NEW INSIGHTS INTO THE GEOCHEMICAL AND GEOCHRONOLOGICAL HISTORY OF THE CHIEF JOSEPH DIKE SWARM AND THE FLOW DYNAMICS OF DIKE EMPLACEMENT

Aside from mapping and some bulk compositional data, few studies on the Columbia River Basalt Group (CRBG) have focused on the main eruptive center: The Chief Joseph dike swarm (CJDS) in NE OR and prior to this study there were no published age dates for the CJDS.

Currently, our data include geochemical data of 147 dikes across the main portion of the CJDS from the OR/WA border in the north to the southern extent of the swarm near the town of Vale, as well as Ar⁴⁰/Ar³⁹ age dates of 22 dikes. Developing a geochronological framework for magma activity at depth is important for answering questions of storage and transport, and for evaluating age relationships between intrusive magmas and subaerial lava flows. This study focuses on using geochemistry, petrography, and age dates to correlate CJDS dikes with units of the Grand Ronde Basalt, Imnaha Basalt, and other main phase formations. The ages for subterraneous magmas of main phase units of the CRBG covered in this study extend the age range of these formations and establish ages for a magma type with great similarities to Eckler-Picture Gorge Basalt, occurring across the swarm including the age of 17.22 ± 0.22 Ma for a dike in the southern portion of the swarm; this establishes contemporaneity of CJDS with Monument dike swarm activity further to the west. This suggests that the first eruptions of the CJDS began with magmas derived from a more depleted and metasomatized mantle with a strong subduction zone signature, which is reflected in the Eckler-PGB basalt, followed by magmas which are more enriched in incompatible trace elements.

This study supports the magma storage model suggested by others where the main magma storage location is in the greater Vale region of eastern OR. Dikes in the Vale region show a wider compositional range and older ages relative to the central and northern areas and combined with a lack of silicic volcanism in those areas suggests little to no storage time of magmas sourcing the dikes there. This suggests that magma traveled from the southern portion of the CJDS in the Vale region to other eruptive locations throughout the swarm. Newly acquired AMS and AARM magnetic data indicate preliminary flow characteristics of dikes in the CRBG and will give insight into regional flow dynamics as well as the mechanics of dike growth that lead to the ~ 20 m wide dikes seen in these dike swarms.