Pre-to-syn-rift tholeiitic magmatism in a transtensive hyperextended continental margin: Onshore and offshore magmatism of the Campos Basin, SE Brazil

Abstract

The wide range of geochemical and isotopic signatures of dyke swarms are essential for defining tectonic settings and isotopic reservoirs, as they represent windows into the Earth's mantle. The Early-Cretaceous Búzios-Cabo Frio dyke swarm (BCFDS) represents the easternmost pre-rift magmatism within the Campos Basin's onshore basement. This magmatism is associated with the break-up of Gondwana and subsequent opening of the South Atlantic Ocean. Whole-rock geochemistry, mineral chemistry and petrography suggest an FC evolutionary process for this low-TiO2 (<1.65%) tholeiitic magmatism. The FC model is supported by petrological and field observations, with augite, Ca-plagioclase, ilmenite, and Ti-spinel phenocrysts. The sub-vertical attitude of the dykes is indicative of emplacement at less than 10 km deep, close to the brittle-ductile transition, supported by the presence of cataclasites along contemporaneous dyke margin faults. Both the dykes geometries and the prevalence of asymmetrical structures are consistent with sinistral oblique extension during the emplacement of the BCFDS. This kinematic interpretation is supported by oblique fault-striae and normal displacement along mapped faults parallel to the dykes. The mean extension direction of the swarm is N50-60°W with 0.1% extension. New Ar–Ar data suggest two discrete episodes of magmatism at ca. 130 and 120 Ma. Low-TiO2 tholeiitic magmatism is present offshore as subaerial, and submarine lava flows. The offshore basalts present slightly younger ages in the ca. 124-111 Ma interval and less enriched geochemical signatures. Only the basalts from most proximal wells display signatures similar to the dykes of the BCFDS. The Sr and Nd isotopic signature of the BCFDS suggests an HIMU-like enriched mantle source possibly represented by enriched, inherited sub-continental lithospheric mantle related to subduction during the Neoproterozoic. The elemental and isotopic signatures variation of the onshore dyke swarms and the offshore lava flows is compatible with the signature of the sub-continental lithospheric mantle (SCLM) beneath different terranes accreted during Late Neoproterozoic Gondwana amalgamation. This conclusion supports the hyper-extended rift-model based on the interpretation of deep seismic lines.