DETRITAL ZIRCON FROM THE YAKUTAT TERRANE: DIFFERENTIATING THE YAKUTAT GROUP AND THE SCHIST OF NUNATAK FJORD

The Yakutat terrane in southern Alaska is currently colliding with North America driving rapid uplift and exhumation of the Chugach-St Elias Mountains. The basement of the Yakutat terrane is composed of Eocene basalt and the Upper Cretaceous and Paleocene Yakutat Group containing a flysch unit of coherent turbidites and a mélange unit containing sandstone phachoids, deformed granitoids, pillow basalt, and marble. The Fairweather fault defines the boundary between the Yakutat terrane and the inboard Chugach Metamorphic Complex, and the Boundary fault separates the Schist of Nunatak Fjord (SNF) from the potentially correlative and less deformed flysch and mélange units of the outboard Yakutat Group (YFM). This study uses detrital zircon U/Pb and Hf isotope data to help define the maximum depositional ages (MDAs) and provenance of the SNF and YFM and to evaluate correlation of the SNF to the adjacent Yakutat Group. Three samples from the SNF yield MDAs between 53.2 Ma and 56.9 Ma and one sample yields an MDA of 68.9 Ma. One sample has a prominent Paleocene peak at 62 Ma and the most prominent peak for the other three samples falls between 72 Ma and 76 Ma. Each of the four samples has a subordinate Jurassic peak between 151 Ma and 194 Ma. There is wide variation in the number of Precambrian grains from the SNF (n=0, n=3, n=18, and n=32), and one sample (n=18) has a prominent peak at 1550 Ma (defined by 6 grains), and another (n=32) has prominent peaks at 1400 Ma and 1740 Ma. Samples from the YFM have MDAs that range from 60.4 Ma to as old as 101 Ma; however, the majority of samples are Late Cretaceous to Paleocene and have remarkably similar grain-age distributions with prominent peaks at ~74 and ~91 Ma and Precambrian populations at ~1380 Ma and ~1710 Ma. We analyzed ¹⁷⁶Hf/¹⁷⁷Hf ratios in 66 zircons from the prominent Phanerozoic populations in the SNF and 146 zircons from the prominent Phanerozoic and Precambrian populations in YFM. The Paleocene and Late Cretaceous zircons from the SNF samples have εHf (t) values ranging from -2.0 to +12.1, and the Late Cretaceous YFM populations have εHf (t) values ranging from -31.3 to +11.9. These data indicate that the SNF is younger than the YFM sediments, have more heterogeneous Precambrian grain-age distributions, and have Phanerozoic zircons with distinctly more juvenile Hf isotope signatures than YFM.