NEW U-PB AGES AND HF ISOTOPIC VALUES FROM THE KAHILTNA ASSEMBLAGE IN THE EASTERN ALASKA RANGE, SOUTH-CENTRAL ALASKA
U-Pb ages from detrital zircons of the eastern Alaska Range Kahiltna assemblage have a maximum depositional age of early Cretaceous (140 Ma-145 Ma), one of the oldest known depositional ages in the Kahiltna assemblage. Significant Mesozoic probability age peaks occur at 145 Ma, 189 Ma, and 200 Ma. Hf isotopic analyses on these zircons show that the eastern Alaska Range Kahiltna assemblage has εHf values between +12.8 to -4 for U-Pb ages between 190 Ma-210 Ma. Zircons between 140 Ma to 153 Ma have εHf values primarily between +8 to +12. Detrital zircon samples from the Kahiltna of the Clearwater Range ~50 km to the south, adjacent to the northern margin of Wrangellia, have a single peak between 148 and 153 Ma, and εHf values from one of these samples clustered between +8 and +12.
The similar ages and positive εHf values of zircons from 140 Ma to 153 Ma within both the Clearwater and eastern Alaska Range Kahiltna assemblages indicates these zircons crystallized from the same juvenile magmatic source. The nearest likely source is the Chitina arc (intrusive ages of 135 Ma-170 Ma) of the Wrangellia composite terrane located ~250 km to the southeast. The eastern Alaska Range Kahiltna assemblage also received sediment from other sources, including inboard terranes, indicated by U-Pb ages between ~202 Ma-220 Ma, and the presence of both juvenile and more evolved εHf values of zircons between 190 Ma-210 Ma. These ages and isotopic compositions are incompatible with sole derivation from outboard terranes such as the juvenile Talkeetna arc (153 Ma-202 Ma).
These data indicate that zircons from both Wrangellia and from more evolved, likely more inboard, terranes contributed sediment into the Kahiltna basin in the Late Jurassic and supports prior interpretations that the Wrangellia composite terrane was separated by a narrow basin from the continent prior to sinistral translation away from the Chitina arc.