Abstract: 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.

Authors: Liverton, T., Thirlwall, M.F., and McClay, K.R.

Keywords: igneous petrology, Regional Geology

NTS Mapsheet(s): 105C

Citation: Liverton, T., Thirlwall, M.F., and McClay, K.R., 2001. Tectonic significance of plutonism in the Thirtymile Range, southern Yukon. In: Yukon Exploration and Geology 2000, Emond, D.S. and Weston, L.W. (eds.), Exploration and Geological Sciences Division, Yukon Region, Indian and Northern Affairs Canada, p. 171-180.

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