BASALTIC VOLCANISM OF THE OREGON HIGH LAVA PLAINS

The High Lava Plains Province (HLP) of southeastern Oregon is a volcanic upland underlain by widespread basaltic lavas and age progressive rhyolite domes and tuffs. The west-younging age progression of HLP rhyolites is an enigmatic feature for which the province is renowned. Basaltic rocks of the HLP are generally primitive (67% Mg#>60), high-alumina (80% Al₂O₃>16 wt%), olivine-tholeiites (70% hypersthene- and olivine-normative). They are enriched relative to MORB in incompatible trace elements, especially fluid mobile elements; ten most primitive analyzed basalts average Ba=136 ppm, Sr=227 ppm, and Pb=1.0 ppm. Variation in trace element ratios of primitive HLP basalts suggests the dominant source component (88-98%) is depleted upper mantle (MORB-source), with a 2-12% contribution of an OIB-source and a minor (<0.2%) subduction component. The isotopic composition of HLP basalts is evolved relative to MORB, ⁸⁷Sr/⁸⁶Sr=0.7031-0.7051 and eNd=1.6-6.7. The plot below depicts the variability of Sr-isotopic composition in space and time. Several trends are clear: late Miocene HLP basalts vary little across the HLP, similar to middle Miocene basalts north and east of the HLP; and Pliocene and Quaternary basalts show more variability, being more depleted in the west and becoming systematically more evolved east of 119°W. The influx of an evolved source under the HLP could reflect westward sublithospheric flow of plume material that had incorporated an ancient lithospheric component. Westward flow originating at the craton margin may also be responsible for age progressive rhyolitic volcanism.