GEOQUÍMICA E AMBIÊNCIA TECTÔNICA DE METABASITOS DO ALTO E MÉDIO VALE DO RIBEIRA, SÃO PAULO E PARANÁ

Abstract

In order to stablish tectonic settings, petro- graphic and litochemical analyses were carried on samples of metabasic rocks from the Upper and Middle Ribeira Valley, between the townships of Apiaí, Iporanga and Barra do Turvo, São Paulo State. The metabasites are considered to be of Mesoproterozoic age and their samples were collected both to the north and south of the Ribeira Lineament. Basic rocks from the Ápiaí Gabbro, the Betari Dyke and another smaller dyke were also sampled, whith the purpose of to compare these magmatisms. Conclusions driven from the analyses are: a) SiO2 contents are very homogeneous; only two samples of metabasites fell out of the basalt field; b) based on their major element composition, ali samples exhibit tholeiitic characteristics; c) the metabasites of the Ribeira Subgroup may be classified into two different assemblages: one, which includes the samples collected to the north of the Ribeira Lineament, is high in Ca, Al and low in P, Ti, Zr, Y, Sr, with peculiarities similar to those found in basalts of modern island arcs; the other, collected to the south of the Lineament, is enriched in Na, Fe, Ti, Zr, Y, Sr, and could be related to the compositions of modern middle ocean ridge basalts; d) the composition of the Apiaí Gabbro also displays similarities with modern middle ocean ridge basalts; e) the diabase dykes are strongly enriched in Ti, P, Fe, Sr, Zr, Y a fact that links them to the HTi basalts whith geochemical features cornmon to the Meso-Cenozoic basaltic magmatism of the Paraná Basin and to other plateau and continental rift basalts.