The Middle Proterozoic Bear River dykes (ca.1.27 Ga) are mafic intrusions that crosscut the Gillespie Lake Group of the Wernecke Supergroup in the Slats Creek and Fairchild Lake map areas (106 D/16, 106 C/13). The dykes are fine-to medium-grained gabbros and basalts with tholeiitic affinities.The most northwesterly dyke was examined in detail. It was emplaced into mainly dolostone, and crosscuts an older fault. A white-weathering aureole along the margins of the dyke consists of calcite-magnetite-serpentine skarn. Within the dyke, hydrothermal effects are dominated by Fe (hematite and magnetite), with local enrichments of Cu (chalcopyrite) and U, a signature characteristic of earlier-formed zones of Wernecke Breccia (1.6 Ga). Alteration of the dyke indicates that a later pulse of hydrothermal fluids was channelled along the dyke or the fault.The Bear River dykes may belong to the coeval, giant radiating Mackenzie dyke swarm of the northern Canadian Shield.

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.

Paleomagnetic results are presented for 154 specimens from 16 sites in the Late Cretaceous Seymour Creek stock, a small granodioritic intrusion emplaced into Paleozoic gneisses and schists of the Yukon-Tanana Terrane (YTT), west-central Yukon. Stepwise demagnetization of the specimens revealed steep characteristic remanent magnetization directions in 2 normal- and 14 reversed-polarity sites with a mean direction of declination D=65.0°, inclination I =-83.6° (alpha 95 = 4.3°, k =73.8). Geological relations suggest that the stock has not been tilted since its emplacement at 68.5 ± 0.2 Ma. The paleopole for the Seymour Creek stock at 55.2°N, 202.5°E (dp =8.3°, dm=8.5°), plots south of the North American apparent polar wander path. This suggests that the YTT has experienced a net 79° ± 36° counter-clockwise rotation, and a nonsignificant 2.4° ± 7.5° anti-poleward translation relative to North America since 68.5 Ma. This result does not agree with the previously reported large poleward translation and minimal rotation estimated for the YTT from paleomagnetism of the coeval Carmacks Group volcanic rocks.

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

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

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Reconnaissance geological mapping in the Coal River map area of southeastern Yukon investigated several small mid-Cretaceous plutons. The intrusions are composed of unfoliated or incipiently foliated, fine to coarse-grained, equigranular and porphyritic, biotite ± hornblende quartz monzodiorite to granodiorite. They are metaluminous to peraluminous and have reduced to oxidized geochemical characteristics. The composition of selected samples is consistent with magma formation from partial melting of infracrustal source rocks.U-Pb ages were obtained for nine plutons from five or six zircon single-grain analyses by the isotope dilution thermal ionization mass spectrometry method with chemical abrasion (CA-TIMS). All interpreted ages are concordant within statistical uncertainty. The plutons range in age from 99.80 ± 0.03 to 97.70 ± 0.03 Ma. Given the primarily unfoliated nature of the plutons, contractional, fabric-forming deformation within the Cordilleran orogeny must therefore have largely ceased at the present level of exposure in the Coal River area by the time of intrusion (ca. 98 Ma).The ages and compositions of the plutons in Coal River map area are consistent with their being part of the Tay River plutonic suite, a northwest-trending belt of coeval and compositionally similar plutons and local volcanic rocks (South Fork volcanic suite) that, when augmented by the addition of the Coal River plutons, extends approximately 465 km with a width of up to 150 km.

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