O REGISTRO DO VULCANISMO CALIMIANO NO ESPINHAÇO CENTRAL (MG): CARACTERIZAÇÃO PETROFACIOLÓGICA, GEOQUÍMICA E GEOCRONOLÓGICA

The Riacho Seco Formation defines the upper sequence of Mato Verde Group that occurs on the western border of the Central Espinhaco range, in northern Minas Gerais, Brazil. It constitutes an important part of the tectono-stratigraphic record of the so called Espinhaco Basin. This formation comprises a major volcano-sedimentary record associated with one of the rifting stages of Espinhaco basin first identified at the northern Aracuai belt; it comprises effusive and pyroclastic volcanic rocks as well associated epiclastic deposits, both of them with well preserved textures and structures. Lithofacies mapping supported by petrography and chemical studies allowed distinguishing four groups of genetically related volcanogenic facies, as follow: a) lava flow lithofacies; b) hydroclastic lithofacies; c) pyroclastic lithofacies; and d) epiclastic lithofacies. The U-Pb age of 1524 ± 6 Ma obtained for zircon on dacite demonstrates this volcanism is related to the second episode rifting of Espinhaco basin, Early Calymmian. The volcanosedimentary succession here studied has a correlate chrono-stratigraphic unit analogous in Northern Epinhaco range, represented by the volcanogenic succession of Bomba Formation (upper part of Pajeu Group). Both situations allow argue favorably for polycyclic evolution of the Espinhaco basin as the first rifting stage was dated at the Early Sthatherian.