COLUMBIA RIVER RHYOLITES: AGE-DISTRIBUTION PATTERNS AND THEIR IMPLICATIONS FOR ARRIVAL, LOCATION, AND DISPERSION OF FLOOD BASALT MAGMAS IN THE CRUST

Despite earlier views, Columbia River province magmatism is now recognized to be associated with abundant rhyolites erupting over a wide area. On the other hand, the view that earliest rhyolites erupted from the McDermitt and other nearby volcanic fields has persisted. Our study concerns unknown or little studied rhyolite occurrences in eastern Oregon and our new data complement recent studies along the Oregon, Idaho and Nevada state borders. Along this southern periphery, ⁴⁰Ar/³⁹Ar geochronology revealed voluminous rhyolite activity started at ~16.5 Ma and lasted until ~15 Ma. Our currently oldest rhyolite ⁴⁰Ar/³⁹Ar ages of 16.53±0.14, 16.69±0.01, and 17.00±0.12 were obtained on the northernmost rhyolites from the greater Dooley Mountain area, near Baker City, NE Oregon. Furthermore, rhyolite activity was most extensive and voluminous from 16.3 to 15.5 Ma and waned thereafter, eventually leading to the two age-progressive rhyolite trends of the SRP, Idaho and the High Lava Plains, Oregon.

Our and literature data suggest following rhyolite age-distribution patterns that in turn have implications for arrival, location, and dispersion of flood basalt magmas in the crust. Earliest rhyolites (≥16.4 Ma) appear in the south and north of the province with a possible initial gap in between. This is consistent with the arrival of the earliest CRBG magmas of the Steens Basalt in the south and Imnaha Basalt in the north. Most voluminous and widerspread rhyolite volcanism around 16 Ma clearly coincides with the most voluminous CRBG unit, the Grande Ronde Basalt (GRB). Although main GRB activity from the Chief Joseph dike swarm seems to have ceased by ~15.9 Ma, enough GRB magmas may have stalled in the crust to support rhyolite centers from 15.9 to 15 Ma. Samples of such late and evolved GRB magmas erupted along with rhyolite in co-mingled eruptions as preserved in the youngest unit of the Dinner Creek Tuff and the Wildcat Creek Tuff but also locally in purely mafic eruptions.

The existence of crustal CRBG reservoirs beneath rhyolites seems inevitable particularly for GRB magmas that then travelled in dikes northward to their main eruption sites. These age-distributions patterns also question the significance of the apparent time-progressive CRBG volcanism.