Petrologia dos enclaves alcalinos da formação Trindade, Ilha da Trindade, Oceano Atlântico Sul

Abstract

The Trindade Island represents the most recent record of volcanism in Brazilian territory, featuring five identified volcanic successions: Trindade Formation, Desejado Formation, Valado Formation, Morro Vermelho Formation, and Paredão Formation, comprising a nephelinitic-phonolitic association. The Trindade Formation constitutes the basal portion of the volcanic succession, consisting of approximately 500 meters thick pyroclastic deposits. These pyroclastic deposits are intruded by necks and dikes of phonolites, often presenting carbonate vesicles and xenoliths of pyroxenites. Petrographic analyses, mineral chemistry data, geochemical signatures, and La/YbN e Sm/LaN ratios were employed to investigate the thermodynamic processes in the magma chamber that resulted in the observed compositional diversity in the Trindade Formation. Petrographic results reveal a variety of textures, including magmatic corrosion, zoning, cyclical twinning, anhedrality. Recrystallization in phenocrysts (macro and microcrystals) of autoliths can also be observed. These autoliths provide information on thermodynamic variations over geologic time in the magma chamber, giving insights into the different types of magmas involved in volcanic activity. This study enhances and validates some previous assumptions, serving as a foundation for future petrogenetic investigations in oceanic islands.