Crystallization history of a massif anorthosite in the eastern Indian shield margin based on borehole lithology

Journal of Asian Earth Sciences 25 (2005) 77–94 www.elsevier.com/locate/jaes

Crystallization history of a massif anorthosite in the eastern Indian shield margin based on borehole lithology Dipankar Mukherjeea,1, Naresh C. Ghosea, Nilanjan Chatterjeeb,* b

a Department of Geology, Patna University, Patna 800 005, India Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Received 20 June 2003; revised 23 January 2004; accepted 23 January 2004

Abstract The Bengal anorthosite occurs as a large tadpole-shaped pluton (250 km2) in the granulite facies terrain of the Proterozoic Chotanagpur Gneiss Granulite Complex at the northeastern edge of the Indian peninsular shield. Its axis of elongation conforms to the general strike (ENE –WSW) of the country rocks. It is bounded by a (Gondwana) basin margin fault in the north and it shows an interfingering contact with the country rocks at its eastern and western edges. Deep drilling, attaining a maximum depth of 622.85 m, reveals a cyclic order of grey, white and mottled anorthosites of variable thickness. The possibility of anorthosite extending further below contradicts the 200 m thickness of anorthosite previously estimated from gravity modeling. Chemical data also indicate a cyclic variation of elemental concentrations and their ratios with depth. In each chemical cycle, the grey plagioclase megacrysts apparently floated over a relatively denser white granular plagioclase with higher anorthite contents. The base of a chemical cycle also contains higher concentrations of transition elements—a feature arising plausibly by sinking of Fe –Ti oxides. The chemical cyclicity possibly indicates derivation of melt in batches and emplacement of the crystal laden-melt by impulses. Minor presence of orthopyroxene in the anorthosite suggests a tholeiitic source. q 2004 Elsevier Ltd. All rights reserved. Keywords: Anorthosite; Granulite; Eastern India; Borehole

1. Introduction One of the major processes of the growth of continental crust in the Proterozoic era is the emplacement of large anorthosite massifs in shield areas. An understanding of geological and geochemical characteristics of massif anorthosite emplacement is, therefore, one of fundamental importance in tracing the evolutionary history of a continental shield. The Chotanagpur granite-gneiss complex (CGGC) forms part of a Proterozoic mobile belt lying to the north of the Singhbhum craton (. 3.2 Ga, Saha, 1994) at the Eastern Indian shield margin. The CGGC has been subjected to three distinct episodes of anorthositic magmatism ranging in age from Late Paleoproterozoic to the Early * Corresponding author. E-mail addresses: [email protected] (N. Chatterjee), dmukherjeegsi@ rediffmail.com (D. Mukherjee). 1 Present address: Geological Survey of India, Kankarbag, Patna, India. 1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2004.01.012

Mesoproterozoic period (Mukherjee and Ghose, 1992) in association with metamorphosed layered mafic – ultramafic sequence, as a product of differentiated gabbro, and massif anorthosite intrusion in amphibolite-granulite country rocks. Most of these anorthosite occurrences in the CGGC spread across the eastern Indian states of Bihar, Jharkhand and West Bengal (Ghose and Mukherjee, 2000) (Fig. 1). The anorthosite occurring near Saltora, known as the ’Bengal Anorthosite’ (Chatterjee, 1937), is the largest body of massif anorthosite (250 km2) at the eastern Indian shield margin. Modeling of gravity data indicated a 200 m thick anorthosite body over a dense 3.3 –5.5 km thick mafic body (Verma et al., 1975). The present study, based on drill core samples from depths up to 622.85 m from the surface, indicates that the anorthosite body continues to greater depths. This investigation aims at establishing the subsurface mineralogical and chemical changes in the borehole profiles of the Bengal anorthosite massif to ascertain its crystallization history.

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Fig. 1. A simplified geological map of the Bengal anorthosite (modified after Bhattacharyya and Mukherjee, 1987) showing location of the transect lines (TR-1, TR-2 and TR-3) and site of the boreholes (X, Y and Z). The geological maps of the three transect lines are shown by arrows. Inset maps (top) (A) Major geological units of the Indian shield and the position of the Chotanagpur Gneiss Granulite Complex (CGGC), Proterozoic basins (inclined bars), Gondwana basins (stippled area: G, Godavari basin; M, Mahanadi basin; D, Damodar graben), Deccan basalt (V), EGMB: Eastern Ghats Mobile Belt, SC: Singhbhum craton, and solid circle: anorthosite occurrences in EGMB and CGGC (Bengal anorthosite). Inset (B) The mobile belt of the CGGC showing location of the Bengal anorthosite at Saltora and other anorthosite occurrences in this belt (solid circle), Gondwana basins along Damodar graben-(stippled), Rajmahal volcanics (V), and Younger Proterozoic basins in the north (inclined bars). The Singhbhum craton along with Dalma volcanics have been demarcated in the south. The area in map C is enclosed in a rectangle.

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2. Previous work Since the early recognition of anorthosite near Saltora in erstwhile Bengal by Holland in 1890, geologists have established it as a large pluton with a massive core surrounded by a banded variety at the margin. The first geological map of the massif was published by Roy (1968, 1977). The pluton shows an intrusive relationship with gneissic country rocks of granulite facies (Chatterjee, 1929, 1937, 1959) and a polymetamorphic character (Sen, 1954). The anorthosite suite shows a wide variation in texture and mineralogy that can be classified into (i) white to greyish-white anorthosite, (ii) dark grey anorthosite, and (iii) mottled anorthosite ranging from anorthosite sensustricto to gabbroic anorthosite (rarely anorthositic gabbro) (Chatterjee, 1959; Roy, 1977; Mukherjee, 2001). The near-peak metamorphic conditions achieved in the calc-gneissic country rocks have been estimated as 7.3 – 7.6 kb and 820– 840 8C (Sen and Bhattacharya, 1993), whereas, the retrograde equilibrium conditions of the anorthosite and associated country rocks have been calculated as 680 ^ 35 8C at 6.0 ^ 1 kb (Sen and Bhattacharya, 1985), and 650 8C at 6.25 ^ 0.5 kb (Bhattacharyya and Mukherjee, 1987), indicating granulite facies metamorphism. Coexisting oxide minerals (ferrian ilmenite – ilmenomagnetite –spinel) occurring as ore pockets in basic granulite indicate a temperature of 630 ^ 50 8C and oxygen fugacity close to 2 17.3 atm (Bose and Roy, 1966). 2.1. Structure and tectonics Previous workers assigned a tectono-magmatic status to the Bengal Anorthosite on the basis of the relationship between the regional structure and flow patterns within the massif, which they interpreted as primary (Roy, 1977; Kumar et al., 1984; Bhattacharyya and Mukherjee, 1987; Mukherjee, 1995; Dastidar et al., 1997). Structural imprints on the anorthosite and associated rocks suggest three phases of deformational episodes. The first episode (D1) is marked by tightly appressed or isoclinal folds (F1) with concomitant development of E –W trending strong regional axial plane schistosity (S1) in conformity with the Satpura orogenic trend of central India. The second deformational episode of regional scale is also manifested by isoclinal folds (F2) on S0 and S1 planes with variable plunge either toward the west or east with E – W axial traces. Broad warps along N – S axial traces represent the third deformational episode (D3). Statistical analysis of structural data near Nandanpur indicates that large plagioclase plates and gneissic xenoliths within the anorthosite massif are strongly oriented in an ESE – WNW direction (strength parameter, C ¼ 1:34)—a feature interpreted as the direction of laminar flow in the intruding pluton near Nandanpur (Mukherjee, 1995). The parallelism of the plagioclase plates/xenoliths and the regional axial trace of F2-fold in different parts of the massif suggests that the stress field active along regional

