Trace, REE and PGE Geochemistry of the Mesoproterozoic Boggulakonda Gabbroic Rocks in the High-Grade...

184 K. S. V. SUBRAMANYAM AND OTHERS

GEOL. SOC. INDIA SPEC. PUBL., NO.2, 2014

Trace, REE and PGE Geochemistry of the MesoproterozoicBoggulakonda Gabbroic Rocks in the High-Grade Terrain

adjoining Nellore Schist Belt, South East India

K.S.V. SUBRAMANYAM , V. BALARAM , U.V.B. REDDY1, PARIJAT ROY

2 and S.S. SAWANT

CSIR-National Geophysical Research Institute, (CSIR), Uppal Road, Hyderabad - 500 0071Department of Applied Geochemistry, Osmania University, Hyderabad - 500 007

2National Centre for Antarctic and Ocean Research (NCAOR), Vasco-Da-Gama, GoaEmail: [email protected]

Abstract: Boggulakonda gabbroic complex (BKGC) (E 80º00'30", N 15º44'04") in the Prakasam Igneous Province (PIP)(southern India) is studied in detail using major, trace, REE and PGE geochemistry to understand its geochemical evolution. Itshows sharp contact relationships with the Precambrian quartzofeldspathic mica schist and granite gneisses. It is one among theseveral gabbroic complexes, occurring to the east of Cuddapah basin, along the eastern margin of Nellore schist belt. It ispredominantly composed of gabbro and gabbronorite with melanocratic to leucocratic variations.

Petrographic studies show cumulus, poikilitic and porphyritic textures, micro-deformation features and symplectites/replacement textures. Magnetite and ilmenites are dominant opaques with subordinate pyrite and chalcopyrite. Cumulus phasesof plagioclase, Cpx, are essential minerals whereas olivine, opx, amphibole and opaques appear as intercumulus phases. Theserocks are classified as sub-alkaline, low Ti-tholeiitic gabbros. The normative compositions indicate that these rocks are quartz,plagioclase and pyroxene normative. The differentiation index (D.I.) determined from the norm compositions are in therange of 11.4 to 20.9 indicating that they are early- middle stage basaltic differentiates. These studies also indicate fractionalcrystalization was a dominant process of their genesis. The major, incompatible trace and REE geochemistry of BKGC rocksshow LILE-LREE enrichment and HREE-HFSE depletion, pronounced Eu-positive anomalies, negative Nb anomalies relativeto Th and low Ti/V ratios. Based on the above and other binary plots, it can be inferred that the BKG rocks are the productsof ~5-15% partial melting of depleted mantle source strongly modified by the melt enrichment and addition of slabderived fluids in a subduction zone environment and emplaced as tholeiitic island arc magmas. BK gabbroic rocks indicate lowabundances of PGE. PPGE’s are greater than IPGE; Pd/Ir and Pd/Pt ratios are greater than primitive mantle and chondrites.PGE geochemistry correlate well with the petrogenesis and tectonic discrimination studies made by using major, trace and REEgeochemistry.

Keywords: Geochemistry, REE, PGE, Gabbro, Boggulakonday, Nellore schist belt, Andhra Pradesh.

~ 2800 sq km (Rao et al., 1988) (Fig.1). The region is locatedin a narrow tract having Eastern Ghat Mobile Belt to the eastand the Proterozoic Cuddapah Basin to the west. The regionhas experienced bimodal magmatic activities that resulted inthe mafic and alkaline complexes closely spaced along thenarrow tract. Generalised description of the Prakasam Igneous

Province (PIP) was documented by earlier workers(Bhattacharya, 1980; Ratnakar et al., 1985; Rao et al., 1988;Leelanandam, 1989; Vijaya Kumar, 1995). Rao et al., (1988)and considered the gabbros and granites of the province to beyounger to nepheline syenites. Madhavan et al., (1989) basedon the field, petrological and chemical evidences indicated thatthe gabbros and alkaline rocks of PIP are genetically not related

INTRODUCTION

The northern segment to the east of Cuddapah basinrepresents a thrust-stacked terrain that hosts bimodal alkalineand tholeiitic magmas (Rao et al., 1988). This NNE-SSWtrending terrain extends ~80 km, showing intense deformation.The alkaline and mafic intrusive rocks are mainly confined tothe junctions between Nellore schist belt and high-grade rocksof granulite facies (Fig.1; after Dharma Rao et al, 2011). Severalmafic or ultramafic-mafic plutonic intrusions are present buttheir interrelationships, if any, are not known. Lithologicalassociations of gabbro-anorthosite-pyroxinite and nephelinesyenite and ferrosyenite intrusions occur as detached bodies ina zone 80 km long and 35 km wide and occupying an area of

GEOLOGICAL SOCIETY OF INDIA SPECIAL PUBLICATIONNo.2, 2014, pp.184-204


TRACE, REE AND PGE GEOCHEMISTRY OF THE BOGGULAKONDA GABBROIC ROCKS, SOUTH EAST INDIA 185

although they may be related in space also probably in time.Based on the occurrence of gabbroic xenoliths within severalalkaline plutons and the occurrence of nepheline syenite andpegmatite dykes in leucogabbros of Pasupugallu gabbro pluton,located southwest of Boggulakonda pluton (BKP), JyothinderReddy and Leelanandam, (1986) demonstrated that gabbrosof the province are older than alkaline rocks. This finding alongwith presence of gabbroic xenoliths in nepheline syenites ofUppalapadu and Purimetla suggests the mafic magmatismpredates alkaline intrusions in the region. Leelanandam andJyothinder Reddy (1990), identified the rare anorthosite dykefrom Pasupugallu gabbro anorthosite pluton containing 91%(by vol.) plagioclase and concluded that the emplacement ofsuch anorthosite dyke is a result of residual liquid derived froma melt parental to the massive anorthosites. Nagaraju and

Chetty (2006) studied the geochemical characteristics oflithological assemblages of Pasupugallu pluton, suggested thatthe rocks belong to low potassium island arc tholeiites and apossible subduction or collision related magmatic origin akinto Wilson cycle.

BOGGULAKONDA GABBROIC COMPLEX (BKGC)

Previous Work

The BKGC (E80º 00' 30" / N15º 44' 04") falling in theToposheet no. 57 M/13 has N-S elongation and sharp contactrelationships with Archean hornblende-biotite-granite gneisshost rocks. Isolated other plutons of Settupalli, Purimetla,Pasupugallu, Ravipadu, Kellampalli, Amudalapalli, Podili,Pedda Cherlo Palle (Fig.1) of PIP often composed of mafic,

Basic dykes (including lamprophyres)

Alkali Granite

Hornblende syenite

Carbonatite

Quartz monzonite

Anorthosite

Gabbro / Norite

Olivine clino-pyroxinite

Nepheline syenite

Granite gneiss

Fig.1. Geological map of the study area showing various gabbroic plutons in the central Prakasam Igneous Complex. The Boggulakonda gabbropluton is located in the map on the top right hand corner. The regional map showing various tectonic units viz., Eastern Ghats, cratonicgranitic gneiss, Nellore Schist Belt and Cuddapah Basin and the Proterozoic Ophiolite Melanges Kandra and Kanigiri in central andsouth Prakasam Igneous Province to the south of Godavari rift (map after Dharma Rao et al., 2011; Geology as published by GeologicalSurvey of India).



alkaline and anorthositic components, whereas the BKP iscomposed of gabbroic rocks and Chimakurti pluton (Fig.1) iscomposed of pyroxinites in the core with a rim (thickness ofthis rim varying from 2 km in south of Chimakurti pluton to0.5 km in east west and northern sides) of gabbros andanorthosites along with an arcuate body of anorthosite on thenorth east of the pluton (Rao et al.,1988). Earlier workers(Leelanandam, 1972, 1989; Rao et al., 1977, 1988; Nag et al.,1984), studied (as summarised above) the areas of north andsouth of Boggulakonda with special attention paid to thealkaline plutonic complexes like Elchuru, Purimetla,Uppalapadu and Ravipadu (Fig.1). Rao et al., (1988) studiedmajor element geochemistry of BKG along with other maficcomplexes of the PIP. Based on field and major element oxidedata Vijaya Kumar and Ratnakar (1995) interpreted PIPincluding BKGC as plutons emplaced into the preheatedcountry rocks hence, lack chilled margins. Further, the authorsopined that Boggulakonda, Purimetla and Pasupugallu gabbrosare formed by fractionation of olivine and plagioclase,emplaced into the country rocks as either crystal mushes andpulsative multiple intrusions or convective stirring of a highalumina olivine tholeiitic type except Pasupugallu pluton whichis of alkali olivine basalt type derived by shallow melting withinthe mantle. Vijaya Kumar and Ratnakar (1995) also estimatedthe liquidus olivine temperatures (To

liq) of gabbros (exceptinga few cumulate rocks) to be 1102oC to 1412oC and all theplutons of the province had experienced olivine fractionationprior to their emplacement and differentiated under differentpartial pressures of CO2 and H2O conditions.

The purpose of this study is to understand the magmaticand tectonic evolution of gabbroic rocks from Boggulakondapluton based on the petrological, major, trace, REE, PGEgeochemical studies. An understanding on the geochemistryof basic igneous rocks from the region like Boggulakondacomplex further can enhance the knowledge of the basicplutonism and its emplacement in an alkaline complex locatedon the continental plate margin. Generation of new trace, REEand PGE geochemical data in the area and synthesis wouldcontribute to the better understanding of the crustal evolution.

