Evolution of a Long-lived Magmatic System Leading to Caldera-forming Eruptions in the Jemez Mountains Volcanic Field, USA

Abstract

Two caldera-forming rhyolitic supereruptions have occurred at the Jemez Mountains volcanic field (JMVF), producing the Otowi (1.60 Ma) and Tshirege (1.25 Ma) members of the Bandelier Tuff. The magma petrogenesis preceding the two supereruptions was examined via systematic studies of mineral chemistry and zircon geochronology of pre-caldera and caldera-forming volcanic products. The Otowi supereruption was preceded by ~10 m.y. of dominantly intermediate to silicic volcanism (i.e., the 10-7 Ma Paliza Canyon, the 8-7 Ma La Grulla Plateau, and the 5.5-2 Ma Tschicoma formations). Mineral disequilibria and compositional diversity suggest that these were products of magma mixing between multiple mafic components formed from AFC processes in a deep “hot zone”, and a middle-crustal-derived silicic component, either in the deep crust or in a mush formed within the shallow crust. The pre-caldera magmas evolved towards cooler and more oxidized conditions as they gradually propagated into shallower crust. This eventually modified the original crust to form a transcrustal magmatic system over several million years. Reactivation of the system at 5.5 Ma after ~0.5 m.y. tectonic-related cooling, led to formation of the long-lived Tshicoma dacitic magma bodies, each characterized by a series of separate melt/crystal domains with >1 m.y. lifespan. Elevated magma supply after ~2 Ma destabilized this system and coalesced isolated melt lenses and recycled plutons in the upper crust to produce the Otowi supereruption. After evacuation of the Otowi magma, rapid influx of new magma under high supply rates re-primed the system in just 350 ka to produce the Tshirege supereruption. This change from slow to fast magma accumulation rates reflects thermal and chemical maturation of the crust, and correlates to elevated magma supply rate, aided by the previous supereruption. Based on the geochemical results, five magmatic cycles with variable longevity were found to describe the system history. It appears that once established, a transcrustal magmatic system may cool off slowly and re-charge rapidly. With this prerequisite, magma supply rate (and/or tectonics) governs two types of magmatic cycles at the JMVF, either producing long periods of low rates of eruption with small rhyolite events and mixed intermediate products, or catastrophic caldera-forming eruptions.