Geologia, petrografia e geoquímica das rochas vulcânicas da área Sul de São Félix do Xingu (PA), Província Carajás

Abstract

The volcanic rocks of south area of São Félix do Xingu, center-south of the Pará State, are inserted in the geologic context of the geocronologic Amazonia Central Province, southeastern of the Amazonian craton. These rocks are dominantly related to the Sobreiro Formation, and subordinate to the Santa Rosa Formation, both belonging to the paleoproterozoico Uatumã group. The Sobreiro Formation is found in lightly hilly and broken topography, with lower levels of 300m. In the study area, the few occurrences of the Santa Rosa Formation also do not exceed levels of 300m. Three distinct facies of the Sobreiro Formation had been identified: 1- Subaerial Lava flow of subalcalina composition facies, this it constitutes the dominant facies of the studied area; 2- Subaerial lava flow of calc-alkaline to shoshonitic composition facies; and, 3- Subaerial volcaniclastic facies. All described samples of the Santa Rosa Formation had been fit in simple facies called, Subaerial lava flow facies. The volcanic rocks of the Sobreiro Formation are individualizaded in wide specter of petrographic lithotype: 1-plagioclase-amphibole microporphyritic-basaltic andesite, 2-plagioclase-clinopyroxene-amphibole-microporphyritic to porphyritic andesite, 3-plagioclase-amphibole-microporphyritic andesite, 4-K-feldspar-plagioclase-clinopyroxeneporphyritic traquiandesite, 5-K-feldspar-plagioclase-amphibole-microporphyritic to porphyritic traquiandesite, 6-plagioclase-amphibole-microporphyritic traquiandesite, 7-amphibole-porphyritic traquiandesite, 8-plagioclase-K-feldspar-amphibole-microporphyritic to porphyritic traquiandesite, 9-amphibole-K-feldspar-plagioclase-porphyritic traquite, and 10-plagioclase-K-feldspar-porphyritic traquite (Facies 1); 11-calc-alkaline to shoshonitic plagioclase-amphibole-clinopyroxene-porphyritic traquiandesite, 12-calc-alkaline to shoshonitic amphibole-K-feldspar-plagioclase-microporphyritic to porphyritic traquiandesite, 13-calc-alkaline to shoshonitic K-feldspar-plagioclase-amphibole-porphyritic traquiandesite, and 14-calc-alkaline to shoshonitic amphibole-K-feldspar-plagioclase-porphyritic traquite (Facies 2); Felsic crystal tuffs, Mafic crystal lapili-tufos and polymictic breccias (Facies 3). The lithotypes of the Santa Rosa Formation vary of plagioclase-K-feldspart-quartz rhyolites to K-feldspart-quartz rhyolites. The geochemical data of these rocks show that the Sobreiro Formation have contents of SiO2 between 52,14 and 69,21% (wt) and K2O/Na2O ration between 0,16 and 1,62. On the other hand, the volcanites of the Santa Rosa Formation form a evolved series with contents of SiO2 between 72,27 and 77,14% (wt) and K2O/Na2O ratios between 1,50 and 2,12. The Sobreiro Formation is main calc-alkaline weakly transitional calcalkaline to shoshonitic magmatic serie, has metaluminous to weakly peraluminous composition and tectonic signature of volcanic arc environment. The Santa Rosa Formation has peraluminous to weakly metaluminous composition and A-type signature with intraplate tectonic affinity. Comparative studies of the volcanic rocks of the south area, edge east of the second great curve of the Xingu River, with the volcanic rocks of the west/southwestern area, edge west, shows to perfect correlations of the Sobreiro and Santa Rosa Formations in both the areas and indicates lateral continuity of these units.