Geologia, petrologia e geocronologia (Pb-Pb) da formação Serra da Bocaina : terreno Rio Apa – Sul do Cráton Amazônico

Abstract

This work presents geological, petrological and geochronological (Pb-Pb zircon) results of Serra da Bocaina, emphasizing the homonymous geological formation that belongs to the Amoguijá Magmatic Arc of Rio Apa Terrene, southernmost of Amazonian Craton, southwestern of Mato Grosso do Sul state. Geological mapping and petrographic features allowed the discrimination of five facies, four ignimbritic: Ignimbrite Breccia (facies 1), Crystal Rich Ignimbrite (facies 2), Co-ignimbritic Tuff (facies 3) and Rheomorphic ignimbrite (facies 4), and an effusive (facies 5). Rocks of facies 1 to 4 consist of distinct pyroclastic products, such as, lithic and pumiceous fragments, crystal clasts, pumice, fiamme, shards and crystallites of various dimensions and compositions, as well as quartz, plagioclase and alkali feldspar phenocrysts, within an ashy matrix. The facies 5 consists of effusive rocks with porphyritic to glomeroporphyritic texture with plagioclase, quartz and alkali feldspar phenocrysts within a quartz-feldspathic groundmass. Apatite, zircon and opaque minerals are accessory phases and sericite, epidoto-clinozoisite, chlorite, clay minerals are secondary phases. The structural frame allowed the identification of two compressional stages of deformation of ductile and ductile-brittle behavior. The older ductile deformation promoted the development of penetrative foliation, stretching lineation and ductile shear zones. Shear sense indicators show a hanging wall dislocation to NWW for the Santa Rosa Shear Zone, which represents the boundary between facies 1 rocks and the Carandá Granite. The younger ductile-brittle deformation is less intense and consists of incipient crenulation and disjunctive cleavages and gentle folding of S1. The whole volcanic succession displays a mineral assemblage of albite + epidoto + chlorite that is consistent with lower grade metamorphism of green schist facies. The Serra da Bocaina Formation rocks have a dominantly acid composition with local intermediate occurrences that can be classified as rhyolite and andesite evolved through fractional crystallization from a medium- to high-K calc-alkaline magma in a volcanic-arc setting. Pb-Pb geochronological results indicate an Orosirian age of 1878 ± 4 Ma suggesting relation with the evolution of the Amoguijá Magmatic Arc.