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J. Earth Syst. Sci. (2018) 127:120 c© Indian Academy of Scienceshttps://doi.org/10.1007/s12040-018-1022-4
Physico-chemical conditions of four calc-alkaline granitoidplutons of Chhotanagpur Gneissic Complex, eastern India:Tectonic implications
B Goswami*, P Roy, A Basak, S Das and C Bhattacharyya
Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata 700 019, India.*Corresponding author. e-mail: [email protected]
MS received 28 February 2017; revised 3 March 2018; accepted 5 March 2018; published online 26 October 2018
Petrography and mineralogy of four calc-alkaline granitoid plutons Agarpur, Sindurpur, Raghunathpurand Sarpahari located from west to east of northern Purulia of Chhotanagpur Gneissic Complex,eastern India, are investigated. The plutons, as a whole, are composed of varying proportions ofQtz–Pl–Kfs–Bt–Hbl±Px–Ttn–Mag–Ap–Zrn±Ep. The composition of biotite is consistent with thoseof calc-alkaline granitoids. Hornblende–plagioclase thermometry, aluminium-in-hornblende barometryand the assemblage sphene–magnetite–quartz were used to determine the P , T and fO2 during thecrystallisation of the parent magmas in different plutons. The plutons are crystallised under varyingpressures (6.2–2.4 kbar) and a wide range of temperatures (896–718◦C) from highly oxidised magmas(log fO2 −11.2 to −15.4 bar). The water content of the magma of different plutons varied from 5.0 to 6.5wt%, consistent with the calc-alkaline nature of the magma. Calc-alkaline nature, high oxygen fugacityand high H2Omelt suggest that these plutons were emplaced in subduction zone environment. The depthsof emplacement of these plutons seem to increase from west to east. Petrologic compositions of thesegranitoids continuously change from enderbite (opx-tonalite: Sarpahari) in the east to monzogranite(Raghunathpur) to syenogranite (Sindurpur) to alkali feldspar granite (Agarpur) in the west. The watercontents of the parental magmas of different plutons also increase systematically from east to west. Nosubstantial increase in the depth of emplacement is found in these plutons lying south and north of themajor shear zone passing through the study area suggesting the strike-slip nature of the east–west shearzone.
Keywords. Calc-alkaline granitoids; mineral chemistry; intensive parameters; Chhotanagpur GneissicComplex; eastern India.
1. Introduction
The mineral assemblages (quartz–alkali feldspar–plagioclase–biotite–amphibole–sphe ne–magnetiteand/or ilmenite) and evolving conditions of thecalc-alkaline magma during crystallisation areclosely related (Speer 1987; Vyhnal et al. 1991;Ague 1997). Particularly, the compositions ofhornblendes and biotites are related to pressure
and temperature and oxygen fugacity of themagma. While total Al-in-hornblende is a functionof pressure (Hammarstrom and Zen 1986; Vyh-nal et al. 1991; Schmidt 1992; Anderson 1996;Mutch et al. 2016), Ti-in-hornblende (Otten 1984)and the compositions of coexisting hornblende–plagioclase (Holland and Blundy 1994) are relatedto the temperature of the magma. Further, com-positions of biotite, sphene (titanite) and Fe–Ti
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oxides are important for the calculation of oxygenfugacity (Wones 1989). Compositions of amphiboleand biotite are also related to the water contentof the magma (Ridolfi and Renzulli 2012). Onthe other hand, the determination of pressure andtemperatures of the calc-alkaline plutons is impor-tant for unravelling the evidence for the ascent (ordescent) of exposed crustal sections.
Although in the Proterozoic ChhotanagpurGneissic Complex (CGC) granitoids are the mostimportant rock groups with a wide distribution ofcalc-alkaline granitoids, only few papers that dealtwith the mineralogical aspects of these granitoidshave so far been published (Ghose 1983, 1992).
This contribution focuses on four calc-alkalinegranitoid plutons (Agarpur, Sindurpur, Raghu-nathpur and Sarpahari) exposed in northeasternPurulia, West Bengal. The occurrences of theseplutons are restricted between the North PuruliaShear Zone (NPSZ) and the northen megalinea-ment (shear zone) and immediate south of theNPSZ. The aims of this paper are to: (i) describethe mineral–chemical evolution of the above fourplutons, (ii) decipher the physico-chemical condi-tions (pressure and temperature of crystallisationand oxygen fugacity and water content) that pre-vailed during the crystallisation of magmas, and(iii) discuss the implications of the data on thegeotectonic interpretation of the plutons of thenorthern part of Purulia.
2. Geologic setting
Eastern Indian shield comprises the CGC(covering ∼100,000 km2 area) in the north, theSinghbhum craton in the south and the east–west (E–W) trending North Singhbhum MobileBelt (NSMB) between these two (figure 1A).The southern boundary of the CGC with theNSMB is tentatively marked by the South PuruliaShear Zone (Basu 1993). The northern and easternmargins of the CGC are covered by the Ganga–Brahmaputra alluvial deposits (figure 1A). In thewest, the CGC is separated from the CentralIndian Tectonic Zone by younger Gondwanasediments.
The CGC is characterised by widespreadPalaeo- to Neo-Proterozoic predominant grani-toid gneisses and migmatites with older enclavesof metapelitic gneisses (khondalites), amphibo-lites, calc-granulites, quartzites, charnockites, lep-tynites and mafic granulites, and younger intrusives
of granites, pegmatites, norites, anorthosites andalkaline rocks (Mahadevan 2002). Ghose (1983)and Banerji (1991) classified the CGC into severallithostratigraphic units, while Singh (1998) arguedthat the formations within the CGC show grada-tional changes and do not, in general, ‘obey thelaw of superposition’ and hence suggested these as‘lithodemic’ group.
A part of the study area (figure 1b) lying inthe northern part of Purulia district, West Ben-gal (eastern part of CGC) was described by Sen(1953, 1956, 1959), Mitra (1992), Goswami andBhattacharyya (2008, 2014) and Maji et al. (2008).Two prominent lineaments: (i) the nearly E–Wtrending NPSZ and (ii) and ENE trending linea-ment (known as the northern megalineament)lying to the north of (i) pass through the area.The migmatitic/composite granitoid gneisses coun-try occupying the northern side of the north-ern megalineament and also the southern sideof the NPSZ contain enclaves of khondalites,marbles, calc-silicate schists/gneisses, para- andortho-amphibolites belonging to amphibolite facies.However, the leptynitic country in between the twolineaments contains minor patches of migmatites,basic granulites, charnockitic rocks and calc-silicaterocks representing amphibolite and granulite facies.Several intrusive bodies of granitoids intrude intothe leptynitic country. These granitoid intrusionshave been traditionally viewed as Mesoproterozoicin age. The four representative plutons which arestudied include the Agarpur, Sindurpur, Raghu-nathpur and Sarpahari intrusions, of which, threeare located between the E–W trending NPSZand the northern megalineament while the Sin-durpur pluton is located south of NPSZ(figure 1B).
Grey-coloured biotite granitoid gneiss, oftengarnetiferous, is the major component of migmati-tic granitoid gneisses country. Enclaves of migma-titic granitoid gneiss sometimes occur within theintrusive granitoids. Numerous lensoidal meta-sedimentary enclaves of garnet–sillimanite gneiss(khondalite), calc granulites, amphibolite, micaschist, etc., are noticed within the migmatitic gran-itoid gneiss. The contact of the metasedimentaryenclaves and the migmatitic granitoid gneiss is gra-dational. Long axes of the enclaves are alignedparallel to the regional foliation. Pyroxene gran-ulite, charnockitic rocks, nepheline syenite gneissesand orthoamphibolites are also interpreted to haveigneous protolith (Goswami and Bhattacharyya2008).
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Figure 1. (A) Generalised geological map of the CGC showing the distribution of Archean–Proterozoic metasedimentaryenclaves and granitic intrusives. Proterozoic Dalma lava in the Singhbhum mobile belt, the Cretaceous Rajmahal volcanicsand Mesozoic Gondwana basins associated with the Damodar Graben are shown for reference. (B) Simplified geological mapof the area of study in northern Purulia.
Major structural grains are represented by adominant foliation trending E–W to NE–SW witha northerly low-to-moderate dip (20◦–40◦). TheENE–WSW alignment of the intrusive granites ismore or less parallel to the trend of the major shearzones.
The rocks of the area bear signatures of at leastthree phases of deformations (D1, D2 and D3).Nearly parallel S1 and S2 foliations are discerniblein quartzites and grey biotite granitoid gneisses.S1 is the dominant foliation of the area. S1 is axialplanar to the tight, appressed intrafolial D1 folds
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(figure 2A). D2 folds are defined by form surfaceof a set of gneissic banding (S1). In mesoscopicscale, D1 minor folds are highly plunging or nearlyreclined. D1 folds are rarely encountered in intraf-olial layers of biotite granitoid gneiss, quartzite andcalc-silicate rocks. D2 folds are tight to close, lowplunging and overturned (figure 2B). The shearmovement represents the D3-deformation. Granitemylonites, augen gneisses and leptynites charac-terised by the formation of ribbon quartz occurall along the NPSZ and northern megalineament.The strike-slip movement with a sinistral senseon the plan is the dominant nature of NPSZ(figure 2C).
The basic granulites show weak foliation (S1),defined by the preferred orientation of orthopy-roxene, which was subsequently folded by D2.Similarly, the long dimension of rod-shaped silli-manites in garnet–sillimanite gneisses got foldedduring D2. This suggests that the metamorphismin the upper amphibolite to granulite grade wasattained during D1-deformation. The country rockattained granulite facies in the eastern part ofthe study area, i.e., around the Sarpahari pluton.The grade of metamorphism gradually decreasedtowards the west and reached amphibolite faciesaround the Agarpur pluton (Goswami and Bhat-tacharyya 2008).
The intrusion/injection of enderbite (opx-tonalite) and/or charnockite (opx-granite), sensustricto, along the S1 plane of the basic gran-ulites (figures 2B and 2D) and migmatitic gneissesis commonly noticed. Primary banding of ender-bite of the Sarpahari pluton was folded duringD2-deformation (figure 2E). The pluton emplacedbefore D2-deformation.
The long axis of the Raghunathpur pluton isENE–WSW. The porphyritic granitoids containstrongly aligned euhedral crystals of pinkish K-feldspar megacrysts (figure 2F) which is interpretedto be due to magmatic flow (Goswami and Bhat-tacharyya 2014). The long dimension of feldspargrains ranges from 3 to 10 cm. The flow foliationof porphyritic granitoids is deflected around thexenoliths of basic granulite (figure 2G). Along thenorthern and southern boundaries of the pluton,microcline megacrysts are lensoid in shape and fer-romagnesian minerals tend to warp around themgiving rise to augen structures (figure 2H). Thisfeature is interpreted to be due to shearing alongthe boundaries.
The Sindurpur pluton occurs as a veryprominent E–W trending elongate batholithic body
(figure 1B). These rocks often contain bands ofenclaves of amphibolites. The megacrysts offeldspar as well as enclaves are usually orientedparallel to the gneissic trend of the country rock(figure 2I) but in some cases, the megacrysts locallyshow haphazard orientation indicating turbulentflow in magma. The general trend of magmatic foli-ation is ENE–WSW. The dip ranges from 70◦ to72◦N. In places, the granitoid rocks show D2 folds(figure 2J). Hence, the pluton emplaced before theD2-deformation episode. The strong preferred ori-entation of the minerals reflects the shearing effectduring the D3-deformation (figure 2K).
The long axis of the Agarpur pluton is E–W(figure 1B). The rock shows impersistent E–Wtrending and steep dipping magmatic flow foliation(figure 2L). The southern boundary of the pluton ismylonitised by the D3-deformation. The magmaticflow foliation and mylonitic foliation are parallelto the gneissosity of the country rock gneisses.The flattened ribbon-shaped quartz grains paral-lel to E–W foliation defines a megascopic fabricindicative of ductile deformation. Joints developparallel to this foliation and late mafic magmainvaded the rock along these joints (figure 2Mand N).
�Figure 2. (A) Intrafolial D1 folds in the granitised quartzite,north of Raghunathpur. (B) Overturned D2-antiform in thebasic granulites, north of Murlu. Note injection of ender-bite along the foliation plane of basic granulite. (C) Shearfold related to D3-deformation in the migmatite east ofJaipur. Note the injection of pink granite along the foli-ation of the host amphibolite. (D) Injection of enderbitemagma in basic granulite at the foothill of Sarpahari. (E)Antiformal D2 fold in enderbite. Note the older enclave ofbasic granulite is folded with the enderbite arising a falseimpression of hook-like geometry. (F) A preferred alignmentof K-feldspar megacrysts in the porphyritic granite of Raghu-nathpur. Location: Bero Pahar. (G) Amphibolite xenolithwithin porphyritic granite of Raghunathpur. Note: swerv-ing of the K-feldspar megacrysts around the xenolith. (H)Augen-shaped megacrysts of K-feldspar at the northern mar-gin of the Raghunathpur granite, near Bero. (I) Parallelalignment of older amphibolite enclave within the Sindur-pur pluton. Note the parallelism of the long dimension ofamphibolite and foliation of the host granite. (J) D2 foldswithin the porphyritic granite of Sindurpur. (K) Effect ofshearing on the megacrysts of the Sindurpur pluton. Note:stretching of the megacryst. (L) Banding defined by thealternate layers of ferromagnesian and quartzo-feldspathicminerals in the Agarpur pluton. (M) Injection of basic mate-rial along the joint planes of the Agarpur pluton. These jointsare parallel to the shear planes. (N) A close view of theprevious exposure (M).
