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Paleoproterozoic Domo of Lava from Iriri Group – Sonho Meu Farm – Northeast of Mato Grosso, Amazon Craton: geology, geochemistry, and geochronology

2012; Sociedade Brasileira de Geologia; Volume: 42; Issue: 3 Linguagem: Inglês

10.25249/0375-7536.2012423471488

2177-4382

Mara Luiza Barros Pita Rocha, Márcia Aparecida de Sant’Ana Barros, Evandro Fernandes de Lima, Ronaldo Pierosan,

Geological and Tectonic Studies in Latin America

The Iriri Group is composed of acid to intermediate effusive and pyroclastic rocks, which crop out in Northeastern Mato Grosso state, Western Vila Rica. In the vicinity of Sonho Meu Farm, there is some part of an acid to intermediate lava dome, which is 2 km long and 250 m wide. The dome has a hypabyssal core that changes laterally to an autoclastic facies rich in pumice and shards with mingling textures, and subsequently to a facies with flow structures. The core has porphyritic texture and phenocrysts of plagioclase and biotite embedded in a medium-grained matrix of the same composition. The pumice-rich facies has a texture similar to ignimbrites and reoignimbrites. In this facies, a porphyritic to glomeroporphyritic texture is observed, with evidence of pumice collapse. The magma mixture is characterized by clusters of hornblende and plagioclase, in the form of millimetric to decimetric enclaves. Geochemical data on the rocks with no evidence of mingling indicate metaluminous to peraluminous high-K calcalkaline compositions. Rare earth elements patterns show a slight enrichment in LREE over HREE. Geochemical data show an enrichment in Rb, Sr, and K2O and a negative Ta-Nb anomaly, which is common in calcalkaline rocks. Concentrations of trace elements are compatible with post-collisional tectonic environments. U-Pb zircon geochronology yields an age of 1.87 Ga. Sm-Nd isotopic studies report negative €Nd values and Archean Nd model ages, indicating contamination by crustal material. This paper suggested that such dome was formed by melting of the lithospheric mantle, modified by previous subduction events, with subsequent magmatic differentiation.