Caracterização do Complexo Pocrane, magmatismo básico mesoproterozóico e unidades neoproterozóicas do Sistema Araçuaí Ribeira, com ênfase em geocronologia U Pb (SHRIMP e LA ICP MS)

Abstract

The study was conducted in the southern sector of the orogen Araçuaí and extreme north of Ribeira Orogen between parallels 19 ° and 21 ° 45'S, covering a region that is distributed by the states of Minas Gerais, Rio de Janeiro and Espirito Santo. The objective was to characterize the Pocrane Complex, Mesoproterozoic basic magmatism and Neoproterozoic units in the Ribeira-Araçuaí System, with emphasis on U-Pb geochronology (SHRIMP e LA-ICP-MS). The Pocrane Complex is the main focus of the Paleoproterozoic basement studies. It is an orthoderived unit composed of banded hornblende-biotite gneiss, relatively little migmatised with mylonitic terms. Commonly shows porphyroclasts of plagioclase and quartz-feldspathic aggregates with corrosion gulfs and / or jagged edges, wrapped in fine array, suggesting that part of the Pocrane Complex has volcanic -volcanoclastic genesis. Chemical study indicate that the rocks of this complex are peraluminous to metaluminous, predominantly calc-alkaline medium to high potassium, and subordinately tol heitic. The great chemical, isotopic and geochronological similarities, beyond geographic proximity, justify correlation between the Pocrane Complex and Juiz de For a Complex. In this context the Juiz de Fora Complex and Pocrane Complex formed in island arc environment. The complexes are contemporary (2,08 to 2,19 Ga), but represent different crustal sections: Pocrane encompasses a more shallow plutonic-volcanic, and Juiz de Fora is only plutonic. The Mesoproterozoic basic magmatism is represented by amphibolitic lenses that occur interspersed in the Pocrane Complex gneiss. The basic magmatism mark two taphrogenic events related to continental rifting; one during Calimian another in Estenian. Still in Esteniano has the deposition of Córrego Uba Quartzite, interpreted as evidence of a precursor basin of Brasiliana Orogeny in the field of Rio Doce Magmatic Arc, the maximum age of deposition for this unit was 1176 ± 21 Ma, suggesting a extensional basin related to the estenian event. The Neoproterozoic units that occur in the region of the thesis are related to the Rio Doce Magmatic Arc, whose plutonic portion is represented by G1 Supersuit, which includes granites (sl), migmatites and gneisses, with biotite and hornblende, and the charnockitic rocks of the Divino Suit. Rio Doce Group was characterized as metavolcano-sedimentary unit composed by arc-related basins and volcanic rock. The plutonic portion of the arc is given by calc-alkaline suites generated in active continental margin environment with strong involvement of Paleoproterozoic continental crust. The ages obtained for the rocks of G1 Supersuit vary between 586 Ma and 621 Ma. The volcano-sedimentary coverage is represented by the Rio Doce Group. Palmital do Sul Formation consists of garnet-biotite schist interpreted as proximal arch, which pelitic protolith. The São Tomé Formation consists of staurolite-biotite schist which represents a phase bit more distal of the arch, with decreasing of sedimentation energy where would be deposited silt-clay sediments. João Pinto Formation marks an increase in energy of sedimentation that occurs in the plataformal marine environment in hot weather, conducive to deposition of quartz sandstone interbedded with micaceous and arcosean sandstone. Pyroclastic deposits represented by tuffs rich in lapilli fraction, with sparse volcanic bombs represent explosive volcanic buildings, associated with the sedimentary deposition of the Rio Doce Group.