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