Lower Paleozoic continental margin rocks of the North American Cordillera, from Yukon to Nevada, include coeval platformal carbonate and basinal clastic strata that are offset along rift‐transfer faults, including the Liard, St. Mary‐Moyie, and Snake River structures. The Dawson fault is a prominent east‐weststriking structure in central Yukon that is interpreted herein to have been active as a rift‐transfer fault by late Cambrian time. This hypothesis is supported by new zircon U‐Pb dates that range from 501.98 ± 0.17 Ma to 497.57 ± 0.70 Ma from alkaline mafic volcanic rocks concentrated along the Dawson fault. The development of a sub‐Jiangshanian unconformity immediately post‐dates this alkaline magmatism and indicates that final continental breakup and establishment of the northern Cordilleran margin occurred by the late Miaolinginan. Alkaline magmatism caused by local decompression partial melting of the mantle may have been triggered by the release of in‐plane tensile stresses during lithospheric rupture and edge‐driven mantle convection. Upper Ordovician alkaline mafic volcanic and plutonic rocks that occur along a northwest‐southeast striking segment of the Dawson fault erupted ∼50 Myr after breakup and represent an example of post‐rift magmatism along a rift‐transfer fault. New bedrock mapping, and geochronological, paleontological, and petrological results from Upper Ordovician rocks indicate that there was localized basin development and punctuated volcanism along the Dawson fault from 453 to 447 Ma. Late Ordovician extension and post‐breakup magmatism in central Yukon is compatible with dextral strike‐slip reactivation of the Dawson rift‐transfer fault associated with counterclockwise rotation of Laurentia.

The early Tertiary magmatic episode in the northern Canadian Cordillera is linked to the restructuring of the Kula-North American plate system from orthogonal to oblique convergence. Resultant volcanism was widespread, and remnant successions outcrop along the eastern margin of the Coast Plutonic Complex (CPC). The Sifton Range volcanic complex of southwestern Yukon is a member of the Paleogene Sloko-Skukum Group, and comprises a 900-m thick, shallow-dipping, volcanic succession dominated by intermediate to evolved lava and pyroclastic rocks deposited in a northwesterly trending half-graben. Locally, the volcanic sequence is intruded by alkali-feldspar granites of the CPCs Nisling Plutonic Suite dated at 57.5 Ma. Felsite sills radiate from the main intrusive body, and together with numerous basaltic to dacitic dykes traverse the volcanic package. Both the felsic volcanic rocks and epizonal granitoids exhibit anomalous enrichments in large-ion lithophile elements indicating crustal contributions during the late-stage petrogenesis of the complex. In addition, the Sifton Range intrusive rocks exhibit modal mineralogy reflective of lower ambient pressures relative to the compositionally similar Annie Ned granites along the Alaska Highway between Stony Creek and Mendenhall, 20 km south of the complex. The amount of post-Eocene uplift (ca. 30 m/Ma) that exposed the contact between the intrusive and corresponding volcanic rocks is constrained by the presence of a calc-silicate bed at an elevation of 1830 m within the upper volcanic stratigraphy.

The Yukon-Tanana terrane in Yukon, Canada, records Late Devonian (ca. 366–360 Ma) rifting and the onset of latest Devonian–Carboniferous arc and back-arc magmatism (ca. 360–325 Ma) in the Northern Cordillera. Detrital zircon U-Pb and Hf isotope analyses indicate that the metasedimentary basement of the Yukon-Tanana terrane was sourced in northwestern Laurentia. Sandstones in Late Devonian–Carboniferous successions generally have robust Late Devonian–Mississippian age peaks, and their Hf isotope signatures are characterized by strongly negative εHft values in Late Devonian zircons followed by progressively more juvenile εHft values in Carboniferous zircons. This Hf isotopic “pull-up” reflects the melting of Precambrian crust related to regional extension in the Late Devonian, followed by progressively more juvenile magmatism as the arc matured through the Carboniferous. Paleozoic rocks of the Tracy Arm terrane in southeastern Alaska, USA (formerly Yukon-Tanana south), have been compared with the Yukon-Tanana terrane in Yukon. Detrital zircons from the metasedimentary basement to the Tracy Arm terrane have distinct Precambrian populations that indicate sources along a different segment of the Laurentian margin compared to basement of the Yukon-Tanana terrane. Magmatism in the Tracy Arm terrane ranges from 440 Ma to 360 Ma and is characterized by an Hf isotopic “pull-down” in the Silurian to Early Devonian, followed by a “pull-up” in the Middle to Late Devonian and a second “pull-down” in the Late Devonian to early Mississippian. Thus, the Yukon-Tanana and Tracy Arm terranes record distinct pre-Carboniferous histories. Interactions between these two terranes are suggested by the influx of exotic early Mississippian clasts and detrital zircons on the Tracy Arm terrane that match sources in the Yukon-Tanana terrane. This paper is available via open access. You can also contact the Yukon Geological Survey (geology@gov.yk.ca) for a copy of this paper.

