THE ALKALINE POST-COLLISIONAL VOLCANISM OF THE CAMPO ALEGRE BASIN, SOUTHERN BRAZIL: PETROGENETIC ASPECTS

Abstract

The Campo Alegre Formation is composed of volcanic rocks related to the silica saturated alkaline series and represent part of post-collisional Neoproterozoic magmatism (595 ± 16 Ma) at the end of Brasiliano-Pan-African Orogenic Cycle. Rhyolites are the most abundant lithological types, and occur associated to basalts, trachytes, trachydacites, auto-breccia, pyroclastic flow deposits, dikes and ignimbrites. Two lithological associations, both of alkaline affinity, but with different contents of major and trace elements (e.g. Mg, P, Zr, Y, Nb, and REE) were distinguished. Their parental basaltic magmas probably represent different fractions of melting from lithpspheric mantle metasomatized by subduction-related fluids during the previous Brasiliano-Pan-African Cycle. The Campo Alegre Basin volcanism is correlated with the Acampamento Velho Formation in the southern portion of this region, in the Sul-rio-grandense Shield