ERUPTIVE HISTORY AND PETROLOGY OF MOUNT-DRUM VOLCANO, WRANGELL MOUNTAINS, ALASKA

Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80 x 200 km) of southcentral Alaska. It lies at the northwest end of a series of large, andesite-dominated shield v olcanoes that show a northwesterly progression of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by vio lent explosive activity probably after 250 ka. Cone lavas range from b asaltic andesite to dacite in composition; ring-domes are dacite to rh yolite. The last constructional activity occurred in the vicinity of S nider Peak, on the south flank of the volcano, where extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosi ve eruption, that destroyed the top and a part of the south flank of t he volcano, produced more than 7 km3 of proximal hot and cold avalanch e deposits and distal mudflows. The Mount Drum rocks have medium-K, ca lc-alkaline affinities and are generally plagioclase phyric. Silica co ntents range from 55.8 to 74.0 wt%, with a compositional gap between 6 6.8 and 72.8 wt%. All the rocks are enriched in alkali elements and de pleted in Ta relative to the LREE, typical of volcanic arc rocks, but have higher MgO contents at a given SiO2, than typical orogenic medium -K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. T he compositional range of Mount Drum lavas is best explained by a comb ination of diverse parental magmas, magma mixing, and fractionation. T he small, but significant, range in Sr-87/Sr-86 ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites and andesites suggests the presence of comp ositionally diverse parent magmas. The lavas show abundant petrographi c evidence of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages. In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents that are not in equi librium with the host rock and indicate mixing between basalt or cumul ate material and more evolved magmas. Incompatible element variations suggest that fractionation is responsible for some of the compositiona l range between basaltic andesite and dacite, but the rhyolites have K , Ba, Th, and Rb contents that are too low for the magmas to be genera ted by fractionation of the intermediate rocks. Limited Sr-isotope dat a support the possibility that the rhyolites may be partial melts of u nderlying volcanic rocks.