STRUCTURAL AND PETROLOGICAL CHARACTERIZATION OF PRE-COLLISIONAL MAGMATISM IN THE SUL-RIO-GRANDENSE SHIELD: THE VÁRZEA DO CAPIVARITA METAMORPHIC COMPLEX ORTHOGNEISSES

Abstract

The Várzea do Capivarita Metamorphic Complex, situated in the region of Encruzilhada do Sul, RS, is part of the Sul-rio-grandense Shield. It encompasses an orthometamorphic sequence of tonalitic composition, as well as pelitic and calc-silicate paragneisses. Ortho and paragneisses are interleaved as 3- to 4 meter thick slabs, in average, where parageneses and microstructures are compatible with granulite-facies metamorphism. An integrated approach consisting of field observations, structural geology and petrography permits to discuss the geological evolution of this complex. Additionally, major and traceelement data were obtained from the tonalitic gneisses in order to investigate the nature of the magmatism which has generated their protoliths. The gneisses exhibit a NNW-striking metamorphic banding , dipping variably toward ENE or WSW, which bears a stretching lineation of generally high rake angle. The original orientation of the banding, later fold effects being suppressed, is interpreted to have been sub-horizontal. The piling of different lithological units, regarding composition and geological environment, along originally sub-horizontal planes leads to their interpretation as a thrust pile. The orientation of high-grade mineral assemblies along the banding of all lithological types indicates that this thrust tectonics has taken place under granulite-facies metamorphic conditions. The tonalitic gneisses are metaluminous to peraluminous and belong to the calcalkaline series. Their composition and trace- element patterns suggest that they are representative of a continental arc magmatism comparable to the one described in the Encantadas Complex, a Paleoproterozoic magmatic arc assembly from the Sul-rio-grandense Shield. Based on their compositional and stratigraphic similarities with the Encantadas Complex, the studied tonalitic orthogneisses are interpreted to be part of the same Paleoproterozoic continental margin magmatism. It is assumed that the granulite-facies metamorphic event is of Neoproterozoic age. Additionally, it would have been synchronous with the thrust event that lead to the tectonic interleaving of rock sequences of different provenance and age that has generated the Complex.