AGE AND TECTONIC SETTING OF ECLOGITES IN THE ST. CYR AREA, YUKON-TANANA TERRANE, CANADIAN CORDILLERA

Variably retrogressed to well-preserved eclogites are found as meter- to tens of meter-scale lenses within quartzofeldspathic rocks in the St. Cyr area, Yukon of the Canadian Cordillera. The quartzofeldspathic schists consist of both sedimentary and igneous protoliths that display a continuous, steep to moderately dipping, northwest-striking foliation. Metasedimentary rocks include garnet- and kyanite-bearing quartzites, garnet-mica schists and feldspar-quartz-mica schists. Felsic meta-igneous rocks include garnet-bearing metatonalites and metatrondhjemites. The eclogites and their host rocks form a coherent slab approximately 30 km long by 6 km thick. Serpentinized ultramafic rocks and associated gabbros and amphibolites are interleaved tectonic slices within the eclogite-bearing quartzofeldspathic crust. We use SHRIMP-RG and LA-ICP-MS U-Pb zircon geochronology and trace element geochemistry to characterize the age and tectonic affinity of protoliths, detrital provenance and timing of high-pressure (HP) metamorphism in the St. Cyr area. Eclogite yields protolith ages of 379 ± 4 and 369 ± 3 Ma. Eclogite boudins have N-MORB and island arc tholeiite protoliths. Provenance ages of metasedimentary rocks range from ca. 1.8 Ga to 370 Ma, similar in age to the oldest rocks in the Yukon-Tanana terrane. Felsic intrusive rocks give magmatic ages between 346 ± 4 and 333 ± 3 Ma, coeval with the Klinkit magmatic succession of the Yukon-Tanana terrane. Late Permian HP metamorphism of the eclogites and their host quartzofeldspathic schists occurred between 274 ± 4 and 266 ± 3 Ma. Zircons from both eclogites and quartzofeldspathic schists give trace element signatures characteristic of eclogite-facies metamorphism, with depleted heavy rare earth element patterns and no Eu anomaly. The association of eclogites with HP arc-derived sedimentary and felsic intrusives indicates a primary association with felsic arcs of the Yukon-Tanana terrane rather than intra-oceanic rocks of the Slide Mountain terrane. Slices of ultramafic and lower grade metabasic rocks now spatially associated with the eclogite-bearing HP unit may represent imbrication of Yukon-Tanana and Slide Mountain terrane material during exhumation of the HP crust.