Re-setting of the Sm-Nd isotopic system during the neo proterozoic in gneiss from Western Goias

The Nd, Sr and Pb isotopic compositions of the Ribeirao gneiss in western Goias demonstrate that these rocks have been through a very complex crustal history. Sm and Nd isotopes show that the sialic material forming these gneisses has been resident in the terrestrial continental crust since the Archaean. Whole-rock Rb-Sr and Pb-Pb isochrones indicate a Paleoproterozoic re-homogenization event (ca. 1.8 Ga) while the whole-rock Sm-Nd isotopic system revels an even younger re-setting event. The Neo proterozoic re-equilibrium of the Sm-Nd isotopic system is contemporaneous with an important regional tectonomagmatic event which was characterized by intense island arc-type magmatic activity giving origin to a series of volcano-sedimentary and orthogneissic units in western Goias (e.g. Arenopolis volcano-sedimentary sequence and Arenopolis, Matrinxa and Sanclerlandia ortho gneisses). The Neo proterozoic isotopic re-homogenization is responsible for the broad range in TDM model ages obtained for the individual samples (3.8-2.8 Ga). None of these model ages, therefore, bear crust formation significance. (author). 22 refs., 8 figs., 3 tabs