The Peralkaline Rhyolite Spectrum in Marie Byrd Land Volcanoes, West Antarctic Rift, and the Case for Polybaric Fractionation

The Marie Byrd Land (MBL) volcanic province is associated with the rise of a large tectonomagmatic dome, over the past 25-30 m.y. Since 14-15 Ma, five volcanoes have produced pantellerites and three others have produced comendites. Together they display a range in SiO2, Al2O3, FeOt, and peralkalinity that is comparable with the full range of compositions in Pantelleria and the Kenyan and Ethiopian dome provinces. The comendite volcanoes occur on the east flank of the MBL dome, adjacent to volcanoes characterized by highly undersaturated phonolites (20-30% ne). The pantellerite volcanoes, with one exception, occur on the west flank, associated with less undersaturated phonolites (6-11% ne). These seem to represent two fairly distinct sub-provinces with perhaps slightly different plumbing systems. Isotopic and trace element data, maintenance of isotopic equilibrium throughout the basalt-felsic spectrum, and the results of major and trace element modeling, all exclude crustal contamination and point to fractional crystallization as the controlling process in the origins of the felsic rocks. The availability of basalts in several stages of evolution, plus a variety of mugearites, benmoreites, and trachytes, have allowed us to model the evolution of felsic rocks in several stages. The best results, together with trace element and petrographic data, and supported by experimental work, suggest that polybaric fractionation in a multi-tiered plumbing system provides the most reasonable explanation of field, petrographic and geochemical characteristics. A plumbing system that favors prolonged kaersutite fractionation at the base of the crust for comendites (east flank) vs. one that favors comparatively short residency and little kaersutite fractionation at the base of the crust for pantellerites (west flank), seems to provide the best explanation for the differences in SiO2 and FeOt, and the wide spatial separation of these two rhyolite species. The models suggest further that, as a final step, peralkalinity evolved by fractionating a high proportion of plagioclase/clinopyroxene in shallow crustal magma chambers under very low PH2O.