Coronas in olivine metagabbros from the Proterozoic Chotanagpur terrain at Mathurapur, Bihar, India

The Proterozoic Mathurapur olivine metagabbros possess several types of coronas due to subsolidus reactions between igneous mineral pairs. viz. olivine-plagioclase, ilmenite-plagioclase and pyroxenes-plagioclase. Microprobe analyses of coexisting primary and coronitic minerals from different corona domains, indicate attainment of equilibrium. Mineral chemical data of primary (reactants) and coronitic (products) minerals from the present metagabbros and also from similar studies in the literature are critically examined to evaluate the chemistry of corona reactions by mixing calculations assuming boundary migration. The mass-balanced corona equations are consistent with the allochemical system and do not show volume imbalance. However, SiO₂ and Al₂O₃ in olivine-plagioclase coronas remain immobile, which indicates that the corona reactions therein proceeded with minimum structural rearrangements of (Si.Al)---O bonds.