U–Pb ages and geochemistry of mafic dyke swarms from the Uauá Block, São Francisco Craton, Brazil: LIPs remnants relevant for Late Archaean break-up of a supercraton

Abstract

The São Francisco Craton (SFC) is an Archaean craton that hosts a significant number of mafic intrusions. Previous attempts to date the igneous emplacement of these rocks are limited to a few dates of poor precision. Here, two dyke swarms from the Uauá Block in the northeastern portion of the craton were dated using U–Pb on baddeleyite (ID-TIMS) and zircon (SIMS). The older dyke swarm trends mainly N–S to NW–SE (but also NE) and comprises norite dykes, whereas the younger dykes are tholeiite and trend NE–SW. The ages of 2726.2±3.2 and 2623.8±7.0Ma date dykes of noritic and tholeiitic compositions, respectively. The older age matches the ages of several Archaean-Large Igneous Provinces (tholeiitic-komatiite greenstone belts and continental flood basalts) worldwide and represents part of a feeder system, but the younger age is similar to Andean style continental margin magmatism in the São Francisco and Zimbabwe cratons. The two dyke swarms are tholeiitic in composition but the noritic dykes are more enriched in both compatible and incompatible elements and have higher (La/Yb)N ratios than the tholeiite dykes. The norite dykes are interpreted as low degrees of melting from enriched refractory mantle sources, whereas the tholeiite dykes represent high degrees of partial melting of more depleted mantle sources; the two mafic dyke swarms may be related to each other by progressive extension of the continental lithosphere. It is suggested that the Uauá Block with its dyke swarms is a small piece of Archaean crust dispersed after the break-up of a major Archaean supercraton.