Petrogenesis of Early Cretaceous volcanic rocks in Santos Basin, offshore SE Brazil: evidence for the Tristan da Cunha plume

ABSTRACT

The Santos Basin comprises the most voluminous hydrocarbon accumulations in its ‘Pre-salt’ reservoirs. Magmatic rocks have been recorded in the basin mostly by the tholeiitic basalts (Camboriú Formation) within its rift megasequence at Hauterivian time. Recent studies have shown that magmatic rocks are also found interbedded with Aptian sedimentary rocks in the basin, related to its post-rift (mostly Aptian) megasequence. This paper presents petrological data obtained from sidewall core samples of magmatic rocks interbedded with Early Aptian coquina in a well drilled in the Santos Basin. The well comprises six magmatic sections of alkaline basalts being overlain and underlain by the Barra Velha limestones. The basalts are mostly plagioclase-phyric, amygdaloidal rocks but vary in texture and modal compositions. Magmatic compositions seem to have been slightly affected by alteration processes. Basalts comprise an alkaline, miaskitic series with relatively evolved compositions (MgO = 5.55 ± 1.50 wt%; Ni < 220 ppm; Cr < 400 ppm). The patterns in the Primitive Mantle normalized multielement diagram displays the typical Nb peak (0.3 < La/Nb_(N)<0.7) of the OIB. Geochemical modelling showed that basalts within five magmatic sections are not cogenetic by differentiation, representing basaltic flows emitted from different vents. Modal batch partial melting model indicates that it is possible, although unlikely, to generate the differences in La/Yb_(N) and Zr/Y ratios of the least evolved samples in the studied well by different partial melting degrees (0.5–11%) from the same mantle source. Binary mixing models using lithogeochemical datapoint to a role for the Tristan da Cunha plume tail in the petrogenesis of the alkaline basalts with no clear evidence for a possible role for the local subcontinental lithospheric mantle. The results of the petrogenetic models are consistent with a shallow MOHO and a high-temperature mantle below part of the Santos Basin in Early Aptian time.

KEYWORDS:

• Petrology
• geochemistry
• Santos basin
• pre-salt
• post-rift
• Tristan da cunha plume

Acknowledgments

The authors thank the financial support of the EQUIMAG Project, ‘Magmatism and tectonic processes of the rift, drift and post-rift phases in the Santos Basin’ Cooperation agreement FAPUR 07/19, ANP 21110-2.

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2201928

Additional information

Funding

This work was supported by the Support Foundation for Scientific and Technological Research of the Federal Rural University of Rio de Janeiro (FAPUR) under Grant 21110-2.