PETROLOGIC EVOLUTION OF ANORTHOCLASE PHONOLITE LAVAS AT MOUNT EREBUS, ROSS ISLAND, ANTARCTICA

Mount Erebus, Ross Island, Antarctica, is an active, intraplate, alkaline volcano. The strongly undersaturated sodic lavas range from basanite to anorthoclase phonolite, and are termed the Erebus lineage (EL). The lavas are porphyritic with olivine (Fo88-51), clinopyroxene (Wo45-53En36-41Fs8-30), opaque oxides (Usp51-76), feldspar (An72-11), and apatite. Rare earth element (REE) contents increase only slightly with increasing differentiation compared with other incompatible elements. The light REE are enriched (La(N)/Yb(N) = 14-20) and there are no significant Eu anomalies. Sr-87/Sr-86 is uniform and low (approximately 0.7030) throughout the EL, suggesting derivation of the basanites from a depleted asthenospheric mantle source, and lack of significant crustal contamination during fractionation of the basanite. Regular geochemical trends indicate that the EL evolved from the basanites by fractional crystallization. Major element mass balance calculations and trace element models show that fractionation of 16% olivine, 52% clinopyroxene, 14% Fe-Ti oxides, 11% feldspar, 3% nepheline, and 3% apatite from a basanite parent leaves 23.5% anorthoclase phonolite. Minor volumes of less undersaturated, more iron-rich benmoreite, phonolite, and trachyte are termed the enriched iron series (EFS). The trachytes have Sr-87/Sr-86 of 0.704, higher than other EFS and EL rocks. and they probably evolved by a combined assimilation-fractional crystallization process. The large volume of phonolite at Mt. Erebus requires significant basanite production. This occurs by low degrees of partial melting in a mantle plume (here termed the Erebus plume) rising at a rate of about 6 cm/yr.