Whitehorse 2016
Technical Program

June 2 (afternoon) & June 3
le 2 juin (après midi) et le 3 juin

SS4:  Structure, magmatism and metallogeny of the evolving North American Cordilleran margin  (Poster)  
Organizers / Organisateurs:  Sarah Gleeson, Murray Allan & Craig Hart
Room / Salle:  Gymnasium
Date:  From 12:00 Thursday & Friday (all day)
Presenter:  Roderick W. Campbell

Early Paleozoic magmatism and stratigraphy of the Kechika group, Pelly Mountains, Yukon

Campbell, R.W., rwc500@mun.ca, Beranek, L.P., Piercey, S.J., Memorial University of Newfoundland, St. John's, NL  A1B 3X5, and Friedman, R.M., University of British Columbia, 2020-2207 Main Mall, Vancouver, BC  V6T 1Z4

Lower Paleozoic strata of the Kechika group (informal) comprise part of the exposed base of the Cassiar terrane in south-central Yukon.  The precise age and stratigraphic architecture of key Cambrian-Ordovician volcano-sedimentary successions in this region are uncertain and therefore limit the ability to test and develop new models for Cordilleran tectonics and metallogeny.  The significance of long-lived lithospheric structures, such as the Liard Line, to regional magmatism and metal fertility are also unclear.  To address these and other questions, a field-based project was initiated to constrain the stratigraphic record, timing, and tectonic setting of Kechika group magmatism in the central Pelly Mountains.  The Cloutier formation represents the centre of a Cambrian-Ordovician volcanic belt and comprises up to 500 m of volcanic and sedimentary rocks in the Cloutier Creek (105F/10) map area.  Three principal lithofacies associations (basaltic, volcanogenic sedimentary, and limestone-argillite) were identified during field studies ~10 km east of the Ketza River mine.  The basaltic facies association consists of primary volcanic products such as pillow basalt, vesicular to amygdaloidal massive basalt, and sediment-matrix basalt breccia.  The volcanogenic sedimentary facies association likely formed by the reworking of primary products and consists of monomictic basalt breccia and conglomerate.  The limestone-argillite facies association occurs near the top of the Cloutier formation and contains limestone, argillite, and minor polymictic conglomerate.  To the northeast and southwest of the Cloutier Creek map area, the Groundhog formation consists of up to 800 m of tuffaceous to black shale and intercalated basalt and gabbro that are laterally equivalent to the Cloutier formation.  A latest Cambrian age is reported for a pyroxene gabbro stock near Groundhog Creek using chemical abrasion (CA-TIMS) zircon U-Pb geochronology.  All mafic rock samples, including the dated gabbro, have alkali basalt (ocean island basalt) geochemical signatures.  This has led to the working hypothesis of the melting of an enriched mantle source during Cambrian-Ordovician extension and rifting along the Cordilleran margin.  Whole-rock trace element and Nd-Hf isotope geochemical data are further consistent with some mafic intrusive bodies having been contaminated by evolved crustal rocks.  Future work will relate the Kechika group with Cambrian-Ordovician stratigraphic units in the northern Cordillera, including the Menzie Creek formation of the nearby Faro District.  Plate-scale comparisons will aid in understanding the development of the rifted Cordilleran margin and the role of lithospheric scale structures.