Paleoproterozoic sources for Cordilleran-type Neoproterozoic granitoids from the Araçuaí orogen (SE Brazil) : constraints from Hf isotope zircon composition.

Abstract

The Araçuaí orogen of SE Brazil exposes numerous amounts of magnesian, metaluminous, calc-alkaline, and medium- to high-K granitoid batholiths that were emplaced during early convergence between the São Francisco and Congo cratons during West Gondwana assembly in the late Neoproterozoic. Previously proposed sources for these pre-collisional granitoids include the Paleoproterozoic basement and mantle-derived magma based on isotopic signatures, abundant microgranular enclaves and contemporaneous mafic intrusions. Zircons from migmatitic basement gneiss, a noritic intrusion and five granitoid intrusions were analysed for U–Pb–Hf isotopes to evaluate their magma source. Paleoproterozoic migmatites represent addition of juvenile crust in an accretionary orogenic setting at ca. 2.2–2.0 Ga evidenced by their positive zircon εHf values. The Hf isotopic composition of zircon from pre-collisional granitoids has a large range of negative εHf values between −28.8 and −1.7 indicating crustal protoliths. This range of unradiogenic Hf isotopic compositions is explained by inefficient mixing of magmas derived from felsic and mafic Paleoproterozoic sources that hybridised during incremental growth of the plutons. Zircon grains of contemporaneous mafic intrusions display slightly higher, yet enriched, Hf isotopic compositions consistent with crustal contamination of mantle magmas. The new Hf isotope data, integrated with other chemical and isotopic evidence, suggest derivation of the pre-collisional granitoids from lower crustal sources of the Paleoproterozoic basement, with only minor contributions from mantle-derived magmas. Thus, magmatism of the Araçuaí orogen related to the assembly of West Gondwana was dominantly a crustalreworking event.