ppm

ppm

ppm

Rati

o1

σR

ati

o1

σR

ati

o1

σ

PLL35-0

1108.1

113

171

0.6

60.1

918

0.0

038

13.0

586

0.2

578

0.4

910

0.0

059

PLL35-0

2110.3

116

159

0.7

30.2

011

0.0

035

14.3

596

0.2

496

0.5

144

0.0

045

PLL35-0

394.4

70

149

0.4

70.1

858

0.0

036

13.5

952

0.3

160

0.5

308

0.0

096

PLL35-0

466.5

65

99

0.6

60.2

067

0.0

037

14.6

213

0.2

710

0.5

120

0.0

055

PLL35-0

6468.6

510

777

0.6

60.1

991

0.0

031

12.7

523

0.2

141

0.4

633

0.0

040

PLL35-0

7106.1

75

156

0.4

80.1

869

0.0

031

14.3

621

0.3

007

0.5

551

0.0

068

PLL35-0

984.0

72

122

0.5

90.1

983

0.0

031

14.5

401

0.2

456

0.5

313

0.0

054

PLL35-1

082.5

70

128

0.5

50.1

941

0.0

041

14.7

290

0.4

259

0.5

503

0.0

114

208P

b/232T

h238U

/232T

h207P

b/206P

b207P

b/235U

206P

b/238U

208P

b/238T

h

Rati

o1

σR

ati

oA

ge

(Ma)

1σ

Age

(Ma)

1σ

Age

(Ma)

1σ

Age

(Ma)

1σ

Conco

rdance

0.1

514

0.0

041

1.7

328

2757

33

2684

19

2575

25

2850

71

95%

0.1

540

0.0

036

1.4

445

2835

29

2774

17

2676

19

2894

63

96%

0.1

851

0.0

067

2.5

130

2705

33

2722

22

2745

41

3433

114

99%

0.1

610

0.0

043

1.5

706

2880

29

2791

18

2665

23

3017

75

95%

0.1

475

0.0

040

1.6

708

2820

25

2662

16

2454

18

2781

70

91%

0.1

486

0.0

039

2.3

667

2717

28

2774

20

2846

28

2801

68

97%

0.1

877

0.0

071

1.9

797

2813

26

2786

16

2747

23

3476

120

98%

0.2

247

0.0

242

2.4

940

2777

35

2798

28

2826

48

4097

399

98%


Table

3.Trace

elem

entcompositionofzirconsfrom

thePala

Lahara

granitesample

PLL35,Singhbhum

craton.

Pb

Ti

YN

bLa

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Hf

Ta

T

(Ti-in

-zir

con)

Spot

no

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

(◦ C)

PLL35-0

1108.1

9.8

4875

7.9

08.2

622.2

1.6

27.1

24.3

00.5

816.6

6.3

975.2

29.0

131

29.7

304

59.6

8551

2.2

7740

PLL35-0

2110.3

43.3

1386

5.3

61.5

614.2

0.6

24.8

76.5

21.0

132.0

10.8

128

47.0

203

41.3

377

69.6

9521

1.9

6889

PLL35-0

394.4

13.3

930

5.0

70.5

98.6

00.1

92.9

43.4

40.6

118.8

6.9

381.9

31.5

139

30.3

298

53.9

9224

1.5

8767

PLL35-0

466.5

22.6

890

4.1

71.2

210.8

0.2

93.1

03.0

30.4

218.2

6.3

376.7

30.2

134

28.9

280

52.6

9362

1.2

0818

PLL35-0

6468.6

19.6

2094

35.0

58.9

131

11.1

45.7

17.1

2.1

445.7

14.8

166

62.5

301

69.8

692

135

8227

11.8

804

PLL35-0

7106.1

399

738

12.2

0.1

49.0

50.1

41.5

12.4

30.3

213.0

5.0

461.9

24.7

112

24.1

244

46.3

9422

1.9

6N

d

PLL35-0

984.0

411

1014

12.4

1.1

910.4

0.4

13.2

14.1

40.8

422.7

8.2

092.7

34.7

151

30.9

302

53.0

9040

1.5

4N

d

PLL35-1

082.5

354

931

12.9

1.0

511.7

0.3

72.9

63.9

50.5

718.1

6.8

182.3

31.1

138

30.2

294

51.0

9321

1.7

6N

d

Ti-in

-zir

con

tem

per

atu

re(a

fter

Wats

on

etal.