Geological Setting

The outcrop of BKGC appears like a heaped rock masswith numerous dissected blocks and two major peaks. The fieldobservations of the BKGC indicate that the emplacement ofthe igneous complex is tectonically controlled, permissiveemplacement evidenced by minor shear zones and lack of hostrock xenoliths within the gabbroic rocks, absence of chilledmargins respectively. The pluton emplacement is post f4deformation since the Surgur group host rocks (hornblende-biotite-granite gneiss) locally swerve around the plutonic bodies

Boggulakonda, Pasupugallu and Chimakurti plutons (Fig.1)which are major plutons with basic/ultrabasic igneous rocksindicating the intrusions to be a later tectonic event formedduring the waning stage of F4 folding (Rao et al., 1988). Alsolack of xenoliths of host rocks within the gabbros and absenceof chilled margins indicate preheated host rocks further confirmthat these are permissive emplacements. Petford, (1996); Best& Christiansen, (2001); Cruden & Mccaffrey, (2001); Bolle etal., (2002); Burg and Gerya, (2008); by means of field studiesand geophysical imaging indicated that granitic and non-granitic plutons have both very variable and comparable shapesand sizes. Hutton, (1988) opined that forceful emplacementof plutons occurs when magma supply exceeds the rate of spacecreation achieved by tectonic opening and magma shouldersaside the country rocks. Whereas, passive or permissiveemplacement occurs when opening rates are adequate enoughin comparison to magma supply rates that magma fills thespace passively / permissively with no shouldering aside.Cole et al., (2005) suggested subsidence of a large tectonicblock into the adjacent magma chamber from where themagma ascent start to fill up the subsidence void created. Inthe present case the absence of chilled margins and lack ofxenoliths around BKP’s emplacement mechanism is believedto be the permissive through the excessive space creationachieved by tectonic opening which may be a deep rooted one.In case of permissive emplacement the crustal interaction isvery minimal, whereas, if it would be a forceful emplacementa good amount of crustal contamination (through wall rockassimilation) and structurally deformed terrain around thepluton (development of very pronounced contact aureole) areexpected but the same is not evidenced in the present case.Field observations on other plutonic complexes indicate thatthe gabbroic bodies (mafic magmatism) are older than thealkaline complexes found in the area (Jyothinder Reddy andLeelanandam, 1986). BKP is devoid of alkaline intrusions.

The region has gained importance as Leelanandam (1990)speculated about the Kandra volcanics as a possible ophiolitecomplex. Dharma Rao and Reddy (2007) revealed that theKanigiri granite is a resultant of partial melting of a mafic source(gabbroic). Dharma Rao and Reddy (2009) reported subductionzone geochemical signatures and ophiolite-like crustallithologies of Kanigiri melange, highlighted the arc-continentcollision tectonics. Dharma Rao et al., (2011) described1.33 Ga (U-Pb zircon age) Kanigiri Melange very close to thearea of present study. Vijaya Kumar et al., (2010) reported1.85 Ga (U-Pb zircon age) for Palaeoproterozoic KOC locatedat the southern tip of Nellore Schist Belt much in detail. Thesestudies have raised many geological questions related toPrecambrian Suture zone into which the present study areaforms an integral part.



PETROGRAPHY

The igneous complex is non-layered as such, but few rolledout boulders on the lesser elevations show layered features.We call them non-layered, since layering should be visible ona pluton scale. The gabbroic rocks of the BKGC are coarse tomedium grained, melanocratic to leucocratic in colour andmassive in appearance. At places phenocrysts of hornblendeand pyroxenes are present in the more melanocratic varieties.These gabbroic rocks exhibit textures like cumulate, poikilitic,subophitic and hypidiomorphic textures (Fig. 2). Plagioclase,clinopyroxene are essential minerals. Orthopyroxene,hornblende, magnetite, minor olivine, biotite and pyrite areavailable in varying proportions and in varying sizes from northto south of the complex.

The microphotographs (Fig. 2) describe the salient aspectsof petrography of BKGC where the rocks range from gabbro-gabbronorite to anorthosite gabbros (leucogabbro). The thinsections of rock units studied under microscope showholocrystalline, hypidiomorphic crystallinity along withorthocumulus, adcumulus, granular, poikilitic, porpheritic andsubophitic textures. The variation in modal assemblagesobserved as plagioclase 70-35%, Cpx 30-10%, Opx-10-5%,hornblende 2-3%, olivine 2-1%, biotite ~1% and opaques 5-1%. Plagioclase occurs as elongate to short equant crystals of1.5 mm to 5 mm in size with carlsbad-albite twins. Plagioclasein these rocks is unzoned, having bent, spindle shaped,overlapped lamellae and corrugated crystal boundaries. Thesemicro deformation features are well depicted in plagioclasecrystals. The fragmented “domino” type porphyroclasts andshear band type fragmented porphyroclasts developed aroundthe site of micro-deformation are controlled by thecrystallographic direction and by flow type (Passchier & Trouw,2005) (Fig. 2; BK-27 c & e). The cumulus and intercumulusappearance of plagioclase, Cpx in BKGC point towards thevariation in depth of melting and crystallisation from early andresidual intercumulus liquid. The augite clinopyroxenes in theserocks are having euhedral to enhedral crystal shapes, varies insize from 3 mm to <0.1 mm. They are brownish to greenishblue in colour and are slightly pleochroic in PPL. Simpletwining is also observed on Cpx crystals at places. Clino-pyroxene hosts the crystallographically oriented plates androdlets of ilmenite, magnetite, titano-magnetites formed bysubsolidus oxidation of titanium and iron. The opaque mineralslike magnetite, ilmenite are euhedral crystals. These primaryminerals in gabbros display evidence of complex late magmaticand sub-solidus reactions evidenced by ore minerals such asmagnetites, ilmenites and pyrites in the form of exsolved blebs,rod lets, skeletal structures and small streaks or veins. Therims and thin lamellae of clinopyroxenes are observed along

the common grain boundaries and on the crystals of olivineand plagioclase, indicate that the progressive reactionbetween the melt and early formed crystal to produce theCpx rims and thin lamellae. Although, amphiboles andbiotites are observed as accessory phases in these rocks,indicative of hydrous conditions during magma crystallization.The gabbroic rocks from central and northern portions ofpluton show cumulus textures whereas the rocks fromsouthern extents are showing poikilitic and porpheritictextures. The reflected light microscopic observations on oreminerals indicate the abundance in the decreasing order ofmagnetite, ilmenite, hematite, pyrite and associatedchalcopyrite. The ore minerals in these rocks occur as blebs,exsolutions and finely disseminated interstitial phases. Theirnature and smaller size (0.01 mm to 0.1 mm) indicate that theyare products of low temperature magmatic activity. Magnetiteis observed at places with shrinkage cracks characteristicmaghematisation.

Paragenetic sequence of silicate minerals and ore mineralsbased on the petrological observations made for Boggulakondagabbroic rocks is as follows. Simultaneous crystallization ofplagioclase, olivine, Opx, and Cpx took place in which theplagioclase and clinopyroxene as cumulates and dominantphases whereas the olivine and orthopyroxene are also cumulusbut accessory phases. Later the Cpx and Opx have crystallizedagain as intercumulus minerals (Opx has sparingly crystallizedfrom the residual melt in the interstitial zones also evidence inthe formation of Opx rims around Cpx). Opaques havecrystallized at the end showing replacement textures andexsolutions textures on the early formed Cpx crystals. Thefollowing paragenetic sequence is more appropriate for oreminerals. Pyrite and chalcopyrite have simultaneouslycrystalized, subsequently magnetite and ilmenite have alsosimultaneously crystallized from the residual melt. Further,the ilmenite exsolutions on the silicate minerals may becontemporaneous during the late stage of exsolution of ilmenitealong with magnetite on magnetites and mafic minerals.Cumulus minerals in these rocks show little or no zoning andhence considered to have been formed in equilibrium with theevolving melt composition.

As a summary of petrographic studies it can be inferredthat the textural variation observed among various sectionsstudied indicate moderate to slow cooling conditions prevailedduring crystallization. The formation of chilled zones due torapid cooling sites at wall rocks around the pluton are absentindicating the emplacement of pluton into the mid/lower crustallevel where the ambient heat flow of the host rocks is highenough to abstain the melts to form chilled zones. Further,textural variations are often the resultants of thermal maturationof melts rather than the geochemistry.



(a) BK-2

0.5 mm

Oliv

0.1 mm

(f) BK-26

mgn

mgncp

(b) BK-24

0.5 mm

Plg

Cpx

(d) BK-24

0.25 mm

Plg

Sphn

Plg

(c) BK-27

0.25 mm

Plg

Opx

Oliv

Cpx

(e) BK-27

0.5 mm

(g) BK-27 0.25 mm

il Skeletal mag

Cpy

Hem

mag(h) BK-10

Oliv

Oliv

0.25 mm

Fig.2. (a) Lobate grain boundaries of enhedral olivine with intercumulus plagioclase, clinopyroxene. (b) Adcumulus growth of plagioclase with

triple junctions having intercumulus clinopyroxene. (c) & (e) Plagioclase lathes showing step faulting, grain boundary granulation alongwith Cpx and plagioclase as domino boudins (porphyroclasts) in a heterogeneous interstitial mineral mass indicating deformation andassociated ductile mass flow. (d) The creep of sphene and other minerals on the phenocryst of plagioclase developing poikilitic texture isvery well evidenced in the microphotograph. (f) Magnetite associated with stringers of chalcopyrite with finely disseminated magnetite.(g) Reflected light microphotograph showing disseminated skeletal magnetite with adjacent grains of hematite having ilmenite exsolutions,intergrowth of chalcopyrite is seen in association with magnetite. (h) Reaction rims of Opx and plagioclase around olivines.



