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Published 2000 | public
Book Section - Chapter

U-Pb geochronology of mid-Cretaceous and Tertiary plutons along the western edge of the Coast Mountains, Revillagigedo Island, and Portland Peninsula, southeast Alaska


U-Pb geochronologic and field studies demonstrate that steeply dipping, highly deformed, sheet-like tonalitic to granodiorite plutons along the western flank of the Coast Mountains in the Ketchikan, Alaska, region were emplaced in two distinct episodes, between 105 and 90 Ma and ca. 65 and 50 Ma. A nondeformed tonalitic body that postdates deformation yields an Oligocene age. U-Pb ages from zircon further suggest that mid-Cretaceous plutons are younger northeastward. In the Ketchikan region, magmatism migrated northeastward and probably reflects a migration of the mid-Cretaceous arc between 105 and 90 Ma. Proterozoic inheritance in zircon indicates the presence or provenance of old crustal rocks. Thus, the assimilation of old crustal material played a role in the petrogenesis of the Coast Plutonic Complex. Mid-Cretaceous plutons intrude all basement constituents on the western flank of the Coast Mountains, including juvenile, mantle-derived material of the Alexander terrane and reworked older continental crustal fragments of the Taku terrane, confirming that the two terranes must have been adjacent at the time of mid-Cretaceous arc magmatism.

Additional Information

© 2000 Geological Society of America. Manuscript accepted by the Society June 20, 1999. Parts of this research were supported by the National Science Foundation grants EAR-86-05386 and EAR-88-034834 (to Saleeby). Additional support (to Rubin) was provided by a Geological Society of America Penrose grant, a Sigma-Xi grant-in-aid, the United States Geological Survey, Alaska Branch, and the United States Forest Service, Ketchikan Ranger District. George Gehrels, Bill McClelland, and Jim Mortensen are thanked for critically reading an earlier version of the manuscript. The manuscript was carefully reviewed by J. Brian Mahoney and Carl Isachsen; their help is gratefully acknowledged. We thank Fred Barker, Dave Brew, Bill McClelland, M. Meghan Miller, Jim Monger, and Margi Rusmore for helpful discussions on northern Cordillera tectonics. Jeff Marshall assisted in mapping part of the area during the summer of 1987.

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August 22, 2023
January 13, 2024