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

Estudos litoquímicos dos granitóides que ocorrem na região do Complexo Costeiro entre as cidades de Guarujá e São Sebastião, no Estado de São Paulo, são apresentados neste trabalho. Como segmento da Faixa de Dobramentos Ribeira, o Complexo Costeiro na região estudada integra setores com rochas paraderivadas a leste e com rochas ortoderivadas a oeste, organizados segundo uma estrutura de cisalhamento dúctil em flor positiva, cujo eixo se orienta ENE-WSW. Importantes corpos de anfibolitos que ocorrem como"boudins" no interior dos kinzigitos predominantes, acompanham esta orientação. Determinações U-Pb em zircões (SHRIMP I) indicam idade de cristalização de 580 - 590 Ma para os protolitos ígneos dos anfibolitos e de 570 Ma para o ápice do evento metamórfico subseqüente. Os granitóides estudados, presentes no setor ortoderivado, contêm entre 68 e 74% em peso de SiO2 e correspondem a monzogranitos e granodioritos de afinidade cálcio-alcalina. Enclaves máficos de quartzo-monzodioritos localmente presentes nestas rochas posicionam-se por sua vez no campo alcalino. O menor conteúdo em SiO2, assim como as diferentes concentrações de certos elementos traço, como Zr e Hf, contribuem para a distinção entre os enclaves e suas rochas hospedeiras e sugerem intensa interação química entre os materiais durante a cristalização. O padrão de Terras Raras com baixos teores em Sr, P-Ti e Ta-Nb, em conjunto com a distribuição das amostras no diagrama R1-R2, sugerem um ambiente sincolisional de arco de ilhas na geração destas rochas.The aim of this paper is to present lithochemical data from the granitoids found between Guarujá and São Sebastião in a segment of the Costeiro Complex in São 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

Lithogeochemistry of Granitoids in the São Sebastião Flower Structure, São Paulo

The aim of this paper is to present lithochemical data from the granitoids found between Guarujá and São Sebastião in a segment of the Costeiro Complex in São 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