Abstract

U-Pb geochronologic studies indicate that the central part of the Coast Mountains batholith in the Ketchikan area contains plutons of Triassic and possibly mid-Paleozoic age in addition to the previously recognized early Tertiary plutons. Zircons from a foliated granodiorite in Burroughs Bay yield an upper concordia intercept of 336 ± 34 Ma, which is tentatively interpreted as the crystallization age, and a lower intercept of 79 ± 7 Ma, which may record extreme Pb loss and growth of metamorphic zircon during amphibolite- to granulite-facies metamorphism. Zircons from a tonalite in Boca de Quadra yield an upper intercept (crystallization age) of 215 ± 15 Ma and a lower intercept (Pb-loss age) of 50 ± 20 Ma. The presence of these Triassic and possibly mid-Paleozoic plutons supports previous interpretations that pre-Tertiary rocks in this part of the batholith may belong to the Yukon-Tanana terrane.

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