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3. Analytical methods
Electron probe micro analyses (EPMA) of therepresentative minerals of these samples were per-formed using a CAMECA SX100 microprobe withwavelength dispersive spectrometers at the Cen-tral Petrology Division, Geological Survey of India,Kolkata. An accelerating voltage of 15 kV wasmaintained with a beam current of 12 nA and 1 µmbeam diameter.
The EPMA of the representative minerals ofthree samples of the Agarpur pluton was performedin a CAMECA SXFive instrument at DST-SERBNational Facility, Department of Geology (Centrefor Advanced Study), Institute of Science, BanarasHindu University. An accelerating voltage of 15 kVwas maintained with a beam current of 10 nA and1 µm beam diameter. Natural and synthetic stan-dards supplied by CAMECA-AMETEK were usedfor precision and correction.
The EPMA of the representative minerals fromtwo samples of porphyritic granitoids of the Raghu-nathpur pluton was performed using a JEOLJXA-8600M microprobe with wavelength disper-sive spectrometer at the IIC (Institute Instru-mentation Centre), Indian Institute of Technology,Roorkee. An accelerating voltage of 15 kV wasmaintained with a beam current of 50 nA. Beamdiameter was fixed at 3 µm. Natural standardssupplied by SPI Suppliers, Structure Probe Inc.,Canada, were used for precision and correction.
The precision of data of major elements for allthe three instruments is ±3%.
4. Petrography
4.1 Agarpur pluton
The pluton is medium to coarse grained, greyto brownish colour in outcrop and shows well-developed magmatic flow foliation. The plutonis composed of pink-coloured granite and alkalifeldspar granite. The major minerals present in therock are microcline, quartz, plagioclase and amphi-bole. The accessory phases are allanite, magnetiteand apatite. The magmatic flow foliation in therock is defined by alternate layers of aggregatesof ferromagnesian minerals with thicker bands ofquartzo-feldspathic mass. The felsic bands containphenocrysts of microcline giving rise to porphyritictexture (figure 3B). The ferromagnesian bandsare composed mainly of a cluster of aggregatesof prismatic and lensoidal grains of amphibole.
Intergrowth of quartz with microcline is notuncommon. Subidiomorphic to xenomorphic pla-gioclases (albite to oligoclase) mainly occur inthe groundmass. Myrmekitic intergrowth is notuncommon. The actinolite (pleochroic from lightgreen to bluish green) and subordinate hornblendegrains are coarse to medium grained, elongated,irregular and ellipsoidal with subidiomorphic toxenomorphic grain boundary. Allanite (idiomor-phic to subidiomorphic) occurs as a cluster of lathsand patches in the ferromagnesian bands. Sphenes(pleochroic from light brown to dark brown; sub-idiomorphic to xenomorphic) are fine to mediumgrained, elongated parallel to the fabric. Mag-netites are idiomorphic, squarish and equant andoccur as the inclusion within biotite and amphibolegrains. Apatite (idiomorphic to subidiomorphic,hexagonal to rounded, ellipsoidal to prismatic)occurs as a discrete phase within groundmass andalso as inclusions within both felsic and maficminerals.
In the mylonitised zones, impersistent, extremelythin ribbons or elongate streaks of quartz runthrough the flesh-coloured feldspar parallel to thedirection of mylonitic foliation (figure 3A). Phe-nocrysts of microcline and quartz survived as ellip-soidal or augen-shaped porphyroclast with elonga-tion parallel to the direction of mylonitic foliation.
4.2 Sindurpur pluton
The porphyritic granitoids of Sindurpur arecomposed mainly of K-feldspar, plagioclase, quartz,biotite, hornblende and minor orthopyroxene,opaque, apatite, muscovite, sphene, garnet, allaniteand zircon and show distinct banding (figure 3C).A preferred alignment of ferromagnesian (biotiteand hornblende) and platy feldspars define themagmatic foliation. Quartz occurs commonly ascoarse, elongate, lenticular grains, deformed orwarped around microcline megacrysts. It occursalso as tiny (medium to fine grained) xenomorphsintimately associated with feldspar and biotite-rich groundmass. K-feldspar (microcline) occursas megacrysts and coarse subhedra along withaggregates of smaller subhedral crystals, embeddedin quartz–plagioclase–biotite-rich medium-grainedgroundmass. Plagioclase (oligoclase) occurs asmedium to coarse, tabular, subidiomorphic grainsforming aggregates with other groundmass min-erals, mainly quartz, biotite, and occasionallyhornblende. Alteration of plagioclase is common.
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Figure 3. (A) Fine ribbon-shaped quartz within the pink granite. (B) Feldspar megacrysts in the felsic bands of pinkgranite. (C) Banding defined by the preferred alignment of felsic minerals in the Sindurpur granite. (D) Deformationof plagioclase megacrysts and formation of a finer groundmass in the porphyritic granite of Raghunathpur. (E) Strongdimensional orientation shown by the quartzo-feldspathic minerals and lenses of ferromagnesian minerals in enderbite ofSarpahari. (F) Relict megacryst of plagioclase within a granulated groundmass.
Medium-sized laths and flakes of biotite formclusters in the groundmass and oriented subpar-allel to foliation. It is intimately associated withhornblende and replaces the latter. Hornblendeoccurs as xenomorphic to subidiomorphic mediumto coarse grains. Orthopyroxene (colourless to light
green) and clinopyroxene may be present as relictpatch within hornblende. Magnetite, limonite, apa-tite and sphene occur as accessories. Allanite,zircon, sphene and garnet, when present, occur intrace amounts as subidiomorphic to idiomorphicgrains.
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4.3 Raghunathpur pluton
The porphyritic granitoids are composed mainlyof K-feldspar, plagioclase, quartz, biotite, horn-blende and accessory opaque, apatite, chlorite,epidote, allanite, muscovite, sphene, carbonate,garnet, scapolite and zircon in variable propor-tions. Although most of the porphyritic granitoidslie in the granite and quartz monzonite fields ofQAP (Quartz–Alkali feldspar–Plagioclase) diagram(after Streckeisen 1976), few samples lie in thetonalite, granodiorite and alkali feldspar granitefields.
Quartz occurs commonly as coarse, elongate,lenticular grains, deformed or warped aroundmicrocline megacrysts. Abundant tabular or(rarely) ellipsoidal megacrysts (1–10 cm long) ofK-feldspar occur in quartz–plagioclase–biotite-richmedium-grained groundmass, often bordered by anarrow rim of very-fine-grained anhedral quartz,microcline, sericitised and turbid plagioclase andlobate myrmekite. Ellipsoid megacrysts have ‘tails’of millimetre-sized anhedral quartz, plagioclaseand microcline which pinch out into more maficmatrix several millimetres from both ends of themegacryst. Medium to coarse, tabular, subidiomor-phic plagioclase (An25−40) forms aggregate withother groundmass minerals. Plagioclase pheno-crysts may show bending of twin lamellae (fig-ure 3D). Biotite occurs as medium-sized laths andflakes forming clusters in the groundmass andoriented subparallel to foliation. It may replacehornblende. Hornblende occurs as xenomorphicto subidiomorphic medium to coarse grains. Insome samples, hornblende appears to be replacedby scapolite (Ma72 Me28) xenomorphs. Metam-ict prisms of allanite are often rimmed by epi-dote. Muscovite, when present, occurs both assubidiomorphic tiny laths and xenomorphic raggedgrains secondary after biotite and plagioclase. Zir-con and garnet, when present, occur in traceamount as subidiomorphic to idiomorphic grains.
4.4 Sarpahari pluton
The main rock type of the Sarpahari pluton isenderbite (opx-tonalite) and charnoenderbite (opx-granodiorite). However, thin bands of enderbite areseen injected along the S1-foliation of the garnetif-erous gneisses and basic granulite country rock(figure 2D). Enderbite in the area is a coarse-grained foliated rock, composed of plagioclase,quartz, biotite, hornblende, minor orthopyroxene
and accessory opaque, zircon and apatite. Thefoliation is defined by alternate thicker layers ofquartzofeldspathic aggregates with thin impersis-tant stringers and layers of ferromagnesian min-erals through the quartzofeldspathic groundmass.Both quartz and feldspar show strong dimensionalorientation and the impersistant layers and lensesof ferromagnesian minerals are oriented parallel tothe foliation (figure 3E). Coarse plagioclase andquartz grains show protoclastic granulation formedby shearing. Such a medium- to fine-sized gran-ulated mass occurs in the interspaces of coarserquartz and feldspar. Plagioclase occurs as coarse,equant to elongated, xenomorphic to subidiomor-phic grains as well as aggregates of fine-sizedsubidiomorphic equant grains. Coarser plagioclasegenerally shows the deformation effect in the formof bending of twin lamellae and also shadowyextinction (figure 3F). Quartz occurs as coarse,equant to elongated, xenomorphic to subidiomor-phic grains as well as aggregates of fine-sized subid-iomorphic equant grains. Some of the quartz grainsare lenticular and such lenticles are more stronglyoriented parallel to foliation. Biotite (pleochroicfrom straw yellow to reddish brown) occurs com-monly as medium-sized laths. Biotites may replaceorthopyroxene. Orthopyroxenes (pleochroic frompale pinkish to pale greenish) and clinopyroxenesoccur as aggregates of ragged grains running alongthe foliation. Coarser ragged grains are occasion-ally marginally crushed. The fine- to medium-sizedgranules occupy the interspaces of the adjacent fel-sic grains. Hornblendes occur as an aggregate ofsubidiomorphic to xenomorphic elongated grainsand are closely associated with orthopyroxene.At places, orthopyroxenes are altered to horn-blende. Sometimes hornblendes form corona struc-ture around opaque minerals. Opaque mineralsoccur as both equant and elongated grains inti-mately associated ferromagnesian minerals. Finezircon (elongated pyramidal grains) and apatite(equant, subidiomorphic grains) occur as inclusionin feldspar as well as the interspaces.
5. Nature of fabric in granitoid plutons
Emplacement of granitic bodies can be consideredeither linked to orogenic processes (syn-tectonic)or may be rift-related anorogenic (Pitcher 1997).A granitoid pluton involved in regional deforma-tion develops a flow fabric in response to tectonicforces. If the granitoid pluton has been emplaced inthe absence of tectonic forces, the magmatic fabric
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reflects the dynamics of emplacement (Hutton1988; Brown and Solar 1998; Paterson et al. 1998;Rosenberg 2004). The mineral fabric of graniterecords the strain field of the magma during thefinal stages of emplacement (Paterson et al. 1998).The fabric defined by the alignment of subhedral toeuhedral zoned plagioclase or mafic minerals suchas hornblende and biotite is considered as meltpresent flowage (magmatic flow or submagmaticflow with <20% melt; Hutton 1988; Paterson et al.1989).
All the four plutons were primarily emplaced asmagmas in the country rocks under amphibolitefacies (Raghunathpur, Agarpur plutons) and gran-ulite facies environment (Sarpahari and Sindurpurplutons) (Goswami and Bhattacharyya 2008).
The intensity of deformation in the interior ofall the granitoid bodies of the present study isdifferent from their margin. The central part ofthe granitoid bodies is massive with the hap-hazard orientation of megacrysts and show weakimprints of solid-state deformation in the formof local shearing. The border zones of all thesegranitoid bodies show the development of augengneisses and mylonites. A detailed study on the ori-gin of the fabric of porphyritic granitoid batholithof Raghunathpur had been published earlier byGoswami and Bhattacharyya (2014).
Typical igneous textures in these granitoidsinclude a porphyritic texture (e.g., Ridley 1992),characterised by idiomorphic to subidiomorphicK-feldspar and plagioclase (e.g., Shelley 1993; Frostet al. 2000), which show preferred orientation.However, anhedral feldspar is present in thesegranitoids due to impingement (Vernon 2004). Pla-gioclase shows lamellar twinning typical of igneousrocks (Shelley 1993). Intergrowth of quartz withinmicrocline is a clear evidence of magmatic crys-tallisation of the Agarpur pluton. Biotitisation oforthopyroxene in enderbite can be taken as anevidence for the presence of a melt phase wherebiotite–quartz were formed according to the reac-tion Opx + melt = Bt + Qtz (Kramers and Ridley1989; Bohlender et al. 1992; Ridley 1992). Elon-gated, bipyramidal, euhedral zircons in enderbitesuggest their igneous origin.
Sheared parts of all of these granioid plutons aremarked by porphyroclasts of plagioclase (albite–oligoclase) and microcline in a matrix of undulosequartz and granulated quartz–feldspar. Formationof ribbons of quartz is commonly found in theshear zones. Occasional bending of twin lamel-lae in plagioclase indicates the limited plastic
deformation of feldspar. In the deformed zones ofthe Agarpur pluton, myrmekite is quite commonand occurs in both, weakly deformed granites aswell as in granite mylonites. Completely recrys-tallised quartz ribbons and myrmekite suggest fab-ric development under intermediate temperature(∼500–400◦C).