2006)

not

calc

ula

ted

for

zirc

ons

show

ing>

90

ppm

Ti

(mark

edan

‘Nd’)

as

the

hig

hT

im

aybe

due

tom

icro

-incl

usi

on

(e.g

.,ru

tile

)(F

uet

al.

2008).

Figure 6. Zircons from Pala Lahara granite gneiss samplePLL35: (a) representative cathodoluminescence (CL)images, (b) concordia diagram for U–Pb dating and (c)chondrite (Boynton 1984) normalized REE patterns of zir-cons. The small circles in the CL images show LA-ICP-MSanalysis spots with serial numbers corresponding to table 2.

Zn and REE. Depletion of elements compatible inplagioclase feldspars (Al, Ca and Sr) and maficsilicates (V, Sc, Ni and Cr) is also notable. Allthese characteristics point to the A-type natureof the Pala Lahara granites (Collins et al. 1982;Creaser et al. 1991; Bonin 2007; Frost and Frost2011; Grebennikov 2014). Interestingly, the 2.81 GaGolabhand rhyolite sample from the adjacent


Malayagiri Group dated by Nelson et al. (2014)shares many of the geochemical features of thePala Lahara granites (table 1 and figure 5a).Various discrimination diagrams have been pro-posed to discriminate A-type granites from othertypes of granite (e.g., Pearce et al. 1984; Whalenet al. 1987). Considering the uncertainty about theArchaean geodynamic processes, these diagramscan at best be used to identify magmatic affinityor petrogenetic processes, rather than establish-ing tectonic setting. The Pala Lahara granites,with high 1000*Ga/Al ratios (>2.6) and elevatedHFSE contents, plot in the ‘Within Plate’ andA-type fields in such discrimination diagrams (fig-ure 8). Biotite and amphibole grains of the PalaLahara granites are ferroan with low Mg# values,similar to those found in typical A-type granites(Papousta and Pe-Piper 2014; Cunha et al. 2016).In the FeO*–MgO–Al2O3 biotite discriminationdiagram (Abdel-Rahman 1994) biotite analysesfrom the Pala Lahara granite plot in the field ofbiotites from anorogenic alkaline igneous rocks (fig-ure 7d). A-type granites usually crystallize fromhigh-temperature magma (Clemens et al. 1986;King et al. 1997). The high average Ti-in-zirconas well as zircon saturation temperatures of thePala Lahara granites is another indication of theirA-type nature. The Pala Lahara granites are oneof the oldest dated A-type granites of the world,similar in age to the 2.75–2.73 Ga A-type granitesof the Carajas Province of Brazil (Feio et al. 2012;Dall’Agnol et al. 2017).

6.2 Source constraints

A-type granitoids have diverse origins. Eby (1992)divided A-type granitoids into two subclasses: A1and A2 types. The A1 type is considered to bederived from differentiation of melts similar in com-position to oceanic island basalts. The A2 type isexplained as representing melts from varied sourcesincluding continental crust and island arc basalts.One of the main criteria of this division is theY/Nb ratio. The Pala Lahara granites, with highY/Nb ratios (>1.2), are characterized as A2 type(figure 8c).

Three main petrogenetic pathways have beenproposed for generation of A-type granitoids: (i)partial melting of crust including tonalites–grano-diorites, amphibolites, charnockites or felsic gran-ulites (King et al. 2001; Gorring et al. 2004; Duet al. 2016), (ii) differentiation of tholeiitic, tran-sitional or alkaline basalt (McCurry et al. 2008; Table

4.Lu–Hfisotope

compositionofzirconsfrom

Pala

Lahara

granitesample

PLL35,Singhbhum

craton.

