SILURIAN SUBDUCTION UNDER THE LAURENTIAN MARGIN IN THE NEW ENGLAND APPALACHIANS
The Braintree Complex intruded the MF in Braintree, VT at 427 +/- 4 Ma and geochemistry suggests an intraplate rift setting (Ratcliffe and Aleinikoff, 2000). In southeastern VT, the Standing Pond Volcanics (SPV) and possible feeder dikes to the SPV found in the MF preserve a back-arc basin geochemistry. A dike in the SPV constrains this unit to be older than 421 +/- 2 Ma (Aleinikoff and Karabinos, 1990).
Thus, numerous isolated units in western New England provide evidence for a west-dipping subduction zone under the Laurentian margin at the end of the Ordovician Taconic orogeny through the Silurian. The limited volume of these arc-related rocks suggests a distal setting from the arc trench and a shallow dip of the subducting slab. This subduction zone was partly coeval with magmatism in the Bronson Hill arc in NH and MA. At ca. 430 to 425 Ma a transition occurred from arc magmatism to back-arc rifting that led to sedimentation in the Connecticut Valley trough. There are two viable models for the Bronson Hill arc. We suggest it formed above the west-dipping subduction zone near the Laurentian margin. However, it may be a peri-Gondwanan arc that was accreted in the Silurian (van Staal et al., 1998).