LATE DEVONIAN – EARLY MISSISSIPPIAN FELSIC MAGMATISM IN THE SELWYN BASIN AND CASSIAR PLATFORM, YUKON, AND IMPLICATIONS FOR THE NATURE OF THE NORTHERN CORDILLERAN MARGIN IN THE MID-PALEOZOIC

Small accumulations of felsic volcanic rocks are present at a number of localities within mid-Paleozoic strata of the North American miogeocline (NAM) of the Selwyn Basin (SB) in central Yukon, and a more extensive sequence of volcanic rocks is associated with high-level syenite bodies in the Cassiar Platform (CP) in central and southern Yukon (the “Pelly Mountains volcanic belt”). The SB occurrences comprise quartz-feldspar-phyric dacites and rhyolites as well as small bodies of quartz-feldspar porphyry, and limited whole rock geochemical data from these units indicate calcalkaline to weakly alkaline compositions. The CP magmatism is strongly alkaline, comprising syenite and quartz syenite intrusions and locally riebeckite-bearing trachyandesite to trachyte eruptive phases. U-Pb zircon dating of seven samples of volcanic and subvolcanic units from several localities within the SB yields preliminary ages in the range of 381+/-1 Ma to 359+/-1 Ma. Quartz syenite from the CP yields a U-Pb zircon age of 362 +4/-2 Ma. Most of these ages are older than igneous ages from the Yukon-Tanana Terrane (YTT) to the west; however they overlap with ages for strongly bimodal volcanic sequences that have been reported from the western Yukon-Tanana Upland in central Alaska, which has been interpreted as a parautochthonous fragment of the western margin of the NAM. Unlike the western Yukon-Tanana Upland magmatism, however, the SB volcanism is restricted to relatively felsic compositions. The SB igneous rocks contain a large component of inherited zircon, consistent with a mainly crustal origin. Geochemical and isotopic data from the CP alkaline rocks are consistent with mantle derivation, likely in a rift setting. We interpret the SB magmatism to reflect either initial arc or more probably back-arc magmatism related to the southwest-facing YTT arc. Slab rollback led to migration of the locus of arc magmatism to the southwest, and the initiation of the main YTT arc development at ~360 Ma. Initiation of the rifting that ultimately led to the opening of the Slide Mountain back-arc ocean is marked by the alkaline magmatism in the CP at ~362 Ma.