PROVENANCE AND AGE DISTRIBUTION OF DETRITAL ZIRCON IN THE UPPER CRETACEOUS-PALEOCENE FLYSCH AND MÉLANGE OF THE YAKUTAT GROUP, YAKUTAT BAY AND RUSSELL FJORD, ALASKA

The Yakutat terrane consists of the well-exposed Yakutat Group, Paleocene-Eocene basalts, and sedimentary cover rocks. The Upper Cretaceous to Paleocene Yakutat Group consists of a flysch unit and mélange unit. The mélange unit has a mudstone matrix with sandstone phacoids, pillow basalts, limestone, metaplutonic rocks, and an abundance of sandstone beds. Detrital zircon from six samples of the flysch, and eight samples from sandstones in the mélange were U/Pb dated by LA-ICPMS. The mélange sandstones have two primary zircon facies: 1) the Russell zircon facies (5 samples) has MDAs that range in age from ~ 60-72 Ma, with primary Phanerozoic populations at ~74 Ma and ~91 Ma and Precambrian populations at ~1380 Ma and ~1710 Ma; 2) the Shelter Cove zircon facies (2 samples) has MDAs of 96 and 101 Ma with primary populations at ~154 Ma, ~170 Ma, and ~196 Ma, and Precambrian populations at ~1760 Ma, ~1850 Ma, ~1910 Ma. The sandstones of the Yakutat flysch have a grain-age distribution essentially identical to the Russell zircon facies of the mélange, and thus these units appear to be related and tectonically mixed. The Maximum Depositional Age (MDA) of the sandstones are constrained by a young population of zircon grain ages: for both the flysch and the Russell zircon facies the MDA ranges from 60 to 72 Ma, suggesting deposition no older than Campanian, Maastrichtian, and Danian. Reported fossil occurrences in the Yakutat Group indicate older JK clastic units also exist in the mélange, but they were not encountered. Raman spectroscopy on Precambrian zircons show that there are two populations with different metamorphic histories: one with considerable disorder and one with little disorder. The population with very little damaged is indicative of grains that have undergone metamorphism and annealing of radiation damage, probably in the Cretaceous. In one sample, core-rim dating was done on twelve zircons to better understand the metamorphic history of the detrital grains. The rim-core dates show that: 1) 1.6 Ga zircons were affected by metamorphism at 1.4-1.3 Ga; and 2) Precambrian grains (1.38, 1.33, and 1.67 Ga) were involved with magmatism and metamorphism at 83-84 Ma. This latter suite of grains provides a constraint on the timing of annealing of radiation damage and crystal disorder revealed in the Raman analysis.