Abstract

Near Aishihik Lake in southwest Yukon, Canada, Paleozoic and older pericratonic metamorphic rocks of the Nisling terrane are overlain by foliated 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, including 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. Petrogenetic relations and geothermobarometry indicate peak metamorphism at pressures of 8 to 10 kbar and temperatures in excess of 720° C in the sillimanite zone. A U-Pb age determination on titanite from the aureole indicates cooling through 600° C at 184 ± 2 Ma, soon after crystallization of the batholith, consistent with emplacement of the Aishihik batholith into the Nisling terrane. Early Jurassic arc magmatism is therefore a characteristic of Nisling and Stikinia. Isotopic and stratigraphic data point to the construction of a single arc across these previously juxtaposed terranes, although it is possible that coeval Lower Jurassic arcs may have developed on both terranes in response to the closure of intervening basins. This magmatism distinguishes Nisling from conterminous North America, which lacks significant Lower Jurassic igneous rocks.

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