Petrologia dos granitóides sintectônicos à Zona de Cisalhamento Transcorrente Dorsal de Canguçu, Encruzilhada do Sul, RS

Abstract

The Quiteria, Arroio Francisquinho and Cordilheira metagranites are syntectonic intrusions in the Dorsal of Cangucu Transcurrent Shear Zone. The Quiteria metagranite shows an I-type signature taking into account its mineralogy, type of enclaves and chemistry, although the initial isotope ratios ( 87 Sr/ 86 Sr = 0.716), negative e Nd . values, lower Zr and V contents suggest a mixed origin involving crustal and mantle derived materials in its generation. Field data, petrograpmc, geochemical and numerical modelling evidence indicate the AFC process as the dominant mechanism during the evolution of this granitoid. The parental magma is represented by microdioritic enclaves, but crustal assimilation of host rocks of calc-alkaline orthogneisses and subordinately paragneisses contributed to the hybrid genesis for these rocks. The emplacement of leucogranites of the Cordilheira Granitic Suite represents the late magmatism syntectonic to this shear zone. These rocks have a similar S-type signature as demonstrated by their mineralogical and chemical compositions, with high SiO, normative corindon (> 2 %) and high initial 87 Sr/ 86 Sr isotope ratios and strong e Nd negative values. This indicates crustal melt as the dominant mechanism in the evolution of this suite. On the other hand, field relations, enclave typology, petrologic and chemical characteristics of these rocks do not indicate an S-type source for these peraluminous rocks. Dominant melting of crustal orthogneisses with xenoliths of metasedimentary rocks may explain their origin.