Petrology of the Eocene alkaline volcanism from the western Rio Grande Rise, South Atlantic Ocean

• Main Author: Guerra, Júlia Taciro Mandacarú
• Other Authors: Janasi, Valdecir de Assis
• Format: Master Thesis
• Language: English
• Published: Biblioteca Digital de Teses e Dissertações da USP 2023
• Subjects: Vulcanismo alcalino; Química Mineral; Petrologia ígnea; Oceano Atlântico Sul; Geoquímica; Elevação do Rio Grande; Igneous Petrology; Mineral Chemistry; Geochemistry; Rio Grande Rise; South Atlantic Ocean; Alkaline volcanism; Gough; Sob’
• Online Access: https://www.teses.usp.br/teses/disponiveis/44/44143/tde-13022023-121718/; https://doi.org/10.11606/D.44.2023.tde-13022023-121718

The Rio Grande Rise (RGR) is an important bathymetric feature in the South Atlantic Ocean. It is widely accepted that it was formed together with the Walvis Ridge at a plume-influenced spreading centre. Despite its potential to clarify the complex tectonomagmatic evolution of both plateaus in the context of the opening of the South Atlantic Ocean, the RGR has been subject to a much lower number of petrological studies, as compared to the Walvis Ridge and Guyot Province. This study focused on the alkaline affinity volcanism from the western Rio Grande Rise aimed at the investigation of its magmatic system through detailed characterization of dredged porphyritic alkalibasalts, a picro-basalt, a basanite, trachyandesites and trachytes. The integrated results of whole-rock geochemistry, petrography, and mineral chemistry (specially clinopyroxene textural and compositional analysis) suggest a complex polybaric transcrustal magmatic system with multiple episodes of magma recharge and evolution strongly controlled by fractional crystallization. Zircon U-Pb dating of a single trachyte sample yielded an age of 46.86 ± 0.33 Ma which reinforces the importance of the western RGR Eocene alkaline magmatism. Pb, Sr and Nd isotope analysis evidence that the signatures of the investigated volcanic rocks are similar to those from EMI flavoured Gough-type lavas from the Walvis Ridge, Guyot Province and Gough Island, what suggests that their mantle sources had common characteristics even after both oceanic plateaus were separated from each other. A Elevação do Rio Grande (ERG) é uma feição batimétrica importante do Oceano Atlântico Sul. Considera-se amplamente que sua origem se deu junto com a Cadeia de Walvis em um centro de espalhamento sob influência de uma pluma mantélica. Apesar do seu potencial para esclarecer aspectos importantes da evolução tectonomagmática complexa dos dois platôs no contexto da abertura do Oceano Atlântico Sul, a ERG foi investigada por um número consideravelmente menor de estudos petrológicos do que a ...