THE PICTURE GORGE AND STEENS BASALT: A TALE OF TWO MAGMATIC TRANSPORTATION AND STORAGE SYSTEMS

The Columbia River Basalt Group (CRBG) is the youngest large igneous province. Dikes for the four main eruptive units (Steens, Imnaha, Grande Ronde, and Picture Gorge Basalt (PGB)) are the Monument, Steens and Chief Joseph Dike Swarms (MDS, SDS, and CJDS, respectively) exposed in SE Washington to N. Nevada.

Lava ages indicate that eruptions began in the MDS and SDS, shortly followed by eruptions from the CJDS. We investigate the MDS and SDS as part of a larger project examining the CRBG dike feeder system to answer questions of magma storage, transport, and whether magmas of different units resided in discrete locations. To date, 41 MDS dikes and several small sills were sampled, and strike was measured, 61 dikes mapped through orthoimagery to determine strike, including 21 dikes from Cahoon et al. (2020). Dike strike is primarily between 295-305 degrees and average width is 5m, up to 14m, often extending for several km. Several sill bodies near the town of Monument show only small alteration halos with no apparent mineralization suggesting short-term storage. Here, geophysical data show prominent anomalies but they do not spatially correlate with dikes. Comparisons to published PGB data indicate close compositional overlap and similar ranges that show significant magma evolution and open system processes, as exemplified by variations of Nb/Zr ratios.

In the SDS, 26 dikes were sampled, and strike measured, with geochemical data of 11 dikes from Moore and Grunder (2020) included here, along with 55 dikes mapped through orthoimagery. Strike was primarily 15-25 degrees, thickness mostly between 2-3m. Dikes have compositions close to published data with 6 dikes matching Lower A, 2 dikes to Lower B, and 6 dikes to Upper Steens lavas.

As published, we see geochemical evidence of assimilation, fractionation, and recharge as based on a Zr/Nb AFC model. One new layered mafic intrusion, 6.5 km long and 250m thick, was sampled and this along with field evidence of large-scale alteration and mineralization of the country rock, circular geophysical anomalies, and one linear anomaly to the south with exposed dikes imply longer term storage of Steens Basalt magmas.

It is likely that PGB and Steens Basalt were sourced and evolved in discrete locations in the Vale-Castle Rock area and near the Oregon-Nevada border to the south, respectively.