GEOLOGY AND AFFINITY OF ALASKA'S FAREWELL TERRANE

The Farewell terrane is a microcontinental fragment in interior Alaska. It was once regarded as a displaced piece of the Canadian margin, but is now regarded as truly exotic. Its nucleus, the Nixon Fork subterrane, includes ~1200-Ma pelitic schist, ~980-Ma rhyolite, ~850-Ma orthogneiss, and an overlying carbonate-dominated platform sequence deposited along a late Neoproterozoic to Devonian passive margin. In Ruby and Medfra quads., basement and cover of the Nixon Fork were deformed and metamorphosed together during the ~285-Ma Browns Fork orogeny. To the east, the Nixon Fork interfingers with the Dillinger subterrane, a Cambrian to Devonian deeper-water succession. Most of the Dillinger sediments appear to have a Nixon Fork source, but Silurian flysch with minor ashfall tuff suggests the proximity of an unidentified arc at that time. Also included in the Farewell terrane is a structurally complex tract called the Mystic subterrane—a catch-all unit in need of modern study. The Mystic outcrop area includes Devonian, Miss., and Penn. carbonates, Devonian and Triassic phosphatic black shale, Devonian barite, Devonian sandstone, Triassic gabbroic sills, undated pillow lavas, undated melange, Silurian?, Carboniferous? and Triassic flysch, and Permian conglomerate—the latter believed to be the fill of a foreland basin related to the Browns Fork orogeny. Which of these really belong in the Farewell terrane remains to be seen. Cambrian trilobites, Ordovician conodonts and brachiopods, and Devonian brachiopods and gastropods suggest that during the early to mid-Paleozoic, the Farewell terrane lay between Siberia and Laurentia, as did the coeval carbonate platform of the Arctic Alaska terrane. Detrital zircon populations from quartzites low in the Nixon Fork platform succession have dominant age clusters at ~2050, ~1375, and ~950 Ma, which are distinct from Laurentian basement ages. The 2050-Ma zircons closely match those from the Carboniferous Nuka Formation of the Arctic Alaska terrane, and are plausibly derived from the ~2050-Ma Kilbuck terrane in southwest Alaska. The Farewell terrane is flanked by the Innoko terrane to the west, Ruby terrane to the north, Yukon-Tanana terrane to the east, and the Kahiltna flysch basin to the south. When and how the Farewell came to be lodged between these objects remains to be worked out.