PHYSICAL, PETROGRAPHIC, AND GEOCHEMICAL CHARACTERIZATION OF ASH FLOW VOLCANISM: THE MID-MIOCENE COLD SPRINGS TUFF OF THE SANTA ROSA-CALICO VOLCANIC FIELD, NEVADA

This is a case study aimed at addressing the source and chemical homogeneity of an explosively derived eruptive unit associated with an intra-continental volcanic field. In the mid-Miocene, the Pacific Northwest (USA) was characterized by abundant flood basalt volcanism and the coeval development of silicic-dominated volcanic fields. The Cold Springs tuff (CST) was deposited at ~15.4 Ma, during the waning stages of ~16.4-14 Ma volcanism in the Santa Rosa-Calico volcanic field (SC), Nevada. Chemically distinct from mapped SC silicic eruptive loci and lava flows, the CST is among the youngest exposed silicic units and most extensive pyroclastic units in the SC. As a result, characterizing the CST is crucial to understanding the evolution of the SC. The CST crops out throughout the central SC and ranges from slightly indurated to extremely welded and appears to consist of multiple cooling units. Field evidence suggests that the CST was derived locally from a source in the western portion of the central SC. CST whole rock major and trace element compositions are variable (~69-77 wt. % SiO₂; ~0.55-0.30 wt. % TiO₂; ~2.0-0.30 wt. % CaO; ~150-30 ppm Sr; ~64-100 ppm La). To better constrain the CST chemically and help determine the influence of lithic fragments and mineral inclusions on its composition, purified glass separates were obtained on three poorly welded CST samples. These purified glass separates are more silicic than their bulk rock counterparts (~2-4 wt. % SiO₂) and exhibit similar variations in other major and trace element concentrations. This variation is associated with the removal of lithic fragments and crystals (mostly feldspar and oxides) during the magnetic separation process. These glass separates also allow us to determine if CST fall deposits are regionally exposed and have been previously documented by other workers. In summary, this study provides the basis for more detailed petrologic work on late stage SC silicic volcanism.