NEW UPPER DEVONIAN U-PB DATE FOR EAGLE BAY FELSIC VOLCANICS IN THE DISTAL CORDILLERAN MIOGEOCLINE, VAVENBY, BRITISH COLUMBIA

Quartz-sericite schist, derived from andesite and dacite lapilli-tuff of magmatic-arc affinity, stratigraphically overlies distal facies of the Cordilleran miogeocline in southeastern British Columbia. The schist was formerly interpreted as lower Paleozoic rift-volcanic rock, associated with breakup of the western margin of Laurentia. New U-Pb isotopic data from magmatic zircon separated from the schist, south of the Thompson River, indicates a Late Devonian age of ~360 Ma. The schist is here correlated with unit A of the Eagle Bay assemblage, which yielded Devonian zircon south of Vavenby, near Adams Lake (Schiarizza and Preto, 1987). The new age-data reveals that the schist occupies the limbs of a large, west-verging fold-nappe. The nappe is cored by Early Cambrian archeocyathan limestone (Tshinakin Limestone), pillowed and fragmental greenstone with transitional MORB affinity, feldspathic grit, and quartz arenite. The nappe is the structural equivalent of the Albert Creek nappe of the Kootenay Arc, with which it was associated before Tertiary extension across the Monashee complex. The Vavenby stratigraphy is correlated with the Hamill-Badshot-Lardeau triad of the Kootenay Arc. The new date establishes the presence of upper Devonian arc volcanics at the stratigraphic position of the enigmatic Broadview Formation of the Lardeau Group in the Kootenay Arc. The Devonian volcanics likely provided the blue-quartz grit characteristic of the Broadview Formation, and thereby indirectly date that formation, which is known to be unconformably overlain by Upper Mississippian conglomerate. This interpretation places the Devonian volcanics in an arc and the Broadview Formation in a back-arc basin developed on the precarious western edge of Laurentia during the Antler orogeny, and is consistent with regionally widespread Devono-Mississippian gneiss sheets.