Paleozoic and Mesozoic high-pressure metamorphism at the margin of ancestral North America in central Yukon
Eclogite and blueschist in the Yukon-Tanana and Slide Mountain terranes in the Yukon and Alaska preserve a partial record of middle Paleozoic, late Paleozoic, and early Mesozoic subduction and subsequent unroofing. Near Faro, eclogites are commonly mylonitized and partially retrogressed. The analysis of Fe-Mg exchange between garnet and clinopyroxene suggests temperatures ( T ) in the 400–500 °C range for eclogite, and jadeite (∼40 mol%) + quartz suggest minimum pressures ( P ) of 11–13 kilobar at that temperature range; clinopyroxene-garnet-epidote-quartz suggest pressure near 15 kilobar ( P H 2 O = P S ). Eclogite near Last Peak, which contains brown hornblende, late-stage oligoclase, and biotite, yielded P-T estimates up to 625 °C at a minimum pressure of 14 kilobar. Eclogite near Ross River yielded temperatures near 525 °C and a minimum pressure near 13 kilobar. The previously proposed Permian-Triassic age of high-pressure tectonism for the entire Yukon-Tanana terrane in the Yukon was inferred from isotopic dating of one eclogite and one blueschist in separate localities. In this study, isotopic 40 Ar/ 39 Ar analysis of white mica from eclogite in the Simpson range yielded a date of 344 ± 1 Ma. Eclogite and blueschist from Ross River yielded white mica 40 Ar/ 39 Ar dates of 267 ± 3 Ma and 273 ± 3 Ma, respectively. White mica in eclogite from Faro was dated at 260 ± 3 Ma. White mica in two eclogites from Stewart Lake, about 2 km apart, yielded 40 Ar/ 39 Ar dates of 228 ± 1 Ma and 346 ± 3 Ma. White mica in Last Peak eclogite, for which a concordant U-Pb zircon date is 269 Ma, yielded an integrated date of 236 ± 1 Ma. Overprinting in the blueschist facies and final cooling in some localities predated eclogite metamorphism in others. The cooling ages indicate significant plate convergence as early as Carboniferous time and either the existence of more than one subduction zone or continuous convergence and episodic exhumation above a single zone. When this is considered together with the range of ages of other high-pressure occurrences, the North American Cordillera is seen to be characterized by multiepisodic and diachronous high-pressure metamorphism.