Heterogeneity of the sub-continental lithospheric mantle and ‘non-juvenile’ mantle additions to a Proterozoic silicic large igneous province

Abstract

Southern Australia, is host to a ca. 1590 Ma Silicic Large Igneous Province (SLIP) that produced approximately 100,000 km3 of felsic and mafic magmas within a 20 myr timeframe. Although a minor component, the mafic magmas within the SLIP present an opportunity to investigate the composition of the mantle sources involved in SLIP formation. The mafic rocks are generally characterised by enrichment in high field strength elements (HFSE) and light rare earth element (LREE), negative Nb-Ta-Ti anomalies, and variable Nd isotopic compositions. Enriched geochemical signatures and lack of correlation with crustal proxy elements in the most primitive magmas (high Mg, Cr, Ni and εNd(1590 Ma)) suggest crustal contamination is not significant in the studied samples and the enrichment is a primary source region characteristic. Th/Nb ratios for the suite are mostly >0.2 and the vast majority of εNd(t) values are in the range 0 to −5. In the first instance this means that a large proportion of the mafic magma added to the crust during the SLIP event is effectively isotopically indistinguishable from that considered consistent with Paleoproterozoic reworking of Neoarchean crust. Therefore, continental growth occurring during this event would not be detected by methods that are solely reliant on isotopic datasets (e.g. detrital zircons or bulk sediment compositions). The mantle source for the magma is interpreted to be dominantly sub-continental lithospheric mantle (SCLM) with varying magma compositions interpreted to represent a heterogeneous SCLM. Enrichment of the SCLM is considered to be related to metasomatism during earlier subduction events with the isotopic composition of the magmas suggesting it may have been as early as the Neoarchean.