U-PB AND HF ISOTOPIC ANALYSES OF DETRITAL ZIRCONS FROM THE TAKU TERRANE, SOUTHEAST ALASKA

In the southeast Alaskan panhandle, amidst the folded, faulted, and accreted terranes, sits the Taku terrane. Previous workers have defined, and re-defined the terrane, and it is currently identified as a package of 3 units; Carboniferous(?)-Permian strata of the Windham Group, Permian strata of the Gastineau Group, and Triassic strata of the Perseverance Group. These strata are structurally overlain to the east by the Yukon-Tanana terrane (YTT) and structurally overlie Jurassic-Cretaceous strata of the Gravina belt to the west. For this study we have collected 583 U-Pb ages and 74 Hf isotope determinations from detrital zircon grains of six samples in locations between Juneau and Ketchikan. Two samples from the Windham Group yield predominantly 360-400 Ma grains (peak age of 383 Ma) and one grain of ~437 Ma. Epsilon Hf values for these grains are highly variable, ranging from +16 to -3. One sample from the Gastineau Group yields a similar age group of 350-395 Ma (age peak of 370 Ma). Two samples of the Perseverance Group have a dominant age group of 340-385 Ma (peak age of 363 Ma) and subordinate age groups of 410-460 Ma and 220-290 Ma. Epsilon Hf values for the 340-385 Ma grains are highly variable, ranging from +10 to -18, whereas the 410-460 Ma grains are all juvenile (epsilon Hf values of +7 to +14). One additional sample of the Gastineau Group yields very different ages, with main age groups of 275-300 Ma (peak age of 292 Ma) and 400-460 Ma (peak age of 435 Ma). Hf isotopic compositions of the 400-460 Ma grains are highly juvenile (+10 to +16), whereas compositions for the 275-300 Ma grains are intermediate (+5 to +10) in value. Collectively these data support previous suggestions that most sediment in the Taku terrane was shed from the Yukon-Tanana terrane, and also demonstrate that some sediment was derived from the Alexander terrane during Permian and Triassic time.