The broadly E–W elongation of these plutonsconcurs with the trend of NPSZ which suggestscontrol of this shear zone in framing the orientationof these plutons. The preferred E–W orientation ofthe feldspar megacrysts and ferromagnesian miner-als with the exception in some places is consideredby us to suggest magmatic flowage differentia-tion. The prolongation of the shearing movementafter solidification of the granitoid magmas accen-tuated the preferred orientation and granulation ofminerals. In such a situation, the textural equilibra-tion is favoured in magmatic rocks if temperaturesremain high in the system (Maaløe 1985; Hunter1987). In the case of the four granitoid plutonsof the present study, the chemical compositionsof the magmatic minerals have not been alteredand hence the physico-chemical parameters derivedfrom the mineral chemistry reveal the originalmagmatic crystallisation.
6. Mineral chemistry
6.1 Feldspar
Representative analyses of K-feldspar andplagioclase in the studied plutons are shown intables 1 and 2.
In the Ab–Or–An ternary diagram (figure 4,after Deer et al. 1992), plagioclase compositionsin the Agarpur pluton range from oligoclase toalbite (An26.9 and An9.3). The compositional vari-ation of plagioclase in the Sindurpur and Raghu-nathpur plutons ranges from andesine to albite.For Sindurpur pluton, the compositional varia-tion ranges from An43.2 to An3.9 and for Raghu-nathpur pluton, it ranges from An47 to An0.4.Such a large variation in Na-content is possi-bly related to alterations and the albite couldbe secondary as seen in petrographic observa-tion. The composition of plagioclase in the Sarpa-hari pluton varies from andesine to oligoclase(An35.6−15.8).
Alkali feldspar in the Agarpur plutoncorresponds to orthoclase with Or content between85% and 100% (figure 4). These alkali feldspars
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Table
1.Electronprobe
microanalysisofK-feldsparandcationic
calculationonthebasisofeightoxygen.
Sam
ple
P18
P19
P20B
Pin
k-c
olo
ure
dbio
tite
Rock
type
am
phib
ole
gra
nit
eP
ink-c
olo
ure
dam
phib
ole
gra
nit
eG
rey-c
olo
ure
dbio
tite
am
phib
ole
gra
nit
e
Plu
ton
Agarp
ur
Analy
sis
63/1
64/1
171/1
172/1
179/1
180/1
189/1
190/1
191/1
30/1
31/1
35/1
36/1
Loca
tion
Core
Core
Core
Rim
Core
Rim
Core
Mid
Rim
Core
Rim
Core
Rim
Min
eral
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
KF
SiO
265.3
964.8
065.3
064.8
266.3
564.9
262.6
362.2
962.8
964.1
764.1
963.5
762.9
1
TiO
20.0
80.1
20.0
60.0
00.0
60.0
00.0
40.0
60.0
40.0
00.0
00.0
00.0
0
Al 2
O3
18.3
818.5
118.3
618.3
717.8
118.3
618.3
018.6
518.4
618.0
918.3
218.3
418.4
8
FeO
0.0
00.0
00.0
60.0
00.0
00.0
60.0
50.0
40.1
50.0
00.0
00.0
00.0
0
MnO
0.0
00.0
00.0
40.0
00.0
00.0
60.0
00.0
30.0
00.0
00.0
00.0
00.0
0
MgO
0.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
0
CaO
0.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
30.0
00.0
00.0
0
Na2O
0.0
00.0
00.5
50.2
30.4
70.4
50.8
21.5
00.8
70.3
10.2
00.4
00.5
2
K2O
14.5
614.8
414.6
913.8
014.3
814.3
314.5
913.2
714.3
215.2
315.1
015.2
615.2
4
Tota
l98.42
98.26
99.05
97.22
99.07
98.19
96.44
95.83
96.74
97.83
97.80
97.56
97.16
Si
12.1
17
12.0
59
12.0
62
12.1
212.2
06
12.0
65
11.9
31
11.8
87
11.9
28
12.0
53
12.0
41
11.9
88
11.9
31
Al
4.0
11
4.0
55
3.9
93
4.0
45
3.8
59
4.0
19
4.1
07
4.1
91
4.1
24
4.0
02
4.0
46
4.0
73
4.1
28
Ti
0.0
11
0.0
16
0.0
08
00.0
08
00.0
05
0.0
08
0.0
06
00
00
Fe 2
00
0.0
10
00.0
10.0
08
0.0
06
0.0
24
00
00
Mn
00
0.0
06
00
0.0
10
0.0
04
00
00
0
Mg
0.0
01
00
00
00
00
00
00
Ca
00
00
00
00
00.0
05
00
0
Na
00
0.1
97
0.0
82
0.1
68
0.1
62
0.3
04
0.5
54
0.3
21
0.1
11
0.0
74
0.1
46
0.1
91
K3.4
42
3.5
22
3.4
61
3.2
93
3.3
74
3.3
99
3.5
47
3.2
32
3.4
66
3.6
49
3.6
12
3.6
71
3.6
88
Cati
ons
19.5
82
19.6
52
19.7
75
19.5
419.6
31
19.7
35
19.9
58
19.9
06
19.9
17
19.8
219.7
73
19.8
78
19.9
38
Ab
00
5.4
2.4
4.7
4.5
7.9
14.6
8.5
2.9
23.8
4.9
An
00
00
00
00
00.1
00
0
Or
100
100
94.6
97.6
95.3
95.5
92.1
85.4
91.5
96.9
98
96.2
95.1
J. Earth Syst. Sci. (2018) 127:120 Page 11 of 33 120
Table
1.(C
ontinued.)
Sam
ple
AD
36a
PR
3E
PR
9B
A11A
BA
4A
BG
24
Bio
tite
am
phib
ole
Mylo
nit
ic
bio
tite
Rock
type
Charn
ock
ite
gra
nit
e
am
phib
ole
gra
nit
eG
ranit
e
Mylo
nit
ic
tonalite
Charn
oen
der
bit
e
Plu
ton
Sin
durp
ur
Sarp
ahari
Analy
sis
15/1
7/1
8/1
85/1
86/1
97/1
38/1
41/1
15/1
9/1
Loca
tion
Core
Core
Core
Core
Core
Core
Core
Core
Core
Core
Min
eral
KF
KF
KF
KF
KF
KF
KF
KF
KF
PF
SiO
263.5
062.9
663.3
064.4
364.4
863.3
963.1
463.2
362.4
664.2
2
TiO
20.0
00.0
00.0
00.0
50.0
00.0
00.0
90.0
30.0
00.0
0
Al 2
O3
18.0
418.2
818.1
818.2
018.0
118.1
218.0
618.0
718.5
018.8
6
FeO
0.0
00.0
30.0
50.0
30.0
30.0
20.0
00.0
00.0
00.0
0
MnO
0.0
20.0
00.0
20.0
00.0
00.0
00.0
00.0
00.0
10.0
3
MgO
0.0
00.0
00.0
10.0
00.0
10.0
00.0
00.0
20.0
00.0
0
CaO
0.0
60.0
20.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
2
Na2O
1.2
80.9
51.0
11.3
10.6
70.7
71.0
10.7
40.8
10.2
6
K2O
14.8
515.2
314.7
915.0
815.9
715.7
416.2
316.2
615.5
616.6
5
Tota
l97.75
97.47
97.36
99.10
99.17
98.04
98.53
98.35
97.34
100.04
Si
11.9
74
11.9
27
11.9
711.9
85
12.0
18
11.9
59
11.9
07
11.9
32
11.8
72
11.8
99
Al
4.0
06
4.0
78
4.0
48
3.9
87
3.9
53
4.0
26
4.0
11
4.0
16
4.1
41
4.1
15
Ti
00
00.0
07
00
0.0
13
0.0
04
00
Fe 2
00.0
05
0.0
08
0.0
05
0.0
05
0.0
03
00
00
Mn
0.0
03
00.0
03
00
00
00.0
02
0.0
05
Mg
00
0.0
03
00.0
03
00
0.0
06
00
Ca
0.0
12
0.0
04
00
00
00
00.0
04
Na
0.4
68
0.3
49
0.3
70.4
72
0.2
42
0.2
82
0.3
69
0.2
71
0.2
99
0.0
93
K3.5
73
3.6
81
3.5
68
3.5
79
3.7
97
3.7
88
3.9
05
3.9
14
3.7
73
3.9
36
Cati
ons
20.0
36
20.0
44
19.9
720.0
35
20.0
18
20.0
58
20.2
05
20.1
43
20.0
87
20.0
52
Ab
11.5
8.7
9.4
11.7
66.9
8.6
6.5
7.3
2.3
An
0.3
0.1
00
00
00
00.1
Or
88.2
91.2
90.6
88.3
94
93.1
91.4
93.5
92.7
97.6
120 Page 12 of 33 J. Earth Syst. Sci. (2018) 127:120
Table
2.Electronprobe
microanalysisofplagioclase
feldsparandcationic
calculationonthebasisofeightoxygen.
Sam
ple
P18
P20B
AD
36a
PR
3E
PR
9
Pin
k-c
olo
ure
dbio
tite
Gre
y-c
olo
ure
dbio
tite
Bio
tite
am
phib
ole
Mylo
nit
icbio
tite
Rock
type
am
phib
ole
gra
nit
eam
phib
ole
gra
nit
eC
harn
ock
ite
gra
nit
eam
phib
ole
gra
nit
e
Plu
ton
Agarp
ur
Sin
durp
ur
Analy
sis
67/1
29/1
39/1
40/1
17/1
18/1
23/1
89/1
100/1
103/1
104/1
Loca
tion
Rim
Core
Rim
Core
Incl
usi
on
Incl
usi
on
Core
Rim
Core
Core
Rim
SiO
267.8
262.9
966.4
665.2
756.7
356.7
056.0
265.6
164.0
863.6
267.1
6
TiO
20.0
00.0
00.0
00.0
00.0
00.0
20.0
00.0
80.0
90.0
70.0
0
Al 2
O3
21.0
223.6
221.4
822.3
325.5
625.5
525.8
321.4
921.7
121.8
819.9
6
FeO
0.0
00.0
00.0
00.0
00.0
90.0
60.0
90.2
80.0
70.0
20.0
0
MnO
0.0
00.0
00.0
00.0
00.0
00.0
50.0
00.0
00.0
10.0
10.0
0
MgO
0.0
00.0
00.0
00.0
00.0
00.0
10.0
10.0
10.0
10.0
00.0
2
CaO
2.0
14.7
02.4
63.8
88.5
78.3
38.4
62.6
63.2
13.1
60.7
9
Na2O
10.7
88.9
810.4
29.6
16.3
06.2
16.0
49.9
39.6
29.1
310.8
3
K2O
0.0
20.1
10.0
30.0
70.1
80.1
00.1
80.1
10.2
30.2
80.0
8
Tota
l101.65
100.39
100.85
101.16
97.43
97.03
96.63
100.17
99.03
98.17
98.84
Si
11.7
04
11.1
02
11.5
811.3
810.4
25
10.4
44
10.3
72
11.5
27
11.4
12
11.4
07
11.8
71
Al
4.2
73
4.9
03
4.4
08
4.5
85
5.5
31
5.5
42
5.6
32
4.4
46
4.5
53
4.6
24.1
55
Ti
00
00
00.0
03
00.0
11
0.0
12
0.0
09
0
Fe 2
00
00
0.0
14
0.0
09
0.0
14
0.0
41
0.0
10.0
03
0
Mn
00
00
00.0
08
00
0.0
02
0.0
02
0
Mg
00
00
00.0
03
0.0
03
0.0
03
0.0
03
00.0
05
Ca
0.3
71
0.8
87
0.4
58
0.7
25
1.6
87
1.6
44
1.6
78
0.5
01
0.6
13
0.6
07
0.1
5
Na
3.6
06
3.0
67
3.5
23.2
49
2.2
45
2.2
18
2.1
68
3.3
83
3.3
22
3.1
74
3.7
12
K0.0
04
0.0
25
0.0
06
0.0
15
0.0
42
0.0
23
0.0
43
0.0
25
0.0
52
0.0
64
0.0
18
Cati
ons
19.9
58
19.9
84
19.9
72
19.9
54
19.9
44
19.8
94
19.9
119.9
37
19.9
79
19.8
86
19.9
11
Ab
90.6
77.1
88.4
81.4
56.5
57.1
55.7
86.5
83.3
82.5
95.7
An
9.3
22.3
11.5
18.2
42.5
42.3
43.1
12.8
15.4
15.8
3.9
Or
0.1
0.6
0.2
0.4
1.1
0.6
1.1
0.6
1.3
1.7
0.5
J. Earth Syst. Sci. (2018) 127:120 Page 13 of 33 120
Table
2.(C
ontinued.)
