Abstract: 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.

Authors: Harris, M.J., Symons, D.T.A., Blackburn, W.H. and Hart, C.J.R.

Map Scale: 1 : 0

NTS Mapsheet(s): 105D

Citation: Harris, M.J., Symons, D.T.A., Blackburn, W.H. and Hart, C.J.R., 1997. Paleomagnetic Study of the mid-Cretaceous Mount McIntyre pluton, Whitehorse map area (105D), southern Yukon Territory. In: Yukon Exploration and Geology 1996, Roots, C.F. (edt), Indian & Northern Affairs Canada/Department of Indian & Northern Development: Exploration & Geological Services Division, 122-130.

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