A STRUCTURAL AND U-PB ZIRCON GEOCHRONOLOGY INVESTIGATION OF THE HARVARD CONGLOMERATE, VAUGHN HILL FORMATION, VAUGHN HILL CONGLOMERATE, AND AYER GRANITE: IMPLICATIONS FOR POST-ACADIAN DEFORMATION IN THE MERRIMACK BELT, EASTERN MASSACHUSETTS

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 also isoclinally 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 underlying Vaughn Hill Formation (VHF), and (3) U-Pb CA-TIMS analysis on the AG, to test this.

The weighted average ²⁰⁶Pb/²³⁸U age of the three youngest detrital zircons of the VHC is 335.2 ± 5.3 Ma, while the youngest 12 zircons of the HC are 413.6 ± 1.5 Ma and the youngest four of the VHF are 470 ± 20 Ma old. 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 ± 0.50 Ma.

Early, cm-scale, steeply plunging isoclinal folds and later, cm-scale, steeply to shallowly plunging open to isoclinal folds are folded by late, m-scale 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 close to 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.