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. 40Ar/39Ar 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. Samples collected on the eastern margin of the sill are 3 to 6 m.y. younger than samples of the same mineral collected at the same elevation from the western margin of the sill, indicating that the western margin cooled earlier. Samples collected at sea level yield 40Ar/39Ar dates on hornblende, biotite, and plagioclase that are consistently younger than those collected at 1- and 2-km elevation. Dates from the center and eastern margin of the sill define isotherm-migration rates (km/m.y.) of 0.8-1.3 at 60-56 Ma (hornblende), 0.8-0.6 at 56-53 Ma (biotite), and 0.4 at 54-50 Ma (plagioclase). 40Ar/39Ar dates from a single sea- level sample collected 15 km farther south indicate no significant variation in dates parallel to the southeasterly strike of the sill for at least 30 km. 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. The 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. Because the relative precision of the 40Ar/39Ar technique is sufficient to detect subtle difference in the cooling ages, its application to spatially distributed samples within a homogeneous plutonic body, as in this study, may yield new insights into the timing of metamorphism, magmatism, and tectonism of young mountain belts.

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