Glass Mountain consists of a 1 km 3 , compositionally zoned rhyolite to dacite glass flow containing magmatic inclusions and xenoliths of underlying shallow crust. Mixing of magmas produced by fractional crystallization of andesite and crustal melting generated the rhyolite of Glass Mountain. Melting experiments were carried out on basaltic andesite and andesite magmatic inclusions at 100, 150 and 200 MPa, H 2 O-saturated with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional crystallization in the origin of the rhyolite of Glass Mountain. Isotopic evidence indicates that the crustal component assimilated at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of mafic andesite (2 to 2.5 km 3 ) periodically replenished the magma reservoir(s) beneath Glass Mountain, underwent extensive fractional crystallization and provided the heat necessary to melt the crust. The crystalline residues of fractionation as well as residual liquids expelled from the cumulate residues are preserved as magmatic inclusions and indicate that this fractionation process occurred at two distinct depths. The presence and composition of amphibole in magmatic inclusions preserve evidence for crystallization of the andesite at pressures of at least 200 MPa (6 km depth) under near H 2 O-saturated conditions. Mineralogical evidence preserved in olivine-plagioclase and olivine-plagioclase-high-Ca clinopyroxene-bearing mag-matic inclusions indicates that crystallization under near H 2 O-saturated conditions also occurred at pressures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geochemical evidence indicate that the andesite underwent fractional crystallization to form the differentiated melts but had no chemical interaction with the melted crustal component. Heat released by the fractionation process was responsible for heating and melting the crust.

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[1] Plagioclase and spinel lherzolite thermometry and barometry are applied to an extensive geochemical dataset of young (<10.5 Ma) primitive basaltic lavas from across Oregon's High Lava Plains, California's Modoc Plateau, and the central-southern Cascades volcanic arc to calculate the depths and temperatures of mantle melting. This study focuses on basalts with low pre-eruptive H 2 O contents that are little fractionated near-primary melts of mantle peridotite (i.e., basalts thought to be products of anhydrous decompression mantle melting). Calculated minimum depths of nominally anhydrous melt extraction are 40-58 km below Oregon's High Lava Plains, 41-51 km below the Modoc Plateau, and 37-60 km below the central and southern Cascades arc. The calculated depths are very close to Moho depths as determined from a number of regional geophysical studies and suggest that the geophysical Moho and lithosphere-asthenosphere boundary in this region are located in very close proximity to one another (within 5-10 km). The basalts originated at 1185-1383 C and point to a generally warm mantle beneath this area but not one hot enough to obviously require a plume contribution. Our results, combined with a range of other geologic, geophysical, and geochemical constraints, are consistent with a regional model whereby anhydrous mantle melting over the last 10.5 Ma in a modern convergent margin and back arc was driven by subduction-induced corner flow in the mantle wedge, and to a lesser extent, toroidal flow around the southern edge of the subducting Juan de Fuca and Gorda plates, and crustal extension-related upwelling of the shallow mantle.

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Melting occurs at progressively greater depths and higher temperatures from west to east across the Cascades volcanic arc in northern California, as demonstrated by compositional variations observed in high-alumina olivine tholeiites. The lavas studied erupted from seven vents defining a 75-km-long, east-west transect across the arc, from near Mount Shasta to east of Medicine Lake volcano. The increase in melting depth across the arc parallels modeled isotherms in the mantle wedge and does not parallel the inferred dip of the slab. The depth of mantle melting at which the high-alumina olivine tholeiites were created is ϳ36 km at the western end of the transect and 66 km at the eastern end. The very high temperatures of dry melting so close to the crust indicate a transitory condition of the mantle.