Dados da Submissão

Título

EDIACARAN DEFORMATION IN THE ITU SHEAR ZONE, SOUTH BRASILIA OROGEN: MICROSTRUCTURES AND Rb-Sr IN SITU DATING

Texto do resumo

The metasedimentary rocks that are intruded by the Itu granite in São Paulo, southeast Brazil, are associated with the Itapira Group, with variable lithotype and metamorphic conditions across the whole group. The highest metamorphic condition is best represented by a paraderived stromatic metatexite composed of quartz, plagioclase, K-feldspar, garnet, cordierite, sillimanite and biotite. Locally, the metatexite is intensely deformed by a subsidiary shear zone from the Itu Shear Zone, parallel to Taxaquara Shear Zone (Ribeira Belt). We investigated the petrogenesis and nature of deformation recorded in the metatexite from the Itapira Group based on conventional petrography, electron backscatter diffraction (EBSD), and in situ biotite Rb–Sr geochronology. The metamorphic conditions are inferred as minimum as upper amphibolite facies with a temperature between 750–850 °C and pressure between 2 and 9 kbar, given petrographic evidence that the rock has crossed the melting reaction sillimanite + biotite + plagioclase + quartz → garnet + cordierite + K-feldspar + liquid. The deformation temperature calculated from quartz opening-angle thermometer based on quartz crystallographic preferred orientations (CPO) data indicates a lower range for deformation between 524 ± 50 to 704 ± 50 °C, the CPO data displays a crossed girdle topology with a small dextral rotation, showing that pure shear was more significant than simple shear during the deformation. Biotite embedded in the metatexite matrix and along shear bands define a robust Rb–Sr isochron age of 570 ± 4 Ma (MSWD = 0.19 and 0.43), bracketing the timing of ductile deformation at lower temperature conditions. The age makes this subsidiary shear zone older than the Taxaquara Shear Zone.

Palavras Chave

Socorro-Guaxupé nappe; South Brasília Orogen; EBSD; Rb-Sr in situ geochronology; Ediacaran.