Geochemical and High Pressure Experimental Studies of Garnet Pyroxenite and Pyroxene Granulite Xenoliths from the Delegate Basaltic Pipes, Australia

Abstract

Subsolidus phase relationships have been determined to pressures of 15–27 kb for a garnet clinopyroxenite, a garnet-plagioclase clinopyroxenite, a spinel-garnet websterite, and a two-pyroxene granulite occurring as xenoliths in the Delegate basaltic breccia pipes. Assuming all the garnet pyroxenite suite xenoliths formed together or last equilibrated together, the experimental data constrain the P-T conditions of their formation to 13–17 kb and 1050–1100 °C; for the pyroxene granulites, pressures of formation of 6–10 kb at temperatures around 1100 °C are indicated. In the case of the spinel-garnet websterite, the texturally implied exsolution of garnet and orthopyroxene from clinopyroxene, and reaction of spinel with clinopyroxene to yield garnet, are shown to be explicable in terms of approximately isobaric cooling of a pre-existing aluminous clinopyroxene+spinel aggregate. The garnet of the garnet and garnet—plagioclase clinopyroxenites cannot, however, have been derived wholly by exsolution processes. New chemical data are presented for the xenoliths studied experimentally and for several similar examples from Delegate and other eastern Australian localities. Consideration of available major and trace element and isotopic data for garnet pyroxenite suite xenoliths from Delegate and elsewhere in the world strongly suggests genetic relationships with their host basaltic rocks. The Delegate examples are interpreted as a series of accumulates from local pockets of alkaline basaltic magma within the Earth's upper mantle, and which have subsequently undergone exsolution and/or recrystallization in response to subsolidus cooling. A similar origin is suggested for the analogous garnet pyroxenites found as layers within western Mediterranean peridotite massifs. The Delegate two-pyroxene granulite xenoliths are considered to be accidental fragments of metamorphic rocks from the deep crust beneath eastern Australia.