Abstract

A quartz monzonite-syenite-alkali granite plutonic complex in eastern Alaska crosscuts the contact of the Alexander terrane and Wrangellia and intrudes the basement rocks of both terranes. Zircon U-Pb data indicate an intrusion age of 309 ±5 Ma (Middle Pennsylvanian) for the pluton, and 40K-40Ar age data for hornblende separates indicate cooling to about 450 °C during Middle Pennsylvanian-Early Permian time. The new field relations and age data demonstrate that Wrangellia and the Alexander terrane were contiguous during the Middle Pennsylvanian. This conclusion provides an important new constraint on paleogeographic reconstructions of the northwest Cordillera, and necessitates reassessment of stratigraphic and paleomagnetic data that were cited as evidence that the terranes evolved separately until the late Mesozoic.

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