EVOLUTION OF PHONOLITE AND TRACHYTE MAGMAS AT THE CRARY MOUNTAINS, MARIE BYRD LAND, ANTARCTICA

The Crary Mountains represent one of four major ranges of alkaline volcanoes in Marie Byrd Land, Antarctica, each of which show an age progression younging away from the province center. In the Crary Mountains > 400 km³ of material was erupted from four NW-SE aligned volcanoes between ~9 and <1 Ma (Wilch and McIntosh, 2002). The deposits consist of lavas interbedded with agglutinated scoria and hydrovolcanic deposits (hyaloclastite, pillow breccias, tuff). Lavas range in composition from mafic to intermediate to phonolite, trachyte, and rhyolite. Basalts are basanite and alkali basalt with olivine, clinopyroxene, plagioclase and titanomagnetite phenocrysts. Phenocrysts in intermediate to felsic compositions include olivine, clinopyroxene, amphibole (pargasite, kaersutite), alkali and plagioclase feldspars, nepheline, apatite, titanomagnetite and aenigmatite. Trachyte is subdivided into nepheline-normative (Ne) and quartz-normative (Qz) types. Phonolite consists of low-alkali (Na₂O + K₂O <13.5 wt.%) high-silica (>60 wt.%) and high-alkali (Na₂O + K₂O >15.5 wt.%) low-silica (<56 wt.%) types. Least-squares mass balance and Rayleigh fractionation models demonstrate the production of Ne-trachyte (F = 19%) by a single differentiation trend by fractional crystallization of basanite. The production of Qz-trachyte and rhyolite is explained by assimilation of granodiorite by mugearite coupled with fractional crystallization. Two fractionation schemes are predicted for the production of phonolite; (1) crystal fractionation of alkali feldspar, amphibole, apatite and magnetite from intermediate benmorite to form low-alkali phonolite (F = 35%) and (2) crystal fractionation of plagioclase, olivine, clinopyroxene, and magnetite from tephrite to produce high-alkali phonolite (F = 35%).

Wilch, T.I. and McIntosh, W.C. (2002). Lithofacies analysis and ⁴⁰Ar/³⁹Ar geochronology of ice-volcano interactions at Mt. Murphy and the Crary Mountains, Marie Byrd Land, Antarctica: implications for Late Miocene ice-sheet history in West Antarctica, in Smellie, J. L., and Chapman, M. R., eds., Ice-volcano Interaction on Earth and Mars: London, Geological Society of London, 237-253.