COMPOSTIONAL DIVERSITY IN MID-MIOCENE MAFIC LAVAS FROM THE SOUTHEASTERN OREGON PLATEAU

Mid-Miocene basaltic volcanism on the Oregon Plateau initiated at ~17 to 16.5 Ma, contemporaneous with the onset of Columbia River Basalt Group volcanism.  While the most extensive volcanism was centered at Steens Mountain, the tholeiitic lavas of the Steens Basalt (SB) erupted from fissures scattered throughout the Oregon Plateau over an ~2 to 3 Ma duration.  Accompanying these voluminous eruptions, smaller volumes of additional tholeiitic basalts and basaltic andesites erupted from loci scattered throughout the northern Basin and Range and southernmost Oregon Plateau.  In the northern Nevada Rift (NNR) of north central Nevada, mid-Miocene tholeiitic lavas are well exposed and often interspersed with intermediate and silicic eruptive products and eruptive loci.  One such location, the Santa Rosa-Calico volcanic field (SC), is located in the southern Oregon Plateau, within the NNR at its intersection with the Yellowstone-Snake River Plain volcanic province.  In the SC, eruption and emplacement of basalt through high-Si rhyolite volcanic products and shallow intrusive bodies initiated at ~16.4 Ma. This magmatism continued for at least ~2 Ma, contemporaneous with the entire duration of Columbia River-Steens flood basalt volcanism.  Mafic lavas sourced in the SC can be broadly divided into two groups.  The first group consists of basalts and basaltic andesites compositionally identical to SB lavas (Mg # ~30-55, ~48-55 wt.% SiO2, >1.5 wt.% TiO2, >0.5 wt.% K2O).  Basalts of the second group are more primitive, and closely resemble small volume, regionally extensive <11 Ma HAOT lavas (Mg # ~59, ~48 wt.% SiO2, ~1.3 wt.% TiO2, ~0.33 wt.% K2O) that typically were erupted from discrete shields and cones throughout the Oregon Plateau.  The chemically distinct group two basalts represent the most primitive mid-Miocene material erupted in this portion of the Oregon Plateau.  The contemporaneous eruption of SB and more primitive magmas suggests either a direct petrogenetic relationship between these magma types or their derivation via distinct sources/processes.  Further study is needed to resolve this issue, although the distribution of primitive mid-Miocene Oregon Plateau basalts suggests a strong link to lithospheric and local extensional tectonic domains.