Spot

no.

176H

f/177H

f1

σ176Lu/177H

f1

σ176Y

b/177H

f1

σ

Age

(Ma)

176H

f/177H

f(t

)εH

f(t)

1σ

TDM

1T

DM

2f L

u/Hf

PLL35-0

10.2

80989

0.0

00089

0.0

01324

0.0

00053

0.0

34395

0.0

01560

2788

0.2

80918

−2.5

3.6

3.1

53.3

6−0

.96

PLL35-0

20.2

81050

0.0

00014

0.0

00975

0.0

00010

0.0

24998

0.0

00373

2788

0.2

80998

0.1

1.8

3.0

53.1

9−0

.97

176H

f/177H

f(t

)re

pre

sents

init

ial

176H

f/177H

fra

tio

of

zirc

on.

ε Hf(

t)is

calc

ula

ted

rela

tive

toa

chondri

tic

rese

rvoir

wit

ha

pre

sent-

day

176H

f/177H

fra

tio

of

0.2

82785

and

176Lu/177H

fra

tio

of0.0

336

(Bouvie

ret

al.

2008).

Pre

sent-

day

176H

f/177H

fra

tio

of0.2

83294

and

176Lu/177H

fra

tio

of0.0

3933

fordep

lete

dm

antl

e(B

lich

ert-

Toft

and

Puch

tel

2010)

and

am

ean

valu

eof176Lu/177H

fra

tio

of0.0

15

for

the

aver

age

crust

(Gri

ffinet

al.

2002)

wer

euse

dduri

ng

calc

ula

tion

ofT

DM

1and

TDM

2(G

a).


Table 5. Representative microprobe analyses of amphiboles from Pala Lahara granite, Singhbhum craton.

Sl. no. 1 2 3 4 5 6 7 8 9 10 11 12

Oxide (wt%)

