NEW PROVENANCE CONSTRAINT FROM UPPER PALEOZOIC STRATA OF THE FAREWELL TERRANE, SOUTHWEST ALASKA

Paleozoic strata of the Farewell terrane are exposed primarily in the Alaska Range of southwestern Alaska and throughout the foothills of west-central Alaska. Three, somewhat distinct stratigraphic successions make up the Farewell terrane and consist of (1) Neoproterozoic–Devonian carbonate rocks of the Nixon Fork subterrane, (2) Cambrian–Devonian carbonate and siliciclastic strata of the Dillinger subterrane, and (3) Devonian–Permian(?) siliciclastic strata of the Mystic subterrane. Some of the more widespread units of the Mystic subterrane consist of Devonian(?)–Pennsylvanian submarine fan deposits and a Permian marginal-marine to nonmarine fluvial-deltaic deposits.

New provenance data from Upper Paleozoic strata of the Mystic subterrane reveal a diverse range of Precambrian and Paleozoic age detrital grains. Three U-Pb detrital zircon samples, collected from the Mystic Pass region of southwest Alaska yield primary age peaks between 300–350 and 420–450 with smaller peaks between 1800–2000 Ma. A fourth and slightly older sample, collected from near the Farewell Lakes-Sheep Creek region, yields a dominant peak at ~450 Ma with minor peak at 600, and occurrences between 1000–2000 Ma. Modal composition trends from these strata reveal pervasive occurrences of mono- and polycrystalline quartz, plagioclase, as well as a very strong lithic volcanic component.

Provenance and faunal data from Neoproterozoic–Devonian strata of the Dillinger and Nixon Fork subterranes have been used to suggest a Siberian-link for the Farewell terrane. However, while the Mystic subterrane does share some provenance trends with Dillinger and Nixon Fork strata, preliminary provenance data potentially suggest a link with Laurentia toward the end of the Paleozoic. The relative abundance of lithic volcanic detritus in these strata may reflect the occurrence of a significant arc source area that was being exhumed throughout much of the depositional history of these strata.