Magmatismo Neoproterozóico no Sul do Estado da Bahia, maciço sienítico serra das araras: geologia, petrografia, idade e geoquímica

Abstract

NEOPROTEROZOIC MAGMATISM IN THE SOUTHERN OF BAHIA STATE, SERRA DAS ARARAS SYENITIC MASSIF: GEOLOGY, PETROGRAPHY, AGE AND GEOCHEMISTRY The Serra das Araras Syenitic Massif (220 km2) is a Brasiliano aged intrusion (single zircon Pb-Pb age of 739 ± 2 Ma), located in the South area of an alignment of alkaline rocks that constitute the Alkaline Province of South Bahia. These rocks are predominantly hypersolvus syenites with biotite, hornblende and occasionally quartz, which gradually evolve into blue quartz-alkaline granites in the core. The massif has been passively emplaced at a Mesoproterozoic geosuture, which was re-activated during the Neoproterozoic. Geochemical data suggest that the syenitic and granitic rocks are cogenetic. The chemical evolution of Araras rocks allows explaining the generation of alkaline granites by fractional crystallization of syenitic magma, with a mafic syenitic cumulates similar to those observed in this massif. The negative epsilon Nd values, associated with the low HREE contents, indicate an enriched mantelic source at the garnet stability zone. The patterns observed to the syenitic rocks, in the multielement diagram normalized by the primitive mantle, and the absence of Nb negative anomalies, indicate an OIB signature and do not suggest any important process of crustal assimilation.