ALLEGHENIAN ASSEMBLY OF CENTRAL NEW ENGLAND

Monazite crystallization ages have been measured in situ using SIMS and EMP on a suite of samples on a transect across central New England. Each major tectonic unit (nappe) displays a distinctive metamorphic history which has been dated in this study. Th/Pb and U/Pb SIMS ages are marred by large, nonsystematic errors but ²⁰⁷Pb/²⁰⁶Pb ages are systematic across strike and are broadly correlative with EMP ages. Low grade (garnet zone) monazites generally display only a single growth episode but high grade (migmatite zone) monazites display up to five growth episodes. The high grade Chesham Pond nappe (migmatite zone) experienced monazite growth at 396±4, 381±4, 370±3, and 353±6 Ma. The last age is similar to the youngest monazites in the underlying (migmatite zone) Fall Mountain nappe and similar to the age of monazites within andalusite and staurolite pseudomorphs in the still lower Skitchewaug nappe and is taken as the age that the migmatites crystallized in the high level nappes when they were emplaced on top of rocks of the Skitchewaug nappe. Similar ages of 400 – 350 Ma are recorded in the vicinity of the Chester dome, VT, but the two terranes must have been separated by considerable distance at this time because of metamorphic dissimilarities and lack of intervening metamorphic rocks of the appropriate age. Most surprisingly, rocks of the low-level “big staurolite” nappe contain monazites that crystallized between ca 335 and 270 Ma. These ages constrain the formation of the dominant fabric in these rocks, as well as the growth of the big staurolite crystals as being Alleghenian. A similar age range is found in the greenschist facies shear zone that forms the base of the “big staurolite” nappe. The “big staurolite” nappe is found along strike from southern Connecticut to northern New Hampshire and, based on these new ages and the metamorphic histories of the nappes, must represent the fundamental suture along which rocks of the New Hampshire stratigraphic series were juxtaposed against rocks of Vermont series. This occurred not during the Acadian as was previously believed but during the Alleghenian orogeny.