U-PB DATING AND RB-SR ISOTOPE GEOCHEMISTRY OF THE EASTERN PORTION OF THE TRÊS CÓRREGOS BATHOLITH, RIBEIRA FOLD BELT, SÃO PAULO

ANTONIO GIMENEZ FILHO, VALDECIR DE ASSIS JANASI, GINALDO ADEMAR DA CRUZ CAMPANHA, WILSON TEIXEIRA, LUIZ EMÍLIO TREVIZOLI JÚNIOR

Resumo


The ages of three facies of syn-orogenic metaluminous granites from the Três Córregos Batholith (Southern Ribeira Belt, São Paulo) were obtained by conventional U-Pb zircon dating. A typical porphyritic hornblende-biotite granite from the Saival massif yielded 605 ± 2 Ma. Two other samples of foliated granites show signs of inheritance, and no precise ages of crystallization were obtained. Their lowest 207Pb/206Pb ages, however, are only slightly discordant, and yield reliable upper limits that are, in both cases, around 608-610 Ma, demonstrating that these more deformed varieties are products of the same magmatic event that formed the bulk of the batholith, and not older orthogneisses, as admitted in some previous works. Calculated to the age of crystallization, the 87Sr/86Sri ratios show a wide spread (0.710 to 0.717). A clear negative correlation, in a batholith scale, with the Sr contents, is attributed to mixing with an old, high Rb/Sr mid-crustal component, and can explain the older isochronic ages typically obtained in these and similar granites. High 87Sr/86Sri , high Sr mafic microgranular enclaves seem to point to a contribution from enriched sources in the continental mantle lithosphere to the Três Córregos magmatism.

Palavras-chave


U-Pb dating; Rb-Sr isotope geochemistry; granite; Ribeira Fold Belt; Neoproterozoic.

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