Ti-rich eocene basaltic rocks, abrolhos platform, Offshore Brazil, 18°S: Petrology with respect to South Atlantic magmatism

Eocene igneous rocks from the Abrolhos Islands and surrounding sedimentary platform, offshore Brazil, 18°S, are largely Ti-rich basalt and diabase (4-6 wt.% TiO2), and cumulate rocks such as wehrlite. Despite high Ti, incompatible-element abundances are relatively low (e.g., K2O ̃ 1 wt.%; P2O5 0.5 %; Zr 225 ppm; Rb 23 ppm; Ba 275 ppm); LREE enrichment yields La/YbN ̃8. Compared to other mafic rocks of the South Atlantic region, such as Mesozoic high-Ti dikes (̃5 wt.% TiO2) and basalts (3-4 wt.% TiO2) of the Serra Geral (Paraná, southern Brazil) province, and high-Ti basalts (̃4 wt% TiO2) of some South Atlantic features (Walvis, southwest Indian ridge), Abrolhos basalts differ by lower incompatible-element concentrations and/or by isotopic compositions that emphasize depleted characteristics (εSr-12; εNd 3) relative to bulk earth. Abrolhos isotopic compositions do, however, match those of some S. Atlantic Islands (e.g., Pb like those of nearby Trindade), and conform generally to Dupal anomaly contours.Abrolhos high-Ti basalts can be modeled as liquids from about 90% crystallization of parent picritic liquid emplaced near the base of the Brazilian crustal margin; no mantle geochemical anomaly or special metasomatism are needed to account for the Ti contents. Isotopic and trace-clement compositions (e.g., Zr, Nb, Y) of the Abrolhos province suggest parentage in a mantle representing a plume of bulk earth or 'enriched' composition that interacted with overlying depleted mantle. © 1989 Oxford University Press.