Paper No. 8
Presentation Time: 10:15 AM
CRUSTAL AFFINITY OF LATE CAMBRIAN-EARLY ORDOVICIAN ROCKS IN THE NORTHERN ALEXANDER TERRANE, NORTHWESTERN CANADA
BERANEK, Luke P., Geological Survey of Canada, 625 Robson Street, Vancouver, BC V6B 5J3, Canada, VAN STAAL, Cees, Geological Survey of Canada, 1500-605 Robson Street, Vancouver, BC V6B 5J3, Canada, MCCLELLAND, William C., Department of Earth and Environmental Sciences, University of Iowa, Iowa City, IA 52242, ISRAEL, Steve A., Yukon Geological Survey, 2099 2nd Ave, Whitehorse, YT Y1A 2C6, Canada, MIHALYNUK, Mitchell G., Victoria, BC v8x 2z3, Canada, GORDEE, Sarah M., University of Tasmania, Hobart, Tasmania, 7001, Australia and JOYCE, Nancy, Geological Survey of Canada, Ottawa, ON K1A 0E8, Canada, luke.beranek@nrcan.gc.ca
Late Cambrian-Early Ordovician mafic volcanic and marine sedimentary rocks of the Donjek assemblage comprise the oldest known units of the northern Alexander terrane in the St. Elias Mountains of NW Canada, and thus provide a proxy for its origin and paleotectonic setting at this time. New field studies confirm that the Field Creek Volcanics (FCV) are a volcanic-epiclastic member of the Donjek assemblage that is composed of mafic lavas, mafic sills, coarse-grained volcaniclastic strata, and fossiliferous marine sediments. The mafic sills typically display peperitic margins. Volcanic and sedimentary facies associations in the FCV are consistent with a shallow marine depositional environment, and field relationships document that magmatism and sedimentation were coeval and spatially related. Sandstone samples from the siliciclastic-gabbro (SG) member of the Donjek assemblage, presumably coeval with the FCV, yield 620-720, 1000-1250, 1450, and 1650 Ma detrital zircon age populations. The depositional age of the SG member is poorly known, but limited by the youngest detrital zircon ages of 590-680 Ma from the sandstone samples. The Donjek assemblage is overlain by an Ordovician-Devonian carbonate platform that is preserved across the St. Elias Mountains. Devonian sandstone samples from the carbonate platform primarily yield 430-465 Ma detrital zircon ages.
Geological evidence argues for the northern Alexander terrane to have crustal affinities with the NE European craton. The age of mafic volcanism and sedimentation recorded by the FCV is consistent with Late Cambrian-Early Ordovician rifting along the Timanian margin of NE Baltica and opening of the Uralian Ocean. New detrital zircon results from the SG member sandstone are similar to the Baltican-Timanian signatures in strata of the Arctic Alaska-Chukotka terrane and Ellesmerian clastic wedge of northern North America, as well as the Kara block, Novaya Zemlya, and Timan-Pechora basin in the Polar Urals. Devonian stratigraphic linkages with the southern Alexander terrane of SE Alaska are implied by the presence of 430-465 Ma detrital zircons in strata overlying the Donjek assemblage. Deformation associated with Devonian sedimentation may be tied to Caledonian-Innuitian orogenesis along northern Laurentia and Baltica.
