Abstract

Volcanic rocks of the Hutshi, Mount Nansen, and Carmacks groups occur in the southwestern Yukon where they unconformably overlie the Yukon Crystalline Terrane and deformed strata of the Whitehorse Trough. The volcanic rocks are faulted and tilted, locally altered, and largely postorogenic. The more basic Carmacks volcanics locally overlie intermediate to acid rocks of the Mount Nansen Group, but are mostly in isolated exposures northwest of the Hutshi and Mount Nansen volcanics.Hutshi – Mount Nansen volcanics of the Miners Range are porphyritic, partly vesicular calc-alkaline andesite flows and flow breccias intruded by calc-alkaline alkali-rich rhyolite and two-feldspar andesite dikes. A low-greenschist metamorphic overprint affects most rocks.Carmacks volcanics, near Carmacks, are flows, epiclastic breccias, and sintered tuffs interbedded with immature volcanic sandstone. One analysed breccia clast is calc-alkaline andesite, but the flows are potassic basalt, trachybasalt, and tristanite.Recent assignments of the Hutshi – Mount Nansen and Carmacks volcanic suites to early and mid-Tertiary ages, respectively, are incorrect as all are late Cretaceous. K–Ar dates for a Hutshi – Mount Nansen whole rock and plagioclase of 72.4 ± 2.5 and 69.1 ± 2.6 Ma and Carmacks whole rocks and biotite of 73.1 ± 2.5, 67.9 ± 2.3, and 68.0 ± 2.2 Ma are concordant among themselves and agree with a Rb–Sr whole-rock date of 72.4 ± 2.1 Ma for rhyolite from the Hutshi Group in northern British Columbia.This widespread late Cretaceous volcanic episode has typical subduction-related volcanic arc chemical polarity: calc-alkaline to alkaline from active trench towards stable craton. There is a dearth of documented early to mid-Cenozoic rocks in the Yukon.

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