SiO2 38.58 39.31 39.53 38.99 38.37 39.06 38.76 39.58 38.93 39.24 39.37 38.87

TiO2 0.64 0.88 0.88 0.68 0.59 0.76 0.61 0.92 0.70 0.65 0.84 0.64

Al2O3 11.90 10.96 11.07 12.01 12.39 11.98 12.21 10.54 11.37 11.72 11.12 11.71

FeO 29.00 29.12 29.05 29.29 28.57 28.44 28.75 29.82 29.43 27.99 29.02 28.40

MnO 0.79 0.69 0.73 0.73 0.70 0.81 0.85 0.80 0.85 0.70 0.72 0.76

MgO 2.46 2.63 2.57 2.47 2.44 2.61 2.52 2.63 2.43 2.62 2.55 2.42

CaO 10.43 10.53 10.68 10.51 10.55 10.47 10.59 10.75 10.40 10.60 10.68 10.26

Na2O 1.52 1.45 1.48 1.53 1.46 1.49 1.34 1.39 1.54 1.47 1.54 1.60

K2O 1.63 1.56 1.51 1.60 1.71 1.71 1.73 1.58 1.50 1.51 1.54 1.70

Total 96.95 97.13 97.50 97.81 96.78 97.33 97.36 98.01 97.15 96.50 97.38 96.36

Si 6.23 6.34 6.34 6.24 6.20 6.28 6.22 6.33 6.28 6.34 6.33 6.33

AlIV 1.77 1.66 1.66 1.76 1.80 1.72 1.78 1.67 1.72 1.66 1.67 1.67

AlVI 0.50 0.42 0.44 0.50 0.56 0.54 0.52 0.31 0.44 0.57 0.44 0.57

Fe3+ 0.30 0.25 0.22 0.28 0.28 0.17 0.33 0.37 0.31 0.15 0.22 0.08

Ti 0.08 0.11 0.11 0.08 0.07 0.09 0.07 0.11 0.08 0.08 0.10 0.08

Mg 0.59 0.63 0.61 0.59 0.59 0.63 0.60 0.63 0.58 0.63 0.61 0.59

Fe2+ 3.62 3.68 3.67 3.64 3.57 3.65 3.53 3.62 3.66 3.63 3.69 3.79

Mn 0.11 0.09 0.10 0.10 0.10 0.11 0.12 0.11 0.12 0.10 0.10 0.10

Ca 1.81 1.82 1.84 1.80 1.83 1.80 1.82 1.84 1.80 1.84 1.84 1.79

Na 0.48 0.45 0.46 0.47 0.46 0.46 0.42 0.43 0.48 0.46 0.48 0.50

K 0.34 0.32 0.31 0.33 0.35 0.35 0.35 0.32 0.31 0.31 0.32 0.35

Fetotal 3.92 3.92 3.90 3.92 3.86 3.82 3.85 3.99 3.97 3.78 3.90 3.86

Altotal 2.27 2.08 2.09 2.26 2.36 2.27 2.31 1.99 2.16 2.23 2.11 2.25

Mg# 13 14 14 13 13 14 14 14 13 14 14 13

Classification Fe-Pr Fe-Pr Fe-Pr Fe-Pr Fe-Pr Fe-Pr Fe-Pr Ha Fe-Pr Fe-Pr Fe-Pr Fe-Pr

Representative microprobe analyses of biotites from the Pala Lahara granite,

Singhbhum craton

Oxide (wt%)

SiO2 33.43 33.78 34.39 34.40 33.58 33.34 33.36 33.53 33.18 33.44 33.31 34.16

TiO2 2.73 3.10 3.26 3.22 3.09 2.98 3.10 3.15 2.89 3.27 3.23 3.22

Al2O3 14.75 14.50 14.91 14.65 15.05 14.89 14.90 14.62 14.82 14.51 14.78 14.33

FeO 32.17 31.47 31.60 31.20 30.93 30.87 30.92 31.15 30.87 30.33 30.36 29.89

MnO 0.58 0.51 0.54 0.49 0.41 0.48 0.49 0.46 0.49 0.53 0.50 0.44

MgO 3.51 3.53 3.34 3.38 3.15 3.12 3.62 3.16 3.23 3.57 3.41 3.55

CaO 0.03 0.00 0.00 0.00 0.01 0.02 0.10 0.04 0.05 0.01 0.02 0.01

Na2O 0.09 0.09 0.10 0.13 0.05 0.06 0.09 0.08 0.08 0.08 0.14 0.08

K2O 8.58 8.78 8.99 8.97 8.75 8.98 8.51 8.91 8.91 8.54 8.93 9.09

Total 95.87 95.76 97.13 96.44 95.02 94.74 95.09 95.10 94.52 94.28 94.68 94.77

Si 5.43 5.47 5.46 5.50 5.46 5.46 5.43 5.47 5.45 5.48 5.45 5.54

AlIV 2.57 2.53 2.54 2.50 2.54 2.54 2.57 2.53 2.55 2.52 2.55 2.46

AlVI 0.26 0.23 0.25 0.25 0.35 0.34 0.29 0.27 0.32 0.28 0.30 0.28

Ti 0.33 0.38 0.39 0.39 0.38 0.37 0.38 0.39 0.36 0.40 0.40 0.39

Fe 4.37 4.26 4.20 4.17 4.21 4.23 4.21 4.25 4.24 4.16 4.15 4.05

Mn 0.08 0.07 0.07 0.07 0.06 0.07 0.07 0.06 0.07 0.07 0.07 0.06

Mg 0.85 0.85 0.79 0.81 0.76 0.76 0.88 0.77 0.79 0.87 0.83 0.86

Ca 0.01 0.00 0.00 0.00 0.00 0.00 0.02 0.01 0.01 0.00 0.00 0.00

Na 0.03 0.03 0.03 0.04 0.02 0.02 0.03 0.03 0.03 0.03 0.04 0.03

K 1.78 1.81 1.82 1.83 1.81 1.88 1.77 1.85 1.87 1.78 1.86 1.88

Altotal 2.82 2.77 2.79 2.76 2.88 2.88 2.86 2.81 2.87 2.80 2.85 2.74

Mg# 16 17 16 16 15 15 17 15 16 17 17 17

Fe-Pr: Ferro Pargasite, Ha: Hastingsite.


