IMPROVED AGES, GEOCHEMISTRY AND STRATIGRAPHIC CONTEXT OF LOWER PALEOZOIC VOLCANIC ROCKS WITHIN THE WESTERN LAURENTIAN CONTINENTAL MARGIN IN THE NORTHERN CANADIAN CORDILLERA

Alkalic volcanic rocks occur sporadically in lower Paleozoic sedimentary strata of the western Laurentian ‘passive’ margin. Lower to middle Cambrian volcanic rocks are associated with rifting and breakup of Rodinia, but late Cambrian and Ordovician volcanic rocks occur within post-rift carbonate and shale successions of the continental margin. The mechanisms for post-rift early Paleozoic magmatism in western Laurentia are not well defined in current rift models. To address this problem, we conducted detailed studies of these volcanic successions at various stratigraphic levels and geographic sites in Yukon and Northwest Territories, Canada, including ca. 500-490 Ma Old Cabin Formation, Late Cambrian Sekwi Formation, Cambrian-Ordovician Dempster volcanics, Early Ordovician Vampire Formation, ca. 484 Ma Menzie Creek Formation, Middle Ordovician Haywire Formation, and ca. 453-445 Ma Marmot Formation. High-precision U-Pb zircon CA-TIMS dates, faunal assemblages, geochemical and isotopic data show that post-rift alkalic magmatism occurred in four stages in the late Cambrian, Early Ordovician, Middle Ordovician, and late Late Ordovician. Post-rift volcanism is locally associated with growth faults indicated by facies changes and potential field geophysical data. Major rift structures that localized post-rift volcanism include the Dawson fault and Liard line, bounding the northern and southeastern edges of Selwyn basin, and the Twopete fault marking the southwestern edge of the basin with Cassiar-McEvoy platform. Intermittent alkalic volcanism and extension in the lower Paleozoic continental margin coincided with counterclockwise rotation of Laurentia indicated by paleomagnetic studies. We propose that this continental rotation facilitated lower Paleozoic reactivation of lithospheric structures first developed during the Neoproterozoic-Cambrian breakup of Rodinia. Dextral transtension along the continental margin may provide a solution to the long-standing conundrum of lower Paleozoic post-rift magmatism in western Laurentia.