QUATERNARY BASALTIC VOLCANISM ALONG THE NORTHWESTERN MARGIN OF THE OWYHEE PLATEAU, SE OREGON

The Owyhee Plateau tectonomagmatic province is located in the Oregon-Idaho-Nevada border region and preserves evidence of complex magmatic processes and mantle reservoir interactions over the past 17 Ma. Quaternary basaltic volcanism is concentrated along the northern and northwestern margins of the Plateau in a number of discrete to overlapping volcanic fields characterized by monogenetic cones and small shields. The focus of this investigation is three young basalt volcano fields, the Saddle Butte field (SdB), Jackie's Butte field (JB), and Scott's Butte field (ScB), that lie near the suggested physical northwestern edge of the Owyhee Plateau. In contrast to the multiple monogenetic shield vents of the SdB and the JB, the ScB is dominated by a single vent complex (Scott's Butte) that preserves evidence of early hydrovolcanic activity followed by emergent central and satellite vent Strombolian and Hawaiian activity. New stratigraphic observations and preexisting K-Ar data, when viewed in the context of more extensive chronologic data for the nearby Jordan Valley volcanic field (JVVF), indicate that the volcanism considered in this investigation is less than approximately 1.2 Ma, with portions of the Saddle Butte field likely less than 100 ka in age. Furthermore, stratigraphic relationships along the Owyhee River canyon document the presence of flows likely emanating from 1.9 Ma and older JVVF vents stratigraphically beneath eruptive products of the SdB. Between volcanic field and between vent major and trace element variability is observed including little fractionated, LIL and HFS element depleted olivine tholeiites (HAOT) in the SdB and JB, basalts with characteristics in common with certain Snake River olivine tholeiites (SROT) and young JVVF alkaline basalts in the JB, and basalts transitional to these varieties in the ScB. Within vent geochemical heterogeneities also are observed, only some of which may be produced via shallow fractional crystallization and/or small differences in degree of melting. The observed geochemical complexities require the presence of heterogeneous lithospheric mantle and lower crustal reservoirs beneath this region and post magma generation differentiation processes that involve mixing of heterogeneous melts and/or melts and solids derived from these reservoirs.