Figure 7. (a) Classification of amphiboles (calcic) (after Leake et al. 1997) from Pala Lahara granite. (b) Classification ofbiotite (after Speer 1984). (c) AlTotal vs. Mg diagram for the biotite grains (after Nachit et al. 1985). (d) FeO*-MgO-Al2O3

discriminant diagram for biotites from different suites. Fields: A: anorogenic alkaline suites, P: peraluminous (includingS-type) suites and C: biotite in calc-alkaline orogenic suites (after Abdel-Rahman 1994).

Namur et al. 2011) and (iii) crustal assimilationand fractional crystallization of basalt (Frost et al.1999; Mingram et al. 2000). The lack of associ-ated mafic to intermediate members and restrictedchemical composition of the Pala Lahara granitespreclude fractional crystallization of basalt as aviable mechanism for their formation. Frost andFrost (2011) divided the A-type granites into eightsubclasses on the basis of major element composi-tions, each class being related to distinct source andpetrogenetic process. According to this scheme, thePala Lahara granites are classified as metalumi-nous calc-alkalic variety, which forms by partialmelting of crust. Further evidence comes fromthe Y/Nb vs. Yb/Ta diagram on which the PalaLahara granites predominantly plot very close tothe average continental crust (figure 9). In the PalaLahara area possible source candidates includemagnesian granitoids similar to the Singhbhum

granite (for example the sample PLL40) and theamphibolites or metasedimentary rocks (metapel-ites or psamopelites) of the adjacent MalayagiriGroup.

Experimental studies have proved to be particu-larly useful in understanding the source of graniticmagmas (Skjerlie and Johnston 1993; Patino Douce1997; Bogaerts et al. 2006). Pala Lahara gran-ites are metaluminous with low Al2O3 contents.Melting of metasedimentary rocks would ratherproduce highly peraluminous S-type magmas bypartial melting (Erdmann et al. 2007). Therefore,metasedimentary rocks are not a suitable sourcefor the Pala Lahara granites. In figure 10, the PalaLahara granite samples are compared with com-positional fields of magmas generated by partialmelting experiments on various crustal lithologies.Compared to the partial melting products of maficsources, these gneisses contain higher K2O and


Figure 8. Plotting of the Malayagiri rhyolite sample (datafrom Nelson et al. 2014) and Pala Lahara granites on theclassification diagrams commonly used for A-type gran-ites: (a) Y vs. Nb (Pearce et al. 1984), (b) 10,000*Ga/Alvs. Zr (Whalen et al. 1987), (c) Nb–Y–Ce (Eby 1992).WPG: within-plate granites, VAG: volcanic arc granites,syn-COLG: syncollisional granites, ORG: ocean ridgegranites.

lower CaO contents. On the other hand, fluid-absent partial melting of tonalitic to granodioriticsource produces magma composition with CaO,K2O and Na2O contents similar to those of the PalaLahara granites. The Al2O3 contents of the PalaLahara granites are lower compared to the exper-imentally produced melts, which can be explained

Figure 9. Yb/Ta vs. Y/Nb plot for the Pala Lahara granites(after Eby 1990). Average 3.5–2.5 Ga Archaean upper conti-nental crust (UC) from Condie (1993) and lower crust (LC)from Rudnick and Gao (2003).

by retention of a high amount of plagioclase in thesource residue.

