U-Pb geochronology of late Cretaceous and early Tertiary plutons in the northern Coast Mountains Batholith

U-Pb geochronologic studies demonstrate that steeply dipping, sheetlike tonalitic plutons along the western margin of the northern Coast Mountains Batholith were emplaced between ~83 and ~57 (perhaps ~55) Ma. Less elongate tonalitic-granodioritic bodies in central portions of the batholith yield ages of 59-58 Ma, coeval with younger phases of the tonalitic sheets. Large granite-granodiorite bodies in central and eastern portions of the batholith were emplaced at 51-48 Ma. Trends in ages suggest that the tonalitic bodies generally become younger southeastward and that, at the latitude of Juneau, plutonism migrated northeastward across the batholith at ~0.9 km/Ma. Old zircons, combined with evolved Nd isotopic signatures for most plutons, record assimilation of continental crustal or supracrustal rocks during the generation and (or) ascent of the plutons. -from Authors