K40/Ar40 Age Relations of the Coast Range Batholith and Related Rocks of the Juneau Ice Field Area, Alaska

K 40 /Ar 40 whole-rock, hornblende and biotite ages were determined for samples of crystalline schists and quartz monzonitic intrusives along a northeasterly traverse from Blackerby Ridge, 4 miles north of Juneau, Alaska, to the Alaska-Canada Boundary. Whole-rock and biotite ages of the quartz monzonite composing the East Marginal Pluton of the Coast Range Batholith, range from 46.9 to 52.8 m.y. Hornblende and biotite ages of migmatitic gneisses range from 50.4 to 56.1 m.y.; 57 to 60 m.y. K 40 / Ar 40 ages were determined for biotite and hornblende from crystalline schists in almandine amphibolite facies terrane on Blackerby Ridge. Apparent hornblende and biotite K 40 / Ar 40 ages increase to the southwest in the crystalline schists on the south margin of the bathohth. Hornblende ages are older than those determined forco-existent biotite from parent migmatitic gneisses. These data suggest that synkinematic metamorphism occurred in Late Cretaceous or early Tertiary time in the West Marginal Belt, and that the quartz monzonite intrusives of the East Marginal Pluton were emplaced during the Eocene period.