IDENTITY OF CRBG LAVAS IN THE GREATER VALE AREA, EASTERN OREGON: IMPLICATIONS FOR VENTING GRANDE RONDE BASALT (GRB) UNITS LOCALLY, WITHOUT LATERAL CONNECTION TO MAIN GRB UNITS

Along the Malheur Gorge corridor of southeastern Oregon between the towns of Vale and Juntura, main phase Columbia River Basalt Group (CRBG) units overlap with flows of Steens Basalt from the south and flows of Imnaha and Grande Ronde Basalt (GRB) from the north. Compositions of local Birch and Hunter Creek Basalt units are consistent with being GRB lavas. Recently, Webb et al. (2018, Geosphere, v.15 n.1) found evidence for vent sites of Hunter Creek Basalt along Malheur Gorge calling into question if any of the flows exposed in Malheur Gorge are from the main eruption area for the GRB in northeastern Oregon. This study aims to answer this question by investigating the most proximal mapped CRBG lavas around latitude 44.3N, from near the town of Brogan, OR to Weiser, ID, located about 50-70 km north/northeast of Malheur Gorge. We sampled along three transects and include data collected by others from elsewhere in the area.

Literature data indicates virtually all GRB lavas, including the local Birch and Hunter Creek lavas, range in SiO₂ from 53-56 wt.% with a few reaching values above 56 wt.%. Lavas are fine-grained and often indistinguishable in the field. Differentiation among units relies on subtle chemical variations in e.g. TiO₂, P₂O₅, MgO along with paleomagnetic orientation. All other main phase CRBG units have SiO₂ values < 53 wt.%, with some rare exceptions for Steens and Picture Gorge Basalt. An initial assessment of our new and published data of CRBG lavas for the Brogan-Weiser corridor indicate numerous samples have SiO₂ values around 49-52 wt.% with a subgroup of samples being as silicic as local GRB units (Birch and Hunters Creek Basalt). Therefore, select sections of flows can be correlated with local GRB units, yet these appear distinct as far as GRB erupted from the main GRB eruption sites in NE Oregon is concerned. The lower silica content samples do compositionally overlap with Imnaha, Picture Gorge, and possibly Steens Basalt. The combination of data suggest that Birch and Hunter Creek lavas of the Malheur Gorge may be locally sourced rather than connected to distal lava flows from the north.