Middle Cambrian extensional tectonism in Yukon, Canada: New age constraints and lithogeochemistry from the Dempster volcanics of the Ogilvie Mountains

Volcanic rocks assigned to the Dempster volcanics of the western Ogilvie Mountains, Yukon, Canada, consist of a diverse package of subaqueous pillow basalts and breccias, gabbroic dikes and sills, and minor felsic rocks that record a distinct phase of early Paleozoic extension along the northwestern margin of Laurentia. Previous age constraints on these volcanics were limited to rough stratigraphic position between the Ediacaran Hyland and Ordovician-Devonian Road River groups. In order to update these age data and elucidate the tectonic significance of widespread volcanic rocks along the northern margin of the Selwyn basin, we conducted fieldwork on previously mapped outcrops of the Dempster volcanics within the western and central Ogilvie Mountains. Two samples of rhyolitic breccias from the western Coal Creek region yield U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) ages on zircon of 718.13±0.28 Ma and 718.12±0.21 Ma, which suggests these localized felsic rocks belong instead to the Neoproterozoic Mount Harper Group. Further east near the Dempster Highway corridor, trilobites and agnostoid arthropods were recovered from syn-volcanic carbonate debris-flow horizons and contain more than 15 genera that assign them to the uppermost Bolaspidella Zone and/or basal Cedaria Zone (Marjuman Stage, Lincolnian Series, Global Series 3). Consistent with other early Paleozoic volcanics in the Selwyn basin, new geochemical data from the Dempster volcanics suggest they are alkaline basalts with characteristically high Nb/Yb ratios that suggest an enriched magma source. These new geochronologic, biostratigraphic, and geochemical data require significant changes to regional map units, including the location of the Dawson fault in the western Ogilvie Mountains, and provide new insights into early Paleozoic volcanism at the northern edge of the Selwyn basin.