Múltiplos eventos tectônicos de um orógeno erodido: caracterização tectono metamórfica do Sistema de Arco Magmático Externo (OMAS) e Terreno Cabo Frio, Faixa Ribeira Central

Abstract

The Outer Magmatic Arc System (OMAS) represents a long-lived magmatic arc that collided with the Inner Magmatic Arc System (IMAS) at ca. 590 Ma (M1 metamorphism), and at ca. 530 Ma with the Cabo Frio Terrane (Angola Block) (M2 metamorphism). The subduction polarity of the second collision and the nature of the magmatism recorded in the first collisional episode at ca. 590-565 Ma is in debate nowadays. Based on U-Pb and Lu-Hf isotopic analysis using LA-ICP-MS in zircon grains from the metasedimentary units of the São Fidélis Group and Búzios Succession, besides the units associated with the syn- and post-collisional stages, an evolutionary model is proposed. The results showed that the rocks of the São Fidélis Group are in a back-arc position of the OMAS and were affected by both M1 and M2 events. The metamorphic rims of the detrital zircon grains yielded ages between ca. 592-553 Ma and 543-533 Ma, both with 176Hf/177Hf rations similar to the inherited cores, indicating recrystallization of preexisting grains, as well new grains are grown in melting phases coeval to the M1 and M2 events. Orthogneiss of the Tinguí Complex resulted from the M1 syn-collisional stage, displayed a crystallization age of 578 Ma and a metamorphic overgrowth age of 529 Ma (M2). The Búzios Succession yielded just the M2 metamorphism, interpreted as a passive margin, associated with the recycling of the São Fidélis Group and OMAS units. M2 event resulted from the closure of the oceanic space between the OMAS and Angola block. Structural analysis of the southern segment of the Cabo Frio Thrust (CFT) demonstrated that the variation in thrust vergence is associated with its geometry, defined by a refolded F2 fold. Folded leucosome dating in the orthogneiss of the Região dos Lagos Complex resulted in age of 523 ± 10 Ma, constraining the time of folding after thrusting. The effect of the M2 was also verified in this orthogneiss, with crystallization age of 1968 Ma and metamorphic age of 522 Ma. Undeformed granites and pegmatites that intrude both terranes showed crystallization ages between 496 and 480 Ma, marking the orogenic collapse, and closing the tectonic evolution of the belt.