Chromite from the Bela Arc-Basin Ophiolite, Pakistan

Abstract

The Bela arc-basin ophiolite (BABO) is a complex of ophiolite and melange outcrops that formed in the tectonic setting of an oceanic volcanic arc and its adjoining forearc basin. The BABO contains typically podiform segregations of chromite, whose microprobe compositions display a variation trend separate from that of the chromian spinel from surrounding peridotites, but similar to that in the latter igneous differentiates (pyroxenites and an andesite). The podiform chromitite and associated dunite were derived by mantle-melt reaction and cumulate-melt crystallization. Both exhibit conformable relations and are cogenetic. Chromite characteristics conform to the suprasubductionzone setting of the BABO, with two components distinguished from the whole-rock geochemistry, field relations, and chromite chemistry—a remnant forearc basin toward the southwest and an adjacent arc-trench gap toward the northeast. Individual ophiolite massifs within the BABO display differences in degree of partial melting and may have originated from different depths within the mantle.