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

This study focuses on the main eruptive system, the Chief Joseph dike swarm (CJDS) in NE Oregon from which the Imnaha flows first erupted ~16.7 Ma, followed by the Grand Ronde Basalt (GRB), which together account for 76% of the total volume of Columbia River Basalt Group (CRBG) lavas. For this study dikes were mainly sampled along the Grand Ronde River in the north, and Big Lookout and San Pedro Mtns., and along the Snake River in the south. Our current data set consists of petrographic and bulk compositional data of 103 dikes and initial ages obtained by ⁴⁰Ar/³⁹Ar geochronology. This project is part of a larger project examining along-strike variations in age, chemistry, and flow direction in the Columbia River dike feeder system, to constrain regional-scale magma transport dynamics in the plumbing system of the LIP.

On bivariate plots against SiO2, samples (treated as a single data set) reveal two distinct magmatic evolutionary trends: a low silica trend ranging from 46.8% to 51.2% SiO2, and a high silica trend ranging from 51.3% to 58.0%; incompatible elements increase along both. The trends are more subtle on plots vs. MgO with the Low-Si trend at higher incompatible elements at a given MgO. Comparison with literature data reveals that the Low-Si trend corresponds with compositions of the Rock Creek (RC) subunit of the Imnaha Basalt and the High-Si with the lowest flows of the American Bar (AB) subunit and the Picture Gorge Basalt (PGB) on the primitive end. On the evolved end, the High-Si trend overlaps with GRB.

In the south, dikes intrude pre-Cenozoic units, and all compositions are observed with no spatial preference. In the north, where dikes intrude GRB, most have GRB or compositions of younger units (e.g. Wanapum Basalt) with some few revealing compositions akin to stratigraphically older units (e.g. lowest AB). Compositional ranges of dikes show only minor changes with latitude. To date, we identified 33 GRB, 5 Imnaha, 5 Saddle Mtn., and 3 Wanapum dikes across the CJDS with ≥ 98% confidence using a Machine Learning CRBG ID model.

Our first new ages indicate that dikes of the Low-Si trend are around 16.55 Ma while samples of High-Si trend have a broader age range with 15.9 ± 0.04 Ma for the lowest (51.4%) silica sample to 16.4 ± 0.05 Ma for the highest (58.0%). This is consistent with the ~17 and 15.9 Ma activity span of the main CRBG.