COMPOSITIONS OF DIKES FROM THE NORTHERN AND SOUTHERN PORTIONS OF THE CHIEF JOSEPH DIKE SWARM

The Columbia River Basalt Group (CRBG) is the youngest continental flood basalt province and recent ages constrain main phase CRBG activity (Steens, Imnaha, Picture Gorge, Grande Ronde Basalt (GRB), Wanapum) from > 17 to 15.9 Ma. Few studies have focused on the main eruptive system, the Chief Joseph dike swarm (CJDS) in NE Oregon. The Imnaha flows first erupted from the CJDS ~16.7 Ma followed by the GRB, together producing 76% of all CRBG lavas. This study uses geochemistry, petrography, and geochronology to correlate select dikes of the CJDS with lava units of the GRB and the Imnaha Basalt – as no age dates exist for the CJDS, establishing age constraints will prove critical to defining and correlating dike activity with eruption durations.

Dikes of interest for this study make up the northern and southern portion of the CJDS. In the north, dikes were mainly sampled along the Grand Ronde River and three locations in the south, Big Lookout Mtn., San Pedro Mtn., and along the Snake River north of Huntington, OR. Northern dikes exclusively cut CRBG lavas, have horizontal columnar jointing or massive texture as well as vertical relief in valleys. Some show vertically oriented vesicle bands along margins suggesting devolatilization during ascent. Dikes in the south have no vertical relief and intrude granodiorite plutons or metamorphic rocks. At an average width of 9 m, southern dikes are wider than northern dikes, which average 5 m. A total of 39 dikes in the south and 31 in the north were sampled.

In the south, we have tentatively identified 15 Imnaha and 18 GRB dikes. The Imnaha dikes have coarse porphyritic texture with plagioclase phenocrysts between 0.3 - 2 cm. The GRB dikes are aphyric or sparsely phyric. Some samples have zoned plagioclase suggesting magma mixing during recharge events in crustal reservoirs. Preliminary geochemical data on 6 dikes allows correlation of 3 with GRB and 2 with Imnaha Basalt. Our data set will consist of petrographic and ICP-MS & XRF data for all 70 dikes, from which ~20 samples will be selected for ⁴⁰Ar/³⁹Ar geochronology. This project is part of a larger collaborative project examining along-strike variations in the age, chemistry, and flow direction in the ~400 km-long Columbia River dike feeder system to constrain regional-scale magma transport dynamics in the plumbing system of this LIP.