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F2-axial trace controlled the flow kinematics, and the massif was intruded syn- to post-tectonically with respect to the F2-folding (Mukherjee, 1993a). Further, because of the juxtaposition of the massif with the E –W trending Damodar graben in the north, and the close proximity of its eastern fringe to the NNE –SSW trending marginal fault of the Bengal basin, a regional tectonic control in the emplacement of the Bengal anorthosite massif was suggested (Mukherjee, 1993a). The domal structure defined by the oval-shaped anorthosite with its axis of elongation parallel to regional E – W foliation possibly indicates simultaneous deformation of both the intruded pluton and the host country rock. The anorthosite massif shows interfingering pattern with the country rocks at its eastern and western edges.

3. Geological setting The Bengal Anorthosite massif is an E– W trending 40 km long elongated body covering an area of about 250 km2, a central width of 8 km and with a long narrow tail at the western end (Fig. 1). In addition to the main massif, there is another regionally parallel small isolated anorthosite outcrop, 2.5 km south of the south-central margin of the main massif (Roy, 1977). Apophyses and interlayering of anorthosite with the host gneisses, and inclusions/rafts of the country rocks (gneisses, metabasics and metasediments) within the massif are ubiquitous. The eastward extension of the pluton abuts against Gondwana trough sediments of the Damodar basin (Damodar graben), and westward the outcrop thins out gradually. The gneissic country rock has been intruded by widespread mafic (basic granulite and amphibolite) and ultramafic (hornblendite) rocks and granites (Mukherjee and Ghose, 1999; Ghose and Mukherjee, 2000). A large number of E – W trending sub-parallel discontinuous bands of metabasic rocks consisting of amphibolites and basic granulites and ranging in thickness from less than a millimeter to tens of meters, occur both within the anorthosite massif as enclaves, as well as in the surrounding country rocks (Roy, 1977). These rocks exhibit similar mineral assemblages both inside and outside the anorthosite massif (Manna and Sen, 1974). On the basis of field relationships, major oxide chemistry and partial trace element data, a non-comagmatic relationship between anorthosite and associated metabasic rocks has been suggested (Roy and Saha, 1975; Bhattacharyya, 1984; Mukherjee, 1993b). The presence of high-grade metamorphic rocks (charnockite, granulite, leptynite, khondalite, migmatite, metabasic rocks and quartzofeldspathic gneisses) together with massif anorthosite and peralkaline rocks in the CGGC (Ghose, 1983, 1992), suggests similarity of geological environment with that of the Eastern Ghats mobile belt in the south (Fig. 1). The Bengal anorthosite massif is composed of a core of coarse grey anorthosite surrounded by medium-grained equigranular white anorthosite. In addition to the detached

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Table 1 Stratigraphic relationship of the Bengal anorthosites and associated rocks of the eastern part of Chotanagpur gneiss-granulite Complex (CGGC) (cf. Ghose and Mukherjee, 2000) Age

Lithology

Recent Alluvium Lower Tertiary (65 ^ 1 Ma) Dolerite Permo-Carboniferous Sandstone with shale partings and (Lower Gondwana) coal seams ------------------------------------Unconformity------------------------------------Mesoproterozoic Pegmatite, aplite and quartz vein Alkali granite Syenite Palaeo-Mesoproterozoic Porphyritic biotite granite (1700– 1400 Ma) Bengal anorthosite massif Metabasic rocks: basic granulite and amphibolite, and Meta-ultramafic rocks: hornblendite Palaeoproterozoic (.1700 Ma) Enclaves of supracrustals viz. metapelites (mica schist, sillimanite schists and khondalite), meta-arenite (quartzite and quartz-magnetite rock) and calc-silicate gneiss, in remobilised basement gneisses consisting of Quartzofeldspathic gneiss, Migmatite, Augen gneiss, Biotite/hornblende gneiss, and Leptynite (unclassified)

outcrops of the grey anorthosite core, there are other isolated pods of grey anorthosite within the massif (Fig. 1). A transitional zone of mottled anorthosite is developed at the interface of the grey and white anorthosites with a gradational contact. The white anorthosite exhibits a conspicuous banded fabric near the metabasic enclaves due to the presence of streaks of mafic minerals. At the surface outcrop, the white, mottled and banded anorthosites form a single mappable unit on the regional scale (Fig. 1). A coarse gabbroic anorthosite is encountered as a very small isolated pocket within the white and mottled anorthosites. Development of a hydrated phase at the contact zone with the country rocks as well as within the metabasic enclaves of the anorthosite massif is noted by the development of abundant biotite. 3.1. Age relationships The major granitic activity in the CGGC has been dated between 1741 ^ 102 Ma at Rajhara and Daltonganj in the western part (Ray Barman and Bishui, 1994; Ray Barman et al., 1990) and 1590 ^ 30 Ma in the mica belt of the northern part (Pandey et al., 1986) by the Rb –Sr isochron method. Minor intrusions of syenite and alkali granite in the massif anorthosite suggest that the emplacement of anorthosite preceded the granitic activity. The metabasic rocks, however, preceded the massif anorthosite emplacement as well as the major granitic activity as evident from

the occurrence of metabasic enclaves within the anorthosite massif and the granites. Indeed, the foliation within the E – W trending metabasic bands inside the massif shows a marked parallelism with the S1-regional foliation of the enveloping country rocks, an observation consistent with the older age of the metabasic bands compared to the anorthosite massif. A younger tholeiitic dolerite dike known as Salma dike, dated at 65 ^ 1 Ma by the 40Ar/39Ar method (Kent et al., 2002), cuts across the entire lithological sequence and extends beyond to the Gondwana sediments of Raniganj coal field in the north. The stratigraphic relationship between the Bengal anorthosite and associated country rocks and their approximate ages are given in Table 1.

