The Amphibolites of the Costeiro Complex in the São Sebastião Region, SP

Abstract

Geochronological, petrographic and lithochemical data have been used to characterize the crustal evolution and thethermochronology of the Costeiro Complex, São Paulo State, Brazil. This part of the Ribeira Fold Belt is composed ofparaderived rocks to the East, and orthoderived rocks to the West, which are organized along a ductile shear zone forminga positive fl ower structure whose axis runs ENE-WSW. The main amphibolite occurrences are found within predominantgarnet-paragneisses along this axis. The age of crystallization of the intrusive basic magmas which originated the amphiboliteswas 580 Ma (U-Pb SHRIMP). They formed tabular intrusions or secondary magma chambers, that locally still preservecontinental tholeiitic geochemical characteristics. The close proximity of the basic magmatism and the metamorphic peakis evident from the 570 Ma, U-Pb age determination of the overgrowths on detritic zircon crystals in the paragneisses.This suggests a tectonic environment where the basic magmas intruded a back-arc sedimentary basin established overcontinental crust. The source areas were probably rocks differentiated from the mantle between the Paleoproterozoic andNeoproterozoic.

Keywords

• Costeiro Complex
• Ribeira Fold Belt
• Brasiliano Cicle