Petrology of Serra Geral (Paraná) continental flood basalts,southern Brazil: crustal contamination,source material,and South Atlantic magmatism

The Serra Geral (Paraná) continental flood basalt (SG-CFB) province in Brazil is associated with the Jurassic-Cretaceous breakup of Gondwanaland and the transition of continental to oceanic magmatism during the opening of the South Atlantic Ocean. A suite of 24 samples representative of the SG-CFB in Rio Grande do Sul, southern Brazil, shows a compositional continuum from basalt (50–53 wt% SiO₂, Mg# 60-45), to basaltic andesite, to andesite, rhyodacite, and rhyolite (73 wt% SiO₂). Certain compositional aspects of the mafic rocks (e.g., TiO₂, K₂O, CaO, Zr/Nb, Zr/Y, Ti/Zr) resemble those of basaltic dikes and flows associated with the opening of the North Atlantic Ocean.Fractionation trends are apparent in MgO variation diagrams and calculations show that basalt-basaltic andesite continuums are largely due to removal of plagioclase and clinopyroxene. These mafic rocks can be categorized as (i) having higher or lower incompatible-element contents (e.g. K₂O 0.6–1.5 wt%; Rb 12–43 ppm; Ba 125–240 ppm) due to incorporation of Brazilian Archean crust or rhyolitic magma by basalt from a particular source material or to partial-melting differences of that source, and (ii) as having higher or lower TiO₂, Sr, Ba, and P contents due to source heterogeneities. Crustal components are obvious in certain basaltic samples, as where K₂O > 1 wt%, SiO₂ > 51%, and TiO₂ 1%, but are insignificant in others (e.g., compositions close to those of South Atlantic basin basalts). Calculations indicate origins for intermediate and silicic rocks by removal of pl, cpx, and Ti-magnetite from basaltic andesite, but crust and magma-mixing affecting basaltic-andesite fractionates were likely also involved. Where contamination is insignificant, Zr, Nb, and Y abundances indicate T-type MORE source material like that for certain S. Atlantic Ocean basalts. Source material was essentially a 91 hybrid of N-type and P-type MORB components. N-type MORB lithosphere followed SG-CFB because decompression due to rifting crust enabled partial melting of uppermost (depleted) mantle.