Sam
ple
BA
11A
BA
13A
BA
4A
PR
28E
Rock
type
Gra
nit
eE
nder
bit
e
Mylo
nit
ic
tonalite
Porp
hyri
tic
bio
tite
gra
nit
e
Plu
ton
Sin
durp
ur
Sarp
ahari
Analy
sis
35/1
36/1
37/1
21/1
22/1
23/1
18/1
10/1
11/1
29/1
30/1
31/1
Loca
tion
Core
Core
Core
Core
Core
Core
Rim
Rim
Core
Rim
Mid
Core
SiO
260.8
659.9
661.6
358.4
058.2
458.3
058.6
759.3
959.7
359.3
759.0
759.7
9
TiO
20.0
00.0
00.0
00.0
00.0
00.0
20.0
00.0
00.0
10.0
00.0
00.0
0
Al 2
O3
24.0
024.1
123.7
425.5
225.3
625.1
925.7
525.6
125.2
425.7
825.5
725.9
7
FeO
0.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
10.0
00.0
0
MnO
0.0
20.0
60.0
00.0
00.0
00.0
17.6
10.0
00.0
00.0
00.0
20.0
2
MgO
0.0
00.0
10.0
00.0
00.0
00.0
10.0
00.0
10.0
10.0
30.0
20.0
0
CaO
6.0
76.1
75.7
17.8
27.9
57.6
57.6
16.4
66.6
57.0
47.0
57.1
5
Na2O
8.2
38.0
58.4
06.9
27.1
67.2
07.6
47.6
07.7
67.5
77.4
87.4
8
K2O
0.0
70.1
60.1
10.2
80.1
80.2
10.1
60.1
20.1
90.1
90.3
00.3
2
Tota
l99.25
98.52
99.59
98.94
98.89
98.59
107.44
99.19
99.59
99.99
99.51
100.73
Si
10.8
98
10.8
32
10.9
83
10.5
47
10.5
35
10.5
710.1
49
10.6
510.6
85
10.5
91
10.5
96
10.5
92
Al
5.0
61
5.1
29
4.9
82
5.4
28
5.4
03
5.3
78
5.2
46
5.4
08
5.3
17
5.4
16
5.4
01
5.4
18
Ti
00
00
00.0
03
00
0.0
01
00
0
Fe 2
00
00
00
00
00.0
01
00
Mn
0.0
03
0.0
09
00
00.0
02
1.1
15
00
00.0
03
0.0
03
Mg
00.0
03
00
00.0
03
00.0
03
0.0
03
0.0
08
0.0
05
0
Ca
1.1
65
1.1
94
1.0
91.5
13
1.5
41
1.4
86
1.4
11.2
41
1.2
75
1.3
46
1.3
55
1.3
57
Na
2.8
58
2.8
22.9
03
2.4
23
2.5
11
2.5
31
2.5
63
2.6
43
2.6
92
2.6
18
2.6
02
2.5
69
K0.0
16
0.0
37
0.0
25
0.0
65
0.0
42
0.0
49
0.0
35
0.0
27
0.0
43
0.0
43
0.0
69
0.0
72
Cati
ons
20.0
01
20.0
24
19.9
83
19.9
76
20.0
32
20.0
22
20.5
18
19.9
72
20.0
16
20.0
23
20.0
31
20.0
11
Ab
70.8
69.6
72.2
60.6
61.3
62.2
63.9
67.6
67.1
65.3
64.6
64.3
An
28.8
29.5
27.1
37.8
37.6
36.5
35.2
31.7
31.8
33.6
33.7
33.9
Or
0.4
0.9
0.6
1.6
11.2
0.9
0.7
1.1
1.1
1.7
1.8
120 Page 14 of 33 J. Earth Syst. Sci. (2018) 127:120
Table
2.(C
ontinued.)
Sam
ple
PR
30B
BG
36
BG
24
Rock
type
Ender
bit
eE
nder
bit
eC
harn
oen
der
bit
e
Plu
ton
Analy
sis
73/1
74/1
28/1
33/1
8/1
10/1
13/1
Loca
tion
Core
Rim
Core
Core
Core
Core
Core
SiO
260.6
660.4
359.2
659.2
360.1
460.3
059.9
9
TiO
20.0
60.0
10.0
00.0
20.0
00.0
00.0
1
Al 2
O3
24.7
125.1
525.6
525.8
024.8
225.3
825.0
5
FeO
0.1
10.0
00.0
00.0
00.0
00.0
00.0
0
MnO
0.0
70.0
00.0
80.0
90.0
30.0
50.0
0
MgO
0.0
00.0
20.0
10.0
10.0
30.0
20.0
5
CaO
6.0
26.1
57.1
97.4
76.3
06.7
56.8
4
Na2O
8.1
67.8
17.0
47.1
67.3
27.4
27.2
9
K2O
0.3
30.2
30.4
50.4
90.3
70.3
80.3
3
Tota
l100.12
99.80
99.68
100.27
99.01
100.30
99.56
Si
10.7
94
10.7
62
10.6
07
10.5
62
10.7
92
10.7
07
10.7
26
Al
5.1
78
5.2
75
5.4
07
5.4
18
5.2
45
5.3
07
5.2
75
Ti
0.0
08
0.0
01
00.0
03
00
0.0
01
Fe 2
0.0
16
00
00
00
Mn
0.0
11
00.0
12
0.0
14
0.0
05
0.0
08
0
Mg
00.0
05
0.0
03
0.0
03
0.0
08
0.0
05
0.0
13
Ca
1.1
48
1.1
73
1.3
79
1.4
27
1.2
11
1.2
84
1.3
1
Na
2.8
16
2.6
97
2.4
43
2.4
74
2.5
47
2.5
55
2.5
27
K0.0
75
0.0
52
0.1
03
0.1
11
0.0
85
0.0
86
0.0
75
Cati
ons
20.0
46
19.9
65
19.9
54
20.0
12
19.8
93
19.9
52
19.9
27
Ab
69.7
68.8
62.2
61.7
66.3
65.1
64.6
An
28.4
29.9
35.1
35.6
31.5
32.7
33.5
Or
1.9
1.3
2.6
2.8
2.2
2.2
1.9
J. Earth Syst. Sci. (2018) 127:120 Page 15 of 33 120
Figure 4. Nomenclature of feldspar of granitoids accordingto Deer et al. (1992) classification scheme. The data sourceof the Raghunathpur pluton: Goswami and Bhattacharyya(2014).
are totally devoid of anorthite content. Orthoclasecomponent of alkali feldspars of the Sindurpurpluton ranges between Or94 and Or88.2. A smallamount of An component (up to 0.5 mol%) maybe present in the alkali feldspars of the Sindur-pur pluton. In the perthitic alkali feldspar of theRaghunathpur pluton, the compositions of alkalifeldspar vary from Or54.1Ab45.3 to Or93.5Ab6.5,while albite of composition Ab95.5Or0.9 exsolvedfrom the alkali feldspar.
Compositional variations of feldspar megacrystsfrom core to rim also attest to their igneous origin(tables 1 and 2).
6.2 Biotite
Representative analyses of biotite of all the fourplutons are given in table 3 and their plots are givenin figure 5.
The range of molar Fe2+/(Fe2+ + Mg) of thebiotite composition for these plutons is high.The highest variation of biotite composition isencountered in the Sindurpur pluton (0.42–0.95).Biotites of the Agarpur pluton are low in AlVI
(∼p.f.u.). The Sindurpur pluton shows a bimodaldistribution in the composition of biotite. Somebiotites are clustered around Fe/(Fe+Mg) values0.5 and other biotites show Fe/(Fe+Mg) ratio∼0.95. Fe/(Fe+Mg) ratio in the Raghunathpurpluton shows clustering around 0.55. TheSarpahari pluton shows bimodal distribution inrespect of Fe/(Fe+Mg) ratio. Biotites of ender-bite samples of this pluton show Fe/(Fe+Mg) ratioaround 0.5, but biotites from granite samples arericher in Fe and show Fe/(Fe+Mg) values around0.75.
Nachit et al. (2005) proposed TiO2–(FeO+MnO)–MgO ternary diagram as a quantitativeobjective tool for distinguishing between primarymagmatic biotites and those that are more orless reequilibrated, or neoformed, by or within ahydrothermal fluid. The magmatic origin of biotitesof the present study is supported by their chemicalcomposition (figure 6).
6.3 Amphiboles
Amphibole is the predominant mafic phase in allthese granitoids. Ferrous and ferric iron in theamphibole formula was calculated using the chargebalance method. Structural formulae of amphiboleswere calculated on an anhydrous basis assuming23 O atoms per half unit cell, with the generalform A0−1B2C5T8O22(OH)2 representing 1 for-mula unit. According to the IMA classificationand Fe3+ calculation proposed by Leake et al.(1997), all the amphiboles are classified as calcicamphiboles [
∑(Ca + Na) on M4 ≥ 1.00 with
Na < 0.5 and Ca ≥ 1.5 on M4]. In the classificationscheme, the calcic amphiboles are further classifiedinto four groups:
(a) (Na + K)A ≥ 0)50 and Ti < 0)50;(b) (Na + K)A ≥ 0 : 50 and Ti ≥ 0)50;(c) (Na + K)A < 0)50 and CaA < 0)50;(d) (Na + K)A < 0.50 and CaA ≥ 0.50.
Representative compositions of amphiboles fromdifferent plutons are given in table 4.
Amphiboles of the Agarpur pluton can beclassified into two groups; however, both of thesegroups may occur in the same rock. Group (a)amphiboles, characterised by (Na + K)A< 0.5 and
120 Page 16 of 33 J. Earth Syst. Sci. (2018) 127:120
Table
3.Electronprobe
microanalysisofbiotite
andcationic
calculationonthebasisof22oxygen.
Sam
ple
P20B
AD
36a
PR
3E
PR
9B
A11A
BA
13A
BA
4A
Rock
type
Gre
y-c
olo
ure
dbio
tite
am
phib
ole
gra
nit
eC
harn
ock
ite
Bio
tite
am
phib
ole
gra
nit
e
Mylo
nit
icbio
tite
am
phib
ole
gra
nit
eG
ranit
eE
nder
bit
e
Mylo
nit
ic
tonalite
Plu
ton
Agarp
ur
Sin
durp
ur
Analy
sis
41/1
54/1
10/1
9/1
84/1
95/1
102/1
110/1
48/1
49/1
31/1
32/1
17/1
Loca
tion
Core
Core
Core
Core
Core
Core
Core
Core
Core
Core
Core
Core
Core
SiO
238.1
338.3
636.8
535.6
631.9
332.6
733.3
633.3
334.5
035.3
035.3
735.6
534.0
7
TiO
22.6
82.2
45.7
65.3
03.1
53.1
33.4
42.6
74.6
45.6
64.4
95.7
23.1
9
Al 2
O3
13.7
713.8
913.5
513.0
012.8
812.9
513.2
114.8
114.3
414.2
014.1
813.5
916.8
3
Cr 2
O3
0.0
00.0
80.0
00.0
10.0
40.0
00.0
30.1
20.0
30.0
00.0
00.0
00.0
4
FeO
20.1
018.0
620.0
820.2
535.2
436.1
033.2
933.0
216.4
818.2
819.8
719.2
219.8
2
MnO
0.0
70.1
70.0
20.0
00.3
90.4
10.2
50.3
50.2
20.3
30.0
10.1
10.0
5
MgO
9.8
19.4
110.3
110.0
40.9
00.9
60.8
40.8
511.9
510.9
09.8
310.4
510.1
4
CaO
0.0
00.3
20.1
30.0
00.0
50.0
00.1
60.1
60.1
40.0
90.1
10.1
70.1
0
Na2O
0.1
00.1
90.1
00.1
30.2
10.0
70.1
70.2
00.1
30.0
00.0
70.1
00.1
6
K2O
9.0
98.0
49.0
89.5
98.5
08.8
67.4
17.4
87.7
69.5
610.0
39.2
79.2
8
Tota
l93.74
90.74
95.88
93.98
93.29
95.15
92.16
92.99
90.19
94.32
93.96
94.28
93.68
Si
5.9
62
6.0
81
5.8
64
5.8
45.7
27
5.7
52
5.9
18
5.8
37
5.7
34
5.7
08
5.7
93
5.7
79
5.5
71
AlIV
2.0
38
1.9
19
2.1
36
2.1
62.2
73
2.2
48
2.0
82
2.1
63
2.2
66
2.2
92
2.2
07
2.2
21
2.4
29
AlV
I0.4
98
0.6
74
0.4
03
0.3
47
0.4
47
0.4
37
0.6
78
0.8
92
0.5
41
0.4
12
0.5
28
0.3
74
0.8
12
Ti
0.3
15
0.2
68
0.6
89
0.6
53
0.4
25
0.4
15
0.4
59
0.3
52
0.5
80.6
88
0.5
53
0.6
98
0.3
92
Fe 2
2.6
28
2.3
94
2.6
72
2.7
74
5.2
86
5.3
15
4.9
39
4.8
36
2.2
91
2.4
72
2.7
22
2.6
06
2.7
1
Cr
00.0
10
0.0
01
0.0
06
00.0
04
0.0
17
0.0
04
00
00.0
05
Mn
0.0
10.0
22
0.0
03
00.0
59
0.0
61
0.0
38
0.0
52
0.0
31
0.0
45
0.0
01
0.0
15
0.0
07
Mg
2.2
87
2.2
24
2.4
46
2.4
51
0.2
41
0.2
52
0.2
22
0.2
22
2.9
61
2.6
28
2.4
2.5
26
2.4
72
Ca
00.0
54
0.0
22
00.0
10
0.0
30.0
30.0
25
0.0
16
0.0
19
0.0
30.0
18
Na
0.0
30.0
59
0.0
31
0.0
41
0.0
73
0.0
24
0.0
58
0.0
68
0.0
42
00.0
22
0.0
31
0.0
51
K1.8
13
1.6
26
1.8
43
2.0
04
1.9
45
1.9
91.6
77
1.6
71
1.6
45
1.9
72
2.0
96
1.9
17
1.9
36
Cati
ons
15.5
81
15.3
31
16.1
09
16.2
71
16.4
92
16.4
94
16.1
05
16.1
416.1
216.2
33
16.3
41
16.1
97
16.4
03
Fe/(F
e+M
g)
0.53
0.52
0.52
0.53
0.96
0.95
0.96
0.96
0.44
0.48
0.53
0.51
0.52
Mg/(F
e+M
g)
0.47
0.48
0.48
0.47
0.04
0.05
0.04
0.04
0.56
0.52
0.47
0.49
0.48
J. Earth Syst. Sci. (2018) 127:120 Page 17 of 33 120
Table
3.(C
ontinued.)
