SILURIAN SUBDUCTION UNDER THE LAURENTIAN MARGIN IN THE NEW ENGLAND APPALACHIANS

Igneous rocks in western New England help constrain the tectonic setting of the Silurian Laurentian margin. In western CT the Harrison and Newtown Gneiss together with the Brookfield Plutonic Suite appear to record arc magmatism during the interval 453 to 428 Ma (Sevigny and Hanson, 1993. 1995), and Grenville age inheritance in zircon suggests contamination by Laurentian crust. The granite at Yale Farm intruded Grenville rocks on the western edge of the Berkshire massif in northwestern CT at approximately 430 +/- 10 Ma (Zartman et al., 1986). The granite of Beckey Quarry intruded Grenville rocks and the Hoosac Formation along the eastern margin of the Berkshire massif in southwestern MA at 434 +/- 5 and 432 +/- 2 Ma (Karabinos et al., 2003). The geochemistry of these two informal granite units suggests that they formed as arc magmas. The Dell Trondhjemite in northwestern MA has an arc geochemical signature (Kim and Jacobi, 1996) and intruded the Hawley Formation at 434 +/- 8 Ma (Karabinos and Aleinikoff, 1991). A felsic sill intruded the Missisquoi Formation (MF) in Bridgewater, VT at 419 +/- 1 Ma (Aleinikoff and Karabinos, 1990).

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).