SIFTON RANGE VOLCANIC COMPLEX AND THE EARLY TERTIARY VOLCANIC-PLUTONIC RELATIONSHIP IN THE NORTHERN CANADIAN CORDILLERA

The early Tertiary marked a period of widespread transpression-related magmatism in the Northern Canadian Cordillera. The Sifton Range volcanic complex (SRVC) of the southwestern Yukon Territory represents the northernmost volcanic centre within the Paleogene Sloko-Skukum Group, outcropping along the eastern margin of the Coast Plutonic Complex (CPC). Deposited in a northwestern trending half-graben, the SRVC comprises a 700-m thick, shallow-dipping, volcanic succession dominated by intermediate lavas and pyroclastic deposits. The volcanic sequence grades imperceptibly into biotite, hornblende alkali-feldspar granitic stock of the Ruby Range batholith dated at 57.5 +/-0.2 Ma. Hornblende, quartz-phyric felsite dykes and plugs radiate from the main body of the pluton and traverse the volcanic package as sub-concordant sills and dykes. Thermodynamic models using MELTS (Ghiorso and Sack, 1995) indicate that the Sifton lavas evolved at moderately wet (1-2 wt.% H2O), upper crustal conditions (1 kbar). Rheological models on crystalline suspensions of identical composition point to the H2O-content, and its effects on suppression of plagioclase saturation as dominant factors in controlling magmatic viscosity. Together with coeval basaltic remnants regionally landward (ex. Braeburn Lodge, Hoole Rv. and Sixtymile Rv. basalts), the Sloko-Skukum volcanics span an evolutionary continuum from more primitive, transitional tholeiitic to more evolved, calc-alkaline lavas. Furthermore, there exists a striking complimentary relationship between rock compositions and styles of emplacement. The transitional tholeiitic lavas exhibit a discontinuous range from basalts to rhyolites, with a significant decrease in the number of rocks of intermediate Si-content. Contrasting this, the calc-alkaline CPC plutons and the majority of associated pyroclastic are intermediate to evolved in composition. The progression is interpreted to represent elevated fO2 conditions and increased interaction between mantle-derived melts and thickened continental crust along the axis of the CPC during the early Tertiary. Intrusive-to explosive-to effusive transition coupled with relatively primitive volcanism away from the paleo subduction zone is attributed to rheological thresholds induced by decreasing magmatic H2O contents.