MILLION-YEAR MELT-PRESENCE IN MOGOLLON-DATIL VOLCANIC FIELD RHYOLITES: CONSTRAINING THE HISTORIES OF THE BURSUM CALDERA RESURGENT DOMES

The Mogollon-Datil Volcanic Field (MDVF) located in southwestern New Mexico is the result of punctuated volcanism and the transition between arc and rift magmatism. Volcanism showcases the transition from caldera forming ignimbrite eruptions during ignimbrite flare-up to basaltic andesite eruptions between 36 Ma and 24 Ma. Here we seek to understand the temporal and spatial relationship of the resurgent domes related to the 28 Ma Bursum caldera. These rocks represent the transition from the last gasps of rhyolite to basaltic andesite volcanism which occurs abruptly between 27-26 Ma. To accomplish this, we use a combination of new zircon U-Pb ages and trace element contents and isotope geochemistry. We test the hypothesis that the resurgent domes are related in space and time but sourced by different pockets of melt extracted as the result of remobilization of cold storage mushes by injection of intermediate composition magmas. Samples analyzed include the Fanney Rhyolite, Deadwood Gulch, and the Apache Spring Tuff which give zircon U-Pb ages for various localities of the Fanney Rhyolite ranging from 28.48-25.23 Ma. We suggest that residual magma was maintained near solidus for greater than 1 m.y after caldera collapse by the injection of new andesite magmas into the system and remained locally viable for at least some time after injection. The magma feeding the resurgent domes contain zircons that are geochemically distinct from the Bursum caldera ignimbrites. Within the whole rock geochemistry two chemically distinct groups of the Fanney Rhyolite exist along with the ~3Ma age range along with suggest the possibility of two eruptive events of the Fanney Rhyolite.