Abstract

Whole-rock Rb–Sr and U–Pb zircon age measurements on intrusive rocks in southwestern Maine indicate igneous activity at 400, 340, and 320 Ma. These plutonic rocks were emplaced into deformed Ordovician to Devonian(?) (and perhaps Hadrynian) rocks of the Shapleigh and Merrimack Groups. Folding of the Shapleigh and Merrimack Group rocks is interpreted to have occurred during the Acadian event, or before. The graphic age of the Webhannet pluton in southwestern Maine sets a minimum time for Acadian deformation in this region.The 320 Ma age of the Lyman two-mica granite pluton of this study coupled with the reported ages of the Milford two-mica granite (275 Ma) and Lake Sunapee two-mica granite (323 Ma) of New Hampshire suggests a spectrum of Hercynian igneous activity in northern New England similar to that of the well established Hercynian intrusive events in the southern Appalachians and western Europe.

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