Granitos peraluminosos da porção central da Faixa Ribeira, Estado de São Paulo: sucessivos eventos de reciclagem da crosta continental no neoproterozóico

Abstract

The Embu Domain, central portion of the Ribeira Fold Belt in the State of São Paulo, includes a large number of Neoproterozoic granitic plutons. A peculiar feature is the abundance of marginal to strongly peraluminous granites, which comprise dozens of minor occurrences and also some batholiths, especially in the region east of the city of São Paulo. The more common petrographic varieties are biotite granite, white biotite-muscovite leucogranite and tourmaline-garnet granites. Recent monazite U-Pb dating has revealed that, in spite of the petrographic similarities, these granites were generated in several successive geological events that occurred over a ca. 200-million-year time interval at the end of Neoproterozoic. The oldest ages, around 780 Ma, are from peraluminous orthogneisses, some rich in mafic minerals and muscovite and in some respects similar to typical"S-type" granites. Preliminary data from the Serra do Quebra-Cangalha batholith yield a U-Pb monazite age of ca. 680 Ma, revealing it to be significantly older than the main manifestation of calc-alkaline magmatism in the Embu Domain (the Agudos Grandes batholith, 610 - 600 Ma in age). The Turvo-type peraluminous leucogranites, dated at 610 ± 2 Ma, are associated with crustal melting during the latter event. Different smaller granitic occurrences located in the Embu Domain east of the city of São Paulo yield U-Pb monazite ages in the 590 - 580 Ma range (Mauá, Mogi das Cruzes, Jaguari massifs). A similar age was obtained for the magmatic crystallization of the Natividade da Serra batholith in the Costeiro Domain (587 ± 5 Ma). These ages are coincident with the peak in granitic magmatism of the Rio Doce orogeny in the eastern portion of the Ribeira Belt. An even younger U-Pb age (~540 Ma) was obtained in a sample from the Santa Branca muscovite-biotite granite; its meaning, however, is still uncertain, given the strongly discordant character of the analytical results.