TERTIARY VOLCANISM IN THE SOUTHERN BUFFALO HILLS, SMOKE CREEK, NEVADA

Tertiary volcanism in the Pacific Northwest is recorded in an extensive history of eruptions from varied tectonomagmatic environments associated with the Cascade volcanic arc, back-arc extension, and hotspot initiation. All three of these environments were active in the tri-state region of southern Oregon, northern Nevada, and northeastern California in the middle Tertiary. Here, in northwestern Nevada, we describe a previously undocumented succession of fissure-fed lavas exposed in the Smoke Creek drainage of the southern Buffalo Hills. The chemistry of these lavas rules out their association with the hotspot-related Steens Basalt, but is more consistent with an older group of Oligo-Miocene volcanic rocks that erupted very near the eastern boundary of the ancestral Cascades. Our intent here is to examine the geochemical characteristics of these lavas to determine if they are more consistent with a genesis associated with arc volcanism or with back-arc extension. The Smoke Creek section includes more than 500 m of lava with more than 60 distinct lava flows that vary from aphyric to moderately plagioclase-phyric. They are typically thin (less than 5 m) with moderately developed lower colonnades and brecciated flow tops. We analyzed 42 samples from both flows and feeder dikes for major- and trace elements using XRF. These lavas differ from typical arc lavas in that they are largely basaltic trachyandesites falling along a mildly alkaline trend of differentiation, consistent with similar Oligo-Miocene lavas found across the breadth of southeastern Oregon and adjacent Neveda. However, the Smoke Creek lavas also fall within the Western Cascades field of Fe-depletion and they display moderate LILE/HFS ratios and small Nb-troughs on MORB-normalized trace-element plots. Such characteristics are consistent with the generation of basalts derived from a hydrated mantle source followed by the modification of these melts by fractional crystallization of magnetite and/or hornblende. Although the mildly alkaline signature of the Smoke Creek lavas is consistent with volcanism in a back-arc setting, the petrogenesis of these lavas also appears to involve a hydrous component associated with mid-Tertiary subduction beneath the Western Cascades volcanic arc.