CRETACEOUS TO PALEOGENE MAGMATISM IN THE YUKON TANANA UPLAND, ALASKA IN THE AFTERMATH OF YUKON TANANA TERRANE ACCRETION AND SUBSEQUENT EXTENSION

The Alaska Division of Geological & Geophysical Surveys (DGGS) is undertaking a multi-year effort to update the geologic maps of the Yukon Tanana Uplands of Interior Alaska stretching from the Yukon border to near Fairbanks between the Denali and Tintina fault systems. The pericratonic Yukon Tanana Terrane and the oceanic Seventymile Terrane were accreted above basinal rocks of the North American margin in the Jurassic. Recent mapping, geochemical sampling, and geochronology have led to refinements in the location of the structural contacts between and within these terranes. A period of Early Cretaceous extension is recorded by ⁴⁰Ar/³⁹Ar cooling ages below ductile detachments. Syn- to post-extensional mid-Cretaceous plutons were emplaced in the footwall of these structures. High-angle, NE- and NW-striking fault zones post-date the mid-Cretaceous magmatism and are interpreted to have been normal faults that may have been reactivated as strike-slip faults in the Cenozoic. Airborne magnetic and gravity data suggest one of these fault zones, the Black Mountain Tectonic Zone (BMTZ), marks a density change within the lower crust. Scattered Late Cretaceous to Eocene magmatism followed across the Yukon Tanana upland. Similar age magmatism is also recorded by the Carmacks volcanics and Rhyolite Creek volcanics in Yukon, Canada. In Alaska, the Late Cretaceous to Eocene magmas were preferentially emplaced along these high-angle faults or erupted within small basins along the fault zones. Northwest of the BMTZ, intrusions often occur in the hinges of map-scale open folds in the metamorphic host rocks. Late Cretaceous magmas include a range of SiO₂ from 51 to 77 percent and have trace element compositions indicating a volcanic arc setting. Porphyry Cu-Mo-Au prospects are associated with multiple Late Cretaceous intrusions southeast of the Black Mountain Tectonic Zone. The Paleogene magmas are bimodal. In easternmost Alaska, the Paleogene felsic magmas are low in Sr, Ba, and Eu and have elevated high field-strength elements and heavy rare earth elements, indicative of a within-plate magma source. The change from arc- to bimodal within-plate magmatism occurred between 64 and 58 Ma. This change likely overlaps initial movement along the sinistral Tintina Fault, which offsets the ca. 64 Ma McQuesten plutonic suite 430 km.