Development of the Long Valley, California, magma chamber recorded in precaldera rhyolite lavas of Glass Mountain (original) (raw)

1991, Contributions to Mineralogy and Petrology

Glass Mountain, California, consists of > 50 km 3 of high-silica rhyolite lavas and associated pyroclastic deposits that erupted over a period of > 1 my preceding explosive eruption of the Bishop Tuff and formation of the Long Valley caldera at 0.73 Ma. These "minimum-melt" rhyolites yield Fe-Ti-oxide temperatures of 695-718~ and contain sparse phenocrysts of plagioclase + quartz + magnetite + apatite + sanidine, biotite, ilmenite, allanite, and zircon. Incompatible trace elements show similar or larger ranges within the Glass Mountain suite than within the Bishop Tuff, despite a much smaller range of major-element concentrations, largely due to variability among the older lavas (erupted between 2.1 and 1.2 Ma). Ratios of the most incompatible elements have larger ranges in the older lavas than in the younger lavas (1.2-0.79 Ma), and concentrations of incompatible elements span wide ranges at nearly constant Ce/Yb, suggesting that the highest concentrations of these elements are not the result of extensive fractional crystallization alone; rather, they are inherited from parental magmas with a larger proportion of crustal partial melt. Evidence for the nature of this crustal component comes from the presence of scarce, tiny xenocrysts derived from granitic and greenschist-grade metamorphic rocks. The wider range of chemical and isotopic compositions in the older lavas, the larger range in phenocryst modes, the eruption of magmas with different compositions at nearly the same time in different parts of the field, and the smaller volume of individual lavas suggest either that more than one magma body was tapped during eruption of the older lavas or that a single chamber tapped by all lavas was small enough that the composition of its upper reaches was easily affected by new additions of crustal melts. We interpret the relative chemical, mineralogical, and isotopic homogeneity of the younger Glass Mountain lavas as reflecting eruptions from a large, integrated magma chamber. The small number of eruptions between 1.4 and 1.2 Ma may have allowed