EVOLUTION OF THE SOUTHEASTERN MARGIN OF THE MERRIMACK BELT IN EASTERN MASSACHUSETTS

The SE margin of the Merrimack belt in east-central MA consists of (1) the Vaughn Hill Fm., deposited after ~463 Ma, (2) the Worcester and Oakdale Fms., deposited between ~438 and ~407 Ma, and (3) the Harvard and Vaughn Hill conglomerates, deposited after ~415 Ma. The Vaughn Hill Conglomerate overlies the Vaughn Hill Fm. and the Harvard Conglomerate overlies the 420 Ma foliated granodiorite at Pin Hill, which may be a chemically altered halo of Ayer Granite. The Vaughn Hill Fm. may be a temporal equivalent of the Tower Hill Fm. in eastern MA and/or the Rye Complex in SE NH, both present along the southeastern margin of the Merrimack belt. Detrital zircon populations of the Vaughn Hill Fm. reflect input from Ganderia, Avalonia, the Nashoba terrane, and young populations of the Laurentian margin. The Worcester and Oakdale Fms. have Ganderian and Laurentian sources. The Harvard and Vaughn Hill conglomerates were derived from some combination of Ganderia, the Nashoba terrane and/or Avalonia. They may have been deposited during the Acadian and/or Neoacadian orogeny and derived from the southeast, or could be as young as Pennsylvanian, and temporal equivalents of the Coal Mine Brook Fm. in Worcester, MA, and the Narragansett and Norfolk basins in eastern MA.

The Worcester and Oakdale Fms. have been folded by two or more generations of east-verging isoclinal folds, locally overprinted by N-trending, horizontal, close to tight chevron folds. The Vaughn Hill Fm. and Conglomerate show steeply plunging, tight to isoclinal folds, overprinted by late, open recumbent folds, similar in style and orientation to folds in the Worcester and Oakdale Fms. The Harvard Conglomerate and granodiorite at Pin Hill are folded by steeply NNW-dipping, shallowly WSW-plunging, tight to open folds and by localized, late, N- and S-dipping, tight folds. The structures in these rocks are different from those in the other rock types, possibly suggesting a separate deformation history, or, alternatively, they could be a result of the higher competency of the rock types on and around Pin Hill. If the Harvard and Vaughn Hill conglomerates are Pennsylvanian, all of the deformation in these, and possibly some of the deformation in other rocks of the SE Merrimack belt, is Alleghanian. However, it is possible that any of the deformation described above is as old as Acadian.