A DETAILED STUDY OF THE COUGAR POINT III IGNIMBRITE, CENTRAL SNAKE RIVER PLAIN

The lowest unit of the Cougar Point Tuff (Cougar Point III, or CPT III) in the area of Bruneau Canyon near Rowland, NV differs from the overlying ignimbrites in a number of ways that may reflect differing magmatic history and/or proximity to its vent. The CPT ignimbrites are spatially and temporally associated with the Bruneau-Jarbidge eruptive center (12.7-10.2 Ma) along the Yellowstone hotspot track. CPT III is similar to other CPT units and to other rhyolite ignimbrites of the Central Snake River Plain in that it is extensive, high temperature, A-type, and pervasively welded and rheomorphic (Bonnichsen et al., 2008, B. Volcanol., 70:315-342). However, it alone contains titanite and unlike other units it contains orthopyroxene rather than pigeonite, both of which suggest different magma chamber dynamics. In comparison to the other CPT units, III contains: ~200 ppm vs. 350-500 ppm Zr; 3.4 vs. 8-12 Zr/Nb; and <0.5 vs. 4-9 Ba/Ce. None of the other CPT pre-ignimbrite airfall and surge deposits contain lithics or pumice larger than 1 cm while we report the first coarse pumice (>10 cm, unflattened) and ash deposit at the base of CPT III, suggesting it may be more proximal to its vent than are the other CPT ignimbrites. Initial analyses show that CPT III apatite contains more REE and has lower F/Cl (average 35 vs. 45-55) than other CPT units. Further determination of the CPT III’s pre-eruptive history and volcanology will help illuminate the magmatic history and vent arrangement of the Central Snake River Plain.