Permian exhumation of the Buffalo Pitts orogenic peridotite massif, northern Cordillera, Yukon

Abstract

We report the results of a geochemical and U–Pb zircon geochronological study aimed at constraining the timing and tectonic setting of the exhumation of an orogenic peridotitic mantle massif in central Yukon within northern Canadian Cordillera. The Buffalo Pitts orogenic massif is inferred to have been exhumed into continental metasedimentary rocks within the pericratonic Yukon–Tanana terrane. Structurally admixed with the peridotite were boudins of metaleucogabbro and metatroctolite. A metamorphic aureole, defined by migmatite with abundant leucosome, characterizes the metasedi mentary wall rocks to the massif. Whole-rock chemical analyses indicate significant light rare-earth element enrichment of the leucogabbro and the metatroctolite, characteristics commonly ascribed to within-plate or rift settings. Crystallization of the leucogabbro occurred at 261.5 ± 2.3 Ma. The metatroctolite yields a similar crystallization age. These ages are coeval with metamorphism of the wall rocks to the orogenic massif, as indicated by leucosome crystallization at 262.3 ± 0.43 Ma. These geochemical and geochronological data are consistent with the orogenic massif having been exhumed within a continental rift at about 262 Ma, giving rise to metamorphism of the upper crustal rocks into which the massif was exhumed, and coeval with rift-related magmatism. Regional considerations suggest that rifting occurred within the back arc of a northeast-facing magmatic arc, represented by the Klondike schist. Coeval eclogite and blueschist along the northeast margin of the Yukon–Tanana terrane may mark the paleo-trench, along which a southwest-dipping slab is assumed to have subducted beneath Yukon–Tanana terrane.