THE MARA ROSA-ARC IN THE TOCANTINS PROVINCE - FURTHER EVIDENCE FOR NEOPROTEROZOIC CRUSTAL ACCRETION IN CENTRAL BRAZIL

The Mara Rosa volcano-sedimentary sequence consists of several NNE-tre nding belts of metavolcanic (metabasalts to metarhyolites) and metased imentary rocks (micaschists, quartzites, cherts), exposed over large a reas of the Tocantins Province, a Neoproterozoic (Brasiliano) orogenic region in the central part of Brazil. The supracrustal belts are sepa rated by terrains dominated by metatonalites and metadiorites and intr uded by several post-orogenic granitic and dioritic bodies. These volc ano-sedimentary and associated metaplutonic rock associations have bee n previously interpreted as a typical granite-greenstone terrain of th e Archaean basement. In this paper we present geochronological and iso topic data for rocks of the metavolcanic-metaplutonic rock association . Samples of a felsic metavolcanic rock from the Posse gold mine, and of a metatonalite yielded U-Pb zircon ages of 862 +/- 8 Ma and 856(-7) (+13) Ma, respectively. These are interpreted as crystallization ages of the igneous protoliths. Titanites from the same metavolcanic rock s ample yielded a concordant recrystallization age of 632 +/- 4 Ma. Sm-N d isotopes for these rocks indicate primitive compositions, with epsil on(Nd)(T) Of +4.6 and +3.7 and T-DM model ages of similar to 1.0 Ga. A syn-tectonic dioritic intrusion has an U-Pb crystallization age of 63 0 +/- 6 Ma, and epsilon(Nd)(T) Of +1.9 and T-DM of similar to 1.0 Ga. Additional Sm-Nd analyses on metasedimentary rocks and post-orogenic g ranites of the Mara Rosa region yield T-DM ages between similar to 1.2 and 1.0 Ga. The isotopic data, combined with preliminary trace elemen t results, suggest that the protoliths of the metavolcanic and metaplu tonic rocks investigated were formed in an island are system, off the coast of the Sao Francisco-Congo continent at around 860 Ma. Recrystal lization and deformational ages of similar to 630 Ma may represent the timing of final ocean closure and continental collision. The rock ass ociations, structural pattern, geochronological and isotopic character istics of the rocks from the Mara Rosa region are very similar to thos e of Neoproterozoic are terrains exposed similar to 300 km to the sout hwest, in the Arenspolis area, proving the regional importance of the Neoproterozoic crustal accretion event in the central part of Brazil.