4. Geophysical signature The Bengal anorthosite massif shows a positive gravity anomaly with an amplitude of 20 mGals over a width of 25 km in the eastern part, decreasing to 4– 5 mGals in the west (Verma et al., 1975; Mukhopadhyay, 1987). This gravity high was explained by invoking the presence of a denser, 3.3 – 5.5 km thick gabbroic layer underlying the anorthosite layer of about 200 m thickness (Verma et al., 1975). The shape of the causative body was interpretated as a tilted laccolith, the northern and eastern sides of which are characterized by a steep slope against the Damodar graben, while the southern side has a gentle slope. The E– W trending gravity anomaly runs parallel to the strike of the anorthosite massif. The continuation of the anomaly zone westward in the basement gneisses beyond the outcrop limit of Bengal anorthosite, suggests that the causative body has a larger extension than the exposed outcrop. The thickness of the mafic body underneath the exposed gneissic country rock toward west possibly reduces to a tabular form (Mukhopadhyay, 1987). The conclusions drawn on subsurface configuration of the magmatic body on the basis of gravity study are questionable, since all the modeling was carried out using generalized rock densities without using wavelength filtering techniques for the gravity data (Morse, 1982). The importance of wavelength filtering of Bouguer anomaly data was demonstrated in the Adirondacks, where gravity highs up to 10 mGals were obtained after filtering (Simpson et al., 1981; Kerr, 1982) indicating the existence of dense mafic bodies in the lower crust. The original gravity data without wavelength filtering failed to detect the dense bodies at depth, and led to a model of thin flat sheet structure for the anorthosites (Simmons, 1964). Gamma ray well logging of the Bengal anorthosite massif indicates a subtle increase in density with depth (Choudhury, 1989). Based on 2D modeling from density logs of boreholes within the anorthosite massif, Choudhury (1989) has suggested a wedge-shaped body with a maximum thickness of 2.32 km in the east with the thickness reducing to 1.55 km in the central part and 1.48 km in the west.

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5. Petrography

5.1. Grey anorthosite

The Bengal anorthosite is characterized by a bimodal granularity through the occurrence of cumulates of subhedral to euhedral plagioclase megacrysts in grey anorthosite, and an equidimensional mosaic of medium to fine granular plagioclase in white anorthosite (Fig. 2A –D). A combination of the two types of plagioclase has given rise to the mottled variety, whereas, the presence of mafic schlierens/inclusions in white anorthosite has developed banded anorthosite in contact with the metabasic rocks. Minor occurrence of gabbroic anorthosite represents a distinctive mineralogical variant. In order to study petrographic relations, a few mineral compositions were determined by wavelength dispersive spectrometry on a JEOL JXA-733 Superprobe at MIT, Cambridge, USA. Salient features of textural and mineralogical variation of the anorthosite suite as observed in the outcrop and subsurface samples are given below.

The very coarse to coarse-grained grey anorthosite is dominantly composed of large tabular megacrysts of dark grey to bluish labradorite (An54 – 63, detemined by electron microprobe analysis) commonly ranging from 1 to 15 cm in length, with subordinate amounts (, 8% by volume) of clinopyroxene (Mg# or Mg/[Mg þ Fe] ¼ 0.62) and hornblende (Mg# ¼ 0.36). Minor amounts of biotite, Fe– Ti oxides and occasionally hypersthene and garnet have also been recorded in some samples. Large plates of poikilitic plagioclase sometimes contain early-formed subhedral crystals of twinned plagioclase, Fe –Ti oxides, amphibole, epidote, and fine inclusions of iron oxide and mica in the form of rods or laths (Fig. 2D). The megacrysts are weakly zoned, sometimes showing reverse zoning. The plagioclase plates are randomly oriented except in places, where they are aligned. In the central part, the rock is coarse (pegmatitic) showing a panidiomorphic texture. The grain

Fig. 2. Cross-polarized light (A,C) and backscattered electron (B,D) images of the anorthosites showing textural relations. (A) Large twinned plagioclase megacryst surrounded by small granular plagioclase showing protoclastic (mortar) texture. (B) Small granular plagioclase (g-plag) showing saccharoidal texture and weak zoning that can be deciphered from the different shades in the crystals—lighter shades have higher An-content; small intergranular, recrystallized albitic plagioclase (dark shade, a-plag) and ilmenite (Ilm) are also present. (C) Large irregular clinopyroxene (cpx) and megacrystic plagioclase (m-plag) surrounded by small granular plagioclase and minor mafic phases. (D) Slightly higher magnification image of the area in C; the plagioclase megacryst on the left (m-plag) shows fine inclusions of mica and iron oxide; the megacryst on the right of the cpx (near top) shows weak reverse zoning; secondary actinolitic amphibole (amph) rims around the cpx.