Zircon Hf isotope data presented here with two-stage model ages (TDM2 = 3.36 and 3.19 Ga) indi-cate an origin through reworking of older crustalmaterial. The εHf values (−2.5 and +0.1) aremuch lower than that of the contemporary depletedmantle and also suggest crustal reworking. Thetonalites/granodiorites of the Singhbhum Graniteor the OMTG are abundant in the nearby areaand appears to have suitable composition as sourcerock. This is supported by the work of Tait et al.(2011), who identified 3.29 Ga juvenile granodior-ites belonging to the Phase-II of the SinghbhumGranite near Keonjhargarh (figure 1). Recent zir-con U–Pb data suggest that the whole area mappedas OMTG are not as old as 3.45 Ga as traditionallybelieved (e.g., Mishra et al. 1999). A part ofthe so-called OMTG is younger and yielded zir-con U–Pb age of 3.33 Ga (Nelson et al. 2014;Upadhyay et al. 2014). Whole-rock Nd isotopedata of the tonalites belonging to OMTG havegiven similar depleted Nd mantle model ages of3.39–3.28 Ga (Sharma et al. 1994), suggestingtheir juvenile character. Considering all theabove-mentioned evidences, we propose Palaeoar-chaean (∼3.3 Ga) juvenile tonalites/granodiorites,similar to those exposed in the central part of theSinghbhum craton, as source for the Pala Laharagranites. In this context, the presence of sodic mag-nesian granitoids (e.g., the sample PLL40), in thePala Lahara area has important implications. The


Figure 10. The Pala Lahara granites compared with composition of experimental melts in SiO2 vs. CaO, Al2O3, K2O andNa2O plots. Fields after Gorring et al. (2004) as compiled from Skjerlie and Johnston (1993) and Patino Douce (1997) forfluid-absent tonalite/granodiorite melting, Carroll and Wyllie (1990) for H2O-undersaturated tonalite melting, Beard andLofgren (1991), Rushmer (1991) and Rapp and Watson (1995) for basalts and mafic amphibolites melting and Beard et al.(1994) for mafic charnockites melting.

sample represents a granitoid body similar to thoseof the Singhbhum Granite (see figure 5 for compar-ison) suggesting presence of suitable source for theA-type granite in the area. The negative Nb, Pand Ti anomalies in the primitive mantle normal-ized plot (figure 5b) shown by the Pala LaharaA-type granites are also inherited from this crustalsource (i.e., Singhbhum Granite). Our inferenceon the nature of source is not dependent on thelimited zircon Hf isotope data. The latter onlysupports the conclusions drawn from regional geo-logical setting and the whole-rock geochemicaldata.

The low Al2O3, CaO and Sr contents andprominent negative Eu anomalies suggest pres-ence of plagioclase, either in the source residue oras fractionating phase (Moyen and Stevens 2006;Watkins et al. 2007). The restricted range of com-positions (e.g., SiO2 and Rb/Sr), however, ruleout significant effect of fractionation. Plagioclaseis stable under low-pressure condition, whereas

garnet and amphibole in the source residue indicatehigh-pressure melting in deeper levels. HREE andY are compatible in garnet and amphibole. Pres-ence of these two minerals and absence of plagio-clase in the source residue increase the La/Yb andSr/Y ratios (Moyen 2009). The flat HREE pat-terns and enrichment of Y coupled with low La/Yband Sr/Y ratios, therefore, reflect absence of garnetand amphibole in the source residue. Pressurescalculated using Al-in-hornblende barometerssuggest upper to middle crustal depth of emplace-ment. The average Ti-in-zircon temperature(803◦C; table 3) and zircon saturation temperature(920◦C; table 1) suggest high-temperature melt.The low MgO contents can be explained by thepresence of orthopyroxene which is a character-istic mineral in the residual assemblage at pres-sure <4 kbar (Patino Douce 1997). In summary,the Pala Lahara granites were produced by high-temperature, shallow dehydration melting oftonalitic/granodioritic rocks.


