RELATING ‘QUABOAGIAN’ AND ‘NORTHFIELDIAN’ GEOCHRONOLOGY TO DEFORMATION/METAMORPHISM FIELDS IN CENTRAL MASSACHUSETTS AND ADJACENT STATES

Until ~1990 high-grade metamorphism/deformation in central New England was considered "Acadian" (410-360 Ma). Subsequently, more restricted Acadian ages in Maine suggested progressive westward deformation from coastal Maine (~423 Ma) to the Connecticut Valley belt (~387.5 Ma), consistent with W transport of fold/thrust nappes. Those ages are scarce in central Massachusetts; mainly 400-390 Ma ages on sillimanite-grade Rangeley rocks below the Triassic Deerfield Basin, in a high-level Acadian allochthon that escaped overprints.

A broad high-grade deformation pattern of early E-W folds/lineations overprinted by NE to N-S folds/lineations extends from the eastern Bronson Hill Anticlinorium through Fitchburg plutons and Massabesic Gneiss, and includes the central Massachusetts-Connecticut granulite-facies zone. A maximum pattern age from overprinting of the Hardwick Tonalite (360±1, 361±2) and Wachusett Tonalite of the Fitchburg plutons (359±1), is supported by many pegmatite and metamorphic ages (367-350 Ma), the youngest in a Pelham Dome pegmatite. The age of folding near Mt. Monadnock, N.H. (metamorphic monazites 359 Ma) is constrained by unfoliated Fitzwilliam- type binary granites (monazite ages 354±1 Ma), associated with an undeformed mafic dike cutting across the youngest folds. In 1998 this intense Late Devonian - Early Mississippian episode was called ‘Neo-Acadian’, though 40-60 m.y. younger than strict Acadian, and presently lacking a plate-tectonic context. Later, a better name 'Quaboagian' was applied, from the native-American ‘Quaboag’, and Quaboag Plantation, MA (from 1656) within the granulite-facies zone.

The western Bronson Hill Anticlinorium is dominated by latest Pennsylvanian – earliest Permian intense deformation and kyanite-grade metamorphism, overprinted on Quaboagian, dated 300-285 Ma by pegmatite zircon, and metamorphic titanite, monazite, hornblende, and garnet. This just postdates coal deposition in Rhode Island (310-300) followed by Alleghanian deformation/metamorphism (~275 Ma) there. Application of ‘Alleghanian’ to this Massachusetts zone is awkward, and a local name, Northfieldian, for colonial Northfield, where first found, is preferable. The tectonic setting is cryptic. Notably the event is coeval with oroclinal bending in then nearby NW Spain.