HAFNIUM ISOTOPE SYSTEMATICS OF PERMIAN TO EOCENE PLUTONS OF THE YUKON TANANA TERRANE IN EASTERN ALASKA

The Yukon-Tanana terrane (YTT) is a complex, intensely deformed mid-to late-Paleozoic pericratonic allochthonous arc assemblage in tectonic contact with parautocthonous North American margin (NAM). We present new geochemistry, U-Pb ages, and radiogenic isotope ratios from Permian to Eocene plutons in the YTT-NAM assemblage in eastern Alaska that span terrane accretion, arc development, regional shortening, and orogenic collapse. Our data describe previously recognized Mesozoic suites but also include two previously unrecognized late Permian plutons (ca. 264–252Ma) and small Eocene stocks. Permian to Jurassic plutons were emplaced only in the YTT whereas Eocene and most mid-Cretaceous plutons were emplaced only in NAM. Late-Cretaceous plutons and associated volcanics were emplaced in both YTT and NAM basement. Permian plutons are evolved (εHf -10) high-K granodiorite to monzogranite. Triassic plutons, defining an E-W band along the southern margin of the YTT in Alaska, are low- to medium-K and bimodal, with median gabbro and granodiorite groups. Older Triassic rocks (ca. 218 – 208 Ma), on an E-W axis through the broader Triassic core, are juvenile (εHf +10). Off-axis, younger rocks (to 200 Ma) are more evolved (εHf +2 to -15). Jurassic rocks outcrop north of the Triassic band and are also bimodal, with low-K gabbro, and high-K to shoshonitic monzonite/syenite. In total, they resemble the latest-Triassic εHf range but are more radiogenic (εHf > 0) in eastern plutons, whereas they are sub-chondritic in a NE-trending syenite body to the west. Mid- and late-Cretaceous rocks are high-K (> 60 wt.%) tonalite to alkali granite without compositional gap. Late Cretaceous rocks range to higher K and La/Yb_N than mid-Cretaceous, which are evolved (εHf -4 to -20). Late Cretaceous εHf is bimodal, correlated spatially; εHf is high (-6 to +3) to the SE, and low to the NW (-8 to -12). Few Eocene plutons were analyzed, all high-K (εHf -2 to -4) syenogranite. No suites definitively have a depleted mantle component, but (early) Triassic comes closest. Triassic to Jurassic magmatism is progressively more crustal; mid- to Late-Cretaceous (±Eocene) are decreasingly so. Permian, syenitic Jurassic, and mid-Cretaceous rocks resemble YTT + NAM gneiss, suggesting similar source components and/or partial melting of pre-existing crust.