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size grades outward, becoming finer with increasing abundance of white granular plagioclase that results in a porphyritic texture. The plagioclase megacrysts often show kinked twin lamellae, microfracture and granulation of crystal margin indicating post-crystalline deformation. Localized clots of clinopyroxene – hornblende –epidote –magnetite or layers of garnet – orthopyroxene – Fe – Ti oxides within the grey anorthosite, are noteworthy features possibly related to metamorphism. Occasional presence of small anhedral granular plagioclase shows two distinct modes of occurrence: one bordering the large plagioclase plates and the other intergranular between euhedral plagioclase laths. The former appears to be the result of post-crystalline peripheral granulation, while the latter may represent intercumulus or recrystallized plagioclase with higher albite content possibly due to metamorphism (as in Fig. 2B). Exsolution lamellae of orthopyroxene occur within augite, which also contains aligned opaque granules across the exsolution blebs. Large euhedral to subhedral crystals of magnetite and ilmeno-magnetite are not uncommon. In addition, sporadic occurrence of pyrite, arsenopyrite and chalcopyrite has also been recorded in grey anorthosite. Actinolitic amphibole occurs as alteration rims around clinopyroxenes (Fig. 2D). At places biotite occurs as broad dark brown patches or elongated streaks as an alteration product of hornblende or clinopyroxene due to late hydration. Small biotite stringers show alteration to greenish chlorite with release of iron to form magnetite. 5.2. White anorthosite The medium-grained, milky white to pale greyish white, equigranular white anorthosite shows a saccharoidal texture (Fig. 2B), and is mainly composed of labradorite (An51 – 73) with subordinate amounts of hornblende (Mg# ¼ 0.43 – 0.44), biotite (Mg# ¼ 0.43 – 0.46), garnet (Gr18Pyr13Alm65) and Fe – Ti oxides in irregular clusters. Minor amounts of clinopyroxene, orthopyroxene, chlorite, epidote, magnetite, and occasionally scapolite have also been recorded. Accessory minerals include apatite, zircon and sphene. The plagioclase has a wider range of composition and higher maximum anorthite content than the plagioclase of grey anorthosite. The generally untwinned nature of plagioclase in white anorthosite is in marked contrast with the ubiquitous twinned megacrysts of grey anorthosite (as in Fig. 2A and C). The twinning in white anorthosite, when present, is of the albite or pericline types. The plagioclase crystals are arranged in a polygonal mosaic with characteristic 1208 sharp boundaries giving rise to ‘triple-point’ contacts. Both normally and reversely zoned plagioclase grains are present (Fig. 2B), but the reverse zoned variety is less abundant. The mafic minerals are present as irregular clusters in a plagioclase matrix. There are two distinct modes of occurrence of amphiboles: (a) rarely occurring stout

prismatic euhedral to subhedral grains of green amphibole, and (b) irregular-shaped deep green to brown amphibole, strongly pleochroic and commonly occurring in intergranular space. The latter alters to pale yellowish green chlorite or brown biotite. Presence of kink band, distortion or discontinuous twin lamellae are imprints of post-crystalline deformation. Symplectitic growth of garnet with clinopyroxene signifies the former to have developed as a reaction product between clinopyroxene and plagioclase—an imprint of granulite facies metamorphism. Development of garnet is thus a late overprint due to prograde decomposition of clinopyroxene and/or hornblende, in the presence of plagioclase. Garnet is also found as corona around plagioclase with an outer rim composed of opaque and amphibole (Mukherjee, 2001), or around ilmenite, whose outer rims consist of amphibole succeeded by plagioclase and garnet. Such corona formation probably reflects a post-consolidation phase of equilibration of white anorthosite. Replacement textures like actinolitic amphibole after clinopyroxene (Fig. 2D), biotite after hornblende and scapolite after plagioclase are either a result of cooling or late hydrothermal metasomatism. The white anorthosite is susceptible to weathering due to decomposition of feldspar into white clay (kaolinite) at lower elevations. 5.3. Mottled anorthosite The mottled anorthosite represents a transition between grey and white anorthosites. It is characterized by a wide variation in the ratio of tabular megacrystic plagioclase (as in grey anorthosite) to small granular plagioclase (as in white anorthosite) that occurs between the megacrysts. Peripheral granulation of the megacrysts has locally given rise to protoclastic (mortar) texture (Fig. 2A). The small granular plagioclases of the mottled anorthosite (An52 – 68) are similar in composition to the plagioclase of the white anorthosite. On the other hand, the large megacrystic plagioclases (An53 – 63) are similar in composition to the plagioclase of the grey anorthosite. The euhedral shape of the plagioclase megacrysts and inclusions of subhedral, twinned plagioclase in the megacrysts, as seen in grey anorthosite, also occur in the mottled anorthosite. Large irregular clinopyroxenes occur occasionally, surrounded by the plagioclase matrix (Fig. 2C and D). Apatite, zircon, sphene and allanite are present as accessory minerals. The compositional and textural similarities of plagioclase described above along with features such as veins of white granular plagioclase in grey anorthosite observed in the field, provide evidence in favor of formation of at least two generations of plagioclase arising from two different magmas. Reverse zoned large tabular plagioclase and normal zoned small granular plagioclase (Fig. 2B and D) may be either due to interaction between the two magmas, or, incomplete metamorphic equilibrium.

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5.4. Anorthosite dike A single occurrence of a white anorthosite dike shows more or less similar mineralogy to that of white anorthosite of the main massif. However, the profuse development of epidote and sphene with little sericite makes it distinctive from the prototype. The dike is fine grained and is characterized by the absences of phenocryst and compositional zoning in plagioclase. The hydrous phases are apparently products of later recrystallization.

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an average of 4.7 kb were calculated using Al-in-hornblende barometry (Anderson and Smith, 1995; Ghose et al., 2003). Whole rock analyses of the metabasic rocks show these to be essentially tholeiitic in composition, and preserve evidence of low-pressure fractional crytallization involving olivine, plagioclase and clinopyroxene (Ghose et al., 2003). The ultramafic rock is nepheline-normative and contains hornblende, clinopyroxene and scapolite.

6. Subsurface studies 5.5. Gabbroic anorthosite The gabbroic anorthosite is a coarse grained, massive, dark brownish-grey rock, and consists dominantly of clinopyroxene and plagioclase with minor amount of orthopyroxene, hornblende, garnet and Fe – Ti oxides. Apatite, zircon and allanite have been observed as accessory phases. The plagioclase is labradoritic in composition (An43 – 52). Amphibole occurs in two distinct modes: (a) euhedral to subhedral prismatic crystals of hornblende, and (b) irregular-shaped marginally developed secondary grains around clinopyroxene. Both biotite and chlorite are found as alteration products of clinopyroxene and hornblende. 5.6. Metamorphosed mafic – ultramafic rocks As discussed earlier the anorthosite massif is associated with numerous occurrences of metabasic rocks, which are present mostly as concordant bodies throughout the CGGC. At Saltora, these metabasic rocks are represented by amphibolites (plagioclase þ hornblende ^ ilmenite ^ quartz) and basic granulites (plagioclase þ clinopyroxene þ orthopyroxene ^ hornblende ^ garnet ^ ilmenite ^ quartz), and occur both in the country rocks and as roughly E– W bands within the massif. Xenoliths of these rocks also occur within the massif. Mineralogical investigation of these metabasic rocks, together with a meta-ultramafic rock (hornblendite), reveals that the compositions of hornblende range from hornblende to pargasitic hornblende, and the plagioclase composition ranges between An40 and An59 (Ghose et al., 2003). Positive correlation of Mg# between clinopyroxene and hornblende, and clinopyroxene and orthopyroxene, indicates that the Fe –Mg exchange KD remained constant between the ferromagnesian minerals reflecting the conditions of the last metamorphic equilibrium. Positive correlation between Mg# of hornblende and An-content of plagioclase in the mafic – ultramafic rocks also indicates that these rocks attained equilibrium conditions of metamorphism (Ghose et al., 2003). The post-peak metamorphic temperatures determined from coexisting ortho- and clinopyroxenes (Lindsley, 1983), and hornblende-plagioclase equilibrium (Holland and Blundy, 1994) are 634 – 781 8C, and pressures between 3.8 and 5.4 kb with

