LATE TRIASSIC – JURASSIC MAGMATISM, METALLOGENY AND TECTONICS IN THE INTERMONTANE TERRANES OF YUKON

Late Triassic to Jurassic granitoid plutons that intrude the Intermontane terranes in the northern Cordillera are considered parts of the early Mesozoic arcs of Stikinia (ST) and Quesnellia (QN). In British Columbia these plutons are associated with prolific Cu-Au porphyry deposits. They extend into southern Yukon where Early Jurassic plutons locally host unusual high-grade Cu-Au mineralization.

In Yukon, the oldest, Late Triassic plutons (ca. 217-214 Ma) comprises mainly small, melanocratic monzodiorite to quartz diorite plutons that were emplaced at high crustal levels into metamorphic rocks of the Yukon-Tanana terrane (YT) and Upper Triassic volcanic rocks of ST. This suggests that YT formed the basement to Mesozoic ST in Yukon. These plutons are geochemically and isotopically juvenile and locally associated with Cu mineralization. The Minto suite (ca. 205-195 Ma) intrudes YT and ST in central Yukon and comprises variably deformed granodiorite that were emplaced at lower crustal depths (6-7 kbar) during crustal thickening and accretion of the Intermontane terranes. High-grade Cu-Au mineralization at Minto and Carmacks Copper is hosted in variably migmatized mafic xenoliths within Early Jurassic granodiorite. A detailed study of the xenoliths used combined LA-ICPMS and CA-TIMS techniques on zircons and Re-Os dating of molybdenite to show that protolith and mineralization are Late Triassic, and that migmatization resulted in upgrading of original porphyry mineralization. Granodiorite and granite of the Long Lake suite (ca. 188-182 Ma) were emplaced at shallower crustal levels (3-5 kbar) during continued convergence and development of the syncollisional Whitehorse trough (WT). Trace element patterns and isotopic analyses of the Minto and Long Lake suites show decreasing subduction influence and increasing crustal contamination consistent with syncollisional emplacement. Early Jurassic magmatism also migrated southward between ca. 205 and 178 Ma in Yukon. The Middle Jurassic Bryde suite (ca. 172-168 Ma) comprises post-collisional, alkalic plutons ranging from monzonite to syenite and granite that were emplaced at high crustal levels into ST, Cache Creek and WT. Their juvenile isotopic signatures combined with regional surface uplift reflect lithospheric delamination or slab breakoff.