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
Published literature data indicate 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.
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 are concerned. The lower silica content samples compositionally overlap best with Imnaha and Picture Gorge Basalt. The combination of data suggest that Birch and Hunter Creek lavas of the Malheur Gorge are likely locally sourced rather than connected to distal lava flows from the north.