REE geochemistry of Late Permian shoshonitic lavas from the Sydney Basin, New South Wales, Australia

Abstract

Nine Late Permian shoshonitic lavas from the southern Sydney Basin have moderately fractionated rare-earth element (REE) patterns and are enriched in light REE (LREE). Petrographic and geochemical data accord with the lavas being related by low-pressure fractionation of plagioclase, olivine and clinopyroxene. Trace-element modelling on the basis of published partition coefficients and possible source compositions suggests that the shoshonitic magmas were generated by 10–15% partial melting of spinel lherzolite which had been enriched previously in LREE. Generation of the shoshonitic lavas in the Sydney Basin was not related directly to Benioff-zone magmatism but the enrichment of the source region in LREE may have been related to a pre-Late Permian subduction event. A compilation of REE data for shoshonitic volcanic rocks from a variety of geographic and tectonic settings indicates that there is no simple relationship between REE content and the thickness of continental crust above the source region.