Idade Pb-Pb E Aspectos Petrológicos Da Mineralização Em Sodalita Azul Do Maciço Nefelina-Sienítico Itarantim, Sul Do Estado Da Bahia

Abstract

Pb-Pb AGE AND PETROOLOGICAL ASPECTS OF THE BLUE SODALITE MINERALIZATION OF THE ITARANTIM NEPHELINE-SYENITE MASSIF, SOUTHERN STATE OF BAHIA The Itarantim nepheline-syenite massif (220 km2) is a late proterozoic intrusion (727±30 Ma; Rb-Sr) of the southern region of the State of Bahia. It is mostly constituted by nepheline-bearing syenites, with biotite or aegirine, and by alkaline dykes. The dykes are more abundant in the biotite-nepheline-syenites and are the most evolved rocks of the massif. Recently, a blue sodalite-syenite dyke has been identified in the southwest part of the massif, hosted by biotite-nepheline syenites. The blue syenites have been exploited as dimension stone. The dyke has magmatic sodalite that crystallized between 720 ± 9 Ma and 732 ± 24 Ma (Pb-PbZr), and cuts the magmatic foliation of the host syenite, imprinting a metasomatism with sodalite formation. Petrographic data indicate that the magmatic sodalite formed during the late stages by a volatile rich magma (500–600o C). The high contents (ppm) of Zr (>5,000), Hf (141), Ta (169), U (74) and F (1200) of the blue syenite suggest that it represents a highly evolved product from differentiation of the nepheline-syenitic magma.