Deepest exposed crust of Brazil-SHRIMP establishes three events

The deepest exposed crust of Brazil is in the western portion of the expose d Precambrian shield of southernmost Rio Grande do Sul State and comprises a bimodal sequence of mafic garnet granulites and metatrondhjemites, interc alated with smaller volumes of metamorphosed pyroxenites, anorthosites, sil limanite gneisses, and banded iron formation. As determined by zircon U/Pb sensitive high-resolution ion microprobe (SHRIMP), this granulite terrain a ccreted from the mantle at the end of the Archean (ca, 2.55 Ga) and was def ormed in high-pressure granulite facies metamorphic conditions in the Paleo proterozoic (ca, 2.02 Ga). A younger event, possibly ca, 900 Ma, caused loc al amphibolite facies retrogression in the complex. Zircons from a metabasa lt and a metatrondhjemite show external morphologies typical of high-grade metamorphic rocks, whereas backscattered electron images reveal internal os cillatory zoning of their igneous precursors. Spot ages from zircon agree w ell with previous model Nd and Sm/Nd mineral isochron ages.