SHRIMP U-Pb age and high temperature conditions of the collisional metamorphism in the Várzea do Capivarita Complex: Implications for the origin of Pelotas Batholith, Dom Feliciano Belt, southern Brazil

Abstract

The Várzea do Capivarita Complex is composed of pelitic gneisses with subordinate calc-silicate rock, marble and rare quartzite. It is part of the neoproterozoic Dom Feliciano Belt, in southern Brazil. The gneisses are associated to veins and tabular leucogranite bodies, which are the product of anatexis of the pelitic gneiss. The paragneisses of the Várzea do Capivarita Complex are tectonically juxtaposed to orthogneisses of the Arroio dos Ratos Complex. This complex is exposed as megaxenoliths in granites of the Encruzilhada do Sul Suite and as smaller fragments in the Quitéria and Cordilheira Granites, all part of the Pelotas Batholith. The metamorphic foliation is oriented to N30°W, with dips between 35 and 55° to the SW and mineral lineation is suborizontal with rake ranging from 15° to 30° and down dip to NW and SE, suggesting deformation associated with a transpressive system. Based on the paragenesis garnet–cordierite–sillimanite-biotite, metamorphism occurred at 720–820° C and pressure of 8–9 kbar, characterizing it as of intermediate pressure and high temperature series. Zircon grains of one sample of garnet–cordierite–sillimanite-biotite gneiss and one of peraluminous leucogranite was dated by the U–Pb SHRIMP method. The paragneiss metamorphic zircon yielded an age of 619 ± 4.3 Ma interpreted as an age of the main metamorphic event, whereas igneous zircon grains from the leucogranite indicates that magma crystallization took place at 620 ± 6.3 Ma. The main metamorphic foliation (S2) of the paragneiss and the leucogranite magmatic foliation (S0) are parallel to each other, indicating that they were formed during the same event. This suggests that anatexis of the paragneisses of the Várzea do Capivarite Complex generated the peraluminous leucogranites right after the climax of the collisional metamorphism.