Zonas de cisalhamento normais no SW do Cráton Amaz?nico: análise microestrutural das rochas miloníticas do Granito Indiavaí

摘要

Ductile shear zones of normal kinematics in continental orogens previously thickened by compressive orogenic events have aroused great interest in the scientific community. In the region of Indiavaí southwestern portion of the Amazonian Craton, central west portion of Brazil, the água Rica and Cristo Rei shear zones affect the rocks of the Indiavaí Granite. Analysis of the structures on a mesoscopic scale reveals a progressive transformation of a granitic protolite with a weak foliation, passing to rocks with a protomillonitic texture, milonitic to ultramillonitic. Microstructural analysis shows a gradual decrease in grain size, leading to an increase in the matrix. Rocks with incipient foliation have no matrix. Recrystallized quartz is observed, wavy extinction in feldspars, intense processes of sericitization in plagioclase, and orthoclase with pertites in flames. In the protomilonites, the matrix, making up 20 to 30% of the rock, is composed of quartz and biotite. Potassium feldspar porphyroclasts are observed, with fractures, faults and undulating extinction. In the milonites the matrix, which makes up 60 and 75% of the rock, is composed of quartz, biotite and muscovite. Quartz porphyroclasts commonly occur in the form of ribbons, which show recrystallization by migration of the grain boundary. Potassium feldspar and plagioclase exhibit wavy extinction and lump recrystallization. In ultramilonites, whereas the matrix predominates, this is basically composed of quartz, muscovite and medium grained biotite. It is estimated that the processes that acted in the development of mylonitic foliation occurred at temperatures between 400–600°C. Analysis of the kinematic indicators reveals that the tectonic movement was normal. It is interpreted that the Cristo Rei and água Limpa shear zones are part of an extensional shear system developed in intermediate crust during the Tonian period.