Sam
ple
PR
28E
PR
30B
Rock
type
Porp
hyri
tic
bio
tite
gra
nit
eE
nder
bit
e
Plu
ton
Sarp
ahari
Analy
sis
1/1
12/1
13/1
17/1
18/1
50/1
55/1
66/1
Loca
tion
Rim
Rim
Core
Core
Rim
Core
Core
Core
SiO
233.9
733.8
933.9
634.5
733.7
736.9
736.1
736.4
8
TiO
24.5
04.8
14.6
84.5
84.5
35.4
25.1
84.9
5
Al 2
O3
15.6
315.9
815.7
115.6
215.6
213.6
213.3
213.1
6
Cr 2
O3
0.0
30.1
10.1
70.0
70.0
30.1
70.0
80.1
6
FeO
26.0
427.2
526.9
126.2
427.3
820.0
820.5
419.3
4
MnO
0.0
40.1
20.1
10.0
30.1
30.1
50.0
00.0
2
MgO
5.7
55.4
45.4
65.8
45.0
110.8
211.1
611.7
0
CaO
0.0
80.0
40.0
30.0
00.0
40.0
00.0
50.0
2
Na2O
0.2
00.1
30.1
40.1
50.1
00.0
20.0
70.0
7
K2O
8.8
08.8
79.0
49.3
69.1
210.0
110.0
010.0
0
Tota
l95.04
96.64
96.21
96.46
95.73
97.26
96.57
95.90
Si
5.6
35
5.5
58
5.5
95
5.6
58
5.6
15.8
31
5.7
73
5.8
28
AlIV
2.3
65
2.4
42
2.4
05
2.3
42
2.3
92.1
69
2.2
27
2.1
72
AlV
I0.6
88
0.6
45
0.6
43
0.6
69
0.6
66
0.3
61
0.2
77
0.3
04
Ti
0.5
61
0.5
93
0.5
80.5
64
0.5
66
0.6
43
0.6
22
0.5
95
Fe 2
3.6
12
3.7
38
3.7
07
3.5
92
3.8
04
2.6
48
2.7
42
2.5
84
Cr
0.0
04
0.0
14
0.0
22
0.0
09
0.0
04
0.0
21
0.0
10.0
2
Mn
0.0
06
0.0
17
0.0
15
0.0
04
0.0
18
0.0
20
0.0
03
Mg
1.4
22
1.3
31.3
41
1.4
25
1.2
41
2.5
44
2.6
56
2.7
87
Ca
0.0
14
0.0
07
0.0
05
00.0
07
00.0
09
0.0
03
Na
0.0
64
0.0
41
0.0
45
0.0
48
0.0
32
0.0
06
0.0
22
0.0
22
K1.8
62
1.8
56
1.9
1.9
54
1.9
33
2.0
14
2.0
36
2.0
38
Cati
ons
16.2
33
16.2
41
16.2
58
16.2
65
16.2
71
16.2
57
16.3
74
16.3
56
Fe/(Fe+M
g)
0.72
0.74
0.73
0.72
0.75
0.51
0.51
0.48
Mg/(Fe+M
g)
0.28
0.26
0.27
0.28
0.25
0.49
0.49
0.52
120 Page 18 of 33 J. Earth Syst. Sci. (2018) 127:120
Figure 5. Classification of biotite of granitoids according toDeer et al. (1992). The data source of the Raghunathpurpluton: Goswami and Bhattacharyya (2014).
Figure 6. The composition of biotites in the 10 TiO2–FeO*–MgO ternary diagram (Nachit et al. 2005). FeO∗ =[FeO + (0.89981 * Fe2O3)]. Symbols are same as those offigures 3 and 4. The data source of the Raghunathpur pluton:Goswami and Bhattacharyya (2014).
Ti< 0.5 apfu (atoms per formula unit) aredominant among the pink granite and mag-nesiohornblende is the dominant species of thegranite with subordinate actinolitic hornblende.
All the amphiboles of the Raghunathpur plutonbelong to Group (a) with (Na + K)A > 0.5,
Ti < 0.5. Magnesian hastingsite is the mostcommon amphibole of this pluton, followed bymagnesian hastingsitic hornblende.
Amphiboles of the Sindurpur pluton belong to allfour groups. While amphiboles of the granite andcharnockite of the Sindurpur pluton are classifiedas hastingsite and magnesian hastingsite, amphi-boles of enderbites belong to edenite species.
Group (c) amphiboles occur more rarely and aredefined by (Na + K)A < 0.5 with TSi < 6.5 apfu,and are classified as tschermakite. Tschermakiterarely occurs in charnockite.
Amphiboles of Sarpahari enderbites belong totwo groups. Group (a) amphiboles are commonand are defined by (Na + K)A > 0.5 with lowTSi < 6.5 apfu and are classified as magnesianhastingsite and magnesian hastingsitic hornblendeor pargasitic hornblende.
Compositions of amphiboles are plotted in the Sivs. Ca+Na+K diagram of Leake (1971) in whichmost of the analyses plot well within the fieldof igneous amphibole while a few analyses frompink granite of Agarpur cross the boundary of theigneous field and enters into the metamorphic field(figure 7).
6.4 Pyroxene
Electron microprobe analyses of representativeclino- and orthopyroxenes are shown in tables 5and 6, respectively. All of the pyroxenes in thisarea are Na-poor and their composition falls in thequadrilateral pyroxene field according to the Mori-moto’s (1988) nomenclature (figure 8). Both clino-and orthopyroxenes are present in the Sindurpurpluton. While clinopyroxene is diopside (En34.21
Fs19.2Wo44.5 to En34.3Fs16.2Wo47.2), the orthopy-roxene of the Sindurpur pluton is hypersthene(En52.5Fs48.8–En50.1Fs46.2) (figure 8). Both clino-and orthopyroxenes are present in the Sarpaharipluton. Clinopyroxene of the Sarpahari pluton isdiopsidic (En34Fs17.5Wo45.7 to En30.7Fs20.8Wo46.2)and orthopyroxene composition in this plutonvaries from En60.2Fs49Wo1.2 to En48.7Fs39.0Wo0.0(figure 8).
7. Physico-chemical parameters of magmas
7.1 Pressure estimates
Al-in-hornblende barometry has been widely usedto calculate pressures of magmatic crystallisation
J. Earth Syst. Sci. (2018) 127:120 Page 19 of 33 120
Table
4.Electronprobe
microanalysisofamphibole
andcationic
calculationonthebasisof23oxygen.
Sam
ple
P18
P19
P20B
Rock
type
Pin
k-c
olo
ure
dbio
tite
am
phib
ole
gra
nit
eP
ink-c
olo
ure
dam
phib
ole
gra
nit
eG
rey-c
olo
ure
dbio
tite
am
phib
ole
gra
nit
e
Plu
ton
Agarp
ur
Analy
sis
65/1
66/1
72/1
160/1
161/1
165/1
166/1
194/1
13/1
15/1
17/1
20/1
38/1
Loca
tion
Rim
Core
Core
Rim
Rim
Core
Rim
Core
Core
Core
Rim
Core
Rim
SiO
247.1
249.9
845.3
743.7
647.7
047.5
644.3
348.0
242.5
648.0
943.1
344.3
442.6
0
TiO
20.0
00.0
00.0
00.2
60.4
50.2
80.1
30.1
10.5
70.0
00.5
60.3
00.9
6
Al 2
O3
6.9
25.1
77.9
89.0
56.2
97.0
28.5
16.7
19.6
14.4
69.6
29.0
610.0
2
Cr 2
O3
0.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
00.0
0
FeO
17.6
416.1
018.1
319.6
816.4
117.4
819.6
518.5
220.8
017.8
920.8
220.4
821.1
8
MnO
0.0
70.1
90.1
80.6
20.4
60.4
30.6
40.5
80.1
70.3
60.1
70.2
50.1
8
MgO
10.7
412.6
39.6
68.9
411.1
09.8
39.0
310.5
98.3
710.5
68.4
48.5
97.7
8
CaO
11.7
711.7
311.7
111.6
011.9
210.6
411.3
511.8
411.3
411.8
411.2
711.2
011.0
5
Na2O
1.0
10.7
51.0
01.3
31.0
20.9
21.2
91.0
61.3
90.6
81.7
21.4
31.6
5
K2O
0.6
10.3
40.5
71.0
70.6
40.4
80.9
20.6
91.1
70.4
41.3
01.0
81.3
7
Tota
l95.87
96.89
94.60
96.31
95.97
94.64
95.85
98.13
95.97
94.32
97.02
96.71
96.77
TSi
7.1
15
7.3
46.9
82
6.6
98
7.2
02
7.2
16
6.7
89
7.1
12
6.5
62
7.4
12
6.5
96
6.7
52
6.5
48
TA
l0.8
85
0.6
61.0
18
1.3
02
0.7
98
0.7
84
1.2
11
0.8
88
1.4
38
0.5
88
1.4
04
1.2
48
1.4
52
TFe 3
00
00
00
00
00
00
0
TT
i0
00
00
00
00
00
00
CA
l0.3
45
0.2
33
0.4
29
0.3
30.3
21
0.4
71
0.3
24
0.2
82
0.3
07
0.2
21
0.3
29
0.3
76
0.3
61
CC
r0
00
00
00
00
00
00
CFe 3
0.3
19
0.4
60.3
19
0.5
02
0.1
0.4
26
0.5
69
0.3
86
0.6
06
0.1
66
0.4
90.5
20.4
72
CT
i0
00
0.0
30.0
51
0.0
32
0.0
15
0.0
12
0.0
66
00.0
64
0.0
34
0.1
11
CM
g2.4
19
2.7
66
2.2
15
2.0
42.4
98
2.2
24
2.0
62
2.3
39
1.9
23
2.4
27
1.9
24
1.9
49
1.7
82
CFe 2
1.9
08
1.5
17
2.0
14
2.0
17
1.9
72
1.7
92
1.9
48
1.9
08
2.0
75
2.1
39
2.1
72
2.0
88
2.2
5
CM
n0.0
09
0.0
24
0.0
24
0.0
81
0.0
59
0.0
56
0.0
82
0.0
73
0.0
22
0.0
47
0.0
22
0.0
33
0.0
23
CC
a0
00
00
00
00
00
00
BC
a1.9
04
1.8
46
1.9
31.9
03
1.9
28
1.7
31.8
62
1.8
79
1.8
73
1.9
55
1.8
47
1.8
27
1.8
2
BN
a0.0
96
0.1
54
0.0
70.0
97
0.0
72
0.2
70.1
38
0.1
21
0.1
27
0.0
45
0.1
53
0.1
73
0.1
8
AC
a0
00
00
00
00
00
00
AN
a0.2
0.0
58
0.2
29
0.2
97
0.2
26
00.2
45
0.1
85
0.2
88
0.1
60.3
56
0.2
48
0.3
1
AK
0.1
17
0.0
63
0.1
11
0.2
10.1
22
0.0
94
0.1
81
0.1
31
0.2
31
0.0
86
0.2
54
0.2
10.2
68
Sum
cat
15.3
16
15.1
21
15.3
415.5
07
15.3
48
15.0
93
15.4
25
15.3
16
15.5
18
15.2
46
15.6
115.4
58
15.5
78
120 Page 20 of 33 J. Earth Syst. Sci. (2018) 127:120
Table
4.(C
ontinued.)
