Tracking deep lithospheric events with garnet-websterite xenoliths from southeastern Australia

Abstract

Pyroxenites provide important information on mantle heterogeneity and can be used to trace mantle evolution. New major and trace element and Sr-, Nd-, and Hf-isotope analyses of minerals and wholerock samples of garnet websterites entrained in basanite tuffs in Bullenmerri and Gnotuk maars, southeastern Australia, are here combined with detailed petrographic observations to constrain the sources and genesis of the pyroxenites, and to trace the dynamic evolution of the lithospheric mantle. Most garnet websterites have high MgO and Cr2O3 contents, relatively flat light rare earth element (LREE) patterns ([La/Nd]CN=0·77-2·22) and ocean island basalt-like Sr-, Nd-, and Hf-isotope compositions [87Sr/86Sr=0·70412-0·70657; εNd(t)=-0·32 to +4·46; εHf(t)=+1·69 to +18·6] in clinopyroxenes. Some samples show subduction-related signatures with strong enrichments in large ion lithophile elements and LREE, and negative anomalies in high field strength elements, as well as high 87Sr/86Sr (up to 0·709), and decoupled Hf- and Nd-isotope compositions [εNd(t)=-3·28; εHf(t) =+11·6). These data suggest that the garnet pyroxenites represent early crystallization products of mafic melts derived from a convective mantle wedge. Hf model ages and Sm-Nd mineral isochrons suggest that these pyroxenites record at least two stages of evolution. The initial formation stage corresponds to the Paleozoic subduction of the proto-Pacific plate beneath southeastern Australia, which generated hydrous tholeiitic melts that crystallized clinopyroxene-dominated pyroxenites at ~1420-1450°C and ~75km depth in the mantle wedge. The second stage corresponds to Eocene (c. 40Ma) back-arc lithospheric extension, which led to uplift of the former mantle-wedge domain to 40-60km depths, and subsequent cooling to the ambient geotherm (~950-1100°C). Extensive exsolution and recrystallization of garnet and orthopyroxene (6ilmenite) from clinopyroxene megacrysts accompanied this stage. The timing of these mantle events coincides with vertical tectonism in the overlying crust.