MAGMATIC PROCESSES THAT GENERATED THE RHYOLITE OF GLASS MOUNTAIN, MEDICINE LAKE VOLCANO, N CALIFORNIA

Glass Mountain consists of a 1 km(3), compositionally zoned rhyolite t o dacite glass flow containing magmatic inclusions and xenoliths of un derlying shallow crust. Mixing of magmas produced by fractional crysta llization of andesite and crustal melting generated the rhyolite of Gl ass Mountain. Melting experiments were carried out on basaltic andesit e and andesite magmatic inclusions at 100, 150 and 200 MPa, H2O-satura ted with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional crystallization in the o rigin of the rhyolite of Glass Mountain. Isotopic evidence indicates t hat the crustal component assimilated at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of ma fic andesite (2 to 2.5 km(3)) periodically replenished the magma reser voir(s) beneath Glass Mountain, underwent extensive fractional crystal lization and provided the heat necessary to melt the crust. The crysta lline residues of fractionation as well as residual liquids expelled f rom the cumulate residues are preserved as magmatic inclusions and ind icate that this fractionation process occurred at two distinct depths. The presence and composition of amphibole in magmatic inclusions pres erve evidence for crystallization of the andesite at pressures of at l east 200 MPa (6 km depth) under near H2O-saturated conditions. Mineral ogical evidence preserved in olivine-plagioclase and olivine-plagiocla se-high-Ca clinopyroxene-bearing magmatic inclusions indicates that cr ystallization under near H2O-saturated conditions also occurred at pre ssures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geoch emical evidence indicate that the andesite underwent fractional crysta llization to form the differentiated melts but had no chemical interac tion with the melted crustal component. Heat released by the fractiona tion process was responsible for heating and melting the crust.