Rb-Sr whole-rock isochrons for four main representatives of the syntectonic to post-tectonic New Hampshire Plutonic Series yield the following ages: Kinsman Quartz Monzonite, 411 ± 19 m.y.; Bethlehem Gneiss, 405 ± 78 m.y.; Spaulding Quartz Diorite, 402 ± 5 m.y.; and Concord Granite, 359 ± 11 m.y. (or 330 ± 3 m.y.?). The Kinsman and Spaulding ages are consistent with a growing number of Rb-Sr ages determined for northern Appalachian granites, which are of Early Devonian or younger geologic age, and represent a major magmatic pulse of the Acadian orogeny. Comparison of Rb-Sr ages and geology implies that the beginning of the Devonian Period can be no later than approximately 415 m.y. and that the entire Devonian time scale needs revision.
The Concord Granite is at least 40 m.y. (possibly 70 m.y.?), younger than other members of the New Hampshire Plutonic Series, and its time of emplacement is therefore probably Late Devonian(?) or Mississippian(?).
Isochron plots imply that, although the members of the New Hampshire Plutonic Series have been derived by anatexis of lower crustal rocks, there is admixture of and contamination by mantle-derived magmatic material.