Basanite to tephrite lavas from Melbourne Volcanic Province, Victoria Land, Antarctica

The Pliocene-Quaternary volcanism of Mt. Melbourne Province consists of predominant alkali-basalts and differentiated products, and subordinate basanite-tephrite lavas. Basanite to tephrite lavas, which are the object of detailed petrological studies in this paper, scoria cones and hyaloclastites outcrop along the western branch of the Ross Sea Rift System from Cape Washington to Mt. Overlord. They show a crystallization order of the liquidus phases: olivine, Ti-salite, magnetite (basanites) followed by apatite, plagioclase and kaersutite in tephritic rocks. Ultramafic mantle xenoliths together with strongly corroded/reabsorbed kaersutite megacrysts (up to 3 cm in size) are included in the lavas. Compositional variations from basanite to tephrite can be accounted for by fractional crystallization of the basanite mineral assemblage. The compositional similarity between kaersutite phenocrysts and megacrysts suggests that kaersutite could also represent a liquidus phase of tephrite magmas, which could have begun to crystallize at relatively high-pressure. The least evolved basanites have a high mg value (0.65-0.67), Ni (363-316 ppm), Cr (443-455 ppm), Co (59-63 ppm) and can be considered virtually primary mantle derived magmas. Basanites show high contents of TiO; (2.8-3.2%), P,C>5 (0,6-0.7%), Th (6 ppm), Zr (260-264 ppm), Nb (61-66 ppm). La (41-48 ppm), Ce (89-94 ppm), Sr (600-637 ppm), Rb (29-30), Ba (407-446), strongly fractionated REE (LaN/YbN 19), and display marked Nb positive anomaly in chondrite normalized spidergrams. On the whole, the genesis of the studied highly alkaline basic melts is compatible with hydrous low-degree partial melting of enriched mantle at high pressure (25-30 Kbar). Model calculations indicate that the least differentiated basanites could be produced by 5% of batch melting (source: olivine 66, orthopyroxene 16, clinopyroxene 12, garnet 4, amphibole 2; eutectic: olivine 10, clinopyroxene 30, garnet 30, amphibole 30) of a metasomatized mantle source made up of 50% primitive mantle plus 50% of amphibole-bearing wehrlite as those represented by the Pello Hill wehrlite xenoliths (Tanzania, East Africa Rift. The marked decoupling between the high content of incompatible elements and the low 87Sr/86Sr (0.70288-0.70345) coupled with high 143Nd/144Nd (0.51296-0.51305) commonly found in basanites of the McMurdo Volcanic Group, implies that such magmas were generated from long-term depleted mantle source which only recently underwent metasomatic enrichments comparable to those observed in the included mantle xenoliths.