WINDY-MCKINLEY TERRANE, WESTERN YUKON AND EASTERN ALASKA: TERRANE ANALYSIS RETURNS TO ITS ROOTS

Arguably, the Cordilleran terrane concept, of which Jim Monger was Canada's earliest and foremost proponent, was initially demonstrated in south-central Alaska. Data collected there in the 70's and 80's defined stratigraphically distinct, fault-bound crustal blocks; paleontological and paleomagnetic data showed some of these ‘terranes' to be far-traveled with respect to their neighbours and to the North American craton. These terranes were proof that the potential inherent in plate tectonic processes for the long-distance dispersal and assembly of rock masses from diverse paleogeographic origins had been realized in the formation of the Cordilleran orogen.

Although the terrane concept has since been applied and refined throughout the Cordillera, the terranes of the Alaska Range have not been re-evaluated in light of new developments. New data from the Windy-McKinley terrane of eastern Alaska and western Yukon have implications for the nature of the Alaska Range terranes and their evolution. Field, geochronological and geochemical data show that it comprises a Late Devonian to Late Triassic metavolcanic and metaclastic schist and gabbro subdivision and an undated ophiolite. Neither subdivision resembles the namesake Windy or McKinley terranes. Instead, the schist-gabbro subdivision resembles Alaska's Pingston and Aurora Peak terranes, Hayes Glacier and Jarvis Creek belts, and the Yanert Fork sequence. This near-continous schist-gabbro belt lies fault-bound between Laurentian or peri-Laurentian rocks in the north and mélange terranes (Windy, Kluane schist?) in the south. Its youngest unit in Alaska (unit Trcs) is deposited on the Laurentian margin north of the Hines Creek fault and on the Upper Devonian Yanert Fork sequence south of the fault. The correlative unit in Yukon contains detrital zircons characteristic of the Insular terranes, implying that it also overlapped onto them.

The ophiolite was thrust above both the Windy-McKinley schist-gabbro subdivision and Yukon-Tanana terrane. It may correlate with the Chulitna terrane of Alaska; both have suprasubduction zone geochemistry. Triassic gabbro of the schist-gabbro subdivision also has a suprasubduction zone signature. These data and proximity to the Windy mélange affirm that these rocks were at the Late Triassic continental plate boundary.