The Origin and Evolution of Large-Volume Silicic Magma Systems: Long Valley Caldera

Despite extensive study, the origin of large-volume silicic magma systems remains poorly constrained. We review the source regions and processes involved in the generation, differentiation, and eruption of caldera-related silicic magma with particular reference to the Bishop Tuff erupted from Long Valley Caldera, California. Nd-isotopic compositions of the earliest-erupted rhyolites (between 2,1 and 1.2 Ma) at Glass Mountain, which may be associated with the Bishop Tuff magma chamber, are consistent with extensive fractional crystallization of basaltic magmas derived from an enriched lithospheric mantle source. In contrast, Nd-isotopic compositions of late Glass Mountain lavas (1.2 to 0.8 Ma) and the Bishop Tuff (0.76 Ma) suggest significant incorporation of continental crust. Shallow-crustal residence histories inferred from Sr-isotopic studies at Long Valley suggest that large-volume silicic magmas reflect either: (1) continuous growth with episodic eruption from a single, large, long-lived magma chamber; or (2) rapid generation and eruption of separate, short-lived magma batches. Residence histories that require several rapid differentiation events (≤104 yr) followed by extended periods (≥105 yr) over which magmas are maintained without significant cooling and crystallization are difficult to reconcile with numerical models of the thermal evolution of shallow-crustal magma reservoirs. Sr-isotopic compositions of the Bishop Tuff and precursor Glass Mountain lavas alternatively may reflect assimilation of wall-rock melts undergoing high-87Sr/86Sr, low-Sr concentration dehydration during convective sidewall fractional crystallization of parental rhyolites. If so, the reported apparent Sr-isochronal relationships may have little or no age significance. Sr-isotopic systematics of the Bishop Tuff also provide insights into the influence of eruption dynamics on compositional gradients recorded in pyroclastic deposits. Sr-isotopic compositions of individual fall and flow pumice are consistent with recent field studies that suggest that the Bishop ignimbrite is intraplinian, and that fall and flow deposits previously interpreted to be sequential are largely coeval. Moreover, the distribution of Sr-isotopic compositions of pumice as a function of stratigraphic height suggests that magmas from different depths with different 87Sr/86Sr ratios were intimately mixed before exiting the vent and can be qualitatively predicted from numerical models of magma withdrawal from zoned crustal reservoirs.