PROVENANCE OF AND AGE OF GRANITOID AND SANDSTONE CLASTS IN CONGLOMERATES OF THE PALEOCENE TO UPPER CRETACEOUS YAKUTAT GROUP, RUSSELL FJORD, ALASKA

The Paleocene to Upper Cretaceous Yakutat Group consists of a flysch facies and a mudstone matrix mélange and several hypotheses have been suggested for long, coast parallel transport of this unit. A conglomerate occurs in two places in Russell Fjord, and the composition and age of clasts bears on tectonic reconstructions. One site (#23) occurs in what is mapped as flysch and one in the mélange (#25), but the conglomerates are nearly identical. They contain clasts of sandstone, greenstone, limestone, marble, chert, and plutonic rocks that are clast supported, and interbedded with sandstones that contain coalified plant fragments. The maximum depositional age (MDA) of the U/Pb-dated zircons from the sandstone is 65.9 ± 1.8 Ma and 65.6 ± 2.2 Ma for the two samples, indicating deposition was Maastrichtian or younger. Grain-age distributions for these two samples yield populations at 69-74 Ma, 92-94 Ma,157-183 Ma, 1365 Ma, and 1710 Ma. The Jurassic population, which is subordinate, may be resolved into component populations at 154 Ma and 182 Ma. Clasts of sandstones and plutonic rocks were dated and analyzed. 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. Three plutonic clasts from site #23 were analyzed for geochemistry in addition to two plutonic samples from tectonic slices (knockers) in the mélange. Based upon the geochemistry, the clasts are granite, trondhjemite, and tonalite, and both knockers are tonalite, and all plot as volcanic arc granites on discrimination diagrams. Two clasts and both knockers were U/Pb zircon dated. 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. Thus, these data suggest that the source region for the granitoid clast and knocker is isotopically homogenous and juvenile. The similarity in MDAs and lithology of the sandstone clast and host mélange sandstone suggests that parts of the mélange may have been reworked.