LATE TRIASSIC TO MIDDLE JURASSIC MAGMATISM IN THE INTERMONTANE TERRANES OF YUKON

A series of Late Triassic to Early Jurassic granitoid plutons intrude the Intermontane terranes (Stikinia, Quesnellia, Yukon-Tanana) in southern Yukon. These are the northern continuation of two paired magmatic belts intruding Stikinia and Quesnellia in BC that converge in southern Yukon and peter out into eastern-central Alaska. In BC, these plutons are host to prolific Cu-Au porphyry deposits. The hairpin geometry of the Late Triassic-Early Jurassic magmatic belts form part of the argument that led to proposal of the oroclinal enclosure of the Cache Creek terrane.

In Yukon, the Late Triassic-Early Jurassic plutons define three magmatic suites: Stikine (216-206 Ma), Minto (204-194 Ma), and Long Lake (192-178 Ma) suites. Plutons of the Stikine suite are generally more mafic monzodiorite to quartz diorite, form small plutons and are restricted to Stikinia. They are inferred to represent upper crustal (4 kbar), subvolcanic intrusions to basaltic andesite of the Lewes River arc (Stikinia). The Minto suite straddles the northern apex of Stikinia/Quesnellia in central Yukon. It comprises variably deformed granodiorite that were emplaced at lower crustal depths (6-7 kbar) and host high-grade Cu-Au mineralization (Minto, Carmacks). These plutons were emplaced during accretion of the Intermontane terranes to western North America and onset of orogenesis in the northern Cordillera. The slightly younger Long Lake suite comprises granodiorite and granite that were emplaced at shallower crustal level (3-5 kbar) during development of the syncollisional Whitehorse trough, following regional exhumation. Trace element patterns for the Minto and Long Lake suites show decreasing subduction influence consistent with syncollisional emplacement.

Middle Jurassic plutons of the Bryde suite (172-168 Ma) are post-collisional and intrude Stikinia, Cache Creek terrane, and Whitehorse trough. These plutons have alkalic tendencies and range from monzonite to syenite to granite compositions. They locally host porphyry Cu mineralization (Mars) and may be product of slab break off. The youngest Jurassic intrusive in the Intermontane terranes of Yukon is the 163-161 Ma McGregor pluton. It has calc-alkalic to alkalic tendencies, ranges in composition from monzonite to granite, and was emplaced at mid-crustal levels (4-5 kbar).