ERUPTION OF REVERSE-ZONED UPPER TSHIREGE MEMBER, BANDELIER TUFF FROM CENTRALVENTS WITHIN VALLES CALDERA

Geologic mapping at 1:24,000 supplemented by major/trace element chemical, detailed petrographic, and microprobe analyses from six stratigraphic sections and selected outcrop within the Valles caldera resurgent dome and Pajarito Plateau reveals important, but previously unrecognized, compositional reversals within uppermost Tshirege subunits of the Bandelier Tuff (ca 1.25 Ma). The significant subunits of uppermost Tshirege, named Qbt3t, Qbt4 and Qbt5, are volumetrically minor compared to lowermost subunits Qbt1 to Qbt3 (≤15 % of total erupted volume). Subunits Qbt3t, Qbt4, and Qbt5 have similar chemical and petrographic characteristics, and form short lobes on the Pajarito Plateau where first described. Contents of incompatible elements Nb, Rb, and Y are unchanged from Qbt3 to uppermost Tshirege, despite large, stepwise decreases in average quartz (38% of phenocrysts in Qbt3, 20% in Qbt3t; 8% in Qbt4 and Qbt5). Significantly, major decreases in average BaO (616 to 359 ppm), TiO₂ (0.37 to 0.24 wt%) and anorthoclase (61% to 9% of K-spar) mark the change from Qbt4 to Qbt5.

Mapping has also discovered a NE-trending elliptical zone of lithic-rich vent breccia exposed on the NW sides of Redondo Peak and Redondito. These exposures occur along the SE edges of the Redondo Creek and Jaramillo Creek grabens and are observable only because of extreme faulting and resurgence. The breccia consists of angular fragments (≤1 m diameter; average about 10 cm) of Paleozoic and Tertiary lithologies hosted in Qbt4 Bandelier Tuff. Landslides mask much of the breccia; there could be two vents or one elongated vent. The vent breccias do not resemble hydrothermal explosion breccias (no mosaic textures, silicified matrix, or highly rounded, large xenoliths). The Qbt4 vent breccias are overlain by lithic-poor Qbt5, typical of Tshirege ignimbrites. We have not identified any Qbt5 vents on the Valles resurgent dome.

A relatively small volume of plagioclase- and 2-pyroxene-phyric magma was injected late into residual Tshirege magma chamber and co-erupted as Qbt4. This primitive magma is distinct from “hornblende latite” magma (<1% by volume) co-erupted early with plinian and Qbt1-Qbt2 ash flows. Petrographic and chemical analyses provide no evidence that primitive magma interacted with Tshirege magma erupted as Qbt5.