Aptian flood basalts in Bacalhau oil and gas field: petrogenesis and geodynamics of post-rift tholeiites in the pre-salt sequence of Santos Basin, Brazil

A post-rift Aptian magmatism is recorded in a 500-m-thick sequence of basalts interbedded with marls in the Bacalhau oil and gas field in Santos Basin, SE Brazil. This magmatic section is within the so-called pre-salt sequence of Santos Basin that comprises the major oil and gas reserves of Brazil. This is the first publication of systematic petrological and geochronological data for the Aptian magmatism in Santos. Whole-rock Ar-Ar integrated ages obtained for these basalts are 116.93 +/- 0.22 Ma, 116.16 +/- 0.10 Ma, 115.21 +/- 0.13 Ma and 109.95 +/- 0.20 Ma and. As such, they are younger than the rift-related Camboriu basalts in Santos as well as the Parana-Etendeka basalts and related dike swarms. The Santos basalts comprise a low-Ti tholeiitic suite with La/Nb-n (2.7-4.2) and La/Yb-n (4.2-5.9) ratios typical of continental flood basalt provinces. The basalts vary in MgO content but show no evidence for cogeneticity by differentiation processes. Lithogeochemical data showed that the Aptian basalts in Santos cannot be related with either the low-Ti, Esmeralda and Gramado suites in Parana-Etendeka or the low-Ti Lumiar, Serrana, and Costa Azul suites in the Serra do Mar Dike Swarm on the basis of lithogeochemical data. No geochemical and isotopic correlation can be done with the Aptian, Alagoas basalts in Campos basin as well. Initial (115 Ma) isotope ratios (Sr: 0.705747-0.706804; epsilon Nd: -5.9 to -2.8; Pb-206/Pb-204: 17.61-18.67; Pb-207/Pb-204: 15.47-15.58; Pb-208/Pb-204: 38.17-38.39; epsilon Hf: + 0.3 to -8.2) indicate derivation from a EM1 mantle component in the SCLM. Modal batch partial melting modelling showed that melting occurred within the garnet stability zone. We propose a geodynamic model for the Aptian in Santos in which the melting of the SCLM is related with either the presence of the Tristan da Cunha mantle plume in Aptian time below Santos or stretching of different portions of the blob-rich SCLM itself. This stretching is due to the remaining heat advected from Tristan during the voluminous Early Cretaceous magmatism that gave rise to the Parana-Etendeka CFB.

Automated summary

A post-rift Aptian magmatism is identified in the Bacalhau oil and gas field in the Santos Basin, SE Brazil, which is part of the pre-salt sequence with major oil and gas reserves. Petrological and geochronological data reveal the age and composition of the basalts, which are younger than other basalts in the region and show characteristics of continental flood basalt. Isotope ratios suggest a connection to the EM1 mantle component in the SCLM and a geodynamic model proposes the presence of the Tristan da Cunha mantle plume or stretching of the SCLM.