PICTURE GORGE BASALT, EASTERN OREGON: EXTENDED DISTRIBUTION AND PETROGENETIC CONNECTIONS TO STEENS BASALT AND STRAWBERRY VOLCANICS

The Picture Gorge Basalt (PGB) is considered a main-phase unit of the Columbia River Basalt Group (CRBG) even though recent work has proposed that PGB is sourced from a discrete, back-arc like mantle. This mantle is interpreted based on the high Ba/Ta ratios and low radiogenic Sr of PGB, to be distinct from other main-phase units, and therefore not directly tied to the Yellowstone hotspot (YHS). Our study focuses on unstudied mid-Miocene basaltic lavas and dikes exposed south and east of known PGB localities to reevaluate PGB distribution and to better understand evolution and possible petrogenetic connection among PGB, Steens Basalt, and the Strawberry Volcanics.

Geographically, samples with PGB composition extend lava flows and dikes of this CRBG unit much further south/southeast- and northeastward closing the exposure gap between PGB and Steens Basalt, and PGB and Imnaha Basalt, respectively. We have identified basaltic lavas with PGB compositions in the western Malheur Gorge below local Grand Ronde equivalents (Birch Creek); here, new compositional data suggest basaltic lavas that were correlated with Imnaha Basalt, are actually PGB. On a Ba/Nb vs La/Ta plot, our samples plot well within the PGB field. Additionally, PGB has been identified at the base of Castle Rock (north of Juntura) and northeast of Burns; and we can extend PGB flows eastward to the base of Dooley Mountain near Baker City.

Geochemically, new isotopic data indicates PGB has a wide range of radiogenic Sr, 0.70309-0.70396. The most primitive PGB dike has an ⁸⁷Sr/⁸⁶Sr of 0.70369 but only 239 ppm Sr suggesting that the higher ⁸⁷Sr/⁸⁶Sr ratio is not the result of crustal contamination. We have identified a ‘transitional’ group characterized by a large-ion lithophile and high-field strength element signature similar to the Strawberry Volcanics, and a heavy rare earth element signature similar to the Upper Steens Basalts. This signature and geochemical variability of individual lavas (as opposed to compositional averages) suggest mantle sources of PGB, Strawberry Volcanics Basalt, and Steens Basalt are gradational rather than distinct. This would be compatible with a common contributing source to PGB, Steens Basalt and the Strawberry Volcanics of a depleted upper mantle that in turn had variably experienced subduction related re-enrichment much earlier.