ASSESSING THE SPACE AND TIME CORRELATION BETWEEN CO-CRBG RHYOLITES AND RHYOLITES OF THE HIGH LAVA PLAINS TREND IN EASTERN OREGON

Rhyolitic volcanism of eastern Oregon occurred in two main episodes. Yellowstone plume impingement correlates with >16–15 Ma silicic volcanism associated with the Columbia River Basalt Group (CRBG), trending N-S from NE Oregon to northern Nevada. Rhyolites of the 12–0 Ma High Lava Plains (HLP) episode are correlated either with plume-spreading beneath cratonic and accreted lithosphere or slab rollback and mantle convection spanning from SE Oregon to Newberry volcano.

The provinces have been interpreted as separate and distinct due to 1) lack of regional silicic centers with ages between the youngest CRBG (~15 Ma) and oldest HLP (~12 Ma) centers, and 2) the perception that nearly all co-CRBG rhyolites do not extend further west than 118°W. However, sparse existing age data, including centers we have dated, indicate silicic units within the eastern HLP, as far west as 120°W, possess ages >15 Ma, demonstrating the provinces spatially overlap. Examples include Horsehead Mountain (15.6 Ma), Swamp Creek (16.0 Ma), Donnelly Butte (15.5 Ma), and the Buchanan Rhyolite complex (16.1 Ma). Of the ~64 silicic centers in the study area (117–119°W and 42.7–44.6°N), ~25 are dated, leaving ~39 undated, most without published compositional data. This study and our other recent dating have produced ⁴⁰Ar/³⁹Ar ages of two Star Mountain units (11.0 Ma and 10.7 Ma) and ages within 16-15 Ma and 12-0 Ma elsewhere. Yet, with many centers undated, it is uncertain if the provinces temporally overlap. Compositionally, all 11 analyzed centers are rhyolite, including two mapped as South Fork Dacite (77 wt.% SiO₂). Less evolved Star Mountain samples (0.29-0.63 Eu/Eu*) have higher Sr (60-179 ppm) and Ba (459-1456 ppm). All other centers contain <120 ppm Sr and <500 ppm Ba, except Dry Creek (81 ppm Sr; 2156 ppm Ba). Some centers (South Fork, Stockade Mountain, Mustang Butte, Iron Point, and Sacramento Butte) are highly evolved (≤0.11 Eu/Eu*), while rhyolites of Dry Creek, Saddle Butte, and an unnamed center are slightly less evolved (≥0.14 Eu/Eu*).

This study aims to determine how co-CRBG and HLP province rhyolites are correlated in space and time. Without a complete geochemical and geochronological record of all silicic centers, it remains unclear whether the provinces are correlated or a result of separate, distinct phases in the regional volcanic history.