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406 ANIKET CHAKRABARTY AND AMIT KUMAR SEN Fig.2. (a) Carbonatite showing the mosaic texture along with triple junctions at places. The bulk mineralogy is dominated by the calcite and apatite. Minute grains of magnetite are shown in the area marked by the circle. (b) Carbonatite with two varieties of amphiboles along with magnetite. (c) and (d) Part of the alkali-pyroxenite intruded by the late stage apatite-calcite vein. The apatite in the alkali-pyroxenite is anhedral compared to the associated carbonatite. The effect of metasomatic alteration is marked by the formation of albite, biotite and shown in the circular area. At places amphibole and biotite are indistinguishable due to intense metasomatic alteration. Most of the alteration is found to be associated with the calcite-apatite veins. (e) and (f) Apatitemagnetite rock showing subhedral apatite grains. The apatite grains are variable in size. Magnetite grains are relatively small and distributed irregularly within the rock. MK: Magnesiokatophorite; R: Richterite; Amph: Amphibole; Ab: Albite; Bt: Biotite; Px: Clinopyroxene; Cal: Calcite; Apt: Apatite and Mt: Magnetite. JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010
ENIGMATIC ASSOCIATION OF CARBONATITE AND ALKALI-PYROXENITE, PURULIA, WEST BENGAL 407 rock types were studied petrographically and twelve samples were selected for whole rock trace element analysis, five each of carbonatite, alkali-pyroxenite and two of apatitemagnetite. To constrain the trace elemental concentrations of the carbonatites, routine analyses of major elements (see Table 2) of the whole rock were carried out on Li-tetraborate pellets at IMP (Institute for Mineralogy and Petrology), ETH Zurich following the procedure given by Nisbet et al. (1979) and Dietrich et al. (1984) using WD-XRF of Axios by PANalytical. The trace element analyses (see Table 3) were carried out by an inductively coupled plasma emission mass spectrometry (ICP-MS, Perkin Elmer, Elan 6000e) using a VG Elemental Plasma Quad instrument at Wadia Institute of Himalayan Geology (WIHG), Dehradun, India. The details of the analytical procedure have been given by Rathi et al. (1996). The precision of the measurements is generally better than ±5% for concentrations =1 ppm. GEOCHEMISTRY Carbonatite Most of the major elements in terms of their oxides do not show any substantial variations except for one sample (No. 2, Table 2) where the SiO 2 content goes up to 5%. Expectedly all the carbonatites are essentially enriched in CaO and P 2 O 5 (Table 2). The major element geochemistry confirms they are calcico-carbonatites or sovites. Selected trace elements (Table 3) of the carbonatite when plotted in a multi element spider variation diagram shows that they are enriched in all the incompatible elements with respect to the primitive mantle (Sun and McDonough, 1989) (Fig. 3a), except for Rb. The Purulia carbonatite is characterized by the lower concentration of Th, Nb and U, whereas the trend of other incompatible elements (Ba, U, REEs and Y) matches well with that of the world-average of the calico-carbonatite of Woolley and Kempe (1989). Such Nb depletion (~20-44 ppm.) is in general characteristics of carbo(hydro)thermal carbonatite (Mitchell, 2005). The Sr concentration of the carbonatite is much higher than the world-average of calico-carbonatite. Replacement of Ca by Sr, in Ca bearing minerals like calcite and apatite resulted higher proportion of Sr in carbonatite. Chondrite-normalized REE patterns of the carbonatites (Fig. 3b) shows that they are enriched in LREE compared to the HREE which in general a characteristics of carbonatites. The average (La/ Yb) (avg) (=53.74) and REE (ΣREE = 1633.57) concentration of carbonatites are falling well within the range of calicocarbonatite (Woolley and Kempe, 1989). Alkali-pyroxenite The alkali-pyroxenite is also characterized by the relative enrichment of all incompatible elements. The major Table 2. Bulk rock major element compositions of the carbonatite (C) Sample No 1 2 3 4 5 Rock type C C C C C SiO 2 1.04 5.28 1.09 1.16 2.14 TiO 2 0.25 0.01 0.27 0.03 0.14 Al 2 O 3 0.06 0.08 0.06 0.07 0.07 Fe 2 O 3 2.63 1.19 2.60 1.39 1.95 MnO 0.40 0.15 0.40 0.19 0.29 MgO 1.64 0.53 1.61 0.73 1.13 CaO 49.91 49.11 48.70 50.63 49.10 Na 2 O 0.25 0.12 0.26 0.19 0.21 K 2 O 0.01 0.01 0.02 0.01 0.01 P 2 O 5 3.23 0.53 3.21 4.88 2.96 NiO 0.01 0.00 0.01 0.00 0.01 H 2 O 0.00 0.00 0.00 0.00 0.00 CO 2 0.00 0.00 0.00 0.00 0.00 LOI 37.81 42.21 38.07 36.90 41.28 Total 97.23 99.23 96.30 96.17 99.28 Fig.3. (a) Primitive mantle normalized spider plots of the studied rocks. Studied carbonatite is compared with the world average of calicocarbonatites. (b) Chondrite normalized REE plot of the studied rocks. The normalizing values are from Sun and McDonough (1989). JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010
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