LATE PALEOZOIC DETRITAL ZIRCON U/PB GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY OF THE ADMIRALTY AND CRAIG SUBTERRANES IN THE KEKU STRAIT REGION, SOUTHEAST ALASKA

The Alexander terrane of Southeast Alaska is a long-lived arc and passive margin system that has been divided into the Craig and Admiralty subterranes on the basis of differences in pre-Permian stratigraphy. Detrital zircon samples from the late Devonian to early Permian Cannery Formation of the Admiralty subterrane and the Permian Halleck Formation of the Craig subterrane in the Keku Strait region test the hypothesis that the subterranes collided during the early Permian. Five detrital zircon samples of the uppermost Cannery Formation yield a single age cluster ranging from 300-350 Ma with peaks at 320 Ma and 341 Ma and juvenile Hf isotopes (eHf between +5 to +15). Two detrital zircon samples from the Halleck Formation contain major peaks at 298 Ma, 430 Ma, and 475 Ma, a subordinate peak at 370-390 Ma, and subordinate Proterozoic grains. Paleozoic grains of the Halleck Formation dominantly record juvenile Hf isotopic values (eHf between +5 to +15) with more intermediate values (eHf between 0 to +10) observed for the 298 Ma peak. Two detrital zircon samples from the early Triassic Burnt Island Conglomerate yield a 220-250 Ma population and grains ranging from 295 to 370 Ma with peaks at 305 Ma, 317 Ma, 331 Ma and 364 Ma. Paleozoic grains of the Burnt Island Conglomerate are broadly similar to those in the Cannery and Halleck Formations but the Ordovician-Silurian and Proterozoic grains common in the Halleck Formation are absent. Hf isotopes of the Burnt Island Conglomerate match those found in the Cannery and Halleck Formation with all zircons >300 Ma yielding eHf values between +5 to +15 and zircons <300 Ma yielding eHf between 0 to +10. Late Paleozoic rocks of the Admiralty subterrane record input from a juvenile Carboniferous magmatic source whereas the Craig subterrane reflects recycling of Ordovician-Silurian zircons from the Alexander terrane basement and Devonian grains from the nearby Banks Island Assemblage. The mixed detrital zircon signature in the Burnt Island conglomerate is consistent with Permian amalgamation of the Craig and Admiralty subterranes.