400 MILLION YEARS OF MAGMATISM IN THE NORTHERN NORTH AMERICAN CORDILLERA: U-PB AND HF-ISOTOPE ANALYSIS OF BEDROCK ZIRCON FROM THE YUKON-TANANA UPLAND, EASTERN ALASKA, USA

Current efforts to better understand the geologic framework of critical mineral resources lead us to conduct campaign-style U-Pb and Hf-isotope analysis of igneous rocks from the Yukon-Tanana Upland terrane of eastern Alaska, USA. We present paired U-Pb and Hf-isotope analyses of zircon separated from 32 igneous rocks ranging in age from ca. 370-55 Ma collected over two summers of field mapping led by the Alaska Division of Geological and Geophysical Surveys. Late Devonian plutonic and volcanic rocks of the Ladue, Lake George, and Fortymile assemblages yield evolved εHf(t) values between -30 and -10 at ca. 370-360 Ma. Permian rocks of the Chicken metamorphic complex and Klondike assemblage yield markedly different εHf(t) values; gabbroic rocks of the Chicken Metamorphic complex yield juvenile εHf(t) values of roughly +10 at ca. 265-260 Ma while metavolcanic schists of the Klondike assemblage range from chondritic values to evolved εHf(t) values between -20 to -15 at ca. 260 Ma. Late Triassic to early Jurassic rocks yield εHf(t) values that vary through time; an early ca. 215 Ma hornblende gabbro yields average εHf(t) value of +12, intermediate rocks of the Taylor Mountain batholith yield near chondritic values from 203-200 Ma, and 185-183 Ma granitoids yield εHf(t) values of -15 to -10. Seven mid-Cretaceous igneous rocks yield a range of εHf(t) values that increase from -15 to -5 between 110-100 Ma. Late Cretaceous magmatism may have tapped distinct sources; 70 and 68 Ma intrusions yield εHf(t) values of approximately -10 and +0 respectively. Finally, Paleocene igneous rocks yield near chondritic εHf(t) values between -10 to +0 from 58-56 Ma. Here, we synthesize our new data with regional datasets to test endmember tectonic models of accretionary orogenesis and assess along-strike variation within and between terranes in the northern North American Cordillera.