ANALYTICAL METHODS

For the present study, eighteen fresh representative gabbroicrocks samples were collected from BKGC starting fromnortheast of the pluton towards southwest After petrographicstudies, all the collected fresh rock samples were processedfor whole rock geochemical analysis at Geochemistry Division,NGRI, Hyderabad. After crushing the rock to minus 30 meshsize, fragments free from secondary associations werehandpicked, dried in the hot air oven at 1000C to eliminate theextraneous moisture. These 30 mesh fragments were furtherpulverised to minus 200 mesh size. Then the powders wereproduced using an agate mortar and pestle. The major elementswere determined making pressed pellets and using XRF (PhilipsMagiXpro-PW2440). All the available major elements datawere standardized against the international rock standard JGb-1, running MRG-1, and JB-2 as control standards (Table 5).For the determination of major and minor elements usingXRF, sample pellets were prepared by pressing the rockpowder with the background of boric acid in collapsiblealuminium cups at 20 tonnes of pressure employing a‘HERZOG’ hydraulic press model TP 2D to give pellets of 40mm diameter. International geochemical standard referencematerials (SRMs) from the United States Geological Survey(G-2, NIST-2709, NIST-2710, NIST-2711), the CanadianGeological Survey (SO-1, SO-2, SO-3, SO-4, TILL-1, TILL-3, LKSD-1), the Geological Survey of Japan (JP-1, JR-1, JR-2, SY-3, JB-2) were used to prepare calibration curves for majorelements (Krishna et al, 2007). Trace, REE (Subramanyam etal., 2013) and PGE (Balaram et al., 2006) were determinedusing ICP-MS (Perkin Elmer SCIEX ELAN® DRC-II). ForTrace and REE determinations, Certified Reference Material(CRM) JGb-1 of Geological Survey of Japan is used, whereasMRG-1 CRM of Canadian Geological Survey and JB-2 CRMof Geological Survey of Japan are used as control standards.For PGE determinations WMG-1 of Canadian GeologicalSurvey is used as CRM and WMS-1 is used as controlstandard. The detection limits of all the trace and RE elementswere found to be in the range 0.001 to 0.010 ng/g and theprecision is better than 5% RSD. The detection limits for PGE’sat 1:50 dilutions are Ru(101)=0.016ng/ml (16ppt),Rh(103)=0.01ng/ml, (10ppt), Pd(105)=0.01ng/ml, (10ppt),Ir(193)=0.005ng/ml,(05ppt) Pt(195)=0.04ng/ml,(04ppt),Au(197)= 0.010ng/ml,(10 ppt), precision =<8% RSD (Balaramet al, 2006).

GEOCHEMISTRY

Major and Trace Elements

The major element data obtained is processed using

SINCLAS software program (Verma et al., 2002) for thefollowing parameters.

1. CIPW norm calculations,2. Fe2O3/FeO ratio re-calculations based on Middlemost

(1989)3. Accurate computation of other petrogenetically useful

parameters such asSalic = q + or + ab + an,Femic = (di-Mg) + (di-Fe) + (hy-Mg) + (hy-Fe) + fo +fa + mt + il + hmC.I.= an + 2.1570577(di-Mg) + fo+0.7007616(hy-Fe)D.I. = q + or + ab + ne + lcSI = 100 MgO/(MgO+FeO+Fe2O3+Na2O+K2O)

where Salic = sum of salic normative minerals;Femic = sum of femic normative minerals,C.I. = crystallization index (Poldervaart and Parker,1964);D.I. = differentiation index (Thornton and Tuttle,1960).S.I. = Solidification Index (Kuno, 1959).

The SINCLAS processed data is used for rock classificationand tectonic discrimination using GCD tool Kit geochemistrysoftware (Janoušek et al., 2006) Table 1. In the gabbroic rocksSiO2 ranges from 47 to 52 wt%. The Al 2O3 from 13 to 17 wt%.The total Fe2O3 from 6.3 to 10.6 wt% and MgO from 6 to 10wt% with Mg number from 60-74. The TiO2 content is in therange of 0.23-0.65 slightly above the primitive mantle (0.23 %Sun and McDonough, 1989). The CaO content ranges from11 to 14 wt% and (Na2O+K2O) from 1 to 2 wt%. The totalalkalies against silica TAS diagram (Fig.3a; after Middlemost,1989) classify these rocks as subalkaline, tholeiitic gabbros.The AFM diagram (Fig. 3b; Irvine and Baragar, 1972) indicatesthe iron enrichment trend for all the samples duringdifferentiation within the tholeiitic field. The progressive ironenrichment further indicates the prevalence of crystalizationat low oxygen fugacities (<fo2). The samples contain nearsimilar proportions of normative diopside (16-21%) andhypersthenes (16.7-18.5%) (Table 2) which has beenconsidered as a characteristic of tholeiitic rocks (Wilkinson,1986).

The variations observed based on mineralogy, the gabbroicrocks have been distinguished as shown in Table 1. The BK-2sample is relatively less differentiated representing primitivecharacteristics. The Differentiation Index (Thornton and Tuttle,1960) calculated based on the normative compositions of therocks indicate that the samples are falling in the category ofearly stage and middle stage basaltic differentiates having D.I.in the range of 11.4 to 20.9 (D.I., >11.4<20.9). 50% of thesamples are falling into early stage basaltic differentiates

19

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Table 1. Major, Trace and REE concentrations and various geochemical ratios used for BK Gabbros

Sl.No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18Rock Type Gabbro Gabbro- Gabbro- Gabbro Gabbro Gabbro Gabbro Gabbro Gabbro Gabbro GabbroLueco- Gabbro- Gabbro- Gabbro Lueco- Gabbro Gabbro-

norite norite gabbro norite norite norite gabbro norite

Sample No. BK-1 BK-2 BK-3 BK-4 BK-5 BK-6 BK-7 BK-8 BK-9 BK-10 BK-11 BK-21 BK-22 BK-23 BK-24 BK-25 BK-26 BK-27

SiO2(%) 51.23 51.34 48.72 52.98 51.87 50.35 50.81 50.96 49.66 49.69 50.12 48.95 49.92 48.98 49.95 47.98 49.85 47.89Al 2O3 15.97 13.88 16.77 16.55 17.87 16.75 16.85 16.97 17.52 16.87 16.8818.96 17.79 17.88 17.72 17.98 17.96 17.68Fe2O3 9.62 7.25 10.62 6.31 6.15 10.27 8.4 9.49 9.59 7.63 7.02 6.22 7.60 7.25 6.45 7.59 7.56 7.65MnO 0.14 0.13 0.14 0.11 0.11 0.15 0.13 0.14 0.13 0.13 0.12 0.09 0.07 0.09 0.1 0.09 0.08 0.07MgO 7.61 10.44 9.75 7.24 6.89 6.9 8.3 7.3 8.11 8.12 8.72 8.21 7.52 7.98 8.16 7.72 6.88 7.88CaO 11.53 14.17 11.21 13.44 13.72 11.1 11.83 11.6 11.32 13.81 13.91 13.88 14 13.94 14.15 14.81 13.52 14.58Na2O 1.57 0.9 1.23 1.45 1.23 1.82 1.5 1.7 1.67 1.45 1.33 1.42 1.69 1.65 1.31 1.64 1.83 2.13K2O 0.27 0.16 0.14 0.11 0.1 0.35 0.38 0.4 0.15 0.12 0.09 0.09 0.15 0.15 0.08 0.15 0.25 0.28TiO2 0.46 0.45 0.3 0.3 0.24 0.65 0.54 0.4 0.27 0.35 0.3 0.25 0.23 0.27 0.29 0.26 0.4 0.23P2O5 0.03 0.04 0.03 0.03 0.03 0.03 0.05 0.03 0.03 0.03 0.03 0.02 0.03 0.03 0.02 0.03 0.06 0.04LOI 1.6 1.3 1.2 1.5 1.8 1.7 1.3 1.1 1.55 1.8 1.48 1.91 1.1 1.8 1.8 1.8 1.7 1.6Total 100.0 100.1 100.1 100.0 100.0 100.1 100.1 100.1 100.0 100.0 100.0 100.0 100.1 100.0 100.0 100.1 100.1 100.0

