Rio Apa Block: A Juvenile Crustal Fragment in the Southwest Amazonian Craton and Its Implications for Columbia Supercontinent Reconstitution

Abstract

The Rio Apa Block, a crustal fragment of 20,000 km2 in the southern portion of the Amazonian Craton, is a Paleoproterozoic terrain, making up the basement of the Neoproterozoic Paraguay Belt. The block is not often considered in the evolution models of the Columbia supercontinent. Different hypotheses of the evolution of the Rio Apa Block consider this segment as part of the Amazonian Craton, and its correlation with other Paleoproterozoic accretionary systems remains controversial. We present new U-Pb and Sm-Nd data that suggest that the Rio Apa Block constitutes a continental arc system built around 1.8–1.7 Ga that can be a correlation element in supercontinent models. Recorded orogenic accretionary events and continental crust reworking divide the block into three main sectors. The western and central sectors comprise parts of the 1.88–1.71 Ga Amoguijá arc. The eastern sector consists of Orosirian high-K calc-alkaline and postcollisional A-type granites. The western segment comprises Orosirian banded orthogneisses of the Porto Murtinho Complex, intruded by initial phase granitoids of the Amoguijá arc. The central sector is characterized by weakly to moderately deformed granites of the Amoguijá Suite, Serra da Bocaina Formation metavolcanic rocks, Serra da Alegria gabbro-anorthosite suite, and Morro do Triunfo gabbro, overlain by the Amolar Group sedimentary rocks. The eastern segment comprises the backarc basin generated Alto Tererê Group, intruded by late to postorogenic granites of the Rio Apa Complex, including A-type granites. Mafic dikes and sill swarms related to a late extensional event intrude the preceding rocks. Two important metamorphic-deformational events are recorded in the opening of the Rb-Sr system in biotite around 1670 Ma, followed by thrust deformation and metamorphism probably related to the ca. 1300 Ma Rondonian-San Ignácio event, as indicated by Ar-Ar muscovite and U-Pb monazite ages.