Origin and evolution of the southeastern Merrimack belt, Massachusetts

Abstract

The Ordovician to Early-Devonian metasedimentary Merrimack belt of the southeastern New England Appalachian Mountains is deformed by multiple generations of folds that may have been a result of the Acadian (~421-395 Ma) and/or Alleghanian (~315-290 Ma) orogenies. The adjacent Nashoba terrane to the southeast was deformed primarily during the Acadian orogeny, and does not show much evidence for Alleghanian deformation and metamorphism, suggesting that the deformation in the Merrimack belt may also be largely a result of the Acadian orogeny. However, existing 40Ar/39Ar geochronology results suggest that the deformation in the Merrimack belt is largely Alleghanian. Potentially Pennsylvanian units in the Merrimack belt were investigated to determine if deformation was a result of the Alleghanian orogeny. Along the southeastern margin of the Merrimack belt in Massachusetts the Harvard Conglomerate is noncomformably on top of the Ayer Granodiorite at Pin Hill, MA. The Vaughn Hill Conglomerate (previously considered the Harvard Conglomerate), is adjacent to the Vaughn Hill Formation at Vaughn Hill. The Vaughn Hill Formation may lie at the base of the Merrimack belt. Detailed structural mapping was carried out on the four units and compared with deformation in selected units of the southeastern Merrimack belt. Detrital zircon U-Pb LA-ICP-MS geochronology was carried out on the metasedimentary units and U-Pb CA-TIMS zircon geochronology on the Ayer Granodiorite, in order to constrain the ages of the units and the deformation. The maximum depositional ages are ~463 Ma for the Vaughn Hill Formation, ~415 Ma for the Harvard Conglomerate, and ~416 Ma for the Vaughn Hill Conglomerate. The Ayer Granodiorite at Pin Hill is ~420 Ma. The Vaughn Hill Formation is interpreted as representing a time of continued deposition after deposition of the metasedimentary units of the Nashoba terrane, and forms the base of the Merrimack belt. Previously, the metasedimentary units of the Nashoba terrane and the Merrimack belt were thought to be unrelated as they are now separated by the Clinton-Newbury fault zone, which juxtaposed upper amphibolite facies rocks of the Nashoba terrane with greenschist facies rocks of the southeastern Merrimack belt. The difference in metamorphic grade can be explained by observed normal movement along the Clinton-Newbury fault zone. The Vaughn Hill Formation contains a significant population of ~560-530 Ma zircon interpreted to have been sourced from peri-Ganderian arc-derived sediment recycled from the Nashoba terrane. The Vaughn Hill Formation also contains a significant population of ~645-630 zircon from peri-Ganderian arcs not recorded in the peri-Ganderian Nashoba terrane. The Vaughn Hill Formation either received this sediment from ~645-630 Ma arcs now exposed in the northern New England Appalachian Mountains via an interconnected basin, or from ~645-630 Ma peri-Ganderian arc sediment that was present between the Merrimack belt and the Laurentian margin at ~463 Ma, and that is now buried and/or subducted. The Harvard and Vaughn Hill conglomerates contain a significant population of ~430 Ma zircon grains interpreted to be sourced from local igneous sources and older zircon grains from recycled Merrimack belt metasedimentary units, and are the youngest metasedimentary units in the Merrimack belt, with the exception of the Pennsylvanian Coal Mine Brook Formation. Based on new results, significant deformation in the southeastern Merrimack belt must be younger than ~415 Ma and may still be Acadian or Alleghanian. Based on new and existing structural analysis and on prior 40Ar/39Ar geochronology data, it is likely that bedding-parallel axial planar cleavage of early isoclinal folds (D1) is related to the Acadian orogeny. These are overprinted by m- to km-scale, close to tight, generally steeply NNW-dipping folds (D2), and subhorizontal to recumbent chevron to rounded folds (D3). D2-3 are likely post-Acadian and may be a result of the Alleghanian orogeny.