NEW AGE AND GEOCHEMICAL CONSTRAINTS ON THE EOCENE GARNET RANGE VOLCANIC FIELD, WEST-CENTRAL MONTANA

The Garnet Range Volcanic Field (GRVF) in west-central Montana lies within the regionally extensive Eocene Absaroka-Challis-Kamloops volcanic belt that spans over 10 degrees of latitude in the North American Cordillera. Volcanic rocks in the GRVF record eruptive activity during a transitional period from Sevier-age shortening to regional crustal extension, and therefore provide an excellent opportunity to investigate the interplay between magmatism and tectonism in this part of the Cordillera. The GRVF’s stratigraphy, geochemistry, and structure remain poorly documented, and previous K-Ar dating of volcanic intervals has been greatly hindered by potassic alteration. We report new field mapping, whole-rock major and trace element geochemical data, and U-Pb LA-ICPMS ages from volcanic successions exposed in the eastern Garnet Range that shed light on the eruptive and deformational history of the region.

Volcanic rocks that crop out in the eastern Garnet Range unconformably overlie Proterozoic–Mesozoic sequences deformed by NW-striking faults and folds. The volcanic deposits thicken eastward to a minimum of ~1 km and are deformed by a system of NW- and NE-striking transtensional faults. Petrographic and whole-rock geochemical data reveal volcanic compositions that range from rhyolite tuff deposits to trachybasalt lava flows; the most abundant rock types are aphanitic and porphyritic trachydacite to trachyandesite lavas with subalkaline geochemical signatures. New U-Pb zircon age determinations suggest voluminous volcanic activity over a relatively short time interval (46 to 47.5 Ma), and significantly revise unreliable K-Ar ages obtained from these rocks. Together, new mapping and radiometric ages suggest that middle Eocene volcanism in the region postdated crustal shortening and predated, or was coeval, with transtensional deformation presumably linked to Lewis and Clark Line tectonism.