Sam
ple
AD
36a
PR
3E
PR
9B
A13A
PR
30B
BG
24
BG
36
Bio
tite
am
phib
ole
Mylo
nit
icbio
tite
Charn
o-
Rock
type
Charn
ock
ite
gra
nit
eam
phib
ole
gra
nit
eE
nder
bit
eE
nder
bit
een
der
bit
eE
nder
bit
e
Plu
ton
Sin
durp
ur
Sarp
ahari
Analy
sis
12/1
24/1
83/1
88/1
101/1
105/1
106/1
24/1
26/1
45/1
64/1
8/1
32/1
Loca
tion
Core
Core
Core
Rim
Core
Core
Core
Core
Core
Core
Core
Core
Core
SiO
239.4
339.9
337.9
137.4
136.8
337.2
737.4
142.7
042.8
540.9
941.2
940.4
841.7
6
TiO
22.3
32.0
81.3
61.8
71.2
51.9
11.7
91.8
61.6
82.0
21.5
11.9
21.7
0
Al 2
O3
11.7
111.6
410.3
010.2
010.0
110.1
310.1
810.3
010.0
911.0
311.1
511.7
611.4
5
Cr 2
O3
0.2
10.1
20.0
00.0
10.0
00.0
50.0
20.0
00.0
00.1
60.1
30.0
00.0
0
FeO
19.2
318.9
432.5
532.6
331.8
232.2
732.5
715.6
215.0
919.2
019.1
517.3
219.2
7
MnO
0.1
28.6
10.8
00.8
10.4
30.4
40.5
00.3
20.2
20.3
00.2
90.2
90.0
0
MgO
8.3
88.6
10.4
80.5
20.5
10.6
00.5
49.9
310.1
28.9
98.9
67.7
29.5
4
CaO
11.2
911.1
29.9
010.1
59.8
89.7
39.9
011.8
811.8
111.2
411.2
411.1
511.2
3
Na2O
1.1
31.3
11.8
21.8
61.5
61.6
11.5
41.3
81.1
91.5
61.3
91.3
91.2
1
K2O
2.0
42.1
51.6
91.8
11.7
31.8
11.7
71.4
41.4
01.7
11.8
31.7
81.7
8
Tota
l95.87
104.51
96.81
97.27
94.02
95.82
96.22
95.43
94.45
97.20
96.94
93.81
97.94
TSi
6.1
16
5.6
88
6.1
85
6.1
11
6.1
94
6.1
38
6.1
38
6.5
86.6
46.2
44
6.2
94
6.4
17
6.2
5
TA
l1.8
84
1.9
53
1.8
15
1.8
89
1.8
06
1.8
62
1.8
62
1.4
21.3
61.7
56
1.7
06
1.5
83
1.7
5
TFe 3
00.3
60
00
00
00
00
00
TT
i0
00
00
00
00
00
00
CA
l0.2
55
00.1
64
0.0
73
0.1
77
0.1
03
0.1
06
0.4
49
0.4
82
0.2
23
0.2
96
0.6
12
0.2
68
CC
r0.0
26
0.0
13
00.0
01
00.0
07
0.0
03
00
0.0
19
0.0
16
00
CFe 3
0.5
64
1.7
06
0.9
28
0.8
37
0.8
72
0.9
50.9
70
00.5
88
0.6
09
00.8
06
CT
i0.2
72
0.2
23
0.1
67
0.2
30.1
58
0.2
37
0.2
21
0.2
16
0.1
96
0.2
31
0.1
73
0.2
29
0.1
91
CM
g1.9
38
1.8
28
0.1
17
0.1
27
0.1
28
0.1
47
0.1
32
2.2
81
2.3
38
2.0
42
2.0
36
1.8
24
2.1
29
CFe 2
1.9
30.1
93.5
14
3.6
21
3.6
04
3.4
95
3.4
99
2.0
13
1.9
56
1.8
58
1.8
32
2.2
96
1.6
05
CM
n0.0
16
1.0
39
0.1
11
0.1
12
0.0
61
0.0
61
0.0
69
0.0
42
0.0
29
0.0
39
0.0
37
0.0
39
0
CC
a0
00
00
00
00
00
00
BC
a1.8
76
1.6
97
1.7
31
1.7
76
1.7
81.7
17
1.7
41.9
61
1.9
61
1.8
35
1.8
36
1.8
94
1.8
01
BN
a0.1
24
0.3
03
0.2
69
0.2
24
0.2
20.2
83
0.2
60.0
39
0.0
39
0.1
65
0.1
64
0.1
06
0.1
99
AC
a0
00
00
00
00
00
00
AN
a0.2
16
0.0
59
0.3
06
0.3
65
0.2
89
0.2
31
0.2
30.3
74
0.3
18
0.2
95
0.2
47
0.3
21
0.1
52
AK
0.4
04
0.3
91
0.3
52
0.3
77
0.3
71
0.3
80.3
71
0.2
83
0.2
77
0.3
32
0.3
56
0.3
60.3
4
Sum
15.6
215.4
515.6
58
15.7
43
15.6
615.6
11
15.6
01
15.6
57
15.6
15.6
28
15.6
03
15.6
81
15.4
92
J. Earth Syst. Sci. (2018) 127:120 Page 21 of 33 120
Figure 7. The plot of amphibole composition in the Si vs.Ca+Na+K discriminating diagram separating the fields ofigneous and metamorphic amohiboles (Leake 1971 in Deeret al. 1997). Note: most of the amphiboles of the presentstudy plot within the field of igneous amphibole. Symbolsare the same as those of figures 3 and 4. The data sourceof the Raghunathpur pluton: Goswami and Bhattacharyya(2014).
of calc-alkaline granitoids and to constrain theemplacement depths of batholiths. This barometrycan be used only when the suitable mineral assem-blages, i.e., quartz + plagioclase + alkali feldspar+ biotite + hornblende + titanite + magnetiteor ilmenite are present in the granitic rocks (e.g.,Vyhnal et al. 1991; Anderson and Smith 1995). Inthis paper, all the selected samples have the abovemineral assemblage. According to Anderson andSmith (1995), this barometry is applicable to horn-blendes which have crystallised at approximatelythe same temperature close to the isothermalsolidus and they recommended measurement of rimcomposition for this study. Moreover, this barome-try is applicable for those hornblendes only whichoccur in contact with quartz and K-feldspar (Steinand Dietl 2001). For each sample, rim compositionof co-existing hornblende and plagioclase was mea-sured. In all cases, hornblende is in contact withquartz and K-feldspar.
Oxygen fugacity plays an important role inthe Al substitution in hornblende. Oxygen fugac-ity controls the Mg # {= Mg/(Mg + Fe)} andFe3+/(Fe2+ + Fe3+) ratios in the magma. Ander-son and Smith (1995) estimated that Mg # ofhornblende in the range from 0.4 to 1, 0.2 and0.4 and 0.4 to 0 indicates, respectively, the high,medium and low oxygen fugacity. A high oxygenfugacity favours incorporation of Fe3+ and sub-stitutes Al in hornblende lattice. So Al-content of
Table 5. Electron probe microanalysis of clinopyrox-ene and cationic calculation on the basis of six oxygen.
Sample BA13A BG24
Rock type Enderbite Charnoenderbite
Pluton Sindurpur Sarpahari
Analysis 28/1 29/1 2/1 3/1
Location Core Core Core Core
SiO2 51.47 51.57 50.36 51.15
TiO2 0.12 0.10 0.08 0.12
Al2O3 1.25 1.26 1.17 1.55
FeO 11.61 9.77 12.31 13.75
Cr2O3 0.04 0.04 0.05 0.04
MnO 0.38 0.45 0.15 0.20
MgO 11.59 11.62 10.19 10.48
CaO 21.37 22.61 21.61 20.98
Na2O 0.35 0.40 0.40 0.38
K2O 0.00 0.00 0.04 0.00
Total 98.18 97.82 96.36 98.65
TSi 1.981 1.983 1.986 1.975
TAl 0.019 0.017 0.014 0.025
M1Al 0.037 0.040 0.040 0.045
M1Ti 0.003 0.003 0.002 0.003
M1Fe2 0.293 0.290 0.357 0.347
M1Cr 0.001 0.001 0.002 0.001
M1Mg 0.665 0.666 0.599 0.603
M2Fe2 0.080 0.024 0.049 0.097
M2Mn 0.012 0.015 0.005 0.007
M2Ca 0.881 0.931 0.913 0.868
M2Na 0.026 0.030 0.031 0.028
Sum 4.000 4.000 3.998 4.000
Q 1.920 1.912 1.918 1.915
J 0.052 0.060 0.061 0.057
WO 45.605 48.354 47.478 45.165
EN 34.415 34.577 31.150 31.391
FS 19.980 17.070 21.371 23.445
WEF 97.368 96.997 96.917 97.124
JD 2.632 3.003 3.083 2.876
AE 0.000 0.000 0.000 0.000
hornblende remains low under high oxygenfugacity. In low oxygen fugacity, on the contrary,the introduction of Fe2+ is favoured in the horn-blende lattice. Additionally, high Fe2+/Fe3+-ratioprevailing during low oxygen fugacity conditionfavours the substitution of Mg by Al during thetschermak substitution. So a low oxygen fugac-ity favours the preferential introduction of Al inhornblende lattice. Therefore, hornblendes withMg #> 0.35 and Fe3+/(Fe2+ + Fe3+)-ratio ≥ 0.25are recommended for geobarometric calculation(Anderson and Smith 1995; Anderson 1997).
But the determination of Fe2+ and Fe3+ bystoichiometric calculations may not reflect the real
120 Page 22 of 33 J. Earth Syst. Sci. (2018) 127:120
Table 6. Electron probe microanalysis of orthopyroxene and cationic calculation on the basis of six oxygen.
Sample AD36a BG24
Rock type Charnockite Charnoenderbite
Pluton Sindurpur Sarpahari
Analysis 1/1 11/1 2/1 20/1 21/1 5/1 6/1 1/1 4/1 5/1
Location Core Core Rim Core Rim Core Rim Core Core Core
SiO2 50.05 50.1 49.74 49.74 49.46 47.53 49.36 49.44 49.69 49.83
TiO2 0.09 0.15 0 0 0.07 0 0.06 0.06 0.04 0.04
Al2O3 1 0.85 0.87 1.05 0.95 0.82 0.77 0.56 0.43 0.55
FeO 30.72 31.35 30.33 31.03 30.69 30.82 30.81 35 35.43 35
Cr2O3 0 0.02 0.12 0 0 0.54 0.07 0 0.04 0
MnO 0.63 0.6 0.61 0.66 0.51 0.51 0.65 0.52 0.55 0.59
MgO 16.24 15.99 16.44 15.89 16.78 15.21 16.31 13.01 12.87 13.15
CaO 0.59 0.55 0.5 0.49 0.51 0.52 0.48 0.36 0.41 0.46
Na2O 0 0.05 0.02 0.02 0 0.02 0 0 0 0
K2O 0 0 0 0 0 0.02 0 0 0 0
Total 99.32 99.66 98.63 98.88 98.97 95.99 98.51 98.95 99.46 99.62
TSi 1.956 1.956 1.954 1.956 1.934 1.932 1.945 1.985 1.988 1.986
TAl 0.044 0.039 0.04 0.044 0.044 0.039 0.036 0.015 0.012 0.014
M1Al 0.002 0 0 0.004 0 0 0 0.011 0.008 0.012
M1Ti 0.003 0.004 0 0 0.002 0 0.002 0.002 0.001 0.001
M1Fe2 0.049 0.065 0.034 0.064 0.02 0.061 0.038 0.208 0.222 0.206
M1Cr 0 0.001 0.004 0 0 0.017 0.002 0 0.001 0
M1Mg 0.946 0.93 0.963 0.931 0.978 0.921 0.958 0.779 0.768 0.781
M2Fe2 0.954 0.959 0.963 0.956 0.984 0.986 0.977 0.967 0.964 0.96
M2Mn 0.021 0.02 0.02 0.022 0.017 0.018 0.022 0.018 0.019 0.02
M2Ca 0.025 0.023 0.021 0.021 0.021 0.023 0.02 0.015 0.018 0.02
M2Na 0 0.004 0.002 0.002 0 0.002 0 0 0 0
Sum 4.000 4.000 4.000 4.000 4.000 3.999 4.000 4.000 4.000 4.000
Q 1.975 1.977 1.98 1.972 2.003 1.992 1.994 1.969 1.971 1.967
J 0 0.008 0.003 0.003 0 0.003 0 0 0 0
WO 1.238 1.152 1.052 1.035 1.058 1.127 1.006 0.779 0.883 0.988
EN 47.409 46.6 48.126 46.702 48.423 45.864 47.54 39.188 38.587 39.312
FS 51.354 52.248 50.822 52.263 50.519 53.009 51.455 60.032 60.529 59.7
WEF 100 99.622 99.848 99.847 100 99.843 100 100 100 100
JD 0 0 0 0.153 0 0 0 0 0 0
AE 0 0.004 0.002 0 0 0.003 0 0 0 0
oxygen fugacity. High oxygen fugacity conditionmay be retrieved from an assemblage of accessoryminerals. Enami et al. (1993) suggested that occur-rences of euhedral titanite and magnetite as earlycrystallising phases indicate high oxygen fugacitycondition. In our present study, the presence ofeuhedral titanite, magnetite and allanite suggestshigh oxygen fugacity of the melt.
Other prerequisites for a correct applicationof the barometers are as follows: the Si-activity ofthe melt must have been SiO2-saturated (Si≤7.5apfu), because the Al-content of hornblende isdirectly related to its Si content; hornblendes withCa ≥ 1.6 apfu (Hammarstrom and Zen 1986;
Bando et al. 2003); amphibole should coexist withquartz (Hammarstrom and Zen 1986) and/or K-feldspar (Stein and Dietl 2001), because theiractivity also influences the Al content of horn-blende; water saturation of the magmatic system(Anderson and Smith 1995); and plagioclase coex-isting with hornblende should range between An25
and An35 (Stein and Dietl 2001).All the analyses of amphiboles chosen for ther-
mobarometric computations satisfy the criteriadescribed in the preceding paragraph.