6.1. Geology of drill sites Drilling in the Bengal anorthosite massif, undertaken by the Geological Survey of India in 1987– 1989, reached a depth of 622.85 m from the surface level of 159.56 m above mean sea level (Mukherjee, 1993b). The drilling was performed to check the possible existence of a mafic body underlying the massif at shallow depth deduced from the gravity data (Verma et al., 1975). The drilling sites were selected after critical assessment of geological and geophysical data, large-scale geological mapping and interrelationship of the anorthosite massif with the country rocks. The drill core data have been utilized to evaluate the thickness of mineralogical variants, and changes in mineralogy and chemistry of the anorthosite massif in the subsurface. The results of the investigation are summarized below. (a) Eastern borehole. A transect-map (TR-1, Fig. 1) in the eastern sector across the Bengal anorthosite reveals the juxtaposition of the anorthosite massif with Gondwana sediments. A detailed geological map of the area around Nandanpur-Sitadangal (1.11 km2, on 1:2000 scale, Mukherjee, 1993b) shows that the grey anorthosite core of the massif contains large pockets of quartzofeldspathic gneiss and a few metabasic rocks as xenoliths within the white anorthosite. The structures in anorthosite defined by orientation of plagioclase megacrysts are consistent with the deformation pattern. A vertical borehole (X) was sunk through the grey anorthosite core near Nandanpur (Fig. 1), close to the regional axial trace of the F2 fold. This borehole reached a depth of 568.40 m from the surface level of 95.79 m above msl (mean sea level), i.e., down to a level of 472.61 m below mean sea level (msl). (b) Western borehole. The transect-map across the exposed westerly tapering end of the Bengal anorthosite (TR-2, Fig. 1) reveals the presence of a large number of concordant metabasic bands with pinch-and-swell structures (Manna and Sen, 1974) within the anorthosite as well as in the country rock. The anorthosite massif is flanked on the northern and southern sides by gneissic country rock. A detailed geological map of northeast of Ledapalash in the central part of this transect (0.21 km2, on 1:2000 scale, Mukherjee, 1993b) reveals that the anorthosite contains xenoliths of gneiss and metabasic rocks showing concordant

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relationships with the regional S2 foliation. Banded anorthosite has developed within the white anorthosite in proximity to the metabasic bands, and the white anorthosite has induced ptygmatic folding within the metabasic rocks. Small pockets (4.5 m £ 15 m) of coarse gabbroic anorthosite occur within the white anorthosite. Based on the above observations a vertical borehole (Y) was sunk through the coarse gabbroic anorthosite (Fig. 1) reaching a depth of 622.85 m from the surface level of 159.56 m above msl, i.e., down to a level of 463.29 m below msl. (c) South-central Borehole. A transect across the central part of the Bengal anorthosite massif (TR-3, Fig. 1) reveals a relatively sharp contact between the white anorthosite and the grey anorthosite core that thins out westward, and the presence of gneissic xenoliths within the white anorthosite. A detailed geological map of the area of south of Katabaid (0.17 km2, on 1:2000 scale, Mukherjee, 1993b), however, reveals absence of a discernible contact between grey and white anorthosite where mottled anorthosite has developed locally, and gneissic xenoliths are present both in the grey and the white anorthosites. Metabasic xenoliths are restricted to the white anorthosite and the enveloping gneissic country rock. The frequency of occurrence of the metabasic xenoliths increases near the southern margin of the massif near Kushthalia (Fig. 1). The white anorthosite is traversed by pegmatite, aplite and quartz veins. Considering the lithological disposition of anorthosite and the country rocks (gneisses and migmatite), a southerly, 458 inclined borehole was sunk (Z, Fig. 1) to cut across the contact between grey and white anorthosite and the gneissic raft within the grey anorthosite. This borehole reached an inclined distance of 297.7 m from the surface level of 108.86 m above msl. This corresponds to a maximum depth of 136 m from the surface or 103.13 m above msl. 6.2. Borehole lithology Lithologs of each borehole (details in Appendix 1 to 3; Mukherjee, 1995, available on request) have been diagrammatically represented by constructing borehole sections on a 1:500 scale (Figs. 3 –5). The thinner units are shown on a 1:100 scale. The salient features of these three borehole sections are given below. (i)

(ii)

The relative thickness of grey, white, mottled and gabbroic anorthosites is variable and is repeated in depth. A combination of grey and white plagioclases has led to the formation of zones, pockets or bands of mottled anorthosite at the interface between the grey and the white anorthosites as may be seen in the upper part of the eastern borehole up to a depth of 34.64 m with intervening bands of metabasic rocks (Fig. 3). The sequence is repeated up to 568.40 m depth attained in this borehole. The gabbroic anorthosite encountered in the western borehole (Fig. 4), at depths of 2.90 – 25.85,

