Paper No. 4
Presentation Time: 8:45 AM


HART, William K., Department of Geology & Environmental Earth Science, Miami University, 114 Shideler Hall, Oxford, OH 45056,

Southeastern Oregon and portions of adjacent Idaho and Nevada have experienced widespread basaltic volcanism since the mid-Miocene flood basalt and voluminous rhyolite event(s) commonly linked to initial manifestation of the modern-day Yellowstone hotspot. As flood basalt and silicic activity waned, and following a short period of extensional graben subsidence and related magmatic activity, diffuse extension and monogenetic basaltic volcanism initiated at ~10 Ma throughout the region. Building on the earlier activity, this “plains style” volcanism created a relatively flat basalt plateau (Owyhee Plateau) with scattered ~N-S trending low displacement normal faults, a few prominent ~E-W structures, and an abundance of low shield cones often forming multi-vent alignments.

This talk concentrates on two periods of heightened Quaternary basalt production, 2.1 to 1.5 Ma and 0.3 to 0 Ma, from the northernmost portion of the Owyhee Plateau (OP) in southeastern Oregon. Each period is represented by a minimum of six monogenetic systems preserved primarily as lava flows and agglutinate, and the low shield cones and fissures from which these materials erupted. Products of these systems encompass the entire range of basalt compositional varieties erupted on the OP over the past 10 Ma, including primitive to fractionated tholeiitic and mildly alkaline. Major and trace element heterogeneity is observed within and between individual systems, heterogeneity that is not simply coupled with the basalt Sr-Nd-Pb isotope characteristics. The isotope signatures are better correlated to eruption age; the older period has limited variability (e.g., 87/86Sr = 0.7053-0.7055), whereas the young basalts are more diverse (0.7038-0.7052), with the <2 ka system yielding the most depleted Sr (0.7038-0.7041) and Nd (0.51287-0.51280) isotope signatures. Moreover, some but not all of the systems exhibit geochemically distinct phases within their eruptive history. Together the spatial, temporal, chemical, and isotopic characteristics and patterns imply complex vertical and horizontal heterogeneities in the basalt source regions, within and between system variations in the type and extent of crustal differentiation processes, and recurrent reactivation of crustal weaknesses to induce magma ascent and focus eruptive loci.