Calibrations of Johnson and Rutherford (1989)and Schmidt (1992) have been more commonlyused because of their experimental derivation. A
J. Earth Syst. Sci. (2018) 127:120 Page 23 of 33 120
Figure 8. Nomenclature of pyroxenes of granitoids according to the IMA classification scheme (Morimoto 1988). Symbols:green open triangle – charnoenderbite, Sarpahari; green box: charnockite, Sindurpur; yellow box: enderbite, Sarpahari.
new and revised Al-in-hornblende geobarometeris recently proposed by Mutch et al. (2016) whohave made an experimental study of amphibolestability in low-pressure granite magmas. TheirAl-in-hornblende geobarometer is applicable togranitic rocks with the low-variance mineral assem-blage: amphibole + plagioclase (An15−80) + biotite+ quartz + alkali feldspar + ilmenite/titanite +magnetite + apatite. They have recommendedthat amphibole analyses should be taken fromthe rims of grains, in contact with plagioclaseand in apparent textural equilibrium with therest of the mineral assemblage at temperaturesclose to the haplogranite solidus (725 ± 75◦C), asdetermined from amphibole–plagioclase thermo-metry. Mean amphibole rim compositions thatmeet these criteria can then be used to calculateP (in kbar) from Altot (in apfu) according to theexpression:
P (kbar) = 0.5 + 0.331 × (Altot) + 0.995 × (Altot)2.
Since the Agarpur pluton is emplaced in relativelyshallower level (see later), this new geobarometer isused for calculation. According to the calibrationof Mutch et al. (2016), the average pressure esti-mates for Sarpahari, Raghunathpur and Sindurpurplutons are 5.4, 5.4 and 4.5±0.5 kbar, respectively,
for the final emplacement level and crystallisation(table 7). However, following the same calibra-tion the average pressure estimate for the Agarpurpluton yields 3.2 ± 0.5 kbar indicating a shallowdepth of emplacement of the studied rocks.
7.2 Temperature estimates
Coexisting hornblende and plagioclase arecommonly used for thermometric calculations incalc-alkaline igneous rocks (Blundy and Holland1990; Holland and Blundy 1994). According toHolland and Blundy (1994), the constrains forusing this thermometer are: (i) temperature shouldbe in the range of 400–900◦C; (ii) amphibolesshould have ANa>0.02 pfu, AlVI<1.8 pfu andSi in the range of 6.0–7.7 pfu; and plagioclasesshould have XAn < 0.90. These compositional con-straints in the studied amphiboles satisfy the useof this geothermometer. The estimate of pressurerequired for computation of this geothermometeris taken from Schmidt’s (1992) pressure calcu-lation. The hornblende–plagioclase geothermome-ter of Holland and Blundy (1994) is based onthe edenite–richterite reaction and is applicablealso to quartz-free igneous rocks. The averagetemperatures of equilibration calculated with theouter margin compositions of adjacent hornblendeand plagioclase in the granitoid rocks of
120 Page 24 of 33 J. Earth Syst. Sci. (2018) 127:120
Table
7.Intrinsicparametersofgranitoid
rocks.
Sam
ple
Rock
type
Pre
ssure
Tem
per
atu
re
Al-in
-horn
ble
nde
Hbl-pla
gT
i-in
-Hbl
Horn
ble
nde
chem
istr
yT
i-in
-bio
tite
Mutc
het
al.
(2016)
Dep
th(k
m)
(usi
ng
Mutc
het
al.
2016)
Sch
mid
t(1
992)
Holland
and
Blu
ndy
(1994)
Ott
en(1
984)
Rid
olfi
etal.
(2010)
Hen
ryet
al.
(2005)
Agarp
ur
plu
ton
P18
Pin
kgra
nit
e2.4
7.9
2.6
896
545
P19
Pin
kgra
nit
e3.5
11.6
2.6
589
808
P20B
Gre
ygra
nit
e4.3
14.2
4.7
885
650
661
Sin
durp
ur
plu
ton
PR
3E
Gra
nit
e5.1
16.8
6.4
747
776
677
PR
9M
ylo
nit
icgra
nit
e5.1
16.8
6.4
724
788
665
BA
4A
Mylo
nit
icto
nalite
703
BA
13A
Ender
bit
e4.5
14.9
5.6
718
801
886
764
BA
11A
Gra
nit
e775
AD
36a
Charn
ock
ite
5.6
18.5
7.2
793
838
775
Raghunath
pur
plu
ton
CB
6Porp
hyri
tic
gra
nit
e5.2
17.2
6.5
763
787
909
760
CB
38
4.9
16.2
6.2
779
808
760
CB
38c
5.7
18.8
7.1
766
738
744
H1
5.3
17.5
6.7
719
634
731
BG
14
5.4
17.8
6.3
796
829
754
BG
13c
6.2
20.5
7.2
735
734
729
Sarp
ahari
plu
ton
BG
24
Charn
oen
der
bit
e6.0
19.8
7.0
780
810
BG
36
Ender
bit
e5.2
17.2
6.6
775
776
904
PR
30B
Ender
bit
e5.1
16.8
6.5
749
789
907
767
PR
28E
Porp
hyri
tic
gra
nit
e734
J. Earth Syst. Sci. (2018) 127:120 Page 25 of 33 120
Table
7.(C
ontinued.)
Sam
ple
logf O
2H
2O
(wt%
)lo
gf H
2O
f H2O
(bar)
H2O
(wt%
)
Bio
tite
Bio
tite
chem
istr
yC
alc
-alk
aline
rock
s
Horn
ble
nde
chem
istr
y
Horn
ble
nde
chem
istr
y
Calc
-alk
aline
rock
s
Mg/(F
e+M
g)
Wones
and
Eugst
er(1
965)
Wones
(1989)
Rid
olfi
etal.
(2010)
Rid
olfi
etal.
(2010)
Wones
(1981)
Moore
etal.
(1998)
Agarp
ur
plu
ton
P18
−11.2
4.2
2
P19
−13.0
6.5
P20B
0.4
8−1
4.0
−11.2
4.2
5
Sin
durp
ur
plu
ton
PR
3E
0.0
4−1
7.5
−14.6
3.6
24174
5.9
PR
90.0
4−1
7.8
−15.3
3.6
94886
4.4
BA
4A
0.4
8−1
3.5
BA
13A
0.4
8−1
3.5
−15.6
−12.2
5.6
2.7
6575
2.8
BA
11A
0.5
4−1
2.5
AD
36a
0.4
8−1
3.5
−13.3
3.5
73681
6.6
Raghunath
pur
plu
ton
CB
60.4
5−1
4.2
−14.2
−12.1
5.2
3.2
61821
5.9
CB
38
0.4
6−1
4.3
−13.8
3.3
92459
6.7
CB
38c
0.4
2−1
5.0
−14.0
3.3
22104
6.3
H1
0.3
8−1
5.5
−15.4
2.8
3672
3.1
BG
14
0.4
1−1
4.8
−13.2
3.5
83799
6.4
BG
13c
0.4
1−1
5.5
−14.8
3.0
61135
4.4
Sarp
ahari
plu
ton
BG
24
−13.6
BG
36
−13.8
−11.8
5.3
PR
30B
0.5
0−1
3.1
−14.6
−12.0
5.0
3.1
21310
4.9
PR
28E
0.2
8−1
5.5
Data
set:
Agarp
ur,
Sin
durp
ur
and
Sarp
ahari
plu
tons:
pre
sent
study;R
aghunath
pur
plu
ton:G
osw
am
iand
Bhatt
ach
ary
ya
(2014).
120 Page 26 of 33 J. Earth Syst. Sci. (2018) 127:120
Figure 9. The P–T and T–log fO2 diagrams of the selected studied amphiboles. The NNO and NNO+2 curves in (B), (C)and (D) are taken from O’Neill and Pownceby (1993). ‘Arc magmas’ areas from Harald and Galliard (2006). (E) A T–H2Omelt diagram (after Ridolfi et al. 2010). The data source of the Raghunathpur pluton, Goswami and Bhattacharyya (2014).
Sarpahari, Raghunathpur, Sindurpur and Agarpurare 768◦C, 760◦C, 746◦C and 891 ± 40◦C, respec-tively (table 7).
Ridolfi et al. (2010) proposed a geothermometerbased upon the composition of magnesian amphi-bole. Only a few amphibole grains are magnesianand crystallisation temperatures (808–909◦C) ofthese amphiboles are given in table 7. Tempera-tures obtained using Ti-content of amphibole arealso given in table 7. A westward decrease in
temperature is co-relatable with compositionalvariation of plutons from enderbite (Sarpahari)to monzogranite (Raghunathpur) to syenogranite(Sindurpur) to alkali feldspar granite (Agarpur)(figure 9A). This observation strongly supportsthat the crystallisation temperature of magmadepends on the composition of the magma, espe-cially silica content. In our study, silica contentis minimum in enderbite and maximum in alkalifeldspar granite.
J. Earth Syst. Sci. (2018) 127:120 Page 27 of 33 120
7.3 Oxygen fugacity estimates
The oxygen fugacity of magma can be calculatedusing the distribution coefficient of Ti from coex-isting magnetite and ilmenite. But this methodcannot be satisfactorily used in plutonic rocksbecause during slow cooling, magnetite becomesTi-free and ilmenite undergoes oxidation and exso-lution (Haggerty 1976). However, biotite is avery good indicator of the oxidation state of themagma from which it is crystallised. The coexis-tence of biotite, alkali feldspar and iron-titaniumoxide minerals in the granites provides the basisfor tentatively estimating oxygen fugacity. TheTi4+ and Fe3+ contents of biotite depend mostlyon the temperature of crystallisation and oxygenfugacity (fO2) and the AlVI depends on the Alactivity. The Fe3+ content and Fet/(Fet + Mg)ratio provide at least the relative informationabout the oxygen fugacity during crystallisation(Wones and Eugster 1965). Since Fe3+ and Fe2+
cannot be estimated by EPMA, a quantitativeestimation of the oxygen fugacity or tempera-ture can be obtained using the experimental workof Wones and Eugster (1965) modified by Sha-bani et al. (2003) for biotites coexisting withmagnetite and alkali feldspar. The crystallisa-tion temperatures of biotites of the present studyhave been obtained from the calibration of Henryet al. (2005), where Ti-content of biotite was used(table 7; figure 9B).
The rocks of the present study show oxygenfugacity between NNO and NNO+2 buffers whichis consistent with the presence of sphene, allaniteand magnetite/ilmenite. In this diagram, the NNOand NNO+2 curves were obtained from O’Neill andPownceby (1993). The Sarpahari enderbites equi-librated at an oxygen fugacity (log fO2) between−13.4 and −14.4 bar, for the temperatures ofcrystallisation interval between 780◦ and 749◦C.In porphyritic monzogranites of the Raghunath-pur, the oxygen fugacity (log fO2) varies between−13.1 and −15.2 bar for crystallisation temper-ature range between 796◦ and 719◦C. The rocksof Sindurpur show more or less similar range ofoxygen fugacity (log fO2) between −13.1 and −15.4bar for a temperature range between 793◦ and718◦C. The oxygen fugacity is quite high for theAgarpur pluton and is about −11.2 bar at atemperature range between 885◦ and 896◦C.
Wones (1989) suggests a quantitativeestimation of oxygen fugacity of granitoid rockswith assemblage sphene + magnetite+ quartz.
Equilibrium expression of Wones (1989) is usedin the present study to estimate the prevailingoxygen fugacity in the different plutons (table 7).Temperatures and pressures estimated fromhornblende–plagioclase thermometry (Holland andBlundy 1994) and aluminium-in-hornblende bar-ometry (Schmidt 1992) were used to calculatelog fO2 of the magma following Wones (1989)expression (figure 9C).
Ridolfi et al. (2010) proposed an empiricaluniversal amphibole sensor to estimate tempera-ture and log fO2 parameters. However, only a fewdata of the present study admit the validity level.The present study depicts (figure 9d) the slightlyincreasing oxygen fugacity trend with decreasingtemperature. Following the calibration of Ridolfiet al. (2010), the calculated range of values oflog fO2 for Sarpahari enderbite rocks are foundto be −11.8 and −12.0 bar. In Raghunathpur,porphyritic granitoids and Sindurpur granites, thelog fO2 are found to be −12.1 and −12.2 bar,respectively, at about 900◦C. According to Ridolfiet al.’s (2010) calibration, the oxygen fugacity (logfO2) of the Agarpur pluton is about −13 bar at808◦C temperature.
7.4 Hygrometric estimates
The assemblage biotite–K-feldspar–magnetite actsas a buffer of oxygen and water fugacities in themagma (Wones and Eugster 1965; Czamanske andWones 1973):
KFe3AlSi3O10(OH)2 + 0.5O2
= KAlSi3O8 + Fe3O4 + H2O.
Annite activity (aann) of biotite reflects thefugacities of oxygen and water in magma. In a crys-tallising magma, the aH2O increases with decreas-ing T due to crystallisation of anhydrous phases.Concomitantly, when fO2 of the magma is buffered,annite in biotite increase through reaction of K-feldspar with magnetite, with resulting growthof the Fe2+/(Fe2++Mg) ratio. At the constanttemperature and constant fH2O, when fO2 decrea-ses, annite of biotite also increases either bya decrease of Fe3+/Fetot or by an increase ofFe2+/(Fe2++Mg) ratios. If fO2 of the magmaand activities of magnetite and K-feldspar areknown, the activity of annite in biotite may beused to derive the water fugacity through thereaction (Wones 1972; Czamanske and Wones1973):
120 Page 28 of 33 J. Earth Syst. Sci. (2018) 127:120
log fH2O = 7409/T + 4.25 + 0.5 log fO2 + log aann
−log aKf − log amag.