33.50 – 34.24, 302.94 – 311.37, 446.05 – 447.26 and 456.22 – 461.77 m, corresponding to 22.90, 0.71, 8.43, 1.21 and 5.55 m thickness, respectively, overlie either mottled or white anorthosite. The gabbroic anorthosites occasionally contain xenoliths of metabasic rocks rich in biotite. (iii) The anorthosite suite is represented entirely by white anorthosite below 466.71 m depth, reaching to 622.85 m in the western borehole with occasional presence of metabasic xenoliths and rarely of plagioclase megacrysts (Fig. 3). The absence of any gabbroic, grey or mottled anorthosite towards the basal part of magma column of over 156 m is noteworthy. A similar feature is observed in the southcentral borehole (Fig. 5). (iv) The grey anorthosite is one of the dominant members present intermittently to a depth of 568.40 m in the eastern borehole (Fig. 3). However, it constitutes the uppermost part of the 107.14 m interlayer with mottled and white anorthosites in the southcentral borehole, and thins out in the western borehole, indicating its limited lateral extent. (v) Correlation of the three lithologs indicates that in spite of repetition of the two principal mineralogical variants, the coarse grained grey anorthosite (color index, M , 8) most likely is limited to the upper part of the pluton, and is succeeded by medium grained, granular, white anorthosite ðM $ 8Þ at greater depths. (vi) The metabasic xenoliths of variable thicknesses within the anorthosite have sometimes developed banded anorthosite or banded metabasic rock in the upper part as a result of an interfingering relationship, and migmatized metabasic rock due to the presence of apophyses of anorthosite at deeper levels (Fig. 4). The metabasic rocks are often rich in biotite. (vii) Xenoliths of metabasic rocks dominate over quartzofeldspathic gneisses. (viii) Minor amounts of intrusive rocks like alkali granite, pink syenite, pegmatite and aplite are encountered along with veins of quartz and calcite. (ix) Xenolithic layers or clots rich in muscovite, occur up to a depth of 55 m in the eastern borehole, possibly due to assimilation of basement gneisses or Al-rich mica schist near the top of the pluton. Garnet persists along the entire length of 568 m depth achieved. (x) Megacrysts of amphiboles coexisting with clinopyroxene, garnet and opaques in minor amounts, occur in white anorthosite between depths of 509 and 605 m in the western borehole. They become less abundant at 622.85 m, the deepest depth attained by drilling. (xi) Alteration of pyroxene to amphibole, amphibole to biotite, and feldspar to epidote/sercite in anorthosite are commonly observed in proximity of country rocks. (xii) Ore minerals are represented by magnetite, ilmenite, pyrite, and rarely chalcopyrite and arsenopyrite as accessory constituents. The sulphides occur between

D. Mukherjee et al. / Journal of Asian Earth Sciences 25 (2005) 77–94

Fig. 3. Lithological section along eastern borehole (X) near Nandanpur. 85

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Fig. 4. Lithological section along western borehole (Y), northeast of Ledapalash. Key as in Fig. 3.

depths of 311 and 567 m, but arsenopyrite is absent below 439 m in the eastern borehole. The subsurface disposition of mineralogical variants of anorthosite and their repetitive occurrence (Figs. 3– 5),

possibly reflect derivation of anorthositic melt and subsequent introduction of the crystal-laden melt into the magma chamber in batches by impulses prior to crystallization. It is further evident that the lighter grey plagioclase-laden mush (i.e. grey anorthosite) has been

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87

Fig. 5. Lithological section along south-central borehole (Z) at Kusthalia. Key as in Fig. 3.

gradually replaced by a relatively denser mush (i.e. white anorthosite) at deeper levels of the pluton. Interaction with the country rocks, hydrothermal alteration, postcrystalline deformation and metamorphism are commonly observed.

6.3. Borehole chemistry Eleven representative samples of anorthosite from the deepest borehole (Y) of the massif (NNE of Ledapalash) were chemically analyzed both for major and trace elements

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by XRF on pressed powder pellets using the wavelengthdispersive XRF technique at the Department of Geology, Leicester University, UK. FeO was determined by titration following digestion of sample in a mixture of analytical grade HF and concentrated H2SO4 in a platinum crucible on a hot plate. Samples were powdered by passing through a tungsten-carbide crusher and subsequent grinding in an agate mortar. Several natural standards, MRLG-1, JP-1, W1, BOB-1, NIM-G and JA-1 for major elements, and MRG1, JG-3, JP-1, BR, W-2 and W-1 for trace elements were also analyzed simultaneously to control the precision and accuracy of analytical data. Details of the precision of data presented are given in Table 7.1 of Mukherjee (1995). The possibility of contamination by Co in WC crusher can not be ruled out. The chemical data of anorthosites reveal a cyclic variation in major and trace element contents, and trace element ratios with depth (Table 2, Fig. 6). The depth profiles of plagioclase-compatible trace elements, K2O, Ba and Rb are very similar. Sr, however, is different and is somewhat similar to Al2O3. The depth profiles of spinelcompatible elements, TiO2, Co and Nb are also very similar. In these rocks, except in the gabbroic variety, MgO, FeO, Ni and V are compatible only in ilmenites, magnetites and other Fe– Ti oxides. Hence, these elements also show profiles similar to TiO2, Co and Nb. The depth profiles of Zr, Ce, Y and P2O5, which are preferentially incorporated in accessory phases such as zircon, allanite and apatite, are also very similar. Such variations reflect the modal content of accessory minerals in different parts of the massif column. The depth profiles of the incompatible element ratios (Fig. 6T – W) possibly reflect the variation in modal proportions of phases along the depth of the column. For example, K2O/Nb reflects the distribution of Fe – Ti oxides containing Nb with respect to plagioclase that contains K2O. At the 294 m level, which may be regarded as the base of a chemically-defined cyclic zone, the low value of K2O/ Nb (Fig. 6T) is possibly related to Fe– Ti oxide accumulation at the base of the zone (Fig. 4). The depth profiles of Zr/Nb, La/Nb and Zr/Y probably reflect the depthdistributions of accessory minerals such as zircon relative to Fe – Ti oxides or allanite. Zircon has indeed been observed both from gabbroic anorthosite at 18 m depth and from mottled anorthosite at 581 m depth. Zr content and Zr/Y are high at these depths (Fig. 6P and W). Considering that basement gneiss assimilation is significant only in the upper part of the massif (e.g. the top 55 m in the eastern borehole, see borehole lithology), and wall rock interaction and metasomatism mainly affected the margins, it is reasonable to assume that the cyclicity of the element concentrations is due to emplacement of magma in batches by impulses. Generally, speaking, in each chemical cycle, the trace element concentrations seem to decrease upward until there is a sharp increase at the beginning of the layer above, possibly related to the arrival of a fresh batch of magma containing crystal mush. The sharp increases,

