Geology, Petrology and Magmatic Evolution of the Felsic Alkaline Rocks of the Vitória Island, São Paulo State, Brazil

This paper presents geological, petrographic, geochemical characteristics and radiometric ages for the felsic alkaline rocks of the Vitoria Island Intrusive Complex, Sao Paulo State, Brazil, and discusses its geochemical evolutionary processes. This intrusive body is about 3 km in diameter and the emerged island areas correspond to its northeastern border. The main intrusion is constituted by nepheline syenite at the central zone, nepheline-bearing syenite at the outer zone, syenite at the border, and quartz syenite at the contact zone with alkali feldspar represented by orthoclase. Clinopyroxene vary from hedenbergite, soda-augite to aegirine-augite, and some crystals have resorption shape and amphibole reaction rim. They are intruded by phonolite, trachyte and lamprophyre dykes. Based on the intrusion relation, the dykes are subdivided into two generations: the first one encompass a radial system and the second a parallel system with NE-SW orientation and sinistral displacement. The alkaline rocks vary from silica undersaturated to oversaturated, show peralkaline to peraluminous compositions and belong to the potassic series. The K-Ar ages for amphibole of the nepheline-bearing syenite and syenite are 89.58 ± 3.18 Ma and 84.35 ± 3.92 Ma, respectively. The rocks are strongly fractionated with high differentiation index (DI) of 88.15 in average, indicating mafic minerals fractionation. K 2 O/(K 2 O+Na 2 O) ratios are low, in average 0.51, in accordance to crystallization of pseudo leucite and potassic feldspar the last observed in thin sections.