NEW CONSTRAINTS ON THE GIANT RADIATING DIKE SWARM AT INITIATION OF THE YELLOWSTONE HOTSPOT THROUGH GEOLOGICAL MAPPING AND HIGH-PRECISION 40AR/39AR GEOCHRONOLOGY OF CONTEMPORANEOUS CALDERAS

Giant radiating dike swarms associated with flood basalt provinces are commonly cited as evidence for impingement of mantle plumes on the base of the lithosphere. At the onset of the Yellowstone-Snake River Plain hotspot in the western United States, three subswarms of the middle Miocene Columbia River giant dike swarm have been described in detail: the Steens, Chief Joseph, and Monument swarms. Recent high-precision ⁴⁰Ar/³⁹Ar geochronology of ignimbrites contemporaneous with these swarms and geological mapping of associated calderas allow the division of the Steens swarm into the distinct High Rock and McDermitt radial trends. Voluminous rhyolitic volcanism in the McDermitt trend began ~16.5 Ma on eruption of the Tuff of Oregon Canyon from the Fish Creek Caldera in southeastern Oregon. Caldera volcanism progressed to the southeast at ~12 cm/y away from Steens Mountain over ~1 My through the main McDermitt Caldera and the Santa Rosa-Calico center. In the High Rock trend to the west, new mapping delineates two overlapping calderas in the Hawks Valley-Lone Mountain region that formed ~16.5 Ma on eruption of the Tuffs of East Creek and Monument Basin. Subsequent caldera-forming eruptions in the High Rock trend occurred progressively to the southwest over ~1 My at a rate nearly identical to the McDermitt trend. In the Chief Joseph and Monument dike swarms to the north of Steens Mountain, a northward time-space progression is loosely constrained by paleomagnetic and geochemical evidence from flood basalts and associated feeder dikes. Though voluminous silicic volcanism associated with the Monument swarm is lacking, at least three ignimbrites are contemporaneous with Chief Joseph swarm basaltic volcanism: the 16.0 Ma Dinner Creek Tuff, 15.8 Ma Mascall Tuff, and 15.8 Ma Tuff of Leslie Gulch. Only the caldera for the Tuff of Leslie Gulch has been mapped in detail; sources for the Dinner Creek and Mascall Tuffs have not been described. Regional reconnaissance mapping and high-precision ⁴⁰Ar/³⁹Ar geochronology of these ignimbrites is essential for evaluating if silicic volcanism in the Chief Joseph swarm mirrors spatiotemporally progressive volcanism in the McDermitt and High Rock trends related to the giant dike swarm, or if the silicic volcanism is less systematic and associated with widespread Miocene crustal melting.