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Stratigraphic and structural relationships within Stikinia, and overlap assemblages of the Whitehorse trough, are investigated in the Teslin Mountain area, southern Yukon. The Middle Triassic Joe Mountain Formation is dominated by a thick sequence of aphyric basalt produced by subaqueous volcanism. The Upper Triassic Lewes River Group displays complex lateral and vertical lithological and facies changes. It illustrates synvolcanic terrane exhumation, with erosion of the volcanic upland leading to deposition of thick volcaniclastic sequences, in parallel with ongoing clastic and carbonate sedimentation in marginal basins. Unravelling the Lewes River Group stratigraphy is critical in understanding the latest stages of Stikinia arc volcanism and the onset of Whitehorse trough marine sedimentation in the Early-Middle Jurassic. Further mapping and analytical work will focus at characterizing the Joe Mountain Formation and Lewes River Group, to determine how Stikinia evolved prior to final amalgamation of the Intermontane terranes with North America.

The Wrangell lavas in the St. Clare province of southwestern Yukon are part of the larger Wrangell volcanic belt that has been active throughout the Late Cenozoic. These lavas have erupted in a transitional tectonic environment that reflects regional transpression along the Queen Charlotte transform-Fairweather-Totschunda Fault System and subduction of the Farallon Plate beneath North America. The volcanic province is composed of subalkaline basalt (31%), basaltic andesite (30%), andesite (21%), dacite (2%) and nepheline normative basalt (16%). The hypersthene normative basalt is (in order of appearance) spinel-olivine-plagioclase ± Fe-Ti oxide ± clinopyroxene phyric, whereas andesite contains plagioclase, Fe-Ti oxide, clinopyroxene, ± orthopyroxene phenocrysts, and dacite and intrusive latite contain phenocrysts of plagioclase, ± clinopyroxene, hornblende, ± biotite, ± sanidine. The nepheline normative rocks, where porphyritic, contain phenocrysts of olivine, plagioclase and hornblende. In the central part of the map area, the lowermost flows are nepheline normative basalt that are interbeddded with clastic sediments and overlain by basaltic andesite, andesite and volcanic conglomerate. This succession is overlain by basalt interbedded with clastic sedimentary rocks and pyroclastic rocks. In the southern part of the map area, alkaline basalt occurs at this stratigraphic level. The uppermost Wrangell lavas are andesitic with minor interbedded volcaniclastic rocks. The hypersthene normative lavas of the St. Clare province are transitional in terms of their Na2O + K2O/SiO2 ratios between alkaline and subalkaline magma series and in terms of their FeO/MgO versus SiO2 ratios between tholeiitic and calc-alkaline series. Chemical composition of these rocks reflects the unique tectonic setting within which they are found.

Paleozoic rocks of the Pelly Mountains, central Yukon, preserve greater than 150 m.y. of sedimentation, magmatism and base-metal mineralization. To identify secular trends in regional tectonics and metallogeny, a multi-year project on the stratigraphy of the Pelly Mountains in the Quiet Lake (105F) and Finlayson Lake (105G) map areas was initiated. Field studies during summer 2015 focused on two stratigraphic intervals: (1) mafic volcanic, volcaniclastic and clastic rock successions assigned to the Cambrian-Ordovician Cloutier and Groundhog formations (Kechika group); and (2) felsic volcanic, volcaniclastic and clastic rock successions assigned to the Devonian-Mississippian Black Slate and Felsic Volcanic formations (Seagull group). Cambrian-Ordovician strata were deposited in a marine environment characterized by episodic mafic volcanism and extensional tectonism. Devonian-Mississippian strata record the transition from an extensional turbidite basin to a metalliferous volcanic rift basin, and resemble key rock assemblages of the Selwyn basin (Earn Group) and Yukon-Tanana terrane (Grass Lakes and Wolverine Lake groups).

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

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The tectonic evolution of the Whitehorse trough in central Yukon is largely preserved by the Early to Middle Jurassic Laberge Group, an ~3000-m thick succession of synorogenic clastic strata that unconformably overlies arc and arc marginal rocks of the Lewes River Group. A two-year project was initiated to test a Sinemurian to Toarcian transgression of basal Laberge Group strata westward across the Whitehorse trough and examine the regional relationships between the timing of Jurassic exhumation, sedimentation, and terrane accretion in the northern Canadian Cordillera. Field studies in 2017 targeted basal Laberge Group strata at seven locations in central Yukon. At each field locality, basal Laberge Group strata are known or inferred to unconformably overlie the Povoas formation and multiple units of the Aksala formation. Pre-Early Jurassic unconformities may indicate variable basin topography due to the complex internal stratigraphy of the Lewes River Group, or that regional exhumation and erosion affected the Whitehorse trough prior to Laberge Group sedimentation.

The history of an eclogite sample and a mica schist sample from the western Teslin zone are discussed in view of garnet zoning profiles. The preliminary metamorphic results support the contention (de Keijzer et al., in press), based on earlier regional and structural arguments, of a structural contact (the "basal thrust" of de Keijzer et al., in press) between the Last Peak eclogite (part of the Anvil assemblage) and metasedimentary rocks of North American affinity to the west of it. Consequently, the eclogite is considered "exotic" with respect to the metasedimentary rocks. The proposed position of the Last Peak eclogite, a few hundred metres above the interpreted basal thrust within the zone of ductile thrusting, explains why it has experienced pervasive amphibolitization (hydration) since fault zones commonly act as conduits for fluid. It is unclear how much of the amphibolite-to-greenschist facies Anvil rocks surrounding the eclogite have experienced earlier high-pressure metamorphism.

The Rackla River area is underlain by normal faulted and gently folded sedimentary strata of the Paleoproterozoic Wernecke Supergroup, Mesoproterozoic Pinguicula Group, Neoproterozoic Hematite Creek Group and Windermere Supergroup, and Paleozoic Bouvette Formation. Gabbro dikes and sills that are likely age equivalent to the ca. 1380 Ma Hart River Sills cut the Wernecke Supergroup rocks. The presence of a mafic volcaniclastic horizon within the Bouvette allows its informal subdivision into a lower and upper member. These volcaniclastic rocks may the distal equivalent to volcanic rocks near the Tiger deposit, located ~20 km to the southwest. Three major angular unconformities are documented in the map area: at the base of the Rapitan Group, the base of the lower Bouvette, and the base of the upper Bouvette Formation.

The Wrangellia Terrane along the northwest margin of North America is an extensive accreted oceanic plateau. These volcanic sequences erupted onto an extinct island arc in less than 5 million years at ca. 230 Ma. Triassic Wrangellia basalts and intrusions form a 1 to 10 km-wide linear belt of mafic and ultramafic rocks extending 300 km across southwest Yukon. A total of 85 samples were collected for geochemical and isotopic analysis from 10 widespread areas along the entire length of the linear belt. Field observations during the summer of 2004, and a synthesis of previous research for the Yukon portion of Wrangellia, are part of a larger research project involving Wrangellia basalts extending from Vancouver Island to central Alaska. The Wrangellia volcanic sequences represent one of the finest examples of an accreted oceanic plateau worldwide. They provide an excellent opportunity to gain a better understanding of the mantle source of oceanic plateaus and to assess the role of accretion of oceanic plateaus in continental growth.