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

Abstract Valles caldera (New Mexico, USA) is the type example of a resurgent caldera and source of the Tshirege Member, Bandelier Tuff, which has been long recognized as a normally zoned sequence of ignimbrites. In this paper, we present geologic, stratigraphic, chemical, and mineralogical data from upper flow units of the Tshirege Member obtained at multiple sites within and east of Valles caldera showing that the upper part of the Tshirege Member is reverse-zoned in chemistry and mineralogy. The key to deciphering these compositional changes in zoning is recognition of flow unit Qbt4u, which we informally name the high Ti–Ba unit or HTBU. The HTBU is widespread within the resurgent dome area of the caldera, but is only found in a small area outside and east of the caldera. The HTBU is the most “mafic” unit in the Tshirege Member and is identified by chemical maxima in Ti, Ba, Sr, P, V, and Th, and by unusually high contents of anorthoclase, plagioclase, orthopyroxene, ilmenite, apatite and zircon with respect to other flow units. The HTBU also contains small (≤ 1 cm) enclaves of quenched andesitic magma consisting primarily of plagioclase, orthopyroxene, clinopyroxene, glass and vesicles. Later erupted Tshirege flow units Qbt5l and Qbt5u are chemically more evolved and contain less plagioclase, orthopyroxene, ilmenite, apatite and zircon than the HTBU, and they are more evolved than flow units erupted before the HTBU (approximately 90% of the Tshirege Member). 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.