Mineralogy and Geochemistry of the Bengal Anorthosite Massif in the Chotanagpur Gneissic Complex at the Eastern Indian Shield Margin

The Bengal anorthosite is a narrow 40 km long, "tadpole-shaped" massif that occurs within granulite facies rocks of the Proterozoic Chotanagpur Gneissic Complex at the eastern margin of the Indian shield. The core of the massif consists of grey anorthosite with coarse-grained cumulus plagioclase megacrysts showing magmatic flow-related alignment and the periphery consists of a mixture of the megacrysts and medium-grained, equigranular white anorthosite. Repetitive graded layers of grey and white anorthosite and cyclic variation of elemental concentrations characterize the massif at depths. These features are consistent with emplacement of the Bengal anorthosite through episodic magma pulses Labradonte (An_{57 58}) is the major constituent with clinopyroxene (Mg# 62), hornblende (Mg# 36-44), ilmenite and occasional orthopyroxene occurring as minor phases. Thermobarometric pressure-temperature estimates of the anorthosites (41-73 kbar and 593-795°C) are similar to earlier studies achieved from the metabasic and gneissic country rocks and correspond with the last high-grade (Grenvilhan) metamorphism. Similar Zr/Nb, Zr/Hf and Th/Ce ratios of the anorthosites and oceanic island basalt possibly indicates derivation from a mantle source. Lower crustal interaction is evident from similar Zr/Y, La/Nb and Th/Ce ratios of the anorthosites and lower continental crust Anatectic upper crustal melts probably contaminated the anorthosites as indicated from an enriched LILE pattern of the anorthosite. Metabasic rocks associated with the anorthosites have lower crustal Zr/Y, Nb/Y and Zr/Nb ratios. Minor gabbroic anorthosites within the massif, rich in iron and incompatible elements, were perhaps derived by differentiation of a coeval mafic parental magma. Proximity of the anorthosite to the Damodar Graben indicates that the Bengal anorthosite may have been emplaced m an extensional tectonic setting.