Petrological characterization of Fe–Ti oxides in metamafic rocks from the Nw borborema province, Ne Brazil

Abstract

The coexistence of Fe–Ti oxide phases constitutes an important petrogenetic tool to understanding the evolution of metamafic rocks. In the NW part of the Borborema Province, NE Brazil, metamafic rocks occur in two distinct zones: the Cariré Granulite and the Forquilha Eclogite. The present work characterizes the sequence of crystallization involving Fe–Ti oxides of these lithotypes and constrains its redox conditions using the ilmenite-rutile oxybarometer. Multiphase association of Fe–Ti oxides of a mafic granulite from the Cariré Zone and a retrograded eclogite from the Forquilha Zone comprise ilmenite, magnetite, rutile, spinel, and hematite. Rutile occurs as inclusions, surrounded by vermicular, xenomorphic or coronitic titanite, within ilmenite crystals. The redox conditions set by the sequence of Fe–Ti oxides crystallization and the rutile-ilmenite oxybarometer are compared to the FMQ (fayalite-magnetite-quartz) buffer, indicating an increase in oxygen fugacity from peak to retrograde conditions. Our characterization stablishes (re)crystallization and oxyexsolution sequences for the Fe–Ti oxides and the rutile-ilmenite oxybarometer in the mafic granulite from the Cariré Zone suggests oxygen fugacity of FMQ – 3.6. The retrograded eclogite indicates a more reduced environment for the Forquilha Zone, in which the low Fe3+ content of ilmenite hampers the estimation of redox conditons. The constrained ΔFMQ is within the range of common redox compositions for subduction-related metamafic rocks, accordingly with the regional geochemical classification reported in the literature to the Cariré and Forquilha zones.