Provenance and age of granite clasts in the Yakutat Mélange, Alaska

The Yakutat Group consists of a flysch facies and a mudstone matrix mélange with a conglomerate occurring in two places in Russell Fjord. One site (#23) occurs in what is mapped as flysch and one in the mélange (#25). They contain clasts of sandstone, greenstone, limestone, marble, chert, and plutonic rocks that are clast supported, and interbedded with sandstones containing coalified plant fragments. The maximum depositional age (MDA) of the U/Pb-dated zircons from the two sandstones are 65.9 ± 1.8 Ma and 65.6 ± 2.2 Ma, respectively, indicating deposition was Maastrichtian or younger. Grain-age distributions yield populations at 69-74 Ma, 92-94 Ma,157-183 Ma, 1365 Ma, and 1710 Ma. The subordinate Jurassic population, may be resolved into component populations at 154 Ma and 182 Ma. A well-rounded sandstone clast from the conglomerate at site #25 was dated and has an MDA of 71.7 ± 2.4 Ma and the overall grain age distribution is identical to sandstone elsewhere in the mélange, suggesting parts of the mélange may have been reworked. Three plutonic clasts from site #23 were analyzed for geochemistry and dated in addition to two plutonic samples from tectonic slices (knockers) in the mélange. The clasts are granite, trondhjemite, and tonalite, and both knockers are tonalite. All plot as volcanic arc granites on discrimination diagrams. One clast has a date of 167.2 ± 2.3 Ma and the other has two zircon populations with modes at 156.2 ± 2.7 Ma and 179.5 ± 2.6 Ma. The bimodal age distribution is unexplained, but might be due to lead loss. The knockers have U/Pb dates of 174.9 ± 2.0 Ma and 173.8 ± 2.1 Ma. εHf(t) values on the 167 Ma clast and 175 Ma knocker range from +10.0 to +14.3 and +9.8 to +12.0, respectively, suggesting a isotopically homogenous and juvenile source area.