Later on, Wones (1981) revised it as
log fH2O = 4819/T + 6.69 + 0.5 log fO2 + log aann
−log aKf − log amag − 0.011(P − 1)/T,
where T is in kelvin and activities are given asmolar fractions. The activity of annite in biotitecan be calculated by various ideal activity mod-els (Mueller 1972; Wones 1972; Czamanske andWones 1973; Bohlen et al. 1980; Holland and Powell1990; Nash 1993; Patino Douce 1993). The activ-ity of K-feldspar is 0.6 for magmatic temperatures(Czamanske and Wones 1973) and activity of mostre-equilibrated magnetites is close to 1. The fH2O
then can be calculated using the Wones’ (1981)equation. The water activity of magma can begiven by
aH2O = fH2O/f0H2O,
where f0H2O is the standard state water fugacity.
On the basis of water solubility models of Burn-ham (1979) and Burnham and Nekvasil (1986), it ispossible to convert the aH2O, fH2O to XH2O, whichcan then be expressed in wt%. We use the empiricalmodel of Moore et al. (1998) for determining theH2O (wt%) from fH2O (vapour) in magmas. Thismodel can be applied between 700◦ and 1200◦Cand 1–3000 bar and can be applied to any natu-ral silicate liquid in that range. For the conversionof fH2O to H2O (wt%), the following regressionequation is derived from the experimental data ofMoore et al. (1998):
H2O (wt%) = −7 × 107(fH2O)2 + 0.0042(fH2O)
+0.5798.
The fH2O of the Sindurpur pluton is estimated tobe 575–4886 bar corresponding to 2.8–6.6 wt% ofwater in the magma (table 7). The Raghunathpurpluton gives a more consistent fH2O values rangingbetween 672 and 3799 bar corresponding to 3.1 and6.7 wt% of water. One sample of the Sarpahari plu-ton has requisite assemblage and gives fH2O valueat 1310 bar corresponding to 4.9 wt% of water inthe magma. Very high log fH2O values are obtainedfrom the samples of the Agarpur pluton and thesevalues are out of range of experimental values ofMoore et al. (1998).
The water content of magma can also beobtained by the empirical calibration of Ridolfiet al. (2010) using magnesiohornblende compo-sition. According to the amphibole hygrometerof Ridolfi et al. (2010), the water contents inthe Sarpahari pluton range from 5.0 to 5.3 wt%(table 7). The water contents of Raghunathpur,Sindurpur and Agarpur plutons were about 5.2,5.6 and 6.5 wt%, respectively. These values are inconformity with the typical values of calc-alkalinemagma crystallisation in an arc-related environ-ment (figure 9E; Ridolfi et al. 2010).
Like temperature, a slight increase in watercontent of magmas of different plutons from eastto west, enderbite of Sarpahari in the east, mon-zogranite of Raghunathpur, syenogranite of Sin-durpur and alkali feldspar granite of Agarpur tothe west is noticed which is correlatable with thechange in magma composition.
8. Nature of magma
In the present study, biotite compositions are usedas effective tools for fingerprinting the magmatype and potentially reflecting the nature of themagma and tectonic environment in which theyare formed (e.g., Nachit et al. 1985; Abdel-Rahman1994). Biotites from different granites are alumi-nous (Altotal = 3.01–3.54 apfu) and ferric (XMg =0.36–0.47), typical of biotites from subalkaline tocalc-alkaline to aluminopotassic granites (table 2;figure 10A). Biotite compositions show a contin-uous increase in Altotal and decrease in Mg fromeastern to western plutons, i.e., from Sarpaharito Agarpur granites. The progressive increase ofthe total Al contents and the Fe2+/(Fe2++ Mg)ratios in biotite (figure 10A) are consistent witha compositional trend of biotite in continentalcollision-related granites (Lalonde and Bernard1993). It indicates the contribution of crustalmaterial (metasediments) during the petrogenesisof the granitoid magmas, either by assimilation oranatexis (Shabani et al. 2003).
Biotites of the Agarpur pluton show FeO/MgOratios in the 1.92–2.05 range and plot well withinthe calc-alkaline field of the MgO–FeO–Al2O3
discrimination diagram (figure 10B).Although the FeO/MgO ratios of biotite
analysed from the Sindurpur pluton are moderatelyenriched in Mg, showing FeO/MgO ratios rangingfrom 1.38 to 2.26, these plot within the calc-alkalinefield in the MgO–FeO–Al2O3 diagram. However,
J. Earth Syst. Sci. (2018) 127:120 Page 29 of 33 120
Figure 10. (A) Altotal vs. Mg (p.f.u.) diagram (Nachit et al. 1985) for biotites of the present study. (B) Al2O3–FeOt–MgO discrimination diagram for distinguishing the nature of the granitoids of northern Purulia (Abdel-Rahman 1994).FeOt = [FeO + (0.89981 * Fe2O3)]. The data source of the Raghunathpur pluton: Goswami and Bhattacharyya (2014).
higher FeO/MgO values (38.85–42.22) are obtainedfrom biotites of Sindurpur and Tilaboni hillocks.In the MgO–FeO–Al2O3 discrimination diagram(Abdel-Rahman 1994), the most biotites from theSindurour pluton fall within the calc-alkaline fieldand some samples plot in the alkaline suite field(figure 10B). Biotites of the Raghunathpur plutonshow the most consistent variation in FeO/MgOratio (2.08–2.96) and all the analysed biotites areplotted on the boundary between calc-alkaline andalkaline granitoid fields in the MgO–FeO–Al2O3
discrimination diagram of Abdel-Rahman (1994)(figure 10B).
An enderbite sample of the Sarpahari plutonshows FeO/MgO ratios of biotites in the range1.65–1.86 which are consistent with the calc-alkaline nature of the magma. But porphyriticgranitoids from the same pluton show relativelyhigh FeO/MgO values (4.49–5.47), consistentwith the peraluminous nature of the magma(figure 10B).
9. Tectonic implications
9.1 Implications of biotite composition
Biotite compositions clearly define the nature ofmagmas from which they have been crystallised.In the MgO–FeO–Al2O3 discrimination diagramof Abdel-Rahman (1994), biotites of the Agarpurpluton plot well within the calc-alkaline field(figure 10B). Most of the biotites from the Sin-durpur pluton also fall within the calc-alkalinefield and some samples plot in the alkaline suitefield, while biotites of the Raghunathpur pluton
are plotted on the boundary between calc-alkalineand alkaline granitoid fields. Similarly, enderbitesof Sarpahari plutons are plotted in the field of calc-alkaline granite (figure 10A). Biotite chemistry offour plutons of the present study suggests that theywere formed from subduction-related calc-alkalinemagma (Abdel-Rahman 1994).
9.2 Implications of pressure estimate
In the present study, the Al-in-hornblendebarometer (Mutch et al. 2016) is used to under-stand the vertical crustal upliftment on a regionalscale, after complete solidification of the magmabecause the calculated pressures probably representconditions during final emplacement (Stein andDietl 2001).
Although different samples from the same plutonmay give different depths of emplacement, a sys-tematic variation of general depth of emplacementis discernable from the eastern to western part ofthe study area. Moreover, calculations of intrin-sic parameters (table 7) enable us to realise thefollowing conclusions:
(i) The Raghunathpur pluton and Sindurpurpluton are separated by E–W trending NPSZand these plutons are located north and southof the shear zone (figure 1B). Slightly deeperemplacement of the Raghunathpur pluton(16.2–20.5 km) relative to the Sindurpur plu-ton (14.9–18.5 km) suggests that negligiblevertical upliftment might have taken placeon the northern block relative to the south-ern block. This is consistent with the field
120 Page 30 of 33 J. Earth Syst. Sci. (2018) 127:120
observation that NPSZ is a sinistral strike-slipshear zone with minute dip-slip component.
(ii) The Sarpahari pluton (16.8–19.8 km) which islocated about 60 km ENE from the Agarpurpluton (7.9–14.2 km) must have been uplifted5–8 km relative to the latter pluton. This, inturn, suggests a weak ENE trend of increasingdepth of emplacement (figure 1B).
9.3 Implication of oxygen fugacity
The oxygen fugacity of magma is related to itssource material and tectonic settings. Sediment-derived S-type granitic magmas are usually redu-ced, while I-type granites are relatively oxidised(Haggerty 1976). Highly oxidised magmas are com-monly associated with convergent plate boundaries(Ewart 1979), while felsic magmas that are formedby fractionation from mantle-derived magmas inrift zones are reduced (Loiselle and Wones 1979).Hammarstrom and Zen (1986) suggest that calcicamphibole crystallised under fairly ‘oxidising’ con-ditions, with fO2 probably buffered between NNOand HM. The oxygen fugacity (log fO2) values cal-culated from the calcic amphiboles by means ofRidolfi et al.’s (2010) formula varied between −11.8and −13.0 bar (figure 9D), and all of the samplesare plotted in the arc magma field described byHarald and Galliard (2006). These values show arelatively high oxidation state (higher than NNObuffer condition) during the late-stage crystalli-sation of biotite and show a trend for magmacrystallisation in oxidising conditions (figure 9C).
9.4 Implication of water activity
The concentration of H2Omelt in the studiedplutons ranges from 5.0 to 6.5% (as estimated fol-lowing Ridolfi et al. 2010) and from 2.8 to 7.01%(as estimated following Moore et al. 1998). Thesevalues are in conformity with the typical valuesof calc-alkaline magma generated in an arc-relatedenvironment (Naney 1983).
10. Conclusions
The fabric of the four granitoid plutons (Sarpahari,Raghunathpur, Sindurpur, and Agarpur) from nor-thern Purulia is interpreted to be primarily ofigneous origin. The marginal parts of these plu-tons were affected by regional shearing. However,the chemical compositions of the magmatic miner-als of the central parts of the plutons, from where
samples were collected, have not been altered dueto shearing at the marginal parts.
The mineral compositions of these plutons wereused to estimate the physico-chemical parametersof the crystallisation conditions of the parent mag-mas and also to establish the chemical nature ofparent magma and the tectonic environment inwhich these plutons were emplaced.
Biotite compositions of all the four plutons of thepresent study suggest that these were crystallisedfrom subduction-related calc-alkaline magma(Abdel-Rahman 1994).
The pressure of emplacement of these plutonswas estimated using Al-in-hornblende barometry(Mutch et al. 2016; table 7). Slight pressure dif-ference of emplacement of Raghunathpur (4.9 and6.2 kbar) and Sindurpur plutons (4.5–5.6 kbar)lying north and south of NPSZ, respectively, sug-gests that vertical upliftment along shear zonewas negligible and the strike-slip movement wasdominant in the shear zone. The Sarpahari plu-ton (P = 5.1–6.0 kbar) which is located about60 km ENE from the Agarpur pluton (P = 2.4–4.3 kbar) must have been uplifted 5–8 km relativeto the latter pluton. This, in turn, suggests aweak ENE trend of increasing depth of emplace-ment.
The average temperatures of equilibrationcalculated by the method of Holland and Blundy(1994) using compositions of adjacent hornblendeand plagioclase in the granitoid rocks of Sarpa-hari, Raghunathpur, Sindurpur and Agarpur are768◦, 760◦, 746◦ and 891 ± 40◦C, respectively.The crystallisation temperatures of biotites of thepresent study have been obtained from the cali-bration of Henry et al. (2005), where Ti-contentof biotite was used and showed a systematic vari-ation from Sarpahari (734◦–767◦C), Ragunathpur(729◦–760◦C), Sindurpur (677◦–775◦C) to Agarpur(661◦C).
The four plutons of the present study havebeen equilibrated at the oxygen fugacities of cal-cic amphiboles (log fO2) between −11.8 and −13.0bar (using calibration of Ridolfi et al. 2010). Theassemblage K-feldspar–biotite–magnetite–quartz–sphene displays consistent oxygen fugacity values(log fO2 = −11.2 to −15.6 bar) (using calibrationof Wones 1989) typical for the calc-alkaline magmacrystallisation.
The water content of the melt (H2Omelt) rangesfrom 5.0 to 6.5% for the four plutons.
The high oxygen fugacity (higher than Ni–NiObuffer) and high water content of the parental
J. Earth Syst. Sci. (2018) 127:120 Page 31 of 33 120
granitoid magma are consistent with theiremplacement in a subduction zone.
Acknowledgements
Major research project grant of the UGC [F.-43-367/2014(SR)] and research grant of the Universityof Calcutta given to B Goswami are gratefullyacknowledged. Thanks are due to Prof. Chala-pathy Rao and Dr Dinesh Pandit (Departmentof Geology, Benaras Hindu University) and DrS Nandy, Sri S K Tripathy and Sri Narahari(EPMA Laboratory, CHQ, GSI, Kolkata) for pro-viding electron microprobe facilities. Painstak-ing reviews by three anonymous reviewers andtheir most valuable suggestions helped to improvethis paper to a great extent. Supportive editorialhandling by Prof. Rajesh Kumar Srivastava andProf. Chalapathy Rao (Chief editor) is gratefullyacknowledged.
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Corresponding editor: N V Chalapathi Rao