Sc(µg/g) 37.6 51.2 28.1 37.7 29.4 36.1 31.5 33.6 26.0 44.0 42.9 44.7 30.2 33.5 47.2 32.3 34.18.0V 213.1 243.0 116.1 172.2 132.6 257.9 169.8 158.1 116.7 202.9 178.3 214.0 148.2 177.1 231.7 164.4 201.558.8Cr 504.2 1732.8 585.4 684.1 972.6 390.9 512.6 544.5 215.9 354.8 740.3 625.2 568.6 550.4 658.5 720.7 730.5 199.3Co 53.1 50.4 66.3 39.1 40.3 51.6 48.0 49.0 57.2 43.4 44.6 51.7 45.5 55.9 42.7 52.8 44.1 48.2Ni 121.9 196.5 231.3 127.6 113.5 110.0 119.9 100.5 94.6 84.4 165.6 142.5 121.3 178.4 125.7 170.6 166.0 132.3Cu 70.2 113.8 79.5 73.5 80.5 54.0 61.6 59.9 37.1 46.3 65.6 99.8 69.5 97.8 39.9108.7 132.4 46.4Zn 107.8 50.9 71.8 74.7 77.9 113.6 56.0 61.0 53.4 49.4 57.5 47.2 53.0 39.7 36.3 47.8 66.8 55.3Ga 13.9 10.0 12.4 14.3 15.4 15.9 13.2 14.5 14.1 13.3 13.0 13.8 16.8 15.8 13.7 16.0 17.5 19.9Rb 2.0 2.3 1.3 1.3 1.9 1.5 3.8 2.1 1.5 1.3 0.9 1.3 1.5 1.6 1.3 1.3 2.1 2.6Sr 454.1 295.2 396.6 406.5 427.0 515.3 405.7 480.4 418.1 349.5 355.9 425.3 540.1 506.7 420.2 512.6 602.5 789.6Y 8.6 9.6 3.5 5.1 4.2 11.8 7.4 8.9 3.7 6.1 5.1 4.9 3.7 4.5 5.1 4.2 5.4 1.9Zr 17.6 25.4 10.4 7.8 9.9 23.6 16.8 16.4 8.2 10.9 7.4 6.6 6.6 6.1 5.8 6.5 7.4 5.1Nb 2.7 1.8 1.2 0.6 0.9 1.1 4.7 0.6 0.6 0.8 0.4 0.3 0.3 0.5 0.4 0.3 0.7 0.8Cs 0.04 0.04 0.04 0.03 0.04 0.03 0.05 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.03Ba 119.1 99.2 122.1 96.9 106.1 157.2 153.8 158.2 106.9 93.0 94.1 169.2 225.8 238.7 138.5 228.6 515.7 558.4Hf 0.6 0.8 0.3 0.3 0.3 0.8 0.5 0.5 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2Ta 0.26 0.23 0.26 0.21 0.15 0.37 0.65 0.14 0.50 0.10 0.23 0.09 0.06 0.09 0.06 0.07 0.10 0.13Pb 1.4 1.4 1.9 1.3 2.3 1.2 1.1 1.0 1.0 0.9 2.5 6.2 6.8 6.3 4.5 4.3 10.3 6.1Th 0.4 0.5 0.3 0.2 0.3 0.2 0.8 0.8 0.2 0.2 0.1 0.1 0.2 0.2 0.2 0.1 0.3 0.3U 0.1 0.1 0.1 0.0 0.1 0.0 0.1 0.1 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.0 0.1 0.1La 4.2 5.1 1.8 1.8 2.1 6.7 6.3 6.0 1.7 1.7 1.3 2.2 2.5 3.2 1.8 2.4 4.0 3.3Ce 8.8 11.4 3.7 3.8 4.0 13.9 12.0 11.9 3.4 3.9 2.9 4.0 4.8 5.6 3.9 4.7 8.0 5.8Pr 1.1 1.4 0.5 0.5 0.5 1.7 1.3 1.4 0.4 0.5 0.4 0.5 0.5 0.6 0.5 0.6 0.9 0.6Nd 5.8 7.3 2.6 2.9 2.7 9.1 6.3 7.4 2.3 3.2 2.5 2.9 2.8 3.3 2.9 3.1 4.9 2.7Sm 1.4 1.7 0.7 0.8 0.7 2.1 1.3 1.7 0.5 0.9 0.7 0.8 0.7 0.8 0.8 0.8 1.1 0.5

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1Table 1 Contd....

Sl.No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18Rock Type Gabbro Gabbro- Gabbro- Gabbro Gabbro Gabbro Gabbro Gabbro Gabbro Gabbro GabbroLueco- Gabbro- Gabbro- Gabbro-Lueco- Gabbro Gabbro-

norite norite gabbro norite norite norite gabbro norite

Sample No. BK-1 BK-2 BK-3 BK-4 BK-5 BK-6 BK-7 BK-8 BK-9 BK-10 BK-11 BK-21 BK-22 BK-23 BK-24 BK-25 BK-26 BK-27

Eu 0.8 0.6 0.5 0.5 0.5 1.1 0.8 1.0 0.5 0.6 0.5 0.5 0.6 0.6 0.5 0.6 1.0 0.7Gd 1.3 1.6 0.6 0.8 0.7 2.0 1.3 1.5 0.6 0.9 0.8 0.9 0.7 0.9 0.9 0.8 1.1 0.5Tb 0.2 0.2 0.1 0.1 0.1 0.3 0.2 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.1Dy 1.5 1.6 0.6 0.9 0.7 1.9 1.2 1.5 0.6 1.0 0.9 0.9 0.7 0.8 0.9 0.8 1.0 0.3Ho 0.3 0.3 0.1 0.2 0.1 0.4 0.3 0.3 0.1 0.2 0.2 0.2 0.1 0.2 0.2 0.2 0.2 0.1Er 0.7 0.8 0.3 0.4 0.4 1.0 0.6 0.8 0.3 0.5 0.5 0.4 0.3 0.4 0.5 0.4 0.5 0.2Tm 0.1 0.1 0.0 0.0 0.0 0.1 0.1 0.1 0.0 0.1 0.0 0.1 0.0 0.1 0.1 0.0 0.1 0.0Yb 0.5 0.6 0.2 0.3 0.2 0.7 0.4 0.5 0.2 0.4 0.3 0.3 0.2 0.3 0.3 0.2 0.3 0.1Lu 0.1 0.1 0.0 0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.1 0.0ΣREE 26.7 32.9 11.9 13.1 12.8 41.3 32.2 34.5 10.9 14.1 11.1 13.8 14.1 16.9 13.5 14.8 23.3 14.8ΣLREE 22.0 27.5 9.8 10.2 10.5 34.6 27.9 29.4 8.8 10.8 8.3 10.9 11.8 14.1 10.4 12.2 19.9 13.5ΣHREE 4.7 5.4 2.0 2.8 2.3 6.6 4.2 5.1 2.0 3.3 2.8 2.9 2.3 2.8 3.1 2.6 3.3 1.2ΣLREE/ΣHREE 4.6 5.1 4.9 3.6 4.6 5.2 6.6 5.8 4.3 3.3 3.0 3.7 5.2 5.1 3.4 4.7 6.0 11.3

Ba/Th 305.32 205.47 435.94 468.00 424.59 924.49 199.25 210.14 663.67 481.94 729.391158.57 1152.01 1236.67 810.14 1644.87 1481.92 1918.91Ba/La 28.32 19.55 66.66 53.32 50.62 23.30 24.60 26.25 62.63 53.98 73.68 77.34 91.05 74.61 76.96 94.24127.40 167.44Nb/Y 0.31 0.19 0.33 0.11 0.22 0.09 0.64 0.07 0.15 0.12 0.08 0.06 0.08 0.11 0.08 0.08 0.13 0.43Nb/Ta 10.49 7.89 4.46 2.67 6.34 2.88 7.23 4.35 1.12 7.36 1.81 2.95 5.05 5.33 6.97 4.76 6.94 6.14Rb/Y 0.23 0.24 0.38 0.24 0.45 0.13 0.51 0.23 0.40 0.21 0.18 0.26 0.39 0.35 0.25 0.32 0.39 1.35Sm/Yb 2.66 2.96 3.46 2.66 2.74 2.88 2.95 3.21 2.61 2.40 2.68 2.60 3.03 2.84 2.63 3.08 3.77 3.93Ta/Yb 0.50 0.41 1.35 0.76 0.62 0.51 1.44 0.26 2.39 0.29 0.84 0.32 0.26 0.32 0.20 0.29 0.331.11Th/Nb 0.14 0.26 0.24 0.36 0.26 0.16 0.16 1.21 0.29 0.25 0.31 0.53 0.68 0.40 0.41 0.41 0.51 0.36Th/Zr 0.02 0.02 0.03 0.03 0.03 0.01 0.05 0.05 0.02 0.02 0.02 0.02 0.03 0.03 0.03 0.02 0.05 0.06Th/Ta 1.52 2.09 1.06 0.97 1.68 0.46 1.19 5.27 0.32 1.87 0.57 1.55 3.44 2.14 2.85 1.96 3.55 2.19Ti/Y 320.48 282.40 511.08 349.36 342.73 329.93 438.96 270.56 439.25 344.26 351.54 303.76 367.99 359.30 338.17 369.27 441.30 728.01Ti/Zr 156.79 106.03 172.57 232.06 144.66 165.42 192.42 145.81 197.88 193.24 241.67 226.29 209.08 265.92 300.79 239.43 323.44 269.94Ti/V 0.84 0.99 1.55 1.04 1.36 0.70 1.76 1.14 1.54 0.89 1.01 0.56 1.21 1.02 0.52 1.09 1.79 4.08(Gd/Yb)N 2.14 2.39 2.66 2.37 2.31 2.28 2.42 2.33 2.32 2.08 2.40 2.43 2.58 2.62 2.46 2.69 3.17 3.18(La/Lu)N 4.17 4.73 4.91 3.04 4.49 4.89 7.44 5.77 4.16 2.56 2.44 3.61 5.42 6.01 3.01 4.91 6.6712.77(La/Sm)N 1.97 1.95 1.75 1.56 2.04 2.08 3.05 2.24 2.01 1.29 1.14 1.85 2.38 2.62 1.44 2.04 2.36 4.57(La/Sm)PM 1.97 1.53 1.77 1.36 2.32 1.39 3.41 1.02 3.05 0.65 2.69 1.06 3.47 1.17 1.91 1.66 2.21 3.21CaO/Al2O3 0.72 1.02 0.67 0.81 0.77 0.66 0.70 0.68 0.65 0.82 0.82 0.73 0.79 0.78 0.80 0.82 0.75 0.82Al2O3/TiO2 34.72 30.84 55.90 55.17 74.46 25.77 31.20 42.43 64.89 48.20 56.27 75.84 77.35 66.22 61.10 69.15 44.90 76.87Mg# 61.03 74.03 64.51 69.43 68.92 57.08 66.17 60.36 62.60 67.81 71.09 72.32 66.20 68.54 71.46 66.82 64.31 67.10