6.3 Implications for Neoarchaean crustal evolutionof the Singhbhum craton

The 2.79 Ga Pala Lahara granites probably markthe final phase of a poorly recognized but signifi-cant tectonothermal event. These granites displayA2 type nature which are generally characteristicof post-orogenic granites, emplaced shortly aftera period of orogenesis (Eby 1992). Small bodiesof similar ∼2.8 Ga leucogranites occur along thesouthern periphery of the Singhbhum craton atRengali and Bhuban (Mishra et al. 2000) (figure 1).Along the western margin of the craton the coeval,A-type Temperkola granites intruded the metased-imentary sequence of the Singhbhum Mobile Beltas well as the ∼3.3 Ga Bonai Granite (Bandhy-opadhya et al. 2001). These occurrences suggestthat the ∼2.8 Ga A-type granitic magmatism is nota localized event, but spread along the peripheryof the Singhbhum craton pointing to extensionaltectonism long after the formation of the maingranitic nucleus and associated sedimentary basinsbetween 3.5 and 3.3 Ga. Indication of this eventin the central part of the Singhbhum craton isrecorded as the U–Pb age of 2790±27 Ma fromCL-dark to patchy pitted zones of zircons of theSinghbhum Granite (Upadhyay et al. 2014). Thesezircons display sieve texture and were interpretedto have recorded a strong fluid-assisted alterationduring metamorphism (Upadhyay et al. 2014).

Several alternative sources of heat have beenproposed for reworking of continental crust in theArchaean. These include arrival of mantle plume,thermal blanketing by greenstone belt volcanicrocks, crustal thickening and hot subduction ora combination thereof (Rey et al. 2003). A-typegranites represent a special case because theyare generally considered as products of crystal-lization from high-temperature (>900◦C) magmas(Clemens et al. 1986; Rutter and Wyllie 1988;Creaser et al. 1991; King et al. 1997, 2001). In orderto produce such high-temperature magma fromcrustal melting an external heat source, capableof substantially raising the crustal temperature,is required. A scenario of coeval mantle-derivedmagmatism associated with extensional tectonismwould apply for formation of such granites (Creaseret al. 1991; Du et al. 2016). Many studies on Pro-terozoic and Phanerozoic examples envisage crustaldelamination followed by asthenospheric mantleupwelling as the source of heat required for suchpartial melting (Gorring et al. 2004; Zhang et al.2007).

In the Singhbhum craton, synchronous maficvolcanism is recorded within the Dhanjori basin(∼2.8 Ga Sm–Nd whole-rock age; Mishra andJohnson 2005) (figure 1). The basin formed withinan intracratonic setting in NE part of the Singhb-hum craton (Gupta et al. 1985). A biotite–hornb-lende-bearing, alkali feldspar A-type graniteintruding the southern part of the Dhanjori basinnear Pitamahali (figure 1) yielded similar zircon U–Pb age of ∼2.8 Ga (Acharyya et al. 2010). LatestPb–Pb baddeleyite dating indicated emplacementof widespread NNE–SSW trending mafic dykeswarm within the Singhbhum craton at 2.80 and∼2.76 Ga (Kumar et al. 2017). We, therefore,suggest that the ∼2.8 Ga A2-type granitic mag-matism in the Singhbhum craton was attendant tosubstantial volume of mantle-derived mafic mag-matism in an extensional tectonic setting. Themafic magma supplied heat to the crust andinduced crustal reworking and generation of high-temperature A2-type granitic magma. An expectedconsequence of this process is mixing between themafic magma with crustal derived melts. However,absence of mafic microgranular enclaves within thePala Lahara granites and their high SiO2, K2O, Thand U contents coupled with very low contents offerromagnesian elements (MgO, Cr, Ni and V) doesnot support this possibility.

