Abstract Santonian-Campanian tuffaceous siltstone, epiclastic siltstone, arkose, and fossiliferous mudstone occur in a 45-m-thick section of well 1-BRSA-37-RJS drilled in the southern Campos Basin, offshore SE Brazil. Well log data and petrographic and lithogeochemical data obtained from cutting samples were used to propose a schematic model of the volcano's sedimentary paleoenvironment. The volcaniclasts in the tuffaceous siltstone and epiclastic siltstone are scoria basalts, typical of spatter cones associated with monogenetic fields worldwide. The combination of these features with the petrographic ones of the arkoses found in the same well interval is likely to be related to volcaniclastic processes taking place on the continental shelf. Ratios between the immobile trace elements of the volcaniclastic rocks can be explained by mixing between sources in the upper continental crust adjacent to southern Campos and the scoria basalts extruded in the monogenetic fields. The little differentiated, olivine-rich basalts extruded from the scoria and spatter cones suggest a rising asthenosphere mantle extending from hundreds of kilometers eastwards till southern Campos in the Santonian-Campanian. This may have resulted in regional discordances in the Santonian, both in the Campos and Santos basins, and also places the petroleum systems in the southern Campos Basin under the thermal influence of such a rising asthenosphere in the Santonian-Campanian.

RESUMO: As rochas sedimentares paleozóicas da Bacia do Parnaíba, no Nordeste do Brasil, foram intrudidas por soleiras volumosas de diabásio toleítico e cobertas por derrames basálticos. Este artigo apresenta dados geoquímicos obtidos a partir de amostras de poços da porção leste da bacia sedimentar. Os diabásios são subalcalinos, toleíticos e agrupam-se em três suites de alto-TiO2 e três de baixo-TiO2 não relacionadas por processos de diferenciação. Os processos petrogenéticos foram investigados com base em modelagem geoquímica e revelaram que as suites toleíticas evoluíram por cristalização fracionada de augita e olivina à exceção de uma, de baixo-TiO2, evoluída por AFC em câmaras magmáticas pequenas na crosta superior. As composições parentais de ambas as suites, de baixo-TiO2 e alto-TiO2, estão relacionadas com fontes mantélicas harzburgíticas variavelmente enriquecidas dentro da zona de estabilidade espinélio representadas pelo manto litosférico subcontinental. Esta fonte mantélica é lateralmente e/ou verticalmente quimicamente heterogênea e a provincialidade geoquímica em escala local parece não ter sido controlada pelo Lineamento Transbrasiliano, mas sim por remobilização do manto litosférico subcontinental possivelmente amalgamado durante processos orogênicos colisionais ocorridos previamente.

ABSTRACT: The sedimentary rocks within the Paleozoic Parnaiba basin in NE Brazil were intruded by voluminous tholeiitic diabase sills and covered by coeval basaltic flows. This paper presents lithogeochemical data of borehole samples obtained from wells located in the eastern portion of the Parnaiba basin. The diabases are subalkaline tholeiitc rocks comprising three high-TiO2 and three low-TiO2 suites that are unrelated by differentiation processes. Fractional crystallization of olivine and augite was the predominantly evolutionary processes within individual high- and low-TiO2 suites as depicted by trace element geochemical modelling, exception being made for one low-TiO2 suite that evolved by AFC. Parental compositions for both low- and high-TiO2 suites are related with variably enriched, spinel harzburgitic sources likely to represent the heterogeneous subcontinental lithospheric mantle underneath the sedimentary basin. The geochemical provinciality of the Parnaiba tholeiitic magmatism seems unrelated with the Transbrasiliano Lineament but may be due to lithospheric mantle amalgamation and remobilization occurred during previous tectonic events.