Trace element and Sr-Nd isotopic constraints on the compositions of lithospheric primary sources of Serra Geral continental flood basalts, southern Brazil

Abstract

Multi-element abundances in twenty-four samples from the Serra Geral continental flood basalt system (Parana Basin) were determined by INAA to assess the trace element signature of primary magmatric sources. Isotopic Sr and Nd data were obtained on a 24-sample composite and three individual samples of the suite to constrain the influences of older enriched material. Twenty basaltic samples have enriched LILE commensurate with most continental tholeiites and, display fractionated REE patterns similar to those in the Columbia River Group and other flood basalt provinces. The mafic units exhibit low-moderate Ti, variable V, Cr and Ni and relatively uniform Co and Sc abundances. Four intermediate to silicic units exhibit higher overall incompatible element abundances, strongly fractionated patterns and depletions of compatible elements consistent with derivation by fractionation of basaltic parents. Isotopic data for two basalts and the composite analysis indicate ^(87)Sr/^(86)Sr = 0.7090 to 0.7105 and εNd = -6.6 to -6.9 requiring magma genesis in an evolved LILE-enriched upper mantle or lower crust. A silicic sample has ^(87)Sr/^(86)Sr = 0.7219 and εNd = -9.2 reflecting a stronger crustal influence. Chemical comparisons within the basaltic members allow the delineation of ten least-evolved compositions having a regionally characteristic trace element pattern and two additional samples (of one flow) representing a second, less fractionated pattern. Trace element models for both types predict magma segregation from lower lithospheric sources having relatively uniform enrichments of incompatible elements via regional metasomatism. The most viable scenarios for basalt magma production require either enriched mantle partial-melt liquids, some of which comingle with crustal components, or partial melting of ultramafic, LILE-enriched lower crust. Either process requires a zone of primary magma extraction in a region where initially uniform source enrichments act independently of subsequent contamination, probably in the crust/mantle transition zone.