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

Abstract

The Bengal anorthosite occurs as a large tadpole-shaped pluton (250 km 2) 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.