PETROLOGY AND EMPLACEMENT OF THE COTTONWOOD WASH TUFF

The Cottonwood Wash Tuff is a very large volume (1500 km³) ash-flow tuff of the Indian Peak Caldera Complex, Nevada-Utah. This tuff was emplaced in the early Oligocene during the Great Basin “ignimbrite flare-up.” The first of several monotonous intermediate ash-flow tuffs from this caldera complex, the Cottonwood Wash Tuff is a single cooling unit of crystal-rich, high K, calc-alkaline dacite with strong negative Nb and Ti anomalies. It has a virtually uniform phenocryst assemblage: plagioclase > hornblende > biotite=quartz > oxides > cpx > apatite, zircon, sulfides +/- sanidine. Average crystallization temperature was 775 C at 2.4 kb, based on hornblende and plagioclase compositions. Although there is a wide range of whole rock compositions ranging from 63 to 69 % SiO₂, the tuff is not systematically zoned—vertically or laterally. Phenocryst compositions are more-or-less uniform—no significant differences have been found in mineral compositions in mafic dacite compared to silicic dacite. Pumices have the same mineral assemblage, mineral compositions, and total phenocryst proportions as the pyroclastic flow. Moreover, the bulk compositional range of the pumices and tuffs are the same, but the silicic end of the spectrum is dominated by pumice fragments. There is no simple correlation between phenocryst abundance and whole-rock composition. The attributes of the tuff are probably not the result of simple glass-phenocryst fractionation. Like other monotonous intermediates, there are no underlying plinian pyroclastic fall deposits, but in four localities pyroclastic surge deposits (less than a meter thick) lie beneath the pyroclastic flow layer. These surge deposits can be divided into two end-members—coarse, phenocryst-rich (45-75% phenocrysts DRE) andesitic layers and finer, phenocryst-poor (14-30%), rhyolitic layers. We suspect that they formed at the front of the pyroclastic flow by mechanical separation and sorting of glass and phenocrysts during small bursts.