GEOCHEMISTRY AND GEOCHRONOLOGY OF THE STEENS AND MONUMENT DIKE SWARM: IMPLICATIONS AND RELATIONSHIPS TO LAVA FLOWS OF THE PICTURE GORGE AND STEENS BASALT

The Columbia River Basalt Group (CRBG) is the youngest and best-preserved example of a large igneous province. Vents and dikes for the four main eruptive units (Steens, Imnaha, Grande Ronde, and Picture Gorge Basalt) are the Monument, Steens and Chief Joseph Dike Swarms located in eastern Oregon, southeastern Washington, and northern Nevada.

Here we report on the geochemistry and geochronology of the dikes and sills of the Monument Dike Swarm (MDS), as well as their relationship to the subaerial lava flows of the Picture Gorge Basalt (PGB) with implications for crustal storage of these magmas. Evaluated data include 48 dikes and several small sills of this study, 21 dikes from Cahoon et al. 2020, as well as 10 new ⁴⁰Ar/³⁹Ar ages of select MDS dikes. With this data, we show that the age range of the Picture Gorge Basalt and its related intrusives span from 17.27 ± 0.05 Ma to 15.61 ± 0.22 Ma. These ages agree with ages determined by Cahoon et al. 2020, thus extending the temporal range of the PGB. Including geochemical and geochronological data of intrusives into our database of PGB compositions, we propose to subdivide PGB into two compositional subunits, Twickenham and Dayville, based on differing trends observed in plots of, e.g., Zr vs Nb, TiO₂ vs SiO₂, Y vs TiO₂ and MgO vs SiO₂. This simplification resolves issues with ages and distribution of Cahoon et al. 2020, this study, and the stratigraphy established by Bailey, 1989.

This study also adds data of 26 newly sampled Steens Basalt dikes to existing dike data of Moore et al. 2018 to evaluate their compositional relationships to subaerial Steens Basalt flows, in part searching for evidence of crustal magma storage locations. Relevant to this, we also report on 16 samples of a newly discovered, compositionally zoned, mafic intrusive body related to the Steens Basalt. This intrusive body is currently estimated to have a minimum extent of 125 km², with compositions including anorthosite, leucogabbro, and troctolite, and geochemically span the range of picritic basalt, hawaiite, and mugearite. Steens Basalt dikes and others intrusives have compositions that match lava flows except in cases where crystal accumulation is likely to dictate bulk compositions.

Bailey, 1989, GSA Spec Paper, 239: 67-84.

Cahoon et al, 2020, Geology, 48: 348-352.

Moore et al., 2018, Geosphere, 14: 2505-2532.