Abstract

A detailed petrochemical study of the Proterozoic "Dalma Epidiorites" of Singhbhum leads to a completely new interpretation of this dominantly volcanic belt. The initial igneous activity is represented by mafic and ultramafic intrusives and lavas of tholeiitic. alkaline, and komatiitic affinities. Interlayered are lithic tuffs and tuffaceous sediments. Also present is an analcite basalt heretofore unreported from the Dalma. This suite is overlain by a high-magnesian vitric tuff horizon, which along with its comagmatic intrusives shows quench textures and its chemistry is closely comparable to peridotitic-pyroxenitic komatiites of Archean greenstone terrains. It is noteworthy that the komatiitic magma in this belt is dominantly pyroclastic. The vitric tuff horizon is followed upwards by a thick sequence of high-iron, low-potassium, pillowed tholeiites with ocean-floor affinities.The proposed evolutionary model of the Dalma belt suggests that stretching of Archean proto-continental crust gave rise to the Dalma graben within the Proterozoic (Chaibasa) basin and unique thermal conditions coupled with a disrupted crust led to a high degree of melting and direct tapping of the mantle producing the komatiitic extrusives at a particular stage of the development of the belt.

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