Mineralogical Evidence of Pre-caldera Magma Petrogenesis in the Jemez Mountains Volcanic Field, New Mexico, USA

The Jemez Mountains volcanic field (JMVF) is the site of the two voluminous, caldera-forming members of the Bandelier Tuff, erupted at 1·60 and 1·25 Ma, following a long and continuous pre-caldera volcanic history (∼10 Myr) in this region. Previous investigations utilizing whole-rock geochemistry identified complex magmatic processes in the two major pulses of pre-caldera magmatism including assimilation–fractional crystallization (AFC) and magma mixing. Here we extend the petrological investigation of the pre-caldera volcanic rocks into the micro-realm and use mineral chemistry and textural information to refine magma evolution models. The results show an increasing diversity of mineral populations as the volcanic field evolved. A range of plagioclase textures (e.g. sieved cores and rims) indicate disequilibrium conditions in almost all pre-caldera magmas ranging from andesite to rhyolite, reflecting plagioclase dissolution and regrowth. Coarsely sieved or dissolved plagioclase cores are explained by resorption via water-undersaturated decompression during upward migration from a deep melting, assimilation, storage and homogenization (MASH) zone. Plagioclase crystals with sieved rims are almost ubiquitous in dacite-dominated magmatism (La Grulla Plateau andesite and dacite erupted at ∼8–7 Ma, as well as Tschicoma Formation andesite, dacite and rhyolite at ∼5–2 Ma), reflecting heating induced by magma mixing. These plagioclase crystals often have An-poor cores that are chemically distinct from their An-rich rims. The existence of different plagioclase populations is consistent with two distinct amphibole groups that co-crystallized with plagioclase: a low-Al, low-temperature, high-fO2 group, and a high-Al, high-temperature, low-fO2 group. Calculation of melt Sr, Ba, La, and Ce concentrations from plagioclase core and rim compositions suggests that these chemical variations are largely produced by magma mixing. Multiple mafic endmembers were identified that may be connected by AFC processes in the MASH zone in the middle to lower crust. The silicic component in an early andesite-dominated magmatic system (Paliza Canyon andesite, dacite and rhyolite, 10–7 Ma) is represented by contemporaneous early rhyolite (Canovas Canyon Rhyolite). A silicic mush zone in the shallow crust is inferred as both the silicic endmember involved in the dacite-dominant magmatic systems and source of the late low-temperature rhyolite (Bearhead Rhyolite, 7–6 Ma). Recharging of the silicic mush by mafic melts can explain observed diversity in both mineral disequilibrium textures and compositions in the dacitic magmas. Overall, the pre-caldera JMVF magmatic system evolved towards cooler and more oxidized conditions with time, indicating gradual thermal maturation of local crust, building up to a transcrustal magmatic system, which culminated in ‘super-scale’ silicic volcanism. Such conditioning of crust with heat and mass by early magmatism might be common in other long-lived volcanic fields.