Granito Coimbra : porção norte da suíte intrusiva alumiador na região de Corumbá (MS) : terreno Rio Apa

Abstract

The rocks studied for this paper belongs, geographically speaking, to the frontier between Brazil and Bolivia, located in Corumbá City region (South Mato Grosso State) and, geologically speaking, in Apa River Terrain. The mapping of Corumbá south portion allowed the identification of Coimbra Granite, amphibolites trapped by dyke forms, in addition to sedimentary rocks from Jacadigo and Corumbá Groups and Pantanal Formation. The macroscopic and microscopic petrographic studies allowed the definition of three facies from Coimbra Granite: Pink Porphyritic Facie, which is the one more voluminous and characterized macroscopically by leucocratic rocks, with a gray pink and pink colors, inequigranulars grains, varying from porphyritic to porphyroclast, with a matrix containing thin to medium grains, classified as syenogranite and monzogranite. The Medium to Coarse Gray Facie are characterized by leucocratics rocks, with gray to dark gray colors, equigranulars and inequigranulars grains, witch varied from medium to thick ones, porphyritic matrix, classified as granodiorites and tonalites. The Pink Thin Facie is represented by aplite dikes, leucocratic, with pink to red pink colors, thin equigranulars and inequigranulars grains, classified as alkalis granites. The Coimbra Granite passed through to two deformation phases, a F1 ductile phase and a F2 ruptile phase. The F1 phase generated a penetrative S1 foliation with N10-20E orientation and a steep-dip with 75-90º for SE. In F1 phases there are shear zones represented by mylonites which have the same attitude from the regional foliation. The F2 phase, which has a ruptile characteristic, was settled in after the Neoproterozoic depositions, and generated big faults and joints. As a product from F2 phase are noted gaps, cataclastic bands, protocataclastic and cataclastic. The fractures are represented by W-E, N-S and NW patterns. The rocks from Coimbra Granite have an intermediate acidic composition, generated in an arc-magmatic environment stem from a calcic-alkaline magmatism, metaluminous and peraluminous with a magnesian with ferrous origin characterized, geochemically, as tonalities, granodiorites, granites and alkaligranites. The geochronological and isotopic data shows the crystallizing age of 1859±4 Ma (U-Pb SHRIMP) for Coimbra Granite with εNd (1,86Ga) of -1,35 and a model age for TDM of 2,27 Ga which indicates a participation of a crustal source in the magma origin, perhaps involving partial fusion process of continental crust. The Coimbra Granite is correlated to Alumiador Intrusive Suite, so is hypothesized that Amoguijá Arc Magmatic extends to the Corumbá (MS) region, in other words, Apa River Terrain extends to the Corumbá region.