USING SINCLAS PROGRAM (after Verma et al, 2002)

DI 20.341 12.906 11.464 20.928 18.487 20.796 18.125 19.958 16.522 14.538 13.697 12.885 15.74315.21 13.873 15.126 18.158 19.056

*Fe2O3 indicate total iron content determined



category and another 50% are falling into the middle stagecategory. The Mg number (Magnesium number is a measureof fractionation, the decreasing Mg number or MgO contentindicates the fractionation of ferromagnesian minerals fromthe crystallising melt) for these samples are in the range of57.04-74.03, supports the early to middle stage differentiationindex as discussed above. Loss on ignition values of thesesamples is in the range of 0.9 to 1.8%. For variation diagramsMg number (Mg# = molar 100*Mg/Mg+FeTotal) has been takenas reference parameter to plot against all major oxides due toits wide range of values and also its importance in fractionalcrystalization of basic magma. The variation diagrams (Fig.

not shown) exhibit typical magmatic trends. There is asystematic increase of SiO2, TiO2, Fe2O3, Na2O, P2O5, MnO,Na2O, and K2O with decreasing Mg number whereas abundantdecrease of CaO is observed. Al 2O3 show only a minimaldecrease with decrease in Mg number. Jacks and Gill (1970)describes evidence of fractional crystallization in a maficmagma system with a strong positive correlation between theplotted compatible and incompatible elements such as K2O vs.La (Fig. not shown). Further, a binary plot of TiO2 vs. Cao/Al 2O3 (Fig.4a) shows flat trend suggesting evidence of olivineremoval from the magmatic system which is in conformity withpetrographic observations (Dungan and Rhodes, 1978, Perfit

Fig.3. (a) Total alkali versus silica diagram for BK gabbros showing sub-alkaline tholeiitic characteristics. (b) AFM diagram after Irvine andBaragar (1971) showing BK gabbros falling along the iron enrichment trend in tholeiite region.

Fig.4. (a) TiO2 versus CaO/Al2O3 variation diagram (Dungan and Rhodes, 1978) depicting olivine removal. (b) Ti (µg/g) versus Zr (µg/g)variation diagram showing dominant early stage Cpx and plagioclase accumulation in BK gabbros (after James et al. 1987).



et al., 1980, Class et al., 1994). Wide variation in Mg number(57-74) and SiO2 (47-52%) are attributed to the fractionalcrystallization of magmas. The fractional crystallization ofclinopyroxene and plagioclase is evidenced by positivecorrelation between Ti (µg/g) and Zr (µg/g) in the binary plot(Fig.4b; after James et al., 1987). These variations areconsistent with fractional crystallization of mafic magma(Wilson, 1989). The discrimination TiO2-10MnO-P2O5

(Fig.5a; after Mullen, 1983) consistently discriminate thesegabbros as boninite-like and island arc tholeiitic rocks.

Primitive mantle normalised trace element distributionpatterns (Fig.6a) show enrichment of LILE such as Cs, Rb, K,

Ba, Sr, Pb and Eu in the range of 400-600 µg/g and depletionof HFSE such as Nb, Ta, Hf, Zr, Ti, TiO2, which are lessfractionated (Table 1). The LILE enrichment and HFSEdepletion clearly indicates the lesser degree of partial meltingor LREE and LILE enriched source, where garnet being left asresidue (Wilson, 1989). The high Ni, Cr, contents of theserocks indicates their tholeiitic parentage which is in the range84-231µg/g and 200-1732µg/g respectively. The highest Crcontent of 1732µg/g is observed in BK-2 sample is anorthopyroxene bearing gabbronorite having En 23.75%, Fs6.1%, 29% Di and 20% Hy in the norm. BK-2 also possesseshigh MgO and CaO of 10.4% and 14% with normal iron content

Fig.5. (a) Tectonic discrimination diagram after Mullen (1983) discriminating gabbros into boninite field and Island arc tholeiites. (b) Ti versusV diagram after Shervas (1982) showing island arc tectonic setting of BK gabbros.

Table 2. Norm compositions of BK gabbros

Sample Quartz Orthoclase Plg Olivine Pyroxene Ilmenite Hematite Sphene Rutile Apatite Total

BK-1 9.69 1.6 50.58 0.00 27.4 0.3 9.62 0.74 0.00 0.07 100BK-2 7.6 0.95 42.2 0.00 40.88 0.28 7.25 0.75 0.00 0.09 100BK-3 5.93 0.83 51.31 0.00 30.59 0.3 10.62 0.35 0.00 0.07 100BK-4 10.7 0.65 52.07 0.00 29.53 0.24 6.31 0.43 0.00 0.07 100BK-5 10.13 0.59 55.13 0.00 27.4 0.24 6.15 0.29 0.00 0.07 100BK-6 8.4 2.07 53.53 0.00 24.16 0.32 10.27 1.18 0.00 0.07 100BK-7 7.42 2.25 52.01 0.00 28.55 0.28 8.4 0.97 0.00 0.12 100BK-8 8.17 2.36 52.89 0.00 26.12 0.3 9.49 0.60 0.00 0.07 100BK-9 6.59 0.89 55.53 0.00 26.75 0.28 9.59 0.30 0.00 0.07 100BK-10 5.48 0.71 53.23 0.00 32.1 0.28 7.63 0.50 0.00 0.07 100BK-11 5.58 0.53 52.55 0.00 33.59 0.26 7.02 0.40 0.00 0.07 100BK-21 3.52 0.53 59.02 0.00 30.11 0.19 6.22 0.36 0.00 0.05 100BK-22 4.6 0.89 55.8 0.00 30.52 0.15 7.6 0.37 0.00 0.07 100BK-23 3.18 0.89 56.67 0.00 31.34 0.19 7.25 0.41 0.00 0.07 100BK-24 5.61 0.47 55.08 0.00 31.69 0.21 6.45 0.44 0.00 0.05 100BK-25 1.62 0.89 56.88 0.00 32.37 0.19 7.59 0.39 0.00 0.07 100BK-26 4.94 1.48 57.14 0.00 27.81 0.17 7.56 0.76 0.00 0.14 100BK-27 0 1.65 57.45 2.48 30.16 0.15 7.65 0.37 0.00 0.09 100

SiO2 < 54 and SiO2 > 45

10 MnO + TiO2 - 10 P2O5 (Mullen, 1983)



of 7.25%. Al 2O3 content (13%) of this rock is slightly lessthan the average of BK gabbros. Boninite-like tholeiiticprimitive character for BK-2 sample is interpreted based on itsmineralogy (olivine, Opx, Cpx and plagioclase assemblages),high MgO, CaO (14%) and low TiO2 (0.4%) contents andunfractionated REE distribution patterns. Ti/Y ratios areindependent of the effects arising due to the fractionalcrystalization and are very useful indicators for the fingerprinting of the magma type (Peate et al., 1992). The Ti/Y (Table1) ratios of BKG range from 270-728, (averaging 382). Ti/Y< 500 for mafic rocks are denoted as low Ti rocks except twosamples. (Viz. BK-3 and BK-27) and hence the BK gabbrosfall into the low-Ti category, might have generated by a low tomoderate degree of the partial melting (Xu et al, 2001). Ti isthe incompatible high field strength (HFS) element which ismoderate to strongly partitioned into refractory residual phasesduring partial melting and into early liquidus mafic phases suchas orthopyroxene and clinopyroxene during crystalization.Contrary to this V, having very unusual geochemical behaviour,exists in 3-5 valence states in terrestrial rocks. But in magmasit readily replaces iron and hence this element is a good indicatorof oxygen fugacity of magmas during crystallization. V alsobehaves as an incompatible HFS element like Ti in maficmagmas. The low Ti/V ratios for the BK gabbros (ranging0.52-4.08 with an average of 1.38) indicate island arc tectonicsetting (Fig.5b) (Shervais, 1982).

C1 Chondrite (Sun and McDonough, 1989) normalisedREE distribution patterns (Fig.6b) show variation with respectto varying MgO contents. ΣREE ranges from 10-41.3µg/g,whereas LREE range is between 8-34 µg/g for these rocks.HREE range (1.2-6.6µg/g) show depletion characteristicsrelative to LREE abundances (Table 1). The HREE also showdepletion characteristics with respect to Chondrite abundances.

The gabbroic rocks of BK show LREE enrichment (La/SmN

=1.1 to 4.6), flat to depleted HREE (Gd/YbN = 2.1 to 3.2) withpositive Eu anomalies (Eu/Eu* = 1.0-2.7). Fractionation andenrichment of LREE relative to HREE may be due to thedominance of cumulus clinopyroxene and plagioclasemineralogy which are the early formed mineral phases in theserocks. The LREE fractionation relative to HREE may be causedby the presence of essential minerals clinopyroxenes,plagioclase and olivine as accessory phases, partitioncoefficients increase by an order of magnitude from La to Luin these minerals. Positive Eu anomaly in the rocks is due topresence of plagioclase (Rollinson, 1993). Similar major, traceand REE patters are also reported by Mondal et al., (2012)from Paharpur gabbros.

PGE Geochemical Characteristics

The PGE and Au contents and PGE ratios of are presentedin Table 3. ΣPGE abundances are low (16.0 to 62.1 ng/g). Theprimitive mantle normalised PGE distribution patterns (Fig.7)show relative enrichment of PPGE (Rh+Pt+Pd) and Cu overIPGE (Os+Ir+Ru) and Ni with positive Rh, Pt and Pd and

Fig.6. (a) Primitive mantle normalised trace element distribution patterns of BK gabbros showing Nb, Ta, Zr, Hf and Y negative anomalies. (b)Cl chondrite normalised REE distribution patters of gabbros showing LREE enrichment with HREE depletion characteristics. the lowtotal REE samples showing high magnitude of Eu-positive anomalies.