however, do not always correspond with the change in rock-type (e.g. from white to mottled variety near the base, and from mottled to white variety in the middle part of the column, Fig. 6) in the lower to middle part of the column. Neither is the normative plagioclase-An% composition constant within a single cycle (Fig. 6A), a feature supported by the presence of both grey and white anorthosites within each cycle. The elemental variations in the upper part of the column at the grey to gabbroic anorthosite transition are, however, consistent with the change in rock-type. For example, the trace element concentrations except for Sr, increase sharply at the base of the gabbroic anorthosite. On the other hand, the normative plagioclase-An%, CaO, Al2O3 and Sr decrease, indicating a lower amount of plagioclase and a higher amount of clinopyroxenes in the gabbroic anorthosite. The gabbroic anorthosites contain higher incompatible elements except the plagioclase-compatible element Sr, than the other anorthosite varieties (Fig. 7), possibly indicating their residual liquid nature. All the anorthosites show Ba, Sr and K peaks (Fig. 7) consistent with their high plagioclase content, and similar incompatible element ratios such as K/Rb, Zr/Nb, Zr/La and Zr/Y (Table 2) possibly reflect a genetic link between the different varieties of anorthosite. The Ti and P peaks (Fig. 7) and high K/Rb ratios of these rocks (684 –4566, Table 2) may reflect the source characteristics. MgO is positively correlated with total-FeO, MnO, TiO2, P2O5 and LREE, and negatively correlated with SiO2, Al2O3 and to some degree, CaO (Fig. 8). Zr and K2O show positive correlations with other trace elements except Sr (Figs. 9 and 10). The latter is negatively correlated with Zr and K2O due to its compatible behavior in plagioclase-rich rocks. These relations are comparable with other anorthosite occurrences such as the Bolangir massif of the Eastern Ghats (Bhattacharya et al., 1998), and are consistent with high degrees of plagioclase accumulation in these rocks.

7. Petrogenesis Since basement rock assimilation, wall-rock alteration and the effects metasomatism are largely restricted to the top and the margins of the massif, the deep borehole sections provide an opportunity to understand the crystallization characteristics in a massif anorthosite suite (Figs. 3 –5). The depth profiles of the major and trace elements indicate a cyclic character (Fig. 6), irrespective of the changes in the rock-types as evident from petrography. The normative plagioclase-An% composition varies within single chemically defined cycle in which both grey and white anorthosites are present. Peaks of Zr, Ce, Ti, Nb and P in depth profiles indicate accumulation of Fe – Ti oxides and other accessory phases like zircon and apatite at the corresponding depths. The plagioclase megacrysts of the grey anorthosite seem to have a tendency to float over

Table 2 Chemical composition of Bengal anorthosites collected from a Deep Borehole, NNE of Ledapalash C8 gabbroic anorthosite

C2 mottled anorthosite

C11 gabbroic anorthosite

C12 grey anorthosite

C15 white anorthosite

C6 mottled anorthosite

C4 mottled anorthosite

C19 mottled anorthosite

C20 white anorthosite

C21 white anorthosite

C22 white anorthosite

45.91 1.94 17.57 2.24 7.79 0.16 4.41 9.97 2.88 1.28 0.91 95.06 10.90 44.50 56 11.7

49.67 0.91 23.84 1.20 3.03 0.06 2.08 10.53 4.04 0.79 0.24 96.39 4.57 47.43 57 26

45.65 2.14 17.38 2.43 6.49 0.12 4.38 11.19 2.69 1.22 0.51 94.20 9.64 47.36 58 98.9

50.70 0.41 28.49 0.50 0.52 0.01 0.27 12.39 3.70 0.65 0.07 97.71 1.08 33.17 65 106

49.93 0.54 22.63 1.28 2.19 0.06 4.10 13.89 2.79 0.67 0.05 98.13 3.71 68.62 67 224.35

45.42 3.26 22.84 1.09 4.52 0.06 2.89 12.39 3.18 0.52 0.05 96.22 6.11 48.36 69 293.79

49.89 1.07 24.79 1.57 1.43 0.04 1.56 11.09 4.07 0.58 0.09 96.18 3.16 49.45 58 326.15

50.96 0.35 25.92

0.02 0.95 11.53 3.60 0.64 0.07 94.04 1.65 53.28 61 403

47.89 0.58 25.84 1.23 1.76 0.06 1.57 12.24 3.37 0.52 0.11 95.17 3.19 49.40 66 459.81

51.08 0.52 27.03 0.73 0.99 0.02 0.98 11.95 4.00 0.44 0.13 97.87 1.83 51.47 62 474.15

48.64 1.02 24.47 1.26 2.77 0.04 2.15 11.61 3.70 0.49 0.34 96.49 4.34 49.54 67 574.2

12.8 872.2 703.7 9.2 7.0 35.9 105.4 8.1 87.1 24.0 23.0 13.6 1.0 67.5 14.7 14.2 31.4 13.6 4.96 4.59 0.97 830

8.7 712.0 899.1 10.5 3.0 17.4 105.6 11.5 31.5 28.0 19.4 9.0 2.3 31.3 6.6 9.1 11.2 3.5 3.48 4.74 1.38 754

14.8 666.7 606.3 20.0 1.3 35.5 257.0 24.1 67.9 24.1 35.7 15.5 1.3 51.0 7.0 13.5 25.3 14.7 3.29 7.29 1.93 684

3.7 412.9 928.0 0.1 b.d.l. 6.5 23.3 b.d.l. 6.3 24.4 8.0 3.2 b.d.l. 20.2 4.2 7.2 4.1 3.2 6.31 4.81 1.71 1458

2.8 317.8 729.7 9.4 11.3 13.8 105.8 b.d.l. 20.2 22.4 26.8 7.0 0.6 19.2 1.8 4.2 11.1 3.6 2.74 10.67 2.33 1986

3.7 316.2 761.6 24.2 2.8 23.8 160.8 10.9 28.5 23.3 27.6 5.9 0.4 35.3 14.1 1.5 15.5 4.9 5.98 2.50 0.11 1167

1.5 468.9 1396.7 10.5 b.d.l. 10.8 57.7 4.8 12.9 25.7 13.8 4.6 b.d.l. 30.6 6.8 6.6 10.1 4.2 6.65 4.50 0.97 3210

3.6 349.7 1255.8 1.8 8.5 6.8 25.0 1.1 4.5 25.3 12.5 3.3 b.d.l. 9.2 1.8 8.5 9.6 3.0 2.79 5.11 4.72 1476

2.3 288.0 1285.2 7.3 16.2 8.8 31.3 1.3 18.6 23.4 9.6 5.2 1.0 28.4 4.2 9.7 14.6 8.9 5.46 6.76 2.31 1877

0.8 342.9 1255.1 3.6 3.6 8.0 29.9 1.7 5.6 25.5 15.7 3.5 0.2 9.6 2.8 8.0 10.0 2.7 2.74 3.43 2.86 4566

3.2 298.8 945.2 12.6 6.6 17.1 84.3 17.2 15.6 24.7 18.3 6.2 b.d.l. 32.6 7.1 13.9 21.5 9.2 5.26 4.59 1.96 1271

Major oxides (wt%)

Trace elements (ppm) Rb Ba Sr Ni Cr Co V Cu Zn Ga Sc Y Th Zr Nb La Ce Nd Zr/Y Zr/Nb La/Nb K/Rb

89

b.d.l.: below detection limit; analyst: N.G. Marsh, Leicester, UK.

