AN EARLY EOCENE LARGE IGNEOUS PROVINCE, NORTHWESTERN NORTH AMERICA, WITH IMPLICATIONS FOR CORDILLERAN TECTONICS AND COEVAL CLIMATIC EXCURSIONS

Late Paleocene to Early Eocene graben-fill volcanic and sedimentary rocks form a broad belt ~2500 km long, from southern Yukon to Wyoming (the Challis-Kamloops belt (CKB)). CKB rocks are preserved in isolated remnants, separated by faulting, and and also due to post depositional/emplacement erosion, most recently by Pleistocene glaciation. The CKB is associated with dextral strike-slip faults, plutonism, exhumation of metamorphic core complexes and is synchronous with the volcanism and accretion of Siletzia. Numerous studies have focused on the petrogenetic/tectonic interpretation of these rocks within a local graben or region. This study is based on a compilation and synthesis of previous work with new geochemistry and high precision geochronology.

Within the CKB, rift-related volcanism comprises thick sequences of basaltic andesite to rhyodacite and rhyolite effusive flows, pyroclastic rocks, isolated domes, and tuffaceous lacustrine deposits. Caldera complexes occur locally (eg. Bennet Lake Volcanic Complex, northern British Columbia and within the Challis volcanic field of Central Idaho). Volcanic rocks are primarily subalkaline with more alkalic rocks occurring in southern British Columbia and Montana, and most show a crustal influence clearly reflected by trace elements and distinct Nd isotopes. Radiometric ages range from 60 to 45 Ma. High-precision CA-IDTIMS ages show that the CKB magmatism initiates at 57 Ma in the north and progressively younger to the south, with most volcanism occurring within a 10.8 Ma window. The exposure volcanic rocks determined from maps covers 35,000 km², a minimal estimate of the originally more extensive blanket. Sequence thicknesses are up to 4 km, but even using an average thickness of 1 km, this extensive and voluminous volcanism over a short period of time represents a large continental igneous province of primarily intermediate compositions. The tectonic implications of the regional characteristics of the volcanism are considered. Further, the onset of this volcanism immediately pre-dates the Paleocene-Eocene Thermal Maximum (PETM) and was coincident with the Early Eocene Climate Optimum (EECO), and its potential contribution to these climatic excursions is evaluated.