U-Pb geochronology of a reversely zoned pluton: Records of pre-to-post collisional magmatism of the Araçuaí belt (SE-Brazil)?.

Post-collisional reversely zoned plutons occurring in the Araçuaí belt frequently show basic cores enveloped by felsic rocks, with intense magma mingling/mixing features. The Venda Nova (VN) pluton presents, in addition to its mafic core composed of gabbronorites surrounded by syenomonzonites, a narrow ring of charnockite and norite. The VN is considered as a marker of the last magmatic phases of the Araçuaí orogenic collapse and bears one of the few high-quality Cambrian paleomagnetic data for the West Gondwana, but no geochronological data is yet available for it. In this paper, we report new in-situ U-Pb geochronological data (SHRIMP II) for the syenomonzonite, gabbro and charnockite of the Venda Nova pluton. These data are discussed and compared to the geochemical and geochronological data available for the post-collisional magmatic rocks of the Araçuaí orogen. The concordant ages of the syenomonzonite (496.9 ± 4.2 Ma, MSWD = 0.0019, n = 14) and gabbro (500.6 ± 3.7 Ma, MSWD = 0.0290, n = 14) are likely the crystallization age of the main plutonic body and corroborate the previous interpretation of paleomagnetic data. In contrast, two age groups were identified for the charnockite: Group I (n = 14, 624.0 ± 3.1 Ma, MSWD = 0.116), which includes zircon borders and rims without evidence of border reaction, and Group II (n = 6, 483.7 ± 7.4 Ma, MSWD = 0.0003) from zircon grains with luminous rims and older cores. Since the Venda Nova charnockite has (i) a distinct geochemical signature compared to other post-collisional charnockite bodies found in the same region, (ii) higher magnetic anisotropy degrees compared to other border units of the same intrusion, and (iii) marked sub-solidus deformation features, the Group I ages are suggested as the crystallization age of these rocks, while Group II ages would be better interpreted as a resetting due to the thermal influence of the syenomonzonite and gabbro intrusions. This implies that the VN is a composite magmatic complex with contributions of magmas from the pre-collisional magmatic arc and from the post-collisional melts originated in the crust and mantle across the Araçuaí lithosphere during orogen collapse.