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Roots, C.F., 1988

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Roots, C.F., 1988, Regional tectonic setting and evolution of the Late Proterozoic Mount Harper Volcanic Complex, Ogilvie Mountains, Yukon: Ottawa, Ontario, Canada, Carleton University, Ph.D. dissertation, xii, 219 p., illust., maps.


The Ogilvie Mountains, near the Yukon-Alaska boundary, include inliers of middle to late Proterozoic strata in the Mackenzie Platform. Among these, the Harper Group preserves a record of crustal extension that correlates with the major unconformity that underlies Windermere Supergroup rocks along the eastern side of the North American Cordillera. The Harper Group comprises three main units: a lower formation of fault-derived conglomerate-sandstone, the lenticular, bimodal Mount Harper Volcanic Complex (MHVC), and an upper clastic succession. Zircons from a felsic flow in the MHVC yield a U-Pb crystallization age of 751 +26/-18 Ma for extrusions near the beginning of Windermere deposition. The MHVC is predominantly tholeiitic basalt, but includes minor felsic flows and dykes. It displays the results of repeated episodes of crustal movement and volcanism. The Complex has been divided into six lithostratigraphic members on the basis of field relations and chemical composition. Pillow flows (Member A) were overlapped by breccias and flows (Member B) that apparently constructed a volcanic island. Pyroclastic and epiclastic breccias (Member C) record the wasting and subsidence of the northern part of this early edifice. In the southern part, subaerially erupted rhyolite flows (Member D) and coeval andesitic floods (Member E) overlie eroded members A and B. Basaltic tuff and sills (Member F) are intercalated with overlying sediments. Whole-rock chemical analyses reveal two magmatic suites. Homogeneous members A and B are relatively primitive, with low (<1.0%) TiO2 and Mg/Mg + Fe ratios up to 0.54. Members D and E are differentiated, with 1.5-3.0% TiO2 and Mg/Mg + Fe of 0.1-0.2; they have continental affinity. Trace-element ratios are similar in the two suites, suggestive of a single magma source. Contemporaneous igneous rocks in the Windermere Supergroup are iron-rich tholeiitic basalt but more enriched in incompatible elements than any member of the MHVC. The MHVC erupted in a half-graben and possibly resulted from penetration of the bounding normal fault to the top of the mantle. The uniquely well exposed Harper Group records initial extension that culminated in crustal separation along the ancient western margin of North America.

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