⁴⁰ Ar/ ³⁹ Ar constraints on the emplacement, uplift and cooling of the Coast Plutonic Complex sill, southeastern Alaska

A >700-km-long and 5- to 15-km-wide aggregation of sill-like Tertiary plutons separates low- to medium-grade (greenschist to amphibolite facies) metamorphic rocks to the west from medium- to high-grade (upper amphibolite to granulite facies) gneisses and granitic plutons to the east along the western boundary of the Coast Plutonic Complex in southeastern Alaska and British Columbia. ⁴⁰ Ar/ ³⁹ Ar analyses of hornblende, biotite, and plagioclase samples from a sea-level traverse and a 1- to 2-km-elevation traverse across one of the sills, near Holkham Bay, southeastern Alaska, yield dates that vary systematically with position. The younger dates calculated for the east side relative to the west side and metamorphic temperatures suggest that the country rock was hotter to the east than to the west when the pluton cooled through mineral blocking temperatures. Rapid isotherm migration rates calculated from the vertically distributed samples and the suggestion of hotter country rock to the east during cooling is consistent with an interpretation that the pluton intruded during uplift of the rocks to the east. -from Authors