U-PB DETRITAL GEOCHRONOLOGY AND HF ISOTOPIC ANALYSES FROM UPPER CRETACEOUS STRATA OF THE NORTHERN TALKEETNA MOUNTAINS, SOUTHERN ALASKA: SEDIMENT DISPERSAL AND MAGMATISM DURING THE FINAL ACCRETIONARY PHASE OF THE WRANGELLIA COMPOSITE TERRANE

Upper Cretaceous, nonmarine strata in the northern Talkeetna Mountains of south-central Alaska record syn- and post-collisional exhumation and sedimentation associated with the final accretionary phase of the Wrangellia composite terrane to western North America. While recent studies have focused on the Upper Jurassic–Lower Cretaceous stratigraphic record of arc accretion, new data is emerging from Upper Cretaceous strata that outcrop along the inboard margin of the Wrangellia. Presented here are U-Pb ages and epsilon Hf isotopic values from detrital and igneous zircons as well as sandstone modal composition trends from Upper Cretaceous synorogenic strata of the Caribou Pass Formation.

Detrital zircon samples (n=1464) from the northern Talkeetna Mountains exhibit peak ages between 103–122, 186–197, 250–260, and 336–362 Ma with isolated ages that fall between 1800–1850, 1950–2000, 2250–2300, and 2600–2650 Ma. Epsilon Hf values (n=235) from Cretaceous zircons exhibit enriched isotopic values (-1 to -10) whereas Jurassic, Triassic, and Devonian–Mississippian zircons have a range of enriched to depleted values (-15 to 15). Phanerozoic peak ages overlap with magmatic sources areas that correspond with Mesozoic arcs of Wrangellia (e.g., Chisana, Chitina, and Talkeetna arcs) and the Yukon Tanana terrane as well as Devonian–Mississippian magmatic provinces of the Yukon Tanana, Quesnellia, Slide Mountain, and Cache Creek terranes. Maximum depositional ages from these strata range from 103–108 Ma and overlap with previously-reported Albian–Cenomanian palynomorph age ranges (Hampton et al., 2007). Sandstone modal composition trends show relatively high abundances of quartz and lithic grains (primarily metamorphic and volcanic fragments) with minor feldspar (Q=61% F=5% L=34%; Lv=28% Lm=57% Ls=15%) and plot within recycled orogen provenance fields. Igneous zircon (n=35) extracted from a mafic dike that cross cut Albian–Cenomanian strata exhibit a U-Pb age of 57.96+0.50 Ma. Epsilon Hf values (n=25) from igneous zircon range from -1 to 6.

Data support a model where Late Cretaceous basins were receiving detritus from both inboard and outboard magmatic provinces (e.g., Wrangellia and Yukon Tanana composite terranes) prior to the onset of latest Cretaceous–Paleocene magmatism in this region.