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Romulo Machado, Universidade de São Paulo (Brazil)
Nolan Dehler , Petrobras (Brazil)
Paulo Vasconcelos, University of Queensland (Australia)
Tiago Karniol, Universidade de São Paulo (Brazil)
Issamu Endo, Universidade Federal de Ouro Preto (Brazil)
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This work presents 40Ar/39Ar ages of the extensional structures situated in the Cajati region, southern part of the Ribeira belt, southeastern region of Brazil. The obtained geochronological data show ages between 600 Ma and 570 Ma. The studied structures were generated under amphibolite facies metamorphic conditions and developed during progressive uplift and regional crustal cooling. Kinematic indicators related to ductile deformation indicate top-down-to E/SE movement. This movement is kinematicaly compatible with extensional structures described in other tectonic segments of the Neoproterozoic terranes situated in the southern Sanfranciscan plate - in the eastern region of Quadrilátero Ferrífero, and in the region of Rio Doce Valley, which represents the internal domain of the Araçuaí belt. These structures have been related to continental-scale extension which occurred during our slightly after the collisional phase of the orogeny, because they are apparently coeval with contractional structures in the belt. We suggest that the shear zones played an important role, in the space and time, in the generation and emplacement of voluminous granitic magmatism associated with the Neoproterozoic belts situated in the south and southeastern regions of Brazil and Uruguay. The alkaline magmatism of the Serra do Mar suite (600-570 Ma) found in the southern portion of the Ribeira belt (Costeiro Granite Belt and Luis Alves Terrane) is contemporaneous with the development of these structures. Recent U/Pb data (in zircon and monazite), obtained in mafic metavolcanic rocks and metarhyolite dikes in the Açungui basin, and an allocthonous ophiolitic complex (Pirapora ophiolite) found in the São Roque Group, suggest the maximum age of about 620 Ma for the development of the extensional structures described in this paper. They are certainly linked to an event much older than the extensional event (520-500 Ma) identified in the northern part of the Ribeira belt and in the external domain of the Araçuaí Orogen.
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