Preservation of Acadian deformation and metamorphism through intense Alleghanian shearing

The Northfield syncline in Massachusetts, USA, preserves the same Acadian succession of FIA trends (foliation intersection axes preserved in porphyroblasts) as that in Southeast Vermont, in spite of the overprinting effects of intense Alleghanian deformation and metamorphism resulting from this syncline being thrust southwards over the Pelham gneiss dome. Therefore, both regions were multiply tectonized about the same succession of directions of shortening prior to the Alleghanian, but all relics of Acadian metamorphism were obliterated in the matrix of the Northfield syncline rocks during southwards thrusting. Within the most intensely foliated rocks at the contact between the Northfield syncline and the Pelham gneiss dome, the compositional zoning within the garnet porphyroblasts was homogenized. The 55 °C increase in the rims of these garnet porphyroblasts against the matrix appears to be a product of shear heating that occurred when these rocks were thrust over the Avalonian rocks of the Pelham dome. The lack of equivalent intense Alleghanian shearing in Vermont suggests north central Massachusetts marks the upper contact of the northwest extremity of Avalon. This extremity was less than 10 km thick if it reached SE Vermont suggesting that these rocks were tectonically wedged into North America rather than simply underthrust, and that some delamination of the upper part of this portion of Avalon occurred in Alleghanian times.