Geochemistry of dolerites from the Parana flood basalt province, southern Brazil

Abstract

The Cretaceous Parana-Etendeka flood basalt magmatism of South America and Africa was accompanied by intense intrusive activity. This study examines the geochemistry and petrogenesis of dykes from two regions of southern Brazil. Dykes from the Ponta Grossa region of Parana. State fed the basalt lava flows of the northern Parana. The dolerites underwent extensive crystal fractionation at pressures of less . than about 5kb, but do not represent parental magmas to the associated rhyolites. The dolerites have received a contribution from a LREE enriched source with time-integrated high RblSr relative to the bulk earth, and yield model Nd and 207PbjlO6Pb ages which are similar to ages obtained from the basement rocks of southern Brazil. This suggests that the distinctive trace element and isotope signature of these dolerites was derived from a source in the subcontinental mantle. The-dolerites of Sao Paulo State are divisible into three magma types, which cannot be related either to one another, or to the flood basalts to the west by any simple petrogenetic process. The geochemical variation within dolerites of the Paraiba and Ubatuba magma types was controlled by assimilation fractional crystallisation and magma mixing processes at shallow levels in the crust In contrast, the basanites of the Sao Sebastiao magma type underwent limited crystal fractionation of olivine and clinopyroxene at pressures of >4 kb. Much of the geochemical variation within this group of rocks is related to variations in the degree of partial melting (estinlated to be 0.1-5%) of a spinel bearing source. The Sao Sebastiao basanites have very similar trace element and isotope compositions to Recent basanites erupted on the South Atlantic island of Tristan da Cunha, and are interpreted as the expression of the Tristan plume at 80 Ma. Selected whole rock samples and mineral separates have been dated by the laser ablation Ar-Ar technique. The Ponta Grossa dolerites yield isochron ages of 130.5±2.9- -134.1±1.3 Ma. The Paraiba and Ubatuba magma types were intruded at between 129.4±O.6 and 135.8±1.1 Ma. A biotite separate from a sample of the Sao Sebastiao magma type yielded a much younger age of 80.1±O.4 Ma, which suggests that these may be related to the alkalic intrusive complexes of Sao Paulo State. These ages imply that the Parana magmatism ocC1Jl'Ied within about 3 Ma at 131-134 Ma, although there is evidence that plagioclase alteration may be responsible for some of the apparent age range. The data suggest that the flood basalt magmatism occurred contemporaneously with continental rifting in the South Atlantic, and are consistent with a model in which the Parana-Etendeka flood basalts represent decompression melts generated during continental rifting above the abnormally hot mantle associated with the Tristan da Cunha mantle plume. This model can also account for the observed volume of magma. A comparison of the major element compositions of flood basalts from various CFB provinces with the predicted composition of decompression melts, suggests that many of these basalts have inherited a distinctive major element signature from the subcontinental lithospheric mantle.