A Structural and U-Pb Zircon Geochronology Investigation of Selected Units in the Eastern Merrimack Belt: Implications for Post-Acadian Deformation, Eastern Massachusetts

Robert Charnock, Colorado School of Mines
Yvette Kuiper, Colorado School of Mines
Andreas Möller, University of Kansas
Robert Buchwaldt, Massachusetts Institute of Technology
J. Christopher Hepburn, Boston College

The Silurian metasedimentary Merrimack belt of the NE Appalachians was deformed by at least two generations of isoclinal folds that were thought to be related to the Acadian orogeny. Along the eastern margin of the Merrimack belt in Massachusetts the Harvard Conglomerate (HC) lies noncomformably on top of the Ayer Granite at Pin Hill (AG). It is tightly folded and was previously thought to be Carboniferous, suggesting that significant deformation in the HC and perhaps elsewhere in the Merrimack belt occurred during the Alleghanian orogeny. We conducted (1) detailed structural analysis, (2) detrital zircon U-Pb LA-ICP-MS analysis on the HC, the Vaughn Hill Conglomerate (VHC) directly south of the HC and the possibly underlying Vaughn Hill Formation (VHF), and (3) U-Pb CA-TIMS analysis on the AG, to test this.

The youngest single grain of the VHC is 335.4 ±4.8 Ma, 347.7 ±4.7 Ma for the HC, and 450.6 ±16 Ma for the VHC. The youngest graphical peak age on a probability histogram is 334.3 Ma for VHC, 431.2 Ma for HC, and 467.2 Ma for VHF.

The zircon age distributions of the VHC and HC are very similar, with major mid-Silurian peaks and minor Proterozoic (and Archean) input, suggesting Ganderian and Laurentian sources and/or recycled Merrimack belt material. The VHF has two late-Neoproterozoic peaks, and minor Archean, Proterozoic, Cambrian and Ordovician input, consistent with metasedimentary units of the Nashoba terrane. The AG has a zircon crystallization age of 420.13 ± 0.11/0.21/0.50 Ma.

Early, cm-scale, steeply plunging tight to isoclinal folds are folded by late, m-scale, open recumbent folds in the VHC and VHF. These late recumbent folds are similar in style and orientation to cm-m-scale folds in the Silurian Worcester Formation of the Merrimack terrane, suggesting that at least some of the deformation in the Merrimack belt occurred during the Alleghanian orogeny. The HC is folded by m-scale, steeply NNW-dipping, shallowly WSW-plunging tight to open folds and by localized and late cm-scale, N and S dipping, tight folds. Thus, the deformation of the HC differs from the VHC and VHF, but the zircon provenance of the HC and VHC is similar while that of the VHF is different. The different deformation styles between the HC and VHC may be a result of competency contrast between the HC and AG in the north versus the VHC and VHF in the south, not a difference in age of the units.