HOT-SIDE-UP AUREOLE IN SOUTHWEST YUKON AND LIMITS ON TERRANE ASSEMBLYOF THE NORTHERN CANADIAN CORDILLERA

Near Aishihik Lake in southwest Yukon, Canada, Paleozoic and older per icratonic metamorphic rocks of the Nisling terrane are overlain by fol iated granodiorite of the Aishihik batholith (186.0 +/- 2.8 Ma), part of the Lower Jurassic Aishihik plutonic suite. Isograd geometry in the Nisling rocks indicates that metamorphism resulted from intrusion of the batholith. Metamorphic minerals define a series of zones, includin g staurolite, staurolite-kyanite, kyanite, and sillimanite, which are parallel to schistosity and dip beneath the batholith. They form a hot -side-up aureole in which metamorphic grade increases upward. Petrogen etic relations and geothermobarometry indicate peak metamorphism at pr essures of 8 to 10 kbar and temperatures in excess of 720 degrees C in the sillimanite zone. A U-Pb age determination on titanite from the a ureole indicates cooling through 600 degrees C at 184 +/- 2 Ma, soon a fter crystallization of the batholith, consistent with emplacement of the Aishihik batholith into the Nisling terrane. Early Jurassic are ma gmatism is therefore a characteristic of Nisling and Stikinia. Isotopi c and stratigraphic data point to the construction of a single are acr oss these previously juxtaposed terranes, although it is possible that coeval Lower Jurassic arcs may have developed on both terranes in res ponse to the closure of intervening basins. This magmatism distinguish es Nisling from conterminous North America, which lacks significant Lo wer Jurassic igneous rocks.