Fig.7. Primitive mantle normalised PGE distribution patterns of BKgabbros showing IPGe depletion and PPGE and Auenrichment.



negative Ir and Ru anomalies. ΣPGE concentrations areranging 2-6 orders high when compared to the chondriteabundances of ΣPGE =28.2 ng/g (Naldrett and Duke, 1980).PPGE enrichment over IPGE is attributed due to low degreeof partial melting since the interstitial sulphides arepreferentially incorporated into the liquid phase during lowdegree of partial melting resulting in high Pd/Ir ratios (Chenand Xia, 2008).

The samples studied show almost very narrow range of Irand Os, whereas Ru contents are slightly higher than Ir andOs. Pt and Pd contents are highest in the range of 1.77 to 12.5ng/g and 9.20 to 40.86 ng/g respectively. Rh contents arevarying from 0.23 ng/g to 1.81 ng/g. Au content is widelyvarying and it is higher than the primitive mantle average of1.2 ng/g, showing a positive anomaly in the PGE distributionpatterns. Out of the ratios mentioned below a few and veryimportant (Pd/Ir), (Pd/Pt), Pd/Ru, Pd/Rh, Ru/Pt, Os/Ir, Ru/Ir,(Pt/Pt*) were taken for further discussion. These ratios forBKPG are higher than chondrite ratios 1.0, 2.73, 0.53, 0.68,0.95, 1.28 and the primitive mantle ratios 1.01, 0.79, 2.75,0.53, 0.67, 0.95, 1.27 respectively except Ru/Pt ratios. The Pt

anomaly (Pt/Pt* = PtN/ [RhN x PdN]-½) provides a measure ofthe deviation of Pt concentration from the general trend of theprimitive unfractionated mantle –normalized pattern of a rock(Kepezhinskas and Marc, 2001). Primitive unfractionatedmantle has a Pt/Pt* value of 1 (Barnes et al., 1988; Garuti etal., 1997). Positive (Pt/Pt* >1) or negative (Pt/Pt* < 1)anomalies indicate Pt fractionation from Rh and Pd as a resultof different petrologic processes. Pt/Pt* values are <0.5 forthe BKG under study. (Pt/Ir)N ratios are ranging 3-19 (19 is ahighest ratio recorded for BK-27 sample and is one of the fewhighest ever reported for mantle rocks (McInnes et al., 1999and Kepezhinskas and Marc, 2001).

The primitive mantle normalised PGE distribution patterns(Fig.7) of BKG show the depletion of Ru and a relativeenrichment of Pt. The relative Pt negative anomalies whencompared to Pd are common features in many mafic rocks(Fryer and Greenough, 1992; Greenough et al., 1993), whichoccurs due to decoupling behaviour of Pt and Pd during earlyfractionation of magmas and also due to their similar partitioncoefficients. (Chen and Xia, 2008). Fig. 8a and 8b graphicallydepict the Pd/Ir and Pt/Pd ratios which are moderately higher

Table 4. Platinum Group Element ratios of BK gabbros

PPGN/ Pt/Pt* IPGE PPGE IPGE/ PPGE/ Pd/Ir Pd/Ru Pd/Rh Pd/Pt Ru/Pt Os/Ir Ru/IrIPGN PPGE IPGE

Mantle 0.99 1.01 14.2 14.3 0.99 1.01 100 0.79 2.75 0.53 0.67 0.95 1.27Chondrite 0.99 1.00 1745 1765 0.99 1.01 1.00 0.79 2.73 0.53 0.68 0.95 1.28BK-1 0.0012 0.50 0.52 3.80 0.14 7.38 40.71 20.55 7.47 5.42 0.26 4.29 1.98BK-3 0.0030 0.36 0.35 6.72 0.05 19.40 115.88 72.94 22.05 12.97 0.18 4.12 1.59BK-5 0.0035 0.52 0.33 6.58 0.05 19.96 99.59 57.73 22.95 9.17 0.16 3.03 1.73BK-7 0.0028 0.34 0.33 3.30 0.10 9.91 49.49 24.46 12.89 10.54 0.43 2.83 2.02BK-9 0.0029 0.59 0.64 6.22 0.10 9.74 21.05 13.67 3.40 3.10 0.23 0.95 1.54BK-11 0.0026 0.32 0.21 3.34 0.06 16.27 82.47 63.58 20.38 13.99 0.22 3.38 1.30BK-21 0.049 0.51 1.46 11.72 0.12 8.04 44.65 11.56 10.01 6.15 0.53 2.15 3.86BK-23 0.0018 0.60 0.43 2.99 0.14 7.02 13.04 20.90 5.67 3.98 0.19 1.03 0.62BK-26 0.0033 0.41 0.25 3.08 0.08 12.22 46.10 23.23 8.91 7.32 0.32 1.73 1.98BK-27 0.0067 1.09 0.14 3.64 0.04 26.11 67.31 50.49 13.77 3.42 0.07 1.19 1.33Average 0.0033 0.53 0.46 5.14 0.09 13.60 48.52 30.06 11.08 6.43 0.33 2.22 1.71Min 0.0012 0.32 0.14 2.99 0.04 7.02 13.04 11.56 3.40 3.10 0.07 0.95 0.62Max 0.0067 1.09 1.46 11.72 0.14 26.11 115.88 72.94 22.95 13.99 0.53 4.29 3.86

Table 3. Platinum Group Element concentration of BK gabbros (Ni and Cu in µg/g PGE’s and ΣPGE are in ng/g)

BK-1 BK-3 BK-5 BK-7 BK-9 BK-11 BK-21 BK-23 BK-26 BK-27

Ni 121.90 231.30 113.50 119.90 94.60 165.60 142.47 176.40 165.97 132.26Os 1.28 0.89 0.73 0.68 0.73 0.51 1.88 0.69 0.39 0.20Ir 0.31 0.23 0.25 0.25 0.80 0.16 0.92 0.71 0.24 0.17Ru 0.79 0.46 0.55 0.64 1.58 0.26 4.50 0.56 0.59 0.30Rh 0.62 0.43 0.40 0.35 1.81 0.23 1.48 0.59 0.44 0.31Pt 4.42 3.83 5.17 2.22 10.31 1.77 12.53 4.36 2.79 6.48Pd 12.70 26.35 25.10 12.38 16.94 13.11 40.86 9.20 10.85 11.73Au 19.95 11.71 8.82 13.02 15.27 8.44 13.57 9.91 7.24 19.85Cu 70.20 79.50 80.50 61.60 37.10 65.60 99.79 97.83132.41 56.69SPGE 20.11 32.20 32.20 16.52 32.17 16.04 62.17 16.11 15.30 19.18



than the chondrite and primitive mantle abundance indicatinglow to moderate degree of partial melting is cause for thegenesis of the BK gabbros. Cu/Pd ratios (Fig.9) show that theBK gabbros are S-undersaturated, S due to its higher partitioncoefficients towards Pd than Cu, available S in the melt will beconsumed by Pd, further if the melt is saturated by S, associatewith Cu, thus reach the saturation of S in the melt. Hence, thehigh ratios of Cu/Pd (3898-21731 averaging 9619) indicate S-undersaturation of magmas (Momme et al., 2002; Barnes etal., 1993). The Ni/Cu vs Pd/Ir plot (Fig. 10) after Barnes et al,(1988) show the samples in the field of high magnesium basalts.The positive correlations among binary plots (Fig. not shown)of Pd vs Pt, Pd vs Ir, Rh vs Ir, Pt vs Ir indicate that the elementsare controlled by same mineral phase. The positive correlationsof various PGEs (ΣPGE, Pt, Pd, Ir, Ru, Rh) with Mg numberindicate the fractionation of PGEs during primary magmaticprocess such as fractional crystallization (Fig. not shown). Ni/

Pd versus Cu/Ir plot (Fig.11) indicate the role of sulphur as amain phase to control the fractionation between the PGEs from

Fig.10. Plot of Ni/Cu versus Pd/Ir showing BK gabbros as high Mg-basalts. Reference fields are from Barnes et al. (1988)

Fig.8. (a and b) Ir versus Pd and Pt versus Pd diagrams showing BK gabbros above the line of chondrite ratio indicating a low to moderatedegree of partial melting of mantle source (Singh et al. 2013).

Fig.9. BK gabbros showing S undersaturated conditions due to higherratios of Cu/Pd, indicating the low degree of partial melting.

Fig.11. Cu/Ir versus Ni/Pd plot after Chen and Xia (2007) for BKgabbros show positive correlation, indicating sulphide mineralphase is essentially controlling the fractionation between Ni,Cu and PGE/



Ni and Cu, further confirms the above observation (Chen andXia; 2007). Binary plot Pd vs Ir and ratio plot Ti/Y vs Sm/Yb(Figs not shown) show that the BKG occupy the field of low-Ti magmas. Relatively high ratios of Pd/Ir w.r.t Pt/Pd in theBKG may be due to retaining of Ir and Pt within the refractoryMSS (mono sulphide solution) and Fe-Pt alloy whereas Pdwould be easily mobilised and removed by the Cu-S melt underrelatively dry melting conditions. The above conditions canbe very well applicable for tholeiites and komatiites.

