Reconstruction of a volcano-sedimentary environment shared by the Porongos and Várzea do Capivarita complexes at 790 Ma, Dom Feliciano Belt, southern Brazil

Abstract

This work investigates the pre-collisional (before ca. 650 Ma) history of the Dom Feliciano Belt in southernmost Brazil through geochronological and zircon oxygen isotope study. U–Pb SHRIMP dating of two orthogneiss samples from the Várzea do Capivarita Complex and one metarhyolite sample from the Porongos Complex yielded crystallisation ages of 786 ± 5 Ma, 780 ± 10 Ma and 787 ± 5 Ma, respectively. The mean oxygen isotope values calculated for the ca. 790 Ma zircon cores from the orthogneisses are 8.41 ± 0.13‰ and 8.68 ± 0.14‰, and 8.75 ± 0.72‰ for the metarhyolite. Such values suggest that zircon crystallised in the more evolved magmas, either from the melting of host rocks and sediments or assimilation of crustal material by mantle-derived magmas. The detrital zircon population was analysed in one additional paragneiss sample from the Várzea do Capivarita Complex, and most of the values cluster at 790–750 Ma. The data spread is centred at ca. 790 Ma, which is the crystallisation age of the interleaved orthogneisses. In our interpretation, such dataset suggests a syn-volcanic origin of the paragneiss protolith and, therefore, a volcano-sedimentary origin of the Várzea do Capivarita Complex. The correspondence of geochronological data and zircon oxygen isotope values for the studied meta-igneous samples suggests that the Várzea do Capivarita and Porongos complexes have shared the same igneous history. Therefore, the samples probably represent one magmatic event at different levels of a single basin at ca. 800–770 Ma. Such results bring first-order information about the meaning of tectonic limits in this Gondwana-related belt and implications for reconstructing the pre-collisional history of the orogen.