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

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 (Camboriu 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.

Automated summary

This study presents petrological data of magmatic rocks in the Santos Basin, which show characteristics consistent with OIB. Geochemical modelling suggests that the basalts in the basin have different sources and are emitted from different vents. The results also indicate a possible role of the Tristan da Cunha plume tail in the petrogenesis of the alkaline basalts, rather than the local subcontinental lithospheric mantle.