PETROGRAPHY AND IDENTIFICATION OF EOCENE ASH-FLOW TUFFS IN THE VICINITY OF THE JARBIDGE MOUNTAINS, NEVADA

Previous studies of Eocene tuffs in north-central NV have focused on their distribution to help shed light on regional paleogeography. In this work, names have been given to widespread ash-flow tuffs. The oldest age recorded in these deposits is 45 Ma (the 45 Ma tuff), but most deposits are ~40 Ma (e.g. the Tuff of Big Cottonwood Canyon). The distinguishing features of these identified deposits are differences in SiO₂, TiO₂, P₂O₅, Ba, Zr, Nb, FeO*, and V, as well as the amount of sanidine, plagioclase, biotite, and hornblende. Geologic mapping indicates that the sources of most of these deposits are calderas that are located west of the study area in the vicinity of Tuscarora and the Bull Run Mts., NV. During eruption, these tuffs flowed eastward, filling paleovalleys that in some cases, extended over 100 km. To augment this prior research, we report new field, petrographic, chemical, and temporal data from ash-flow tuffs in the vicinity of the southern Jarbidge Mountains, NV. Ash-flow tuffs from this study crop-out best in a broadly E-W trending corridor between Charleston reservoir, NV and NV Route 225. Deposits are typically highly welded, sometimes associated with basal fall deposits, and at one location, include a breccia facies that resembles those associated with caldera collapse. These deposits are plagioclase and biotite-rich, with subordinate amounts of sanidine, quartz, and hornblende. In thin section, plagioclase crystals are often broken and zoned and glass shards have undergone various amounts of devitrification. Thus far, one sample has been dated by ⁴⁰Ar/³⁹Ar geochronology and yields an age of 40.81 ± 0.20 Ma (normalized to 28.02 Fish Canyon tuff), which falls within the general age range of most of the previously identified tuffs. Major and trace element geochemistry allows for the correlation of this sample to the ~40.7 Ma Tuff of Coal Mine Canyon, which is well exposed to the south and east of the study area. Geochemical data also allow for other tuffs from this study to be correlated with at least three other previously named Eocene tuffs: the tuff of Big Cottonwood Canyon, the Plagioclase-biotite tuff(s), and the 45 Ma tuff. Future work, including detailed geologic mapping, is needed to help constrain the relationship between the distributions of these units in the study area and elsewhere in NE NV.