for a copy of this paper please contact the Yukon Geological Survey; geology@yukon.ca.

Late Triassic to Early Jurassic age (~220-185 Ma) intrusions comprise one of the most widespread and volumetrically significant plutonic suites in central and western Yukon, and eastern Alaska, but have received very limited study thus far. A new research project has been initiated that will examine the temporal, geochemical and petrotectonic evolution of this magmatic event, and the nature and origin of associated Cu, Au and PGE mineralization. The intrusions are mainly hornblende- and biotite-bearing granodiorites and quartz monzonites, although granitic phases and rare ultramafic phases (as at Pyroxene Mountain) are also present. Several bodies of coarse-grained muscovite granite that are included within the suite have been recognized in southwestern Dawson, and central and western Stewart River map areas. Most intrusions give preliminary U-Pb zircon and titanite ages of ~195 Ma to ~185 Ma, although scattered bodies give ages up to 218 Ma. Geochemical studies completed thus far indicate that most intrusions are metaluminous and formed in a volcanic arc environment, although some of the muscovite-granite phases in western Yukon are peraluminous and trend into the anorogenic (within-plate) granite field on various tectonic discriminant plots. Dating studies at Minto and Williams Creek indicate that copper-gold mineralization in both areas is hosted in part by deformed intrusions dated at ~194 Ma and is crosscut by massive, post-mineralization Granite Mountain batholith dated at ~190 Ma. The mineralization is therefore intimately associated with the Triassic-Jurassic magmatism, and we tentatively interpret the deposits as deformed copper-gold porphyries.

This paper is available via open access. You can also contact the Yukon Geological Survey (geology@gov.yk.ca) for a copy of this paper.

for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.

This field trip crosses southwestern Yukon and examines the geology of the Intermontane and Insular terranes, and synorogenic sedimentary basins that developed during Mesozoic terrane accretion.

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In the southeastern Yukon Territory, Quaternary continental alkaline basalts have erupted across an important crustal suture, the Tintina Trench, which separates the accreted terranes of the Canadian Cordillera from the ancestral North American craton. The lavas from the Rancheria region from the west side of the Tintina Trench are basanites (BASAN), alkaline olivine basalts (AOB), and hypersthene-normative basalts (HYN). They display fractionated rare earth element (REE) profiles and are enriched in light rare earth elements (LREE) and high field strength elements (HFSE). The compositional spectra of the Rancheria alkaline magmas appears to represent the progressive melting of an amphibole-bearing garnet lherzolite. The involvement of amphibole in the petrogenesis of the Rancheria alkaline magmas indicates that these magmas were generated within the lithosphere. At the eastern end of the Rancheria suite, on the east side of the Tintina Trench, the AOB from Watson Lake have higher Zr contents than Rancheria AOB to the west of the Trench. The high Zr contents of the Watson Lake AOB are similar to those observed in the Hoole Eocene tholeiitic basalts, on the east side of the Tintina Trench, further to the north. The Eocene basalts from the Hoole River region are olivine tholeiites which have experienced closed-system crystal fractionation of olivine at low pressure. The estimated primary magma for these Eocene basalts appears to have been derived by partial melting of an incompatible-element enriched lithospheric mantle source, during which garnet was not a residual phase. The Nb-Zr systematics of the Watson Lake basalts indicate that they may be derived by mixing between melts produced by melting of an amphibole-bearing residue and a lithospheric mantle similar in composition to that of the Hoole basalts. Therefore, these compositional differences in the alkaline basalts across the Tintina Trench appear to reflect the juxtaposition of chemically distinct continental lithospheric mantles, indicating that the Tintina Fault is a steep lithospheric suture.