Nature and evolution of the sub-continental lithospheric mantle of Antarctica: Evidences from ultramafic xenoliths of Melbourne Volcanic Province Victoria Land, Antarctica)

Ultramafic inclusions are commonly found in the basanite to tephrite lavas of the Melbourne Volcanic Province (Victoria Land, Antarctica). Most nodules are mantle Iherzolites, harzbugites and dunites, although olivine websterites are also present. The Iherzolites, harzburgites and dunites are characterized by protogranular to por-phyroclastic texture with local polygonization, variable grain size (0.1-1 cm) and a typical four phase assemblage consisting of olivine (65-91%), orthopyroxene (6-30%), Cr-diopside (1-12%), and Cr-spinel (0.1-3%). Olivine, orthopyroxene and clinopyroxene are generally unzoned, the two latter phases sometimes showing mutual exsolution lamellae. Kink-banding is often well developed in olivine and orthopyroxene. Bulk and mineral chemistry of the Iherzolite-harz-burgite-dunite nodules outline a variably depleted com¬positional range (about 3 to 0.3 times chondritic for incompatible elements), related to different degrees and/or numbers of melting episodes. Evidence of metasomatic effects from incompatible element-rich fluids at mantle depth is revealed by the presence of pargasitic amphibole either as dispersed crystals or in veins, sometimes associated with glassy patches (pyrometamorphic textures). Further evidence can be found in the variable and anomalously high contents of La, Ce, Zr, Nb, Th, Sr, Ba, Rb, K (up to 20 times chondritic) and to a lesser, extent Na, observed in many peridotitic xenoliths. ' " Thermobarometric evaluations indicate equilibration conditions in the spinel-peridotite stability field (P = 21-8 kbar; T = 1200°-820°C) and a minimum provenance depth of about 25 km, just below the Moho discontinuity which is found in the area at the same depth, according, to geophysical data. They thus represent xenoliths ofasub-continental lithospheric mantle accidentally incorporated by volatile-rich alkaline basic magmas during their ascent. The partial re-equilibration of some xenoliths at low P-T values requires an uprising of mantle material of about 20-25 km from deeper to shallower levels in the lithosphere. This conforms well to the Moho upwelling from 40-45 km under the Transantarctic Mountains to a minimum depth of 20 km beneath the Ross Sea Rift system. The inferred geothermal regime of the area fits a "dynamic rift" geotherm where the advective heat is provided by basic magmas invading the upper lithosphere for the last 38 Ma