Ultramafic and mafic rocks of the Union complex, Maine, U.S.A. are interpreted as remnants of Iapetus oceanic crust based upon geological, geochemical, and structural considerations. These rocks were emplaced along a cryptic Acadian suture during North American–Avalonian collision. U–Pb analyses of zircons from the quartz diorite portion of the Middle Road unit yield a primary (crystallization) age of 410 ± 7 Ma marking a maximum age for collision and disappearance of Iapetus ocean floor. Other studies have shown a minimum time of peak Acadian deformation at 385–400 Ma, strongly suggesting that the Acadian event occurred in a period of less than 20 Ma in this portion of the northern Appalachians.

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