Coriolano Dias Neto

Os anfibolitos do complexo costeiro na região de São Sebastião, SP

Geochronological, petrographic and lithochemical data have been used to characterize the crustal evolution and the thermochronology of the Costeiro Complex, Sao Paulo State, Brazil. This part of the Ribeira Fold Belt is composed of paraderived rocks to the East, and orthoderived rocks to the West, which are organized along a ductile shear zone forming a positive flower structure whose axis runs ENE-WSW. The main amphibolite occurrences are found within predominant garnet-paragneisses along this axis. The age of crystallization of the intrusive basic magmas which originated the amphibolites was 580 Ma (U-Pb SHRIMP). They formed tabular intrusions or secondary magma chambers, that locally still preserve continental tholeiitic geochemical characteristics. The close proximity of the basic magmatism and the metamorphic peak is 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 over continental crust. The source areas were probably rocks differentiated from the mantle between the Paleoproterozoic and Neoproterozoic.

Litoquímica dos granitóides da estrutura em flor de São Sebastião, São Paulo

The aim of this paper is to present lithochemical data from the granitoids found between Guaruja and Sao Sebastiao in a segment of the Costeiro Complex in Sao Paulo, State, Brazil. This part of the Ribeira Folded Belt is composed of para-derived rocks on the eastern side, and ortho-derived rocks on the western side of the belt, organized in a ductile shear zone forming a positive flower structure, whose axis runs ENE-WSW. Large boudins of amphibolite occur within predominant kinzigitic gneisses along this axis. The intrusive basic protoliths of the amphibolites have a 580 Ma (U-Pb SHRIMP) crystallization age. SiO2 contents of the granitoids vary between 68 and 74%, corresponding to monzogranites and granodiorites. Some outcrops display mafic enclaves of quartz-monzodiorites. The granitoids show calc-alkaline affinity while their mafic enclaves tend towards an alkaline affinity. The lower content in SiO2 coupled with coherent differences on some trace element concentrations (e.g. Zr-Hf) contribute to clearly distinguish the enclaves from their country rocks and to suggest intense chemical interaction among the materials during crystallization. The REE pattern, the depletion in Sr, P-Ti, Ta-Nb, and the distribution of the samples on the R1-R2 diagram suggest a syn-collision island arc environment for generation of these rocks.