STRUCTURAL HISTORY OF THE REUBENS HILL COMPLEX IN EASTERN MASSACHUSETTS

The Reubens Hill Complex (RHC) is a package of metamorphosed intermediate to mafic composition igneous rocks in eastern Massachusetts located at the northern end of the Rocky Pond Slice, an enigmatic block bounded by splays of the Clinton-Newbury Fault (CNF) between the Nashoba terrane to the east and the Merrimack belt to the west. The Nashoba terrane is a Cambro-Ordovician arc-backarc complex metamorphosed under amphibolite facies conditions and the Merrimack belt consists of Silurian to Early Devonian greenschist-facies metasedimentary rocks. The origin of the RHC and its relationship with adjacent domains were investigated through detailed mapping.

The RHC consists of a fine-grained, biotite- and chlorite-rich, meta-igneous schist in the south and a medium- to coarse-grained chlorite- and hornblende-rich amphibolite in the north. The schist has previously been interpreted as a submarine flow based on the presence of pillow-like structures. However, the only pillow-like structures observed in the complex are highly-altered anastomosing quartz-rich veins.

The foliations in the RHC dip moderately to steeply to the NNE in the SW and become more northwesterly in the NE. Tight to isoclinal NE-plunging asymmetric NW-side up folds and NW-plunging mineral lineations defined by quartz, feldspar, hornblende, and chlorite indicate predominantly NW-side-up movement throughout the complex. Decimeter-scale NW-plunging tight to isoclinal S-folds indicate a sinistral component.

Along the southeastern margin of the complex, the Vaughn Hill Formation (VHF), a greenschist-facies metasiltstone, is much more extensive than previously thought and deformed by regional close