U-PB GEOCHRONOLOGY OF LATE DEVONIAN THROUGH LATE PENNSYLVANIAN DEFORMATION AND HIGH-GRADE METAMORPHISM IN CENTRAL MASSACHUSETTS AND ADJACENT NEW HAMPSHIRE, WITH SPECULATIONS ABOUT BROADER TECTONIC SETTINGS

That the study of west-central New England geology is now complete and that all tectonism is 'Acadian', are untrue, as demonstrated by U-Pb geochronology since 1990. The need for field, fold and fabric research, and improving laboratory approaches, is enormous and continues. Using existing local data and known tectono-metamorphic relationships, we divide activity into four post-Taconian episodes and consider their broader settings.

'Acadian', 420-385 Ma, corresponds to a deformation front progressing across Maine, involving Avalon arrival, NW-spreading clastic facies, volcanism, plutonism, tectonics, including the W-directed Connecticut Valley fold/thrust nappes, and metamorphism. There is related geochronology, but scattered, and rocks are heavily overprinted [3 igneous zircon (IZ), 3 I monazite (Mo), 2 metamorphic (Mt) Mo].

'Belchertown', 381-371 Ma, relating to a thermal spike associated with the Belchertown Quartz Monzodiorite intrusion, with offshoots into SW N. H. [ 5 IZ, 3 MtMo]. Amphibolite xenoliths attest to local Acadian.

'Quaboagian', 370-348 Ma (Late Dev. - Early Miss.), is the heart of deformation/metamorphism with mantle-related magmatism, crustal melting, granulite-facies metamorphism, and broad pattern and sequence of deformation features. First, SE-directed thrusting and E overfolding of older structures, including a unique tectonic window in the Pelham dome; second, a large dextral shear regime [6 IZ, 2 IMo, 14 MtMo, 1 Mt titanite, 2 MtZ]. Quaboagian shear, especially dextral, could relate to a plate reconstruction with NW South America near Newfoundland at 370 Ma and an ocean to SW; near Florida at 340 with 'Paleotethys' to NE; thus an image of 'side-swipe tectonics' for this episode (or Meguma arrival?).

Quiet interval 348-305 Ma, [1 pegmatite IZ, 1 MtMo]

'Northfieldian', 305-285 Ma (Late Pennsylvanian), formed a narrow, tectonically bounded, longitudinal belt of kyanite-staurolite-grade metamorphic overprint and pegmatites near the Connecticut Valley [ 2 IZ, 2 IMo, 7 MtMo, 6 Mt titanite, 3 MtZ, 1 Mt Sm-Nd Gar, 2 Mt Rb-Sr Hnbld]. Westphalian-Stephanian coals were deposited in SE New England at this time, themselves overtaken by up to sillimanite-grade metamorphism during final Alleghanian deformation (270-250 Ma), equating with arrival of W. Africa.