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 small volume intermediate composition composite cones to caldera forming ignimbrite eruptions during ignimbrite flare-up to basaltic andesite eruptions between 40 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 last gasps of rhyolite during the transition to basaltic andesite volcanism occurring abruptly between 27-26 Ma. To accomplish this, we use a combination of new zircon U-Pb crystallization ages and trace element contents with whole rock isotope geochemistry. We tested 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. U-Pb ages indicated that zircon growth with the Fanney Rhyolite was coeval with the last ignimbrite eruptions. Trends in zircon trace elements (Hf, Eu/Eu*, Yb/Gd, etc.) through time show an increasing Th/U ratio and decreasing Yb/Gd ratio showing rejuvenation of the melt. Hafnium vs Eu/Eu* suggests two groups of zircons, one being rejuvenated, and the other stored cold. The age (Ma) vs Hf also show two distinct groups of zircons. We interpret this diversion to record isolated melt pockets recording different thermal histories within a melt-rich cap of re-mobilizing intermediate composition mush column. We suggest that multiple rejuvenation events occurred that were sourced from different compositions of magma. This created the range of crystallization ages and variety of REE ratios we observe in the samples.