ABSTRACT 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, I describe a previously undocumented succession of fissure-fed lavas exposed in the Smoke Creek drainage of the southern Buffalo Hills. Ar-Ar dating of two lava flows from the Smoke Creek stratigraphic section yield ages of 16.54 + 0.19 to 16.05 + 0.17 Ma. The Smoke Creek lavas are therefore partly contemporaneous with the dikes and lavas found in the Northern Nevada rift (NNR) system, and contemporaneous or slightly younger than the hotspot-related Steens Basalt that forms the oldest lavas of the Columbia River Basalt Group in southeastern Oregon. The intent of this research is to examine the geochemical characteristics of the Smoke Creek lavas to determine if they are more consistent with a genesis associated with hotspot volcanism, volcanic-arc volcanism, or with volcanism associated with back-arc extension.
The volcanic section exposed in the Smoke Creek drainage in the southern Buffalo Hills is composed of mildly alkaline basalts, basaltic trachyandesites, trachyandesites, and trachydacites that were erupted from a prominent northerly trending dike swarm. The upper portion of the Smoke Creek stratigraphic section contains evidence of mildly explosive activity from one or more eruptive centers present locally. Comparisons of chemically analyzed lava flows from the region indicated that, despite their similarity in age and fissure-related genesis, the Smoke Creek lavas have a different chemical signature than those associated with the NNR located ~ 200 km to the east. Instead, the Smoke Creek lavas have some chemical similarities to the most evolved lavas of Steens Basalt. However, the Smoke Creek lavas differ from the Steens Basalts in that they are dominated by more differentiated rock types that display a clear trend of Fe-depletion on the AFM diagram. Such calc-alkaline trends of Fe-depletion have often been attributed to the fractional crystallization of hornblende and/or magnetite from H2O-enriched basaltic melts, as expected in subduction zone settings. Since the location of the Smoke Creek dike swarm lies in the back-arc region, but very near the projected eastern boundary of the Miocene Cascade volcanic arc, the this calc-alkaline trend of Fe-depletion might be the product of a hydrated subduction component in the Smoke Creek lava source.
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