Abstract
The Mara Rosa volcano-sedimentary sequence consists of several NNE-trending belts of metavolcanic (metabasalts to metarhyolites) and metasedimentary rocks (micaschists, quartzites, cherts), exposed over large areas of the Tocantins Province, a Neoproterozoic (Brasiliano) orogenic region in central Brazil. The supracrustaJ belts are separated by terranes dominated by metatonalites and metadiorites and intruded by several post-orogenic granitic and dioritic bodies. This metavolcanic/metaplutonic rock association has been previously interpreted as a typical Archaean granite-greenstone terrane and, therefore, part of the sialic basement to the Brasiliano supracrustal fold belt. In this paper we present the first reliable geochronological and isotopic data for rocks of the Mara Rosa sequence and associated metaplutonic rocks. 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 +13/-7 Ma, respectively. These are interpreted as crystallization ages of the igneous protoliths. Titanite from the metavolcanic rock sample yielded a concordant recrystallization age of 632 ± 4 Ma Rb-Sr geochronological data for orthogneissic, metasedimentary and mylonitic rocks indicate the closure of Rb-Sr isotopic system at ca. 600 Ma after isotopic re-homogeneization. Sm-Nd isotopes for these rocks indicate primitive compositions, with e Nd (T) of +4.6 and +3.7 and T DM model ages of ca. 1.0 Ga. A syn-tectonic dioritic intrusion has an U-Pb crystallization age of 630 ± 6 Ma, and e Nd (T) of +1.9 and T DM of ca. l .0 Ga. Additional Sm-Nd analyses of metasedimentary rocks and post-orogenic granites of the Mara Rosa region showed T DM ages between ca. 1.2 and 1.0 Ga. The isotopic data, combined with preliminary trace element results, suggest that the protoliths of the investigated metavolcanic and metaplutonic rocks formed at ca. 860 Ma in an island arc system, off the coast of the Sao Francisco-Congo continent. Recrystallization and deformation ages of ca. 630 Ma could represent the timing of final ocean closure and continental collision. The rock associations, structural pattern, geochronological and isotopic characteristics of the rocks from the Mara Rosa region are very similar to those of Neoproterozoic arc terrains exposed ca. 300 km to the southwest, in the Arenopolis area, proving the regional importance of the Neoproterozoic crustal accretion event in the central part of Brazil.
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