IDENTIFYING TUFF MARKER BEDS USING U-PB GEOCHRONOLOGY AND PETROGRAPHY IN THE CENTRAL MCCULLOUGH RANGE, NEVADA

Unknown white marker beds were discovered in a currently unmapped area of the central McCullough Range of Southern Nevada. These beds were hypothesized to be one of two regionally significant tuffs, the Peach Springs Tuff (18.5 Ma) or the Tuff of Bridge Springs (15.2 Ma). To identify the unknown marker beds, samples were collected and dated in two locations. In one location, the tuff unconformably overlies crystalline Precambrian basement, mainly consisting of a K-feldspar gneiss. In the second location, the tuff conformably overlies a coarse basal conglomerate unit mainly consisting of crystalline basement and Paleozoic limestone clasts. Both samples are rhyolitic in composition and have a phenocryst content ranging between 10% and 15%. The phenocrysts within these samples are dominated by blocky sanidine with minor amounts of plagioclase and biotite. The two units are light in color, one is white while the other is light pink, and both exhibit low degrees of welding. Zircons were dated from the collected samples using the LA-ICP-MS method for U-Pb geochronology. The first sample resulted in a ²⁰⁶Pb/²³⁸U weighted mean age of 18.4 ± 0.5 Ma (2σ error, MSWD = 0.7). The second sample resulted in a ²⁰⁶Pb/²³⁸U weighted mean age of 18.2 ± 0.4 Ma (2σ error, MSWD = 0.9). These ages and the appearance and phenocryst assemblage of the units indicate that the observed tuff beds are the Peach Springs Tuff, and thus the base of the volcanic section is exposed in the McCullough Range. This can be helpful for future mapping efforts of the region.