Tectonic boundaries, crustal weakness zones and plume-subcontinental lithospheric mantle interactions in the Serra do Mar dyke swarm, SE Brazil

Abstract

Tholeiitic basalts of Early Cretaceous age (ca.132 Ma) integrate the Serra do Mar Dyke Swarm in Southeastern Brazil. The dolerites are included in a transitional tholeiitic series that can be divided in a low-TiO (sub 2) and high-TiO (sub 2) suites on a geochemical basis. Parental compositions have La/YbN and La/NbN > 1 pointing to a predominantly contribution from the subcontinental lithospheric mantle in the generation of the two suites. However, the occurrence of a high-TiO (sub 2) dolerite in the swarm with La/NbN=0.3 and La/YbN=9.9 ratios can indicate a contribution of a fertile sublithospheric mantle in the generation of the Serra do Mar Dyke Swarm likely to be represented by the Tristan da Cunha plume. This paper proposes that the Tristan da Cunha plume may have melted locally in places where the Gondwanaland lithosphere had undergone greater thinning due to the reactivation of crustal weakness zones nearby major terrane boundaries amalgamated in Proterozoic times resulting in the generation of restricted volumes of plume-derived tholeiitic basalts.