Cordilleran Section - 113th Annual Meeting - 2017

Paper No. 2-5
Presentation Time: 10:25 AM


MOSOLF, Jesse G., Montana Bureau of Mines and Geology, Montana Tech, 1300 West Park Street, Butte, MT 59701,

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.