New Mexico Geological Society Annual Spring Meeting — Abstracts

Mt. Erebus, the most active volcano in Antarctica, dominates the alkaline volcanic complex of Ross Island, and contains an unusual anorthoclase-phyric phonolite (anorthoclase phonolite) lava lake. Most of the rock exposed on the flanks is anorthoclase phonolite, with more basic rocks outcropping at Fang Ridge and along the coast. One hundred new samples provide a comprehensive coverage of Mt.
Erebus and volcanic stratigraphy of selected areas is based on detailed geologic mapping and K/Ar ages.

Geochemical analyses of 58 representative samples have disclosed a continuous sodic differentiation series consisting of basanite, nepheline hawaiite, nepheline mugearite, nepheline benmoreite, benmoreite, phonolite, trachyte, and quartz trachyte. Four new compositions have been identified: slightly evolved basanite (SiO₂: 43 wt.%; MgO#: 54), kaersutite benmoreite, kaersutite phonolite, and peralkaline quartz trachyte (64 wt.% SiO₂, 12-13 wt.% alkalies). Smooth trends occur for major and trace elements on si1ica variation diagrams. K/Rb ratios decrease from 450 in basanites to 340 in anorthoclase phonolites and 350-260 in kaersutite phonolites and trachytes.

Geochemical data supports the origin of the lavas by fractional crystallization from a basanite-nepheline hawaiite-nepheline benmoreite-anorthoclase phonolite, had a crystallate dominated by titaniferous augite, olivine, plagioclase, Fe-Ti oxides, and apatite, and probably evolved in the main magma chamber. A second series, consisting of basanite, nepheline hawaiite, benmoreite, kaersutite phonolite, and trachyte, may have had kaersutite as a major fractionating phase. It is grossly similar to the DVDP suite, also developed on Ross Island, and may have evolved in subsidiary magma chambers with lower temperatures, higher water contents, and crustal contamination.