Thermochronology in South America

Abstract

The Capiru Group (Neoproterozoic) is a 4 km-thick succession deposited on the northern margin of the Curitiba microplate, Southern Ribeira Belt, Brazil. The Capiru Group successions are deformed within the Brasiliano - Pan African Ribeira Belt, which culminated in the amalgamation of Western Gondwana. Even though the rocks are metamorphosed and deformed, preserved sedimentary structures and outcrop exposures allowed the interpretation of depositional environments and stratigraphic analysis. The Capiru Group metasedimentary rocks were studied by field mapping, measured sections and bulk-rock chemical compositions. Six formations were identified, termed from oldest to youngest as: Santana (proximal to distal marine shelf), Juruqui (proximal to distal deltaic), Rio Branco (carbonate platform), Morro Azul (lagoonal to shallow marine), Morro Grande (proximal to distal deltaic/marine) and Bocaina formations (estuarine to shallow-marine). Two main basin stages were identified: a passive continental margin stage, with increasing tectonic quiescence followed by a marine regression, and a syn-orogenic stage at the Curitiba microplate margin.Chemical and mineralogical Fe- and Al-rich compositions suggest continental denudation during the passive continental margin stage, with low sediment input rates from a relatively low-gradient relief, highly weathered continental source, and stability since Mesoproterozoic time. Sedimentological and stratigraphic similarities were found among Capiru Group passive margin successions and units deposited on the Río de Plata and Southern Paranapanema margins.The paleoenvironmental interpretations, such as greenhouse conditions and high weathering rates alternated with depositional gaps, indicate climatic changes during the Capiru Group deposition, with far-field or Phantom Glacial influence related to the regional discordances. These features are important for the paleogeographic reconstructions that must consider a tropical or equatorial position ∼650 - 635 Ma for the Curitiba microplate.During deposition of the Bocaina Formation, the arkosic composition of sandstones and lithic conglomerate lags suggest source rejuvenation during the syn-orogenic stage, prior to the collisional thrusting that led to the Western Gondwana assembly.