Reassessing the PT conditions of Neoproterozoic collisional metamorphism and partial melting in southernmost Brazil

Abstract

Partially equilibrated textures and partial melting products are key features in the study of high-grade metamorphic complexes, but their complexity limits the advancement of knowledge about the tectono-thermal history of ancient orogens. This difficulty is apparent in the wide interval of conditions (720–1000 °C/3–10 kbar) determined by previous studies in a high-grade complex formed during the amalgamation of West Gondwana in southernmost Brazil. In order to understand the orogen, we reanalyse the metamorphic evolution of the Várzea do Capivarita Complex through field data, petrography, whole-rock and mineral chemistry, and thermodynamic modelling. Peak metamorphic conditions (M1) are constrained with diverse mineral assemblages (Bt-Crd-Sil-Grt-Ilm/Bt-Sil-Grt-Ilm/Bt-Grt-Ilm) and overlap at 800 °C/4.5 kbar, while the partially equilibrated domain (Crd-Hc-Sil) indicates post-peak conditions (M2) of 680 °C/2.8 kbar. Models predict leucosome volume up to 20%, in agreement with field and petrographic estimates, attesting the migmatitic character. Leucogranitic veins correlating petrographically and compositionally with leucosome and melt estimates reinforce this character and the thermodynamic modelling, even with low correlations, which are explained by inherited minerals. Thus, M1 and M2 conditions indicate exhumation from depth of 16 to 10 km and high geothermal gradient, which are interpreted as due to anomalous mantle heat flow and rapid exhumation facilitated by large volumes of melt.