EOCENE HIGH-K, CALC-ALKALINE MAGMATISM WITHIN THE EASTERN LEWIS AND CLARK FAULT ZONE, WEST-CENTRAL MONTANA, USA

Eocene magmatism in the northern Cordillera was synchronous with regional transtension in the North American plate and the exhumation of mid-crustal rocks in the Anaconda, Bitterroot, Clearwater, and Priest River Metamorphic Core Complexes (MCCs). Large-magnitude extension was relayed between these en echelon MCCs through dextral displacement on the WNW-trending Lewis and Clark Fault Zone (LCFZ). In this study, important kinematic and geochemical constraints on Eocene crustal evolution within the eastern LCFZ comes from detailed geologic mapping, 81 whole-rock geochemical analyses, and 25 U-Pb LA-ICPMS ages within the relatively understudied Avon, Garnet Range, and Crater Mountain volcanic fields in west-central Montana.

Volcanic sequences rest on a sub-Eocene angular unconformity and are deformed by NW- and NE-striking, high-angle, oblique-slip faults indicative of dextral transtensive shearing. Intertonguing sequences of dominantly trachyandesite to trachydacite lava flows (SiO₂ = 57-68 wt %; 46.01±0.23 to 48.21±0.17 Ma) are the oldest extrusive volcanic rocks exposed. Rhyolite lavas (SiO₂ = 74-80 wt %; 39.83±0.19 to 40.62±0.16 Ma) and tuff deposits (SiO₂ = 57-68 wt %; 39.24±0.16 to 39.79±0.12 Ma) overlie the older lavas in the Avon and Crater Mountain volcanic fields, respectively. These volcanic sequences all have a high-K, calc-alkaline affinity and exhibit a “subduction signature” of enriched large-ion lithophile elements and depleted high field strength elements. Petrography, combined with major and trace element data, suggest that assimilation-fractional crystallization significantly modified the source magmas.

The new field and analytical results reported herein support passage of the Kula-Farallon slab window beneath west-central Montana during the Eocene. Upwelling of hot asthenosphere beneath the slab window heated and melted a lithospheric mantle enriched by metasomatism during the Late Cretaceous through Paleocene low-angle subduction of the Farallon plate. Magmatism was synchronous with the exhumation of MCCs (~53-40 Ma) and dextral transtensive deformation within the LCFZ induced by thermal softening of the lithosphere and differential traction between the subducting Kula and Farallon plates.