Another way of looking at an Alkaline Province

Abstract

The Mesozoic Alkaline Magmatism on the South American Platform is related to the Gondwana breakup and the opening of the South Atlantic Ocean. This event, known as the Mesozoic Activation phase, was responsible for five magmatic pulses, among them the Late Cretaceous-Eocene Alkaline Magmatism, which implemented ten alkaline provinces, eight of which are in the south-central portion of Brazil. Some authors claim that the reactivation of old zones of weakness from the Precambrian basement, Brasiliano orogeny, may have been responsible for the structuring of few bodies in these provinces. This work used geophysical processing techniques to interpret magnetic and gravity data and 2.5 D forward modeling to contribute with the knowledge on the tectonic control over the Goiás Alkaline Province. The geophysical data processing show a tectonic control over the Goiás Alkaline Province, along the Brasiliano and Mesozoic structures, and there was no predominance of alkaline bodies along the Azimuth 125. We find that the Brasiliano orogeny structures limited the crustal block that contains this Province and Gondwana breakup possibly created and reactivated these structures. We discuss the continuity of large structures, at different depths, related to the Tocantins Province, mainly in the Brasília belt, and some of them can be observed down to approximately 20 km depth. The forward gravity modeling beneath the Goiás Alkaline Province shows a thicker crust and we propose that the primitive lithospheric mantle below this province was not altered during the Brasiliano orogeny stages, as in the adjacent crustal blocks, thus allowing for generation of alkaline rocks. Along the modelled section the Arenópolis Magmatic Arc was separated into two different crustal blocks; the western arc whitin the Goias Alkaline Province, and the eastern arc where the mantle was altered due to its collision with the São Francisco Paleocontinent.