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The paleomagnetic signature of the mid-Cretaceous Mount McIntyre Pluton, west of Whitehorse, was evaluated to develop a better understanding of the motion history of Cordilleran terranes through time. Excluding an anomalous result from the Carmacks Group, all previous tectonic estimates for terranes in the Yukon have been extrapolated from Alaska, British Columbia or the northwestern United States. The Mount McIntyre Pluton (109 Ma) and the adjacent Whitehorse Pluton (112 Ma), are granitic bodies that intrude Triassic and Jurassic sedimentary strata of the Stikine Terrane. Paleomagnetic measurements on samples from 20 granitic sites yielded three clusters of ChRM directions. Each cluster is specific to a geographical area of the Mount McIntyre pluton. The two most northeasterly sites are from a region of mixed igneous rocks between the Mount McIntyre and Whitehorse Plutons and thus are not considered. The other 11 northern sites give a well-defined mean ChRM direction that is steeply down and northeast. Seven sites in the southern part of the pluton gave a well-defined ChRM direction that is directed steeply down and to the northwest. The mean paleopole for the southern sites give an estimate of ~3900 km of northward or poleward translation with no rotation. In contrast, the paleopole for the northern sites in the Mount McIntyre Pluton suggests a poleward translation ~1600 km with ~80° of clockwise rotation. The motion must have occurred between 109 Ma and 45 Ma because earlier studies have shown that Stikine Terrane was fixed with respect to North America by Eocene time. The southern sites of the Mount McIntyre pluton support an estimate of ~2300 km of northward displacement between ~70 Ma and 45 Ma as derived from the ~70 Ma Carmacks Group volcanics. The result from the northern sites is statistically similar to the value determined for the Whitehorse Pluton, as do several other igenous units in southern British Columbia. Geobarometric estimates, made to determine the nature of any post-crystallization tilting of the pluton, were inconclusive. The contrast in tectonic motion estimates for the northern and southern portions of the Mount McIntyre pluton can be accommodated by a large fault between the portions, but a more definitive explanation requires the accumulation of more paleomagnetic sampling, age dating and structural information.

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Two distinct but undeformed suites of granitic plutons intrude deformed siliciclastic rocks in western Dorsey Terrane. A calc-alkaline hornblende-bearing gabbro to granodiorite stock has been dated at 181.5 Ma (by the Rb/Sr method). The second suite consists of highly evolved late-orogenic granites of the Thirtymile stock and Hake Batholith, which are approximately 100 Ma. The penetrative fabric of the metasedimentary rocks indicates generally eastward-vergent layer-parallel shear. The deformation of the siliciclastic rocks is thus constrained at older than 181 Ma. The absence of resetting of the Rb-Sr isotopic ratios of the Jurassic pluton indicates that the mid-Cretaceous magmatism was emplaced at a shallow crustal depth. Since the Jurassic pluton has both a 'juvenile' Sr isotopic ratio of 0.7045 and chemistry indicative of a largely mantle-derived source, a subduction-related setting for magma generation is likely. The spatial relationship of craton-derived clastic rocks and these plutons requires that subduction had an eastward polarity.

Map presents an interpretation of the bedrock geology of the northern Whitehorse trough as extrapolated from field observations and a reflection seismic survey.

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