Additional evidence of crustal heating comesfrom the work of Mahapatro et al. (2011). Theseworkers reported 2.8–2.7 Ga spots within 3.2–3.1 Ga metamorphic monazites from pelitic gran-ulites of the Bhuban area through U–Th–Pb chem-ical dating. The younger monazite age population(2781±16 Ma) was suggested to be formed bymetamorphic reheating. At present the age ofthe mafic volcanic formations occurring along theperiphery of the Singhbhum craton (Dhanjori, Sim-lipal, Malangtoli and Jagannathpur) is not well-constrained. Testing the validity of the connectionbetween these mafic magmatism and ∼2.8 GaA-type granites requires further geochronologicaldata from the former.

Dasgupta et al. (2017) suggested that theprotoliths of the charnockite gneiss, migmatitichornblende gneiss and granite gneiss from thecentral part of the Rengali Province was the 2860–2780 Ma A-type granites. These authors proposedwithin-plate syncollisional setting for the emplace-ment of these granites. This proposal can be ruledout on the fact that outside the Rengali Province,the ∼2.8 Ga A-type granites/felsic volcanic rocks,e.g., those of the Temperkola and Pitamahali


area occurring along the western and SE marginrespectively of the Singhbhum craton (figure 1),show negligible deformation (Bandyopadhyay et al.2001; Acharyya et al. 2010). Occurrence of domi-nant NNE–SSW trending 2.8–2.76 Ga mafic dykeswarm within the Singhbhum craton (Kumar et al.2017) also does not support a syncollisional set-ting. On the contrary, the Pala Lahara, Bhubanand Rengali granites of the Rengali Provinceoccurring along the southern margin of the Singhb-hum craton, are strongly deformed, frequentlygneissic and/or mylonitic (Sarkar et al. 1990;Mishra et al. 2000; Mohanty et al. 2008). Mon-azite U–Th–Pb chemical dating of the associatedsupracrustal assemblages indicated that multiplepost-2.8 Ga metamorphic and deformation eventsaffected the Rengali Province, one of them beingthe docking of the Eastern Ghats Province withthe Singhbhum craton (Chattopadhyay et al. 2015;Ghosh et al. 2016). These events caused exhuma-tion of granitic gneisses and granulites from themiddle and lower crustal parts of the Singhbhumcraton (Mahapatro et al. 2010; Bose et al. 2015,2016).

7. Conclusions

• The 2.79 Ga Pala Lahara granites, occurring atthe southwestern margin of the Singhbhum cra-ton, are strongly deformed high-silica, K-rich,ferroan granites with elevated LILE and HFSE.

• The Pala Lahara granites display features ofA2-type granites.

• Geochemical characteristics (high SiO2, very lowMgO, Mg#, Cr, Ni and V, negative Eu anomaly,flat HRREE pattern, low Sr/Y ratio) indicatethat these granites were derived from shallowpartial melting of tonalitic to granodioritic crust,probably the ∼3.3 Ga Singhbhum Granite.

• The presence of similar A2-type granites andrhyolites along the southern, western and NEmargin of the Singhbhum craton indicates apoorly-recognized, but significant ∼2.8 Gatectonothermal event. A scenario of post-orogeniccrustal reworking, possibly attendant to mantle-derived mafic magma emplacement into thecontinental crust within an extensional tectonicsetting, is suggested.

Acknowledgements

Insightful comments from two anonymous journalreviewers and effective editorial handing by Prof.

Rajesh Srivastava helped to improve the qualityof the paper. SD acknowledges Ministry of EarthSciences, Government of India research GrantMoES/P.O.(Geosci)/45/2015. AT has received aPh.D. research fellowship from Indian School ofMines. The laboratory facilities in the Departmentof Applied Geology, IIT(ISM), funded throughDST FIST Level II Project No. SR/FST/ESII-014/2012(C), are also acknowledged. The researchis also supported by the MOST Special Funds ofthe State Key Laboratory of Geological Processesand Mineral Resources (MSFGPMR01).

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Corresponding editor: Rajesh Kumar Srivastava

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Early Neoarchaean A-type granitic magmatism by crustal