Petrogenesis of the Rifted Southern Victoria Land Lithospheric Mantle, Antarctica, Inferred from Petrography, Geochemistry, Thermobarometry and Oxybarometry of Peridotite and Pyroxenite Xenoliths from the Mount Morning Eruptive Centre

Abstract

The lithospheric mantle beneath West Antarctica has been characterized using petrology, whole- rock and mineral major element geochemistry, whole-rock trace element chemistry and Mossbauer spectroscopy data obtained on a suite of peridotite (lherzolite and harzburgite) and pyr- oxenite xenoliths from the Mount Morning eruptive centre, Southern Victoria Land. The timing of pyroxenite formation in Victoria Land overlaps with subduction of the Palaeo-Pacific plate beneath the Gondwana margin and pyroxenite is likely to have formed when fluids derived from, or modi- fied by, melting of the subducting, eclogitic, oceanic crustal plate percolated through peridotite of the lithospheric mantle. Subsequent melting of lithospheric pyroxenite veins similar to those repre- sented in the Mount Morning xenolith suite has contributed to the enriched trace element (and iso- tope) signatures seen in Cenozoic volcanic rocks from Mount Morning, elsewhere in Victoria Land and Zealandia. In general, the harzburgite xenoliths reflect between 20 and 30% melt depletion. Their depleted element budgets are consistent with Archaean cratonization ages and they have mantle-normalized trace element patterns comparable with typical subcontinental lithospheric mantle. The spinel lherzolite mineral data suggest a similar amount of depletion to that recorded in the harzburgites (20-30%), whereas plagioclase lherzolite mineral data suggest <15% melt deple- tion. The lherzolite (spinel and plagioclase) xenolith whole-rocks have compositions indicating <20% melt depletion, consistent with Proterozoic to Phanerozoic cratonization ages, and have mantle-normalized trace element patterns comparable with typical depleted mid-ocean ridge man- tle. All peridotite xenoliths have undergone a number of melt-rock reaction events. Melting took place mainly in the spinel peridotite stability field, but one plagioclase peridotite group containing high-sodium clinopyroxenes is best modelled by melting in the garnet field. Median oxygen fuga- city estimates based on Mossbauer spectroscopy measurements of spinel and pyroxene for spinel- facies conditions in the rifted Antarctic lithosphere are -0� 6 Dlog fO2 at Mount Morning and -1� 0 6 0� 1( 1r) Dlog fO2 for all of Victoria Land, relative to the fayalite-magnetite-quartz buffer.