DISCUSSION

Petrogenesis

Petrographic studies indicate the dominance of cumulusplagioclase and clinopyroxenes and also vide variation inmagnesium numbers (57-74), indicating the fractionalcrystalization as a dominant magmatic process for thegeneration of BKG. The presence of accessory Opx and olivine

in the samples, iron enrichment trend shown by AFM diagramare indicative of the tholeiitic nature of the magma. Thenormative compositions occupy the line joining the diopsideand hypersthene indicating the magma type to be tholeiitic innature. The differentiation index (D.I.) indicate that thesegabbroic rocks are early to middle stage differentiates ofbasaltic magma. Ti (µg/g) vs Zr (µg/g) positive correlationreveal the Cpx and plagioclase crystallization. The positivecorrelation of MgO content with CaO and Al2O3 contents furtherconfirms Cpx and plagioclase fractionation whereas the olivineremoval trend is also observed by a very narrow range of CaO/Al 2O3 ratios. CaO/Al2O3 ratios (0.65-1.02, average 0.77) arevery near to upper mantle ratios (0.8) and chondrite (0.9) (Sunand McDonough, 1989) indicating lesser degree of partialmelting. Fractionation of olivine is evidenced by a flatrelationship observed between Mg number and Ni (µg/g),whereas the proportionate increase of Mg number and Cr (µg/g) indicate the accumulation of Cpx, Cpx fractionation is in

Table 5. Precision of ICP-MS data for CRM JGb-1 used as standard for BK Gabbros (in µg/g) CV after Govindaraju (1994)

Element Certified 1 2 3 4 5 6 7 8 Avg Std RSD%value DevJGb-1

Sc 36.6 36.600 38.720 37.548 39.011 37.806 40.728 39.976 40.728 38.88963 1.521256 3.912V 640.0 640.000 652.853 644.864 646.001 635.802 683.970 678.817 683.970658.2846 20.49862 3.114Cr 59.3 59.300 59.590 60.144 60.246 59.908 62.642 62.197 62.64260.83363 1.412976 2.323Co 61.6 61.600 63.043 61.796 60.984 61.864 62.909 64.284 62.90962.4235 1.053737 1.688Ni 25.4 25.400 26.145 25.403 26.173 25.078 27.109 26.452 27.10926.10863 0.773408 2.962Cu 86.8 86.800 87.839 87.406 86.343 86.500 87.861 87.836 87.86187.30575 0.657276 0.753Zn 111.0 111.000 114.147 111.044 110.948 109.763 116.496 116.761 116.496 113.3319 2.964997 2.616Ga 18.9 18.900 18.950 19.183 18.783 18.790 19.806 19.172 19.80619.17375 0.418248 2.181Rb 4.0 4.000 4.065 3.981 4.042 3.987 4.158 4.043 4.1584.05425 0.070459 1.738Sr 321.0 321.000 326.609 320.647 322.252 317.066 332.824 331.273 332.824325.5619 6.179369 1.898Y 10.8 10.750 10.965 10.824 10.905 10.814 11.142 11.011 11.142 10.94413 0.148225 1.354Zr 33.5 33.500 35.191 33.684 33.679 32.888 35.147 35.162 35.14734.29975 0.953983 2.781Nb 2.8 2.800 2.884 2.783 2.796 2.789 2.864 2.886 2.8642.83325 0.04508 1.591Cs 0.3 0.270 0.292 0.292 0.286 0.288 0.287 0.283 0.2870.285625 0.006989 2.447Ba 63.0 63.000 65.004 62.861 64.902 62.185 65.065 65.263 65.06564.16813 1.256449 1.958La 3.7 3.740 3.795 3.714 3.912 3.486 3.861 3.903 3.861 3.7840.140491 3.713Ce 7.9 7.860 7.971 7.819 8.206 7.433 8.208 8.232 8.2087.992125 0.28208 3.529Pr 1.1 1.140 1.191 1.157 1.194 1.097 1.236 1.224 1.2361.184375 0.049842 4.208Nd 5.7 5.650 5.847 5.654 5.893 5.518 5.901 5.940 5.901 5.7880.15723 2.716Sm 1.5 1.490 1.612 1.535 1.538 1.519 1.637 1.526 1.6371.56175 0.057797 3.701Eu 0.6 0.630 0.668 0.659 0.696 0.647 0.661 0.678 0.661 0.6625 0.019661 2.968Gd 1.6 1.630 1.667 1.603 1.710 1.661 1.749 1.687 1.749 1.6820.052642 3.130Tb 0.3 0.310 0.330 0.302 0.329 0.311 0.322 0.319 0.322 0.318125 0.009761 3.068Dy 1.5 1.530 1.549 1.535 1.566 1.531 1.609 1.578 1.6091.563375 0.032859 2.102Ho 0.3 0.320 0.327 0.312 0.343 0.314 0.340 0.345 0.3520.331625 0.015334 4.624Er 1.1 1.070 1.130 1.054 1.147 1.063 1.176 1.129 1.1761.118125 0.049654 4.441Tm 0.2 0.150 0.151 0.154 0.151 0.149 0.156 0.151 0.1560.15225 0.002712 1.782Yb 1.0 0.970 1.017 0.984 1.058 0.966 1.032 1.005 1.032 1.0080.032789 3.253Lu 0.2 0.150 0.162 0.147 0.172 0.142 0.166 0.174 0.1660.159875 0.012005 7.509Hf 0.9 0.880 0.877 0.885 0.929 0.856 0.957 0.922 0.9570.907875 0.38576 4.249Ta 0.2 0.170 0.172 0.168 0.168 0.172 0.182 0.168 0.1820.17275 0.005946 3.442Pb 1.9 1.900 1.967 1.904 2.082 1.900 2.120 2.066 2.1202.007375 0.099821 4.973Th 0.5 0.530 0.530 0.529 0.529 0.532 0.521 0.580 0.529 0.5350.01847 3.452U 0.2 0.150 0.150 0.150 0.160 0.149 0.161 0.149 0.1610.15375 0.005751 3.740



turn a controlling factor of Ni and Cr abundances. The variationbetween Mg number and CaO/Al2O3 ratios affiliate the gabbrosto island arc tholeiites. The chondrite normalised REE patternsshow LREE enrichment over HREE. This supports the lowdegree of partial melting with garnet left in the residue. Thepreclusion of garnet as a residual phase during partial meltingis further supported by the superchondrite ratios obtained for(Gd/Yb)N =2.4 and Al 2O3/TiO2 = 25 (Walter, 1998 andreferences therein).

The positive correlations between PGE’s and Mg numberindicate fractional crystallization as a major magmatic processby which the gabbros have been generated (Fig.12). The lowΣPGE concentrations of BKG, high and variable super-chondritic Pd/Ir and Pd/Pt ratios are comparable to the primitiveMORB like characteristics. The Ir depletion which is acharacteristic feature of primitive MORB is the reason for highPd/Ir and Pd/Pt ratios; otherwise the relatively high Pdconcentrations and negative Pt anomalies are characteristicsof primitive MORB tectonic setting (Fryer and Greenough,1992; Rehkamper et al., 1999). The BKGs record the PGEabundances in the order Ir<Ru<Rh<Pt<Pd. PGE ratios suchas Pt/Ir, Pd/Ir and Pd/Pt are due to the fluid inducedmatasomatism and melt enrichment through fluids derived fromthe down going slab (Rehkämper et al., 1997; Alard et al., 2000;Lorand et al., 1993; McInnes et al., 1999). These consistentlyhigher ratios of BKGs w.r.t primitive mantle and chondrite ratios

indicate their fractionation is controlled by same sulphidemineral phase.

Tectonic Discrimination

Trace and REE distribution patterns and ratios BKGs showLILE, LREE enriched and HREE and HFSE-depletedsignatures with positive Eu-anomalies indicating plagioclaseaccumulation. Negative anomalies of Nb, Ta, Ti and Zr whencompared to Th are considered to be the characteristics featuresof mafic rocks produced from the subduction zone tectonicsetting (Wilson 1989; Bektas et al 2007; Pearce and Stern, 2006;Munker et al, 2004). The low Nb/Ta (1.1-10.4 av. of 5.26,whereas the primitive mantle av. 17.4; Sun and McDonough,1989), signifies the depletion of Nb/Ta in the BKGs.

The REE distribution patterns show that BKGs arerelatively enriched in LREE and depleted in HREE withpronounced positive Eu-anomalies indicating plagioclaseaccumulation. The two fold LREE, LILE enrichment, relativelyflat HREE, HFSE and Nb negative anomalies with respect toTh and immobile/incompetent element ratios, are characteristicsof BKGs suggesting subduction zone environment for theirgenesis. This geochemical signature characteristic for the islandarc tectonic setting is explained as below. Rutile is a commonaccessory mineral in eclogites, having a strong geochemicalaffinity to hold Nb and Ta, which substitutes for Ti. Thus, it isplausible to assume that fluids released from a subducted slabwill be depleted in Nb, Ta and other HFSE elements. Therefore,the negative Nb and Ta anomalies are attributed to the presenceof rutile in the dehydrating slab, with the understanding thatthe geochemical signature of the fluids released from the slabremains unchanged when the fluid percolates through the mantlewedge until it reaches the zone of melting in a subduction zonetectonic environment. Hence, we suggest that retention of Nband Ta by rutile in the dehydrating slab is the explanation forthe negative Nb and Ta anomaly in subduction related arcmagmas. Eiggns et al. (1997) found significant deviations ofNb/Ta, Zr/Hf ratios from chondritic ratios while studies onmantle derived basaltic magmas indicate greater incompatibilityof Nb over Ta and Zr over Hf during mantle melting. Thissupports the notion that the cause of Nb and other HFSEdepletions in subduction zone magmas is due to thegeochemical depletion of the mantle wedge by prior meltingevents also.

