Petrologia do Granito Chasqueiro, região de Arroio Grande, sudeste do Escudo Sul-Rio-Grandense

Abstract

Chasqueiro Granite comprises an 400 km2 N50ºEtrend elongated body located at the southernmost portion of the eastern Sul-Rio-Grandense Shield. It consists of monzo to syenogranite, light gray and leucocratic porphyritic texture with megacrystals K-feldspar in a coarse equigranular hypidiomorphic matrix, composed of quartz, K-feldspar, plagioclase, biotite, hornblende, opaque minerals and accessories. The modal megacrystals reach proportions between 30.0 and 60.0%, and range from 4 to 8 cm, with mafic minerals of the matrix showing a subvertical magmatic foliation and a tectonic foliation prominent in the borders marked by intense ductile deformation and protomylonite generation. Another important aspect of the Granite Chasqueiro is the obiquitous presence of microgranular mafic enclaves of dioritic composition in the granite, having different shapes and sizes. The Chasqueiro Granite is high-K calc alkaline, metaluminous to slightly peraluminous and characterized by a narrow SiO2 variation, with characteristic signature of granites generated in post-collisional tectonic setting. Geochronological analyzes obtained by the U-Pb method (LA-ICP-MS) and isotope geochemistry of Lu-Hf zircon data (LA-ICP-MS) presenting, respectively, crystallization age 574 ± 3 Ma and negative εHf, which suggest a granite relationship with deformational event D2 and supporting a magmatic crustal sources with subordinate mantle component.