Nine Rb/Sr apparent ages are reported for igneous rocks of the Yukon Crystalline Terrane. The oldest age (144 m.y.) is from the Triassic? Klotassin quartz diorite and is thought to be a hybrid age that probably reflects the effects of younger intrusives on rocks at least 190 m.y. old. Five ages of about 100 m.y. presumably reflect the cooling of the Coffee Creek quartz monzonite. K/Ar ages for this event are slightly younger than the Rb/Sr ages, suggesting slow cooling. Rb/Sr ages of 53 and 67 m.y. were obtained for the Ruby Range batholith and an age of 61–67 m.y. for the Nisling Range alaskite. The Rb/Sr ages obtained generally confirm recently determined K/Ar ages. There is a regional decrease in initial Sr87/Sr86 ratios, southwestward across the Yukon Crystalline Terrane. This may mean that Precambrian rocks extend under the Yukon Crystalline Terrane, but are absent under the adjoining Coast Plutonic Complex.

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