Development of a continental volcanic field: Petrogenesis of Pre-caldera intermediate and silicic rocks and origin of the Bandelier magmas, Jemez Mountains (New Mexico, USA)

Abstract

The Miocene-Quaternary Jemez Mountains volcanic field (JMVF) is the site of the Valles caldera and associated Bandelier Tuff. Caldera formation was preceded by <10 Myr of volcanism dominated by intermediate composition rocks (57-70% SiO2) that contain components derived from the lithospheric mantle and Precambrian crust. Simple mixing between crust-dominated silicic melts and mantle-dominated mafic magmas, fractional crystallization, and assimilation accompanied by fractional crystallization are the principal mechanisms involved in the production of these intermediate lavas. A variety of isotopically distinct crustal sources were involved in magmatism between 13 and 6 Ma, but only one type (or two very similar types) of crust between 6 and 2 Ma. This long history constitutes a record of accommodation of mantle-derived magma in the crust by melting of country rock. The post-2 Ma Bandelier Tuff and associated rhyolites were, in contrast, generated by melting of hybridized crust in the form of buried, warm intrusive rocks associated with pre-6 Ma activity. Major shifts in the location, style and geochemical character of magmatism in the JMVF occur within a few million years after volcanic maxima and may correspond to pooling of magma at a new location in the crust following solidification of earlier magma chambers that acted as traps for basaltic replenishment. © The Author 2007. Published by Oxford University Press. All rights reserved.