Radiometric age (Late Ordovician) of the Quincy, Cape Ann, and Peabody Granites from eastern Massachusetts

The zircon ages show only mild internal discordancies and most diffusion models used to discuss U-Th-Pb isotopic systematics would give a true age about equal to that of the Pb²⁰⁷/Pb²⁰⁶ value. When plotted on a concordia diagram, the zircon data indicate a time of emplacement of 450 ± 25 m.y. for all three granite bodies. This Late Ordovician age for post-tectonic granites suggests that revisions in the commonly accepted geologic interpretation of eastern Massachusetts may be required. Previously, because of their massive, undeformed nature, the alkalic rocks were generally regarded as being Mississippian or Devonian in age and younger than the Early to Middle Devonian Acadian orogeny.

The amphibole K-Ar and whole-rock Rb-Sr systems have responded in a complex way to postcrystallization disturbances. We interpret the pattern of ages for the Quincy Granite to reflect a late Paleozoic(?), low-temperature alteration that accompanied extensive faulting, and the patterns of ages for the Cape Ann and Peabody Granites to reflect a Devonian heating possibly related to contact metamorphism by a nearby mafic pluton.