The low ratios of Ti/V discriminate these gabbros as islandarc tholeiitic magmas. The arc tholeiites show both the tholeiiticand boninitic characters (BK-2) with mineral assemblages andgeochemistry as discussed in earlier sections have beenspeculated to be generated in fore-arc and back-arcenvironments during the early stages of arc system development(Taylor, 1995). These characteristics are very well established

Fig.12. Multiple plots of Mg number versus variouis PGEs for BKgabbros indicating positive trend lines. These correlations areindicative of progressive fractional crystallization as adominant magmatic process.



in ancient arc systems (Mandal et al, 2012). The low-Ticharacteristics and high Cr concentrations of BKGsdiscriminate them into the Island-arc basalts. The low tomoderate degree of melting of a basaltic slab could generatesuch Ti/Cr ratios ranging from 1.4-9.9 averaging 4.25. Alsothe variation in Mg number and CaO/ Al 2O3 ratios affiliate thegabbros as island arc tholeiites (Fig.13) (after Prefit et al., 1980and Classs et al., 1984).

Geochemical signatures of mantle wedge modification arealso treated as proxies for subduction related magma genesis.The mantle wedge above the subduction zone can be influencedby components derived from the down going slab (1) thecirculating fluids derived by the dehydration of the subductingoceanic crust or subducted sediments and (2) addition of meltsgenerated by the fluxing of subducted sediment or of theMORB portion of the slab (Zhao and Zhou, 2007). The BKgabbros show the melt related enrichment evidenced by thelow ratios of Nb/Y and Rb/Y (Fig.14) (LILE/HFSE ratios such

as Th/Zr, Rb/Y, Ba/Nb and Ba/Th ), since the mantle sourcesmodified by the slab melts reflect with lower ratios of Th/Zr,Rb/Y, Ba/Nb and Ba/Th since the LILEs will get mobilized bythe fluid phase than LREE and HFSE due to their low ionicpotential (charge/radius ratio). These rocks demonstrate theaqueous fluid addition also during subduction evidenced bythe gradually increasing ratios of Ba/Th on the vertical axesand low ratios of Th/Nb (Fig.15), since the LILE (Ba/Th) aremore hydro-soluble. But when the relationship between Ba/La vs. Th/Yb is examined (Fig. 16) elevated ratios of Ba/Laare obtained and this can be explained as the Ba (LILE) ismore soluble in fluids than La (REE), the elevated ratios Ba/La, confirms the fluid influence on the source region of mantleduring partial melting. On the other hand, considering the ratioof Th/Yb, where both Th and Yb remain constant due to theirimmobility in fluids. Hence, the relation between Ba/La and

Fig.13. Mg number versus CaO/Al2O3 plot (after Prefit et al. 1980;Class et al. 1984) depicting IAB signatures for the BKgabbros.

Fig.14. Nb/Y versus Rb/Y relationship showing low ratios of Rb/Yindicating the melt related enrichment of BK gabbros in asubduction zone tectonic setting.

Fig.15. Th/Nb versus Ba/Th for BK gabbros showing higher ratiosof Ba/Th indicative of aqueous fluid addition to the meltgenerated by subduction.

Fig.16. Ba/La versus Th/Yb binary plot show the addition of slabderived melts during subduction process thus increasing theLILO budget of BK gabbros.



Th/Yb indicates only fluid induced metasomatism of the mantlesource during melting.

Depth of Melting and Source of Melting

Incompatible tracer element ratios such as La/Sm (7.3-1.76av. 3.3 Primitive Mantle= 1.54) vs. Sm/Yb (3.9-2.4, av. 2.9Primitive Mantle=0.9) are affected by the process of partialmelting and depth of melting. La/Sm will be high when extentof melting is low and Sm/Yb will be high when melting is deep.The observed average high La/Sm values for BKGs indicatethat these are the products of low to moderate degree of partialmelting around 5-15% (Fig.17).

Partial melting in the intermittent depths of spinel lherzolitesand garnet lherzolite field of mantle source does not changethe Sm/Yb ratio since, both Sm and Yb have similar partitioncoefficients in mafic magmas, whereas it may decrease La/Smratio and Sm contents of melts (Aldanmaz et al., 2000, Zhaoand Zhou, 2007). The Sm/Yb ratio can be used to estimate thedepth of melting since this ratio is unaffected by fractionalcrystallization and these two elements are in equilibrium withgarnet (Mckenzie and O’nions, 1991; Shen Y., and ForsythD.W., 1995: Zi et al, 2008).

Figure 18 highlights the nature of mantle source insubduction process. Subduction processes enrich Th withrespect to Ta and consequently increase Th/Yb ralative to Ta/Yb, as subduction components in general carry Th but no Taor Yb. Crustal contamination may also increase Th/Yb relative

Fig.17. Binary plot of Ta/Yb and Th/Yb to place the BK gabbros inenriched mantle array region. The samples are showing cleartrends of subduction component and fractional crystallizationalso.

Fig.18. Plot of Sm/Yb vs La/Sm for the BK gabrroic rocks. Mantlearray defined by depleted MORB mantle (DM, McKenzieand O'Nions, 1991) and primitive mantle (PM, Sun andMcDonough, 1989). Melting curves for spinel lherzoliteOl53+Opx27+Cpx17+Sp11) and garnet peridotite (Ol60+Opx20+Cpx10+Gt10) with both DMM and PM compositions are after(Aldanmaz et al. (2000). Numbers along lines represent thedegree of partial melting.

Fig.19. Nd versus Ce binary plot (Nd/Ce ratio ranging between 0.05-0.39 averaging 0.16; PM 0.76) (after Ahmad and Tarney,1991). Heavy lines show the chondrite ratio after Sun andMc Donough (1989). Dashed line show the calculateddifferent extent of melting of garnet lherzolite source(mineralogy and melting proportions from Hanson, 1980).Melting calculations were performed using batch meltingequations. Garnet lherzolite mineralogy and meltingproportions from Hanson (1980); Primiive mantle (PM) valueis taken from Sun and McDonough (1989). Archaean uppercrust (AUC) and Archaean total crust (ATC) are from Taylorand McLennan (1985). The solid black diamonds are the dataof BK gabbro rocks.



to Ta/Yb because of higher abundance of Th relative to Ta incontinental crust (Farahat 2006). Yb is a common denominatorwhich has the effect of largely eliminaitng variations due topartial melting and fractional crystalization process, thusbringing out the source compostion as a petrogeneitc indicator(Wilson, 1989). In the Th/Yb versus Ta/Yb diagram, the BKgabbros plot along the enriched mantle source array suggestthat the BK gabbors were derived form an enriched mantlesource region under subduction process. The Fig. 19 Ta/Ybversus Th/Yb indicate simultaneous increase of Th and Ta inan enriched mantle source array with a clear subductioncomponent and fractional crystlisation.

Nd vs. Ce plot (Fig.19) indicate that the mantle derivedmelts affected by progressive crustal contamination or AFClie along the lines intersecting the Nd axis and do not passthrough the origin. In Nd Vs. Ce diagram the trend line followsthe origin, above the chondrite line and majority of the samplesoccupy place between calculated >10% partial melting ofprimitive mantle (Sun and McDonough, 1989), which indicatesthe source of these gabbros was enriched in LREE relative tochondrite ratios before melt generation or the magmas wouldhave contaminated by a LREE rich source before fractionalcrystalization (Horan et al., 1987; Ahmad and Tarney, 1991;Kumar and Ahmad, 2007). This further corroborates thedepth and extent of melting depicted by the incompatible /compatible ratios such as La/Sm and Sm/Yb to produce theBK gabbros.

CONCLUSIONS

1. Boggulakonda gabbroic rocks are composed of plagioclaseand clinopyroxenes as essential mineral phases. They arethe products of sub-alkaline low-Ti tholeiitic maficmagmatism. Early stage fractional crystalization of abasaltic parentage is evidenced by the studies.

2. Trace and REE studies indicate LREE, LILE enriched

geochemical characteristics. They are the products of lowto moderate (5-15%) degree of partial melting of a depletedmantle source (transitional garnet lherzolite and spinellherzolite source) which is strongly modified with meltenrichment and addition of fluids derived by the subductedslab and emplaced in island arc tectonic setting.

3. The preservation of both tholeiitic (dominantly) andboninitic (sample BK-2) character indicate that they arethe products of early stage of subduction. The LREEenriched melt derived from the down going slab has beenmodified by the addition of aqueous fluids resulted inincreased LILE budget during subduction.

4. PGE abundances and distribution patterns also support thefractional crystalization as dominant magmatic process forthe genesis of these gabbros under a low to medium degreeof partial melting of mantle and the melt derivation from adepleted mantle source.

5. Highly fractionated IPGE and PPGE patterns and PGEratios indicate their genesis from a primitive MORB likemelts undergone subduction zone processes. PGEabundances increasing in the order Ir<Ru<Rh<Pt<Pd andhigher ratios of Pt/Ir, Pd/Ir and Pd/Pt w.r.t primitive mantleand chondrites may be due to the fluid inducedmatasomatism and melt enrichment through fluids derivedfrom the down going slab.

Acknowledgements: The authors are grateful to DirectorCSIR-NGRI for his kind permission to publish this work. Wealso acknowledge the help and motivation received fromGeological Society of India for this publication. The authorsare thankful to CSIR, New Delhi and NGRI Hyderabad forproviding necessary funding for field and analytical workunder MLP-6201-28(VB) project funds. Sri K.V. Anjaiah, Geo-chemistry Division CSIR-NGRI, Hyderabad is thanked forimmense help during field work. The authors are thankful tothe anonymous reviewer’s valuable comments.

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