PETROGENESIS OF A PHONOLITE-TRACHYTE SUCCESSION AT MOUNT SIDLEY, MARIE-BYRD LAND, ANTARCTICA

The 1.5 Ma evolution of the Late Pliocene (5.7 to 4.2 Ma) Mt Sidley vo lcano, Marie Byrd Land, is examined using major and trace elements, Sr , Nd, O and Pb isotopic data. A large (5 km x 5 km) breached caldera e xposes lavas and tephras, deep within Mt alkaline rock series are dist inguished: (a) a strongly silica-under-saturated basanite to phonolite series; (b) a more silica-saturated to -oversaturated alkali basalt t o trachyte series. Rock compositions in both series fall within a narr ow range of Sr-87/Sr-86(i) (0.7028-0.7032), Nd-143/Nd-144(i) (0.51285- 0.51290) and delta(18)O (5.0-6.0 parts per thousand), and with Pb-206/ Pb-204 (> 19.5), suggest an asthenospheric source containing a strong mantle plume component. Partial melting models require less than or eq ual to 2% melting to produce primary basanite and greater than or equa l to 5% melting to produce alkali basalt from the same mantle source. The differentiation of diopside, olivine, plagioclase, titaniferous ma gnetite, nepheline and/or apatite from basanite to derive 35% mugearit e, crystallization of a similar mineral assemblage from alkali basalt is modeled for compositions in the trachyte series. However, many trac hytes have variable (87)/Sr-86(i) (0.7033-0.7042), low Nd-143/Nd-144(i ) (0.51280-0.51283), high delta(18)O (6.5-8.4 parts per thousand) and are silica oversaturated, suggesting they are contaminated by crust. T he trachytes evolved by a two-step assimilation-fractional crystalliza tion of alkali basalt by calc-alkaline granitoids within the middle cr ust where high assimilation to crystallization rates (high-r AFC) prod uced trachytic magmas characterized by depletions in Ta and Nb relativ e to K and Rb. The second step involved further fractionation of these magmas by low-r AFC within the upper crust to produce another suite o f trachytes showing extreme incompatible element enrichment (e.g. Zr > 2000 p.p.m and Th > 100 p.p.m).