Geochemistry and Geochronology of the Steens and Monument Dike Swarm: Implications and Relationships to Magmatic Storage and to Lava Flows of the Picture Gorge and Steens Basalt

Author

Rachel Anne Sweeten, Portland State University

Sponsor

Portland State University. Department of Geology

First Advisor

Martin Streck

Term of Graduation

Spring 2024

Date of Publication

5-21-2024

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Geology

Department

Geology

Language

English

Subjects

Columbia River Basalt, Layered Mafic Intrusion, Picture Gorge Basalt, Steens

Physical Description

1 online resource (xi, 131 pages)

Abstract

The Columbia River Basalt Group (CRBG) is the youngest and best-preserved example of a continental flood basalt 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 compositions and timing 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. We evaluate data from 48 dikes and several small sills of this study, 21 dikes from Cahoon, as well as 10 new ⁴⁰Ar/³⁹Ar ages of select MDS dikes. With these data, we show that the age range of the Picture Gorge Basalt and the MDS span from 17.27 ± 0.05 Ma to 15.61 ± 0.22 Ma. These ages agree with ages determined by Cahoon et al., thus extending the temporal range of the PGB. Based on new geochronological and geochemical data (e.g., Zr vs Nb, TiO₂ vs SiO₂, Y vs TiO₂ and MgO vs SiO₂) of intrusives, we propose to subdivide PGB into two compositional subunits, Twickenham and Dayville. This simplification resolves issues with ages and distribution of Cahoon et al., this study, and the stratigraphy established by Bailey.

This study also adds data of 26 newly sampled Steens Basalt dikes to existing dike data. We evaluate compositional relationships of these dikes 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 area of 125 km², with lithologies including anorthosite, leucogabbro, and troctolite. Steens Basalt dikes and others intrusives have compositions that match lava flows except in cases where crystal accumulation is likely to dictate bulk compositions. This new intrusive data acquired from both the Picture Gorge and Steens Basalt was integrated with available geochemical, geophysical, and geographic data to determine a model of Columbia River Basalt Group mid- and upper- crustal magma storage.

Rights

© 2024 Rachel Anne Sweeten

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Persistent Identifier

https://archives.pdx.edu/ds/psu/42396

Recommended Citation

Sweeten, Rachel Anne, "Geochemistry and Geochronology of the Steens and Monument Dike Swarm: Implications and Relationships to Magmatic Storage and to Lava Flows of the Picture Gorge and Steens Basalt" (2024). Dissertations and Theses. Paper 6673.