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404 ANIKET CHAKRABARTY AND AMIT KUMAR SEN to the low-temperature fluids derived from a fractionated magma dominated by CO 2 also containing fluorine and H 2 O in variable proportions. Occurrence of small bodies of carbonatite and alkaline rocks like nepheline-syenite is known since last two decades along 100 Km long Northern Shear Zone (NSZ) traced from Khatra in Bankura, West Bengal to Tamar in Jharkhand through Beldih, Medinitanr, Kutni, Chirugora, Sushina, and Tamakhun (Fig. 1) (Kumar et al. 1985; Basu, 1993, 2003; Chakrabarty et al. 2009).These rocks are intruded within the Chandil formation of 1.5-1.6 Ga age and lies in the close proximity of the Chotanagpur Granite Gneissic Complex (CGGC). The alkaline rocks exposed in different areas along the NSZ are Beldih (carbonatite, alkali-pyroxenite, and apatite-magnetite), Kutni (carbonatite, apatite), and Sushina (nepheline syenite gneiss) (Singh et al. 1977; Ghosh Roy and Sengupta, 1988; Basu, 1988, 1989, and 1990). The exposure of the carbonatite is found only at Beldih (Purulia district, W.B.), the present area of investigation and henceforth will be termed as ‘Purulia carbonatite’. The Geological Survey of India had also reported other occurrences of carbonatite from Mednitanr and Kutni areas based on drill-core samples (Basu, 1993). This shear zone is also known for hosting different mineralization like Nb, apatite-magnetite, REE. The usual silicate rock association of magmatic carbonatite such as nephelinite, melilitolite, ijolites, urtites, kimberlites is absent in Beldih. Instead, the carbonatite is associated with the alkalipyroxenite and a large apatite-magnetite ore body; a case similar to the famous Khibina Complex of Russia (Zaitsev et al. 1998). Such similarity invokes a possible genesis of Beldih carbonatite as carbothermal residua of unknown parentage (Woolley and Kjarsgaard, 2008). In view of such possibilities, the genesis of the Purulia carbonatite and their possible genetic (?) lineage with the other silicate rocks present along the NSZ is important and leads to the present investigation. The investigation is based on detailed petrography and trace element geochemistry of the Purulia carbonatite. Three major rock types are found in this area and are carbonatite, alkali-pyroxenite and apatite-magnetite rock. Carbonatites PETROGRAPHY AND MINERALOGY The carbonatite is a light colored, medium-grained rock composed essentially of subhedral to euhedral calcite (~ 90% by volume, see Table 1 for detail modal compositions) with appreciable amounts of apatite (Table 1), altogether exhibiting mosaic texture. The accessory mineral Fig.1. (a) Regional geological map of the study area. (b) Generalized stratigraphic succession of the Singhbhum region showing the status of the Chandil Formation along with the rocks of Singhbhum Group. JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010
ENIGMATIC ASSOCIATION OF CARBONATITE AND ALKALI-PYROXENITE, PURULIA, WEST BENGAL 405 Table 1. Modal compositions of carbonatites (C), alkali-pyroxenite (Pxn) and apatite-magnetite (Apt-Mt) rocks (volume %). It must be noted that the calcite, apatite, biotite and albite are present as veins within alkali-pyroxenite along the zones of metasomatic alteration caused by the nearby carbonatite intrusion Sample No 1 2 3 4 5 1 2 3 4 5 1 2 Rock Type Carbonatite (C) Pyroxenite (Pxn) Apt-Mt Minerals Calcite 90 90 86 94 84 3 5 × 8 2 Apatite 4 3 5 5 6 1 3 × 2 1 95 97 Amphibole 3 5 3 1 5 6 8 8 10.5 7 Biotite 1 1 3 × 2 3 2 1 3.5 2 × × Clinopyroxene × × × 84 78 87 70 85.5 × × Magnetite 1 1 2 × 2 1 2 2 1 1 5 3 Ilmenite 1 × 1 × 1 0.5 1 2 1 × × × Niobo-rutile × × × × × 0.5 × × 1 × × × Albite × × × × × 1 1 × 3 1.5 × × phases include amphibole, biotite, phlogopite, and ilmenite (Table 1). Overall, the mineral grains exhibit mosaic texture (Fig. 2a, b). Higher concentration of amphibole, biotite, and phlogopite at places resulting formation of the continuous and/or discontinuous bands of dark green color. Two varieties of amphibole grains are noticed; one with dark green colour showing dark brown pleochroism (magnesiokatophorite, Fig. 2b) and the other with light green colored amphibole showing pale-green to light green pleochroism (richterite). A detailed work on the alkali-amphiboles by Chakrabarty et al. (2009) has suggested a shallow depth of intrusion of this carbonatite and also advocated a sudden change in pressure during its emplacement. Alkali-pyroxenite The alkali-pyroxenite is a dark colored medium grained rock juxtaposed with the carbonatite. The subhedral clinopyroxene grains are making up about 85% of the rock by volume (Table 1) and give rise to hypidiomorphic texture. The euhedral to subhedral calcite and apatite grains are at places giving rise to mosaic texture similar to that of associated carbonatite (Fig. 2c). Three distinct mineralogical assemblages viz. primary magmatic, vein-filling and post magmatic are observed in alkali-pyroxenite. Such mineralogical assemblages indicate micro scale metasomatic changes consanguineous to the carbonatite intrusion. The primary magmatic mineralogical assemblage is dominated by the diopsidic pyroxene (Fig. 2d) with significant aegirine component (Ae 34.42-37.29 -Hd 12.67-13.32 -Di 52.91-49.39 ) and sodiccalcic amphibole of magnesiokatophorite type. The alteration of the primary mineralogical assemblages by the late stage vein-filling assemblages indicates post magmatic alteration similar to the sodic fenitization during carbonatite intrusion. The alteration assemblage is represented by the amphibolization (katophorite/taramite) and biotitization of the magmatic or early pyroxene and that of the vein filling assemblage by the calcite-apatite-albite-biotite-ilmenitemagnetite. The alkali-pyroxenite is characterized by the presence of late stage apatite-calcite vein which give rise to carbo(hydro)thermal carbonatite (Chakrabarty, 2009). Apatite-magnetite In general, presences of number of apatite-magnetite lenses of variable dimensions have been reported along NSZ. These lenses are intruded mostly within the country rock represented by mica schist, calc-silicate, metabasite, quartzite and biotite gneiss which are often sheared (Basu, 1993). Clayey alterations around the apatite-magnetite lenses are very common. In Beldih, the strike length of apatite bearing rock is about 300meters with a maximum width of about 60 m and shows tapering towards east and west. The rock is light coloured and dominantly composed of fluor-apatite (Table 1, Fig. 2e). In general the apatite grains are surrounded by opaque of magnetite. However, at places remnant apatite grains under microscope are colorless and showing parallel extinction with imperfect basal cleavage which appears to be cross fractured (Fig. 2f). The shape of the apatite crystals or pseudomorphs is highly variable from elliptical to hexagonal prismatic (Fig. 2e). At places marginal overgrowth of the apatite grains are marked yellow colour under the microscope. This rock is extensively affected by alteration. In many places the meteoric water reacted and subsequently dissolved the apatite grains. Further, there are evidences of re-precipitation of secondary apatite giving rise to colloform texture (Fig.2f). ANALYTICAL TECHNIQUE Approximately fifty whole rock samples of different JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010
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