MAGMATISMO ALCALINO INTRAPLACA NEOPROTEROZÓICO NO SUL DO ESTADO DA BAHIA: O BATÓLITO NEFELINA-SIENÍTICO ITARANTIM

Abstract

The Nepheline-Syenitic Itarantim Batholith, located at the southern area of the Alkaline Province of South Bahia, shows a Rb-Sr age of 727 ± 49 Ma, has a pear shape, and is constituted by syenites and nepheline-syenites. Around this body there is an aureole of fenites. The magmatic syenites (pyroxene-syenites with nepheline and biotite-nepheline-syenites) show difuse contacts and are disposed along a NE-SW trend. The identified geochemistry evolution allows explaining the diversity of pyroxene-syenites through fractional crystallization. However, the biotite-nepheline-syenites, which evolve to lower SiO 2 contents and expressive by of Na 2 O (up to 9.6%) and Al 2 O 3 (up to 23.5%) increase reflect, probably, crystallization in an open system which is strong influenced by peralkaline fluids, as attested by the presence of fenites and enrichment of Zr (up to 5,100 ppm), Hf (up to 141 ppm), Th (up to 117 ppm), U (up to 74 ppm), Ta (up to 169 ppm) and Nb (up to 1372) in the dykes. The Nd and Sr values put the batholith rocks at the mantle array displaying an OIB geochemistry signature. The T DM model ages of 1.1 Ga are the same observed for the toleitic magmas emplaced in this area. These data suggest that the Itarantim anorogenic magmatism is controlled by a NE-SW geosuture, and represents the rift segment which corresponds wiler the West-Congo Craton, where the alkaline magmatism is expressed by ryolites, syenites and carbonatites.