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Proveniência dos arenitos da Formação Águas Claras, Província Mineral de Carajás, SE do Cráton Amazônico
(Universidade Federal do Pará, 2012-08-06) COSTA, Maria Arlete Matos da; NASCIMENTO, Marivaldo dos Santos; http://lattes.cnpq.br/1122362218673413
The Águas Claras Formation is a Precambrian lithostratigraphic unit of the Carajás Mineral Province and comprises sandstones, siltstones and pelites deposited in a geological and paleogeographic setting that encourage discussions about the evolution, the tectonic mechanisms and sedimentary environments of southeastern Amazoian Craton. This silicilastic succession is exposed in the central portion of the Carajás Province, where the sedimentary facies attest braided fluvial, deltaic to shallow marine environments. Petrography and geochemistry study of quartz-arenite, greywacke and subarkose improve provenance analysis of the Águas Claras. These sandstones are composed by mono-and polycrystalline quartz, feldspar and lithic fragments, whose modal data indicates sources from continental blocks. Accessory minerals include zircon, tourmaline, muscovite and opaque. Diagenetic vidences elucidate mineralogical changes attributed to the eo, meso and telodiagenetic regimes, including: mechanical infiltration of clays, generation of pseudomatrix, silica cement, development of sporadic sutured and concave-convex contacts (pressure dissolution) and illitization of kaolinite. Intraestratal dissolution of feldspars and heavy minerals favored the richer in quartz sandstones, production of ultra-stable assembly and precipitation of authigenic minerals. The illitization processes is controlled by the amount of kaolinite and feldspars in framework´s sandstones. Telodiagenetic regime effects the porosity and permeability of the sandstones and can be considered significant, particularly at the top of the outcrops. The CIA index indicates strong weathering of sediments and the A-CN-K diagram infer granitic and volcanic sources, as well as, suggested by detrital tourmaline data (dravite and shorlomite), standard REE, Eu anomaly, and Ce anomaly, which shows weathering of amphibolite, basalt, gabbro and granite. La-Th-Sc and Th-Sc-Zr/10 diagrams suggest sources rocks formed in an continental island environment, although the deposition of the sandstones of the Águas Claras Formation was developed in an environment of intracratonic basin, involving slow exhumation of the basement, consisting by granites, gneisses and granulites of the Archean complex Pium and Xingu. The relief of the source rocks and the subsequent weathering processes led to the quartz-feldspathic-rich sands that still preserve important information of the primary sources of sediment.