This study utilised U-Pb geochronology, and 40Ar/39Ar and (U-Th)/He thermochro-nology to delineate arc magmatism, metamorphism, and exhumation of the pericratonic Yukon-Tanana terrane in the McQuesten map area of west-central Yukon, Canada. SHRIMP U-Pb ages delineate Mid to Late Paleozoic arc magmatism and fit key units into the regional lithotectonic framework of the terrane. The juxtaposition of unmetamorphosed and predomi-nantly undeformed Devono-Mississippian rocks in the northwest of the study area with polydeformed and up to amphibolite facies metamorphosed rocks in the southwest suggests a crustal-scale discontinuity, the Willow Lake fault, bounds the two domains. The asymmetric distribution of 40Ar/39Ar ages across the fault suggest it is extensional, and was active in the Early Jurassic. Zircon (U-Th)/He ages delineate erosion of rocks in the northwest through the upper crust during the Late Triassic and Late Jurassic to Early Cretaceous followed by Mid-dle Cretaceous erosion of the southwestern domain and possibly fault reactivation.

This study utilised U-Pb geochronology, and 40Ar/39Ar and (U-Th)/He thermochro-nology to delineate arc magmatism, metamorphism, and exhumation of the pericratonic Yukon-Tanana terrane in the McQuesten map area of west-central Yukon, Canada. SHRIMP U-Pb ages delineate Mid to Late Paleozoic arc magmatism and fit key units into the regional lithotectonic framework of the terrane. The juxtaposition of unmetamorphosed and predomi-nantly undeformed Devono-Mississippian rocks in the northwest of the study area with polydeformed and up to amphibolite facies metamorphosed rocks in the southwest suggests a crustal-scale discontinuity, the Willow Lake fault, bounds the two domains. The asymmetric distribution of 40Ar/39Ar ages across the fault suggest it is extensional, and was active in the Early Jurassic. Zircon (U-Th)/He ages delineate erosion of rocks in the northwest through the upper crust during the Late Triassic and Late Jurassic to Early Cretaceous followed by Mid-dle Cretaceous erosion of the southwestern domain and possibly fault reactivation.

This study utilised U-Pb geochronology, and 40Ar/39Ar and (U-Th)/He thermochro-nology to delineate arc magmatism, metamorphism, and exhumation of the pericratonic Yukon-Tanana terrane in the McQuesten map area of west-central Yukon, Canada. SHRIMP U-Pb ages delineate Mid to Late Paleozoic arc magmatism and fit key units into the regional lithotectonic framework of the terrane. The juxtaposition of unmetamorphosed and predomi-nantly undeformed Devono-Mississippian rocks in the northwest of the study area with polydeformed and up to amphibolite facies metamorphosed rocks in the southwest suggests a crustal-scale discontinuity, the Willow Lake fault, bounds the two domains. The asymmetric distribution of 40Ar/39Ar ages across the fault suggest it is extensional, and was active in the Early Jurassic. Zircon (U-Th)/He ages delineate erosion of rocks in the northwest through the upper crust during the Late Triassic and Late Jurassic to Early Cretaceous followed by Mid-dle Cretaceous erosion of the southwestern domain and possibly fault reactivation.

The St. Cyr klippe hosts well preserved to variably retrogressed eclogites found as sub-meter to hundreds of meter scale lenses within quartzofeldspathic schists in the Yukon-Tanana terrane, Canadian Cordillera. The St. Cyr area consists of structurally imbricated, polydeformed, and polymetamorphosed units of continental arc and oceanic crust. The eclogite-bearing quartzofeldspathic schists form a 30 by 6 kilometer thick, northwest-striking, coherent package. The schists consist of metasediments and felsic intrusives that are intercalated on the tens of meter scale. The presence of phengite and Permian age zircon crystallized under eclogite facies metamorphic conditions indicates that the eclogite was metamorphosed in situ with its quartzofeldspathic host. I investigated the metamorphic evolution of the eclogite-facies rocks in the St. Cyr klippe using isochemical phase equilibrium thermodynamic (pseudosection) modeling. I constructed P-T pseudosections in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O for the bulk-rock composition of an eclogite and a host metatonalite. In combination with petrology and mineral compositions, St. Cyr eclogites followed a five-stage clockwise P-T path. Peak pressure conditions for the eclogites and metatonalites reached up to 3.2 GPa, well within the coesite stability field, indicating the eclogites reached ultrahigh-pressure conditions. Decompression during exhumation occurred with a corresponding temperature increase. SHRIMP-RG zircon dating shows that the protolith of the eclogites formed within the Yukon-Tanana terrane during early, continental arc activity, between 364 and 380 Ma, while the metatonalite protolith formed at approximately 334 Ma, during the Little Salmon Cycle of the Klinkit phase of Yukon-Tanana arc activity. Both the eclogites and the metatonalites were then subducted to mantle depths and metamorphosed to ultrahigh-pressure conditions during the late Permian, between 266 and 271 Ma. The results of our study suggest portions of the Yukon-Tanana terrane were subducted to high-pressure and ultrahigh-pressure conditions. This is the first report of ultrahigh-pressure metamorphism in the accreted terranes of the North American Cordillera. Petrological, geochemical, geochronological, and structural relationships link the eclogites at St. Cyr to other eclogite localities in Yukon, indicating the high-pressure assemblages form a larger lithotectonic unit within the Yukon-Tanana terrane.

<p dir="ltr">The Intermontane terranes represent one of the largest composite accreted terranes that built the northern Cordillera. To better understand the interactions between the continental margin of Laurentia and the Intermontane terranes, this study analyzes twelve detrital zircon samples (n=3232) from a Neoproterozoic (?) to Cretaceous metasedimentary stratigraphic section exposed in central Alaska. Distinct detrital zircon populations have been identified and are interpreted to represent four stages in the geologic development of this part of western North America. Stage 1 extends from the Neoproterozoic (?) to the Early Paleozoic, and is characterized by Proterozoic and Archean detrital zircon populations that correlate with Laurentian sources of sediment. We interpret Stage 1 to represent deposition along the northwestern continental margin of Laurentia. Stage 2 extends from the Silurian (?) to the Devonian and is characterized by a dominant Devonian and Silurian detrital zircon population. We interpret Stage 2 to have been deposited in a backarc basin coeval with active volcanism as the Yukon-Tanana terrane was rifted away from the Laurentian continental margin as the Slide Mountain Ocean opened. Stage 3 extends from the Mississippian to the Jurassic and records a shift back to sediment sources with abundant Proterozoic and Archean zircon. We interpret this stage to represent deposition of Laurentian detritus along the eastern margin of the Slide Mountain Ocean basin. Stage 4 is represented by the Lower Cretaceous strata of the Manley basin that contain one major Late Triassic to Early Jurassic detrital zircon population. We interpret this population to be sourced from the syn-collisional and post-collisional Late Triassic to Early Jurassic plutons and related sedimentary basins of the Intermontane terranes that were exhumed and eroded during the closure of the Slide Mountain Ocean and the subsequent collision with the Laurentian continental margin. We interpret the Manley basin as a syn- to post-collisional extensional basin associated with regional detachment faults that formed because of crustal thickening in the collisional zone. From a regional perspective, an extensive clastic wedge prograded northward away from the zone of crustal thickening and can be identified in a series of Mesozoic sedimentary basins that are discontinuously exposed over 1500 km in southern Alaska. Results of our study better delineate the tectonic processes that set the framework for the construction of the Late Mesozoic and Cenozoic Cordilleran orogen.</p>