Abstract

The pericratonic Aishihik metamorphic suite occurs outboard of accreted oceanic and oceanic-arc terranes in southwest Yukon. It consists of a structurally lower unit of feldspathic quartz mica schist and a heterogeneous upper unit of carbonaceous quartzite, micaceous quartzite, marble, and metaigneous rods. The metaigneous rocks include volcanic protoliths and crystallized between 351.5 ± 2.0 and 343.8 ± 0.8 Ma, suggesting correlation of the upper unit with the Nasina assemblage. The lower unit may be correlative with the pre-Mississippian Nisling assemblage. The metaigneous rocks and Early Jurassic plutons are characterized by inherited zircon components with Early Protcrozoic average ages. The Aishihik faatholitfa, a quartz diorite magmatic arc pluton, intruded the metamorphic suite at 186.0 ± 2.8 Ma at a depth of about 30 km. The batholith and metamorphic suite were intruded discordant, high-level (<6 km depth) plutons of the Long Lake plutonic suite, which crystallized at  Ma. These data, together with cooling ages, require rapid exhumation of the Aishihik metamorphic suite at about 186 Ma. Exhumation coincided with the deposition of a molasse across the oceanic Whitehorse Trough, suggesting that the west margin of the trough lay above the pericratonic belt prior to and during uplift. Cooling ages of 160–165 Ma indicate a second burial and heating event, probably linked to the obduction of the Cache Creek terrane. Subsequent erosion, recorded by molasse of the Tantalus Group, and associated isostatic rebound resulted in exhumation and slow cooling through Middle Jurassic time.

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