THE PERALKALINE FELSIC ROCK SPECTRUM IN MARIE BYRD LAND, AND THE CASE FOR POLYBARIC FRACTIONATION (Invited Presentation)

The Marie Byrd Land (MBL) province includes five volcanoes that have produced pantellerites, and three that have produced comendites since ~14 Ma. Together they display a range in SiO₂, Al₂O₃, FeO_t, and peralkalinity that exceeds the range of compositions in Pantelleria, Kenya and Ethiopia. Pantellerite volcanoes occur mainly in the western part of the province, widely separated from the more centrally located comendites. Isotopic and trace element data, maintenance of isotopic equilibrium throughout the basalt-felsic spectrum, major and trace element modeling, all indicate <1% crustal contamination and point to fractional crystallization (FC) control in the origins of the felsic rocks. Basalts in several stages of evolution, together with mugearites, benmoreites and trachytes, allow modeling of felsic rock evolution in several stages. The results suggest that polybaric FC in a multi-tiered plumbing system provides the best explanation of field, petrographic and geochemical characteristics of these rocks. 

The most common basaltic rock, and the only one associated with pantellerites and comendites, is basanite (normative nepheline = 3-10%). Modeling suggests that fractionating kaersutite from basanite at the base of the crust will yield a silica-oversaturated magma. Peralkalinity evolves by fractionating a high proportion of plagioclase/clinopyroxene in shallow crustal magma chambers under low P_H2O. Conditions that yield pantellerite in some volcanoes vs. comendite in others are uncertain. Prolonged fractionation of kaersutite at the base of the crust, for comendites, vs. a comparatively short residency and less kaersutite fractionation for pantellerites, could explain the higher SiO₂ and lower FeO_t of comendites, and the wide spatial separation of these two rhyolite species. However, FC at low fO₂ for pantellerites and higher fO₂for comendites cannot be ruled out. Modeling so far has yielded ambiguous results. The evolution of peralkaline phonolite, sometimes from the same edifice as pantellerite, probably takes place when basanite magma rises rapidly to a shallow crustal reservoir and differentiates to phonolite. Multiple small magma chambers seem to be required to produce these diverse rock types, essentially contemporaneously, from the same edifice.