Petrographic characterization and preliminary geochronologic data from volcanic rocks of the Iriri formation, southern-central portion of the Amazon Craton, Aripuana - Mato Grosso

The Moriru area is located in the south-central portion of the Amazon Craton, near the border of Mato Grosso and Amazonas states. The research was carried out using eight drill cores, 200 meters deep in average. The description of these drill cores allowed the identification of felsic lavas and pyroclastics inter layered with mafic flows volcanoclastic and sedimentary rocks. Granitic intrusions are completely altered by hydrothermal metamorphism. Later stage dikes were identified. The pyroclastic successions is characterized by ignimbritic rocks generated by pumice flows. The ignimbrites are enriched in pumice and phenocrysts and are classified as crystal enriched ignimbrite. The content of fine particles is low and the welding grade ranges from good to medium. The composition of ignimbrites is rhyodacite to dacite. U-Pb zircon ages are 1801+- 11 Ma for ignimbrites and 1848+-9 Ma for mafic volcanics. However, for the mafic rocks age may represent some inherited zircon. More data from these rocks are necessary before we can make a definitive conclusion. The air fall deposits are represented by ash tuff enriched in vitroclasts. The volcanogenic sedimentary deposits occur inter layered with pyroclastic deposits and are represented by volcanoclastic conglomerates and immature tuffaceous sandstones. The volcanic particles in this sequence, partially maintain igneous characteristics, which suggests small amount of reworking and a possible proximity to the source. Sulfides occur disseminated in sub-volcanic and in late-stage quartz-carbonate-chlorite veins. (author)