Abstract

The deepest exposed crust of Brazil is in the western portion of the exposed Precambrian shield of southernmost Rio Grande do Sul State and comprises a bimodal sequence of mafic garnet granulites and metatrondhjemites, intercalated with smaller volumes of metamorphosed pyroxenites, anorthosites, sillimanite gneisses, and banded iron formation. As determined by zircon U/Pb sensitive high-resolution ion microprobe (SHRIMP), this granulite terrain accreted from the mantle at the end of the Archean (ca. 2.55 Ga) and was deformed in high-pressure granulite facies metamorphic conditions in the Paleoproterozoic (ca. 2.02 Ga). A younger event, possibly ca. 900 Ma, caused local amphibolite facies retrogression in the complex. Zircons from a metabasalt and a metatrondhjemite show external morphologies typical of high-grade metamorphic rocks, whereas backscattered electron images reveal internal oscillatory zoning of their igneous precursors. Spot ages from zircon agree well with previous model Nd and Sm/Nd mineral isochron ages.

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