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SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total Fe2O3(t) M0 Norm plag-An% Depth (m)

90 D. Mukherjee et al. / Journal of Asian Earth Sciences 25 (2005) 77–94 Fig. 6. Variation of normative anorthite% of plagioclase, major oxides (wt%), trace elements (ppm) and incompatible element ratios in the Bengal anorthosites with depth along Western Borehole (Y), northeast of Ledapalash. Symbols: open square, white anorthosite; solid square, grey anorthosite; cross, mottled anorthosite (showing close affinity with white anorthosite), and diamond, gabbroic anorthosite.

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91

Fig. 7. Primitive mantle-normalized (McDonough and Sun, 1995) incompatible element contents of the Bengal anorthosites. Symbols are as in Fig. 6.

relatively denser transition element-rich white anorthosite. For example, the section of the borehole between depths of 295 and 105 m that corresponds to one chemically defined cycle (Fig. 6), the uppermost part consists of grey anorthosite, followed by mottled anorthosite in the middle, and banded and white anorthosites constitute the lower part

(Fig. 4). Density of plagioclase decreases from anorthitic to albitic compositions. Hence, the more albitic plagioclase forming the grey anorthosite would float over the less albitic plagioclase forming the white anorthosite. It is also observed that the transition elements decrease upward in the same 295 –105 m part of this section (Fig. 6). Since

Fig. 8. Variation of MgO with major oxides and LREE in the Bengal anorthosites along the western borehole section. Symbols are as in Fig. 6.

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Fig. 9. Variation between Zr and other incompatible elements in the anorthosites along the western borehole section. Symbols are as in Fig. 6.

Fig. 10. Variation between K2O and other incompatible elements in the anorthosites along the western borehole section. Symbols are as in Fig. 6.

D. Mukherjee et al. / Journal of Asian Earth Sciences 25 (2005) 77–94

Fe –Ti oxides are the primary carrier of transition elements in these rocks, the decrease of transition elements upward may be due to Fe– Ti oxides sinking to the base. Variation of MgO with other oxides and LREE (Fig. 8) is comparable to similar variations in other anorthosite massifs such as Bolangir in the Eastern Ghats (Bhattacharya et al., 1998). Plagioclase compositions do not seem to have been significantly affected by metamorphism. Unzoned or weakly zoned plagioclase of the Bengal anorthosites may imply slow cooling and fractionation of magma under equilibrium and near uniform thermal condition (Raedeke and McCallum, 1980). Alternatively, it may imply strong adcumulus growth and zone refining (McBirney, 1987) leading to re-equilibration of plagioclase by the passage of new impulses of magma with crystal-mush from below (Mukherjee et al., 1999). Broadly similar incompatible element ratios of the different types of anorthosites (Table 2) indicate that they may be genetically related, and minor presence of cumulates of orthopyroxene and clinopyroxene in addition to dominant plagioclase and Fe – Ti oxides possibly indicates a tholeiitic affinity of the parent liquid.

8. Conclusion The salient features with respect to textural, mineralogical and chemical variations in the three borehole sections and their inter-relationship, together with the surface geological features of the Bengal anorthosite massif are summarized below. (i)

(ii)

(iii)

(iv)

The Bengal anorthosite massif continues beyond 472.61 m below msl. This implies that the thickness of the massif is greater than 622.85 m, considering the surface elevation of borehole site and deepest level attained. Such a voluminous sub-surface extension of the Bengal anorthosite massif contradicts its estimated thickness of 200 m ascertained by surface gravity data modeling (Verma et al., 1975). The anorthosite massif contains xenoliths of older country rocks (e.g. gneisses, metabasic and metasediments) in the upper part, and younger intrusives viz. alkali granite, syenite, pegmatite and aplite, and thin veins of quartz and calcite. Hydrous alteration effects are readily observed at the contact with the country rocks. The massif is composed of a coarse pegmatitic grey anorthosite containing grey plagioclase megacrysts, and a medium grained equigranular white plagioclase-rich white anorthosite. A transitional mottled anorthosite showing porphyritic texture has developed at the interface between the grey and the white anorthosites. The incompatible element ratios of the different types of anorthosite are broadly similar indicating that the different varieties are genetically related. The high P

93

and Ti of the anorthosites may reflect the source characteristics, and minor cumulates of orthopyroxene, clinopyroxene and Fe –Ti oxides in addition to dominant plagioclase indicate a tholeiitic affinity of the parent melt. (v) Uniformity of composition of the plagioclase crystals indicated by weak zoning support slow cooling and crystallization under equilibrium conditions. Alternatively, it may imply adcumulus growth and zone refining, leading to re-equilibration of plagioclase by the passage of new impulses of crystal laden-magma from below. Metamorphic effects may have also contributed to the weak zoning in plagioclase. (vi) Borehole chemistry indicates that major and trace elements, and incompatible element ratios show a cyclic variation with depth. (vii) Within each chemically defined cycle, the coarse pegmatitic grey anorthosite with lower anorthite content of plagioclase apparently floated over white anorthosite containing a relatively denser and granular white plagioclase with higher anorthite content. The denser white anorthosite also contains higher concentrations of transitional major and trace elements near the base, possibly related to sinking of Fe– Ti oxides.

Acknowledgements We are thankful to the Deputy Director General, Geological Survey of India, Eastern Region, for entrusting the senior author to undertake the drilling programme on Bengal anorthosite Project. The assistance received from the senior officers and the field staffs, especially the surveyors are gratefully acknowledged. We also express our sincere thanks to Prof. L.D. Ashwal for twice reviewing the manuscript, Prof. R.A. Wiebe, who reviewed the original manuscript, and Dr R. W. Kent of Leicester, for constructive comments. We express our gratitude to Prof. A.D. Saunders for XRF analyses of samples. One of the authors (NCG) appreciates the funding from the University Grants Commission, New Delhi.

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