Basaltic and Alkalic Rocks of Southern Brazil: A Review

Abstract

General domal uplift may have begun in early Mesozoic in the South Atlantic preceding volcanism by at least 50 m. y. Rifting and the start of basaltic activity began 147 m. y. ago: diabase dikes filled in rifts parallel to the coast and alkalic volcanism took place at Jacupiranga and Anitápolis 122–138 m. y. ago. The Paraná basalts include a complete range of types, but chemically, they are largely alkalic or high alumina. Rüegg [1] divides them into four general geochemical areas and concludes they originated by fractional crystallization from an olivine tholeiite magma, underwent differentiation and crystal fractionation at some depth, show some degree of contamination, and ascended slowly or in stages to the surface.

Alkalic rocks are divided into five sets: (1) Jacupiranga and Anitápolis nodes, 119–138 m.y. with high nepheline types of the urtite-ijolite series as well as carbonatites and pyroxenites; (2) Tunas and Lajes groups, 65–110 m.y., with rock types similar to (1); (3) Minas Gerais-Goiás belt, 64–91 m.y., with the most K- and rare earth-enriched compositions of all; (4) Sao Paulo-Rio de Janeiro littoral belt, 51–80 m.y., salic nepheline syenites with an Na to alkali ratio of about 65: (5) Atlantic Islands, 0.2–3.6 m.y. with high Na to alkali ratios and rock types including tannbuschites, nepheline and phonolite pyroclastics and flows, and ankaratrites. (1) coincides with triple junction development; (2) with actual plate separation and deposition of marine sediments.