THE LAGOA DA VACA COMPLEX: AN ARCHAEAN LAYERED ANORTHOSITE BODY ON THE WESTERN EDGE OF THE UAUÁ BLOCK, BAHIA, BRAZIL

Abstract

The Lagoa da Vaca Anorthosite Complex is a metamorphosed layered igneous body composed of plagioclase-and amphibole-rich bands. Mineral chemical data indicate that anorthosites and associated peridotites are genetically unrelated. A whole-rock Pb-Pb isochron yielded an age of 3,160 ± 65 Ma, implying that this anorthosite complex is the oldest one ever found in the Sao Francisco Craton. Since the complex presents a model u value of 8.8 ± 0.62, compatible with its derivation from the subcontinental lithospheric upper mantle or from the depleted mantle, in both cases followed by crustal contamination, and crops out on the western edge of the Archaean Uaua Block, close to a major ductile shear zone that separates the Uaua Block from the Caldeirão migmatite-quartzite-orthogneiss belt, its emplacement is likely to have occurred in a continental setting, most possibly similar to present-day passive continental margin.