Recent mapping in the ‘Windy McKinley’ terrane of Stevenson Ridge area of western Yukon defined two subdivisions of the terrane, an imbricated ophiolite and a succession of predominantly fine-grained, variably carbonaceous and calcareous clastic rocks extensively intruded by Middle Triassic gabbro. Further work in 2007 has revealed a third subdivision of felsic metavolcanic and carbonaceous clastic rocks, also spatially associated with voluminous gabbro. The two subdivisions of the terrane containing gabbro are reminiscent of the two subdivisions of the Delta district of Alaska, and gabbroic rocks from the two areas are coeval and geochemically similar. If the Stevenson Ridge successions correlate with those of the Alaska Range, the mineral potential of the Stevenson Ridge area would be appropriately increased.

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.

Rocks assigned to the Windy McKinley Terrane occur in Stevenson Ridge and Kluane map areas of western Yukon. Based on new mapping in Stevenson Ridge area, rocks mapped as Windy McKinley Terrane have been divided into three fault-bound assemblages: 1) a structurally lowest assemblage of muscovite-quartz schist, calcsilicate schist, and minor marble, carbonaceous quartzite and schist, pebble meta- onglomerate and granitic meta-plutonic rocks; 2) an imbricated ophiolitic assemblage of meta-chert, probably intrusive greenstone, leucogabbro, variably serpentinized dunite and harzburgite, and mafic greywacke; and 3) an assemblage of fine-grained clastic and calcareous rocks intruded and variably hornfelsed by voluminous Early Triassic (Mortensen and Israel, 2006) gabbro. Assemblage 1 probably correlates with Yukon-Tanana Terrane. Assemblage 2 more strongly resembles the Chulitna Terrane of Alaska rather than either the Windy or McKinley terranes as originally defined. Assemblage 3 resembles part of McKinley Terrane, as well as the Aurora Peak and Pingston terranes. These terrane re-assignments have implications for the area¿s mineral potential.

The Yukon-Tanana Terrane (YTT) is a terrane of pericratonic affinity which occupies an intermediate position between continental margin rocks of Ancestral North America (Cassiar Terrane, Selwyn Basin) to the east and arc and oceanic terranes accreted in Mesozoic time to the west (Quesnellia, Stikinia and Cache Creek). It consists of polydeformed and metamorphosed Paleozoic metasedimentary and meta-igneous rocks which have isotopic and provenance ties to Archean and Proterozoic cratonic source regions, comparable to those of sedimentary strata from northwestern Canada, but whose paleogeographic evolution with respect to the Laurentian craton is enigmatic. The YTT is host to significant base metal occurrences, including the Wolverine and Kudz Ze Kayah deposits in the Finlayson Lake district, in the part of the terrane which lies northeast of Tintina Fault.

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

The Tummel fault zone, a northwest-trending belt of rocks of uncertain age and/or tectonic affinity, separates Paleozoic miogeoclinal strata of Cassiar Terrane from Yukon-Tanana Terrane metavolcanic and metasedimentary rocks. Northeast of the fault, Cassiar Terrane comprises pelitic and semipelitic rocks with rare amphibolite, which are correlated with the Kechika Group. These are overlain by carbonate correlated with the Askin Group. Southwest of the fault, in Yukon-Tanana Terrane, Devono-Mississippian siliciclastic rocks are overlain by Mississippian arc volcanic rocks. Granodiorite and diorite of the Telegraph Plutonic Suite (348-350 Ma) intrude the siliciclastic rocks. Foliated greenstone, leucogabbro intrusions, serpentinite and chert occur in the Tummel fault zone. The Early Cretaceous Glenlyon Batholith intrudes strata of Cassiar Terrane. Contact metamorphism recognized across the Tummel fault zone is interpreted to have been imposed by the Glenlyon Batholith. If correct, this interpretation requires that post-mid-Cretaceous displacement across the Tummel fault zone has been minimal (~5 km).

Geological mapping in Wolverine Lake area has outlined new Yukon-Tanana Terrane stratigraphy, constrained the stratigraphic position of the Wolverine Lake volcanogenic massive sulphide (VMS) deposit, and clarified the relationship of Yukon-Tanana Terrane to the Campbell Range belt. Yukon-Tanana Terrane comprises two stratigraphic successions separated by an angular unconformity. Beneath the unconformity are polydeformed felsic and mafic meta-volcanic rocks, carbonaceous meta-clastic rocks, marble and granitic orthogneiss. The Kudz Ze Kayah VMS deposit occurs in felsic meta-volcanic rocks of this sequence. Yukon-Tanana Terrane rocks above the unconformity are deformed by only one phase of deformation and consist primarily of carbonaceous meta-clastic rocks and quartz- and feldspar-phyric felsic meta-volcanic rocks. The Wolverine VMS deposit occurs in this succession, associated with siliceous exhalite and baritic magnetite iron formation. Meta-basalt of the Campbell Range belt, included previously in Slide Mountain Terrane, overlies the upper succession of Yukon-Tanana Terrane with sharp contact. This contact has been observed at several localities and it appears depositional. There is no evidence that it is a terrane boundary fault.

Paleomagnetic and geobarometric results are reported here for the Early Jurassic Granite Mountain batholith, an ~600 km² granodiorite intrusion in the Yukon-Tanana Terrane. Paleomagnetic analyses of 331 specimens from 24 sites yield magnetite-borne characteristic remanent magnetization (ChRM) directions. Eight northerly and westerly sites display a mean direction A (D=337, I=69°; a 95 =7.6°, k=54). Another 11 sites appear to be lightning-struck, or exhibit unstable remanence, and 3 sites in a fault-bounded block to the southeast carry an anomalous northeast upwards ChRM direction B. Two sites in a Cretaceous andesite porphyry dyke and its contact zone give a westerly, steep-down ChRM direction, C. Aluminum-in-hornblende geobarometry at 10 sites defines emplacement depths of ~16 km in the north and west, and ~19 km in the southeast. The batholith has probably not been significantly tilted since its emplacement, but it may be subdivided into two separate intrusive phases or structural blocks that have experienced differential uplift. Ambient temperatures at 16-19 km were too high for magnetite to record an enduring remanence, so the observed ChRMs likely record uplift of the batholith through ~15 km depth, at 180-170 Ma. Both A and C ChRM directions are similar to those expected for Early Jurassic and Late Cretaceous reference poles, respectively, suggesting that the Yukon-Tanana Terrane is not far-traveled with respect to cratonic North America since Early Jurassic time.

Paleozoic platformal and basinal strata of Cassiar Terrane are separated from rocks of Yukon-Tanana Terrane to the west by an unexposed fault in southeast Glenlyon map area. Quartzite, marble, phyllite, and amphibolite are grouped in Cassiar Terrane, and no rocks of Slide Mountain Terrane are recognized. The mid-Cretaceous Glenlyon batholith contains pendants of Cassiar Terrane and is intruded by at least five andesite dykes. West of the fault, the Yukon-Tanana Terrane includes: (1) mafic volcaniclastic rocks with preserved primary textures; (2) coarse-grained quartz-feldspar grit; and (3) metasiltstone and semi-pelitic schist. The grit is intruded by foliated hornblende granodiorite, likely of early Mississippian age. Small outcrops of tectonized serpentinite were tectonically emplaced into Yukon-Tanana Terrane, and a positive magnetic anomaly parallel to the fault suggests an unexposed extension to the southwest. Two mylonite localities and evidence of brittle cataclasis up to 1 km on either side of the presumed buried fault suggest a complex structural history along this terrane boundary.