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

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.