Eruption of reverse-zoned upper Tshirege Member, Bandelier Tuff from centralized vents within Valles caldera, New Mexico

The HTBU is the host ignimbrite of a complex vent breccia exposed for over 3 km along the NW faulted face of Redondo Peak, the Valles caldera resurgent dome. Breccia clasts consist of angular to subrounded fragments ≤ 1 m in diameter of Permian red beds, Miocene basin fill sediments, Miocene to Pliocene volcanics and Quaternary ignimbrite (Otowi Member, Bandelier Tuff) that underlie Valles caldera. Flow units Qbt5l and Qbt5u overlie the HTBU vent breccia but these later units do not contain unusual quantities of lithic fragments. Significantly, small mudstone lithic fragments (generally ≤ 200 μm in diameter) in the HTBU vent breccia contain reaction rinds of secondary garnet (andradite) along margins with ignimbrite host. We argue that the tiny andradite crystals (≤ 50 μm) formed by reaction of degassing volatiles in the vent area with relatively Fe-Ca-rich vent breccia fragments at temperatures approaching 800 °C and pressures less than a few MPa. Geologic relations and the restricted distribution of upper Tshirege flow units Qbt3t to Qtb5u indicate they were erupted in a short period of time (weeks to months) from a centralized vent near Redondo Peak, and from other buried vents as far east as the eastern ring fracture. We also conclude that late injection of andesitic melt into the residual Bandelier magma chamber stimulated the eruption of upper Tshirege Member pyroclastic flows from different locations and depths of the chamber resulting in a small but significant reversal in chemical and mineralogical zonation.