DETRITAL ZIRCON U-PB GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY OF THE ALEXANDER TERRANE IN SOUTHEASTERN ALASKA

The Alexander terrane consists of Neoproterozoic through Triassic strata that extend along the Cordilleran margin from southern Alaska to southern British Columbia. In southeast Alaska, the terrane consists largely of Neoproterozoic through Silurian arc-type plutonic and volcanic rocks, Lower Devonian conglomeratic strata, Middle Devonian through Permian strata that formed in stable marine settings, and Triassic rift-related strata. U-Pb geochronologic and Hf isotopic analyses have been conducted on detrital zircons from ten sandstones of Ordovician, Silurian, Devonian and Triassic age. U-Pb ages (n=2324) are mostly 390-490 Ma (age peaks at 424-448-475 Ma), with subordinate age groups at 530-610 Ma (peak at 572 Ma), 272-310 Ma (peak at 288 Ma), and 350-382 Ma (peak at 366 Ma). Hf isotopic data (n=214) from these grains are highly juvenile, with epsilon Hf values between +10 and +15 for most grains. These ages and Hf isotopic compositions are an excellent match for nearby plutonic rocks (data from Cecil et al., 2011), suggesting mainly local derivation. Most samples also include older grains that exhibit a nearly continuous age distribution from 990 to 2140 Ma, with age peaks at 1031, 1470, 1635, 1774, 1945, and 2073 Ma. Hf isotopic compositions of these grains are quite variable, with epsilon Hf values that range from juvenile to moderately evolved. These data provide an interesting comparison with U-Pb and Hf information from Alexander strata to the north (in the St Elias Mountains, data from Beranek et al., 2012, 2013) and to the south (coastal British Columbia, data from Tochilin et al., in prep). Both regions yield similar early Paleozoic age peaks (414-448-480 Ma to the north; 407-440-476 Ma to the south), whereas Hf isotopic compositions for these grains are much more evolved. Both regions also yield similar broad distributions of 1.0-2.1 Ga grains with variable Hf isotopic compositions. These relations collectively suggest that the Alexander terrane consists of a Neoproterozoic-early Paleozoic magmatic arc that was built on both juvenile crust (SE Alaska) and older continental crustal material (St Elias and coastal BC). The available age and Hf isotopic data are consistent with suggestions that the Alexander terrane was in proximity to Baltica during Neoproterozoic-early Paleozoic time.