Evidence for the Existence of Oceanic Mantle in the Subcontinental Lithospheric Mantle of Eastern Australia.

J M McCarron, S Y O'Reilly, W L Griffin and M D Norman (all at GEMOC, School of Earth Sciences, Macquarie University, Nth Ryde 2109)

Investigations into the composition of the subcontinental lithospheric mantle (SCLM) in recent years have focussed on the concept that the SCLM is, in part at least, composed of oceanic mantle which has been subducted at various times throughout the Earth's evolution. The presence of oceanic mantle as a constituent of the subcontinental mantle of Eastern Australia is supported here by major element analysis of peridotite xenoliths and theoretical calculations based on experimental data.

An "oceanic trend" for melt extraction from oceanic mantle at shallow depths in terms of major element composition (mg number, modal olivine, Ca/Al, Si) has been defined (Boyd, 1989). This trend defines a path from fertile to residual oceanic mantle. Comparisons of this trend with mantle xenoliths from Archaean cratons (eg. Kaapvaal Craton, South Africa) demonstrates no correlation of the Archaean data with the oceanic trend thereby implying a different origin for mantle beneath Archaean cratons and oceanic regions. Comparison of mantle peridotite xenolith data from Eastern Australia (Mt Gambier in South Australia and Mt Quincan in North Queensland) with this oceanic trend on the other hand illustrates close adherence of the majority of our data for mantle produced in shallow, oceanic environments. This suggests that the mantle beneath Mt Gambier and Mt Quincan is oceanic in origin.

A second approach to determining the existence of oceanic mantle within the SCLM has been to calculate the major element composition of mantle residue after polybaric MORB extraction using the data of Niu and Batiza (1991). They calculated the major element compositions of primary melts parental to mid-ocean ridge basalt (MORB) using experimental data. Although still in the early stages the results from this approach provide further evidence that the SCLM of eastern Australia has an oceanic component. Plots of CaO against CaO/Al2O3 and Al2O3 from the present data set against the theoretical mantle residue compositions show good correlations, particularly CaO vs Al2O3. Higher degrees of melting (~16-26%) for mantle beneath Mt Gambier and lower degrees of melting (~6-16%) for mantle beneath Mt Quincan at pressures dominantly >20kb are suggested by the degree of CaO and Al2O3 depletion in these xenoliths.