Petrology and geochemistry of amphibolites around Kuilapal, West Bengal, eastern India

Abundant small lenses and bands of different types of amphibolites belonging to amphibolite facies occur within Proterozoic (1638 +/- 38 Ma) Kuilapal granitoid gneiss body (12.8 Km x 6.4 Km) and outside it in the metapelitic country. The different petrographic types are: (A) Pl-hb-Mc amphibolite, (B) Ap-mt-hb-M amphibolite, (C) Monomineralic hb-UM amphibolite, (D) Tr-act-UM amphibolite and (E) Mi-hb-MC amphibolite. Petrographic and geochemical studies suggest that all these types of amphibolites except (E) are orthoamphibolites which are similar in chemical composition to tholeiitic ferrobasalt and exhibit trace element characters corresponding to ocean floor basalt. The parent basaltic magma for these amphibolites appears to show signature of a magmatic history connected with Dalma basalts derived from iron-enriched mantle source. However, the parent basaltic magma of the Kuilapal area, in all probability represents an earlier phase of basic magmatism than Dalma basalt (1547 +/- 20 Ma). The age of emplacement of ferrotholeiitic magma in the present area should be earlier than 1638 +/- 38 Ma, which is the age of Kuilapal granitoid rocks. The chemical composition of tr-actinolite amphibolite, ap-mt-hb amphibolite and pl-ap-mt-hb amphibolite have similarity with such igneous rocks as pyroxenite, ap-mt pyroxenite and gabbroic rocks respectively. The type (E) amphibolite represents metamorphosed impure marly sediment. EPMA data of hornblendes show systematic increase of Si and Mg and decrease of Fe from almandine zone to sillimanite zone metamorphism. Hornblende-plagioclase geothermometry suggests the variation of temperature from about 730 degrees C to 795 degrees C from garnet/staurolite to sillimanite zone.