Lithodependence of partly transported weathering horizons above a migmatite-diabase contact in Central Bahia State, Brazil

Abstract

In the semi-arid region of Central Bahia, the weathering cover above a concealed migmatite diabase contact consisting of a loose sandy clayey surface layer and an indurated gravelly laterite shows distinct chemical and mineralogical relationships to the underlying parent rocks. High concentrations of Fe, V, Mn, Cu, Co and Ni characterize the weathering material above the diabase. In the overlying laterite layer, a chemical dispersion of most of these elements towards the migmatite can be observed. On the other hand, the weathering material above the diabase shows abundance of primary quartz, zircon and rutile derived from the migmatite. The evolution of the weathering cover is believed to be controlled by colluvial intermixing and vertical homogenization of partly transported weathering material, being followed by lateral chemical dispersion during laterite formation. Thus, the weathering horizons obliterate the chemical signal of the parent rocks increasingly from the bottom to the top of the profile. Similar profiles can be expected in most savannah-type regions and, therefore their identification is relevant for geochemical exploration.