Dual origins for pantellerites, and other puzzles, at Mount Takahe volcano, Marie Byrd Land, West Antarctica

Abstract Mt. Takahe is a large, late Quaternary trachyte shield volcano that rises through 2000 + m of the West Antarctic ice sheet. It is composed mostly of ne -trachyte, hy - ol -trachyte, and qz -trachyte flows, with subordinate basanite, intermediate rocks, and pantellerites. All rock types can be adequately modeled by fractional crystallization of basanite - the only basaltic rock exposed here. The ne -trachytes can be explained by a single stage of low-pressure fractionation near the base of the upper crust. Models of oversaturated rocks require a period of evolution at a depth of ~ 35 km, below the stability field of plagioclase, where fractionation of kaersutite and associated high pressure minerals will yield silica oversaturated residual magmas. This is then followed by a period of fractionation at a depth of ~ 3 km, where peralkalinity and Fe-enrichment are acquired. Pantellerite compositions span virtually the entire spectrum of peralkalinity, Fe-enrichment, LILE–enrichment, and SiO 2 values, and seem to represent a range of residence times in upper crustal vs., upper mantle magma chambers. Mt. Takahe is unusual among Marie Byrd Land volcanoes for its geochemical anomalies. These include the lowest 143 Nd/ 144 Nd ratios in West Antarctica, and unusually high but unpredictable Ba values. These anomalies are believed to originate in a pre-85 Ma subduction melange at the base of the lithosphere, which seems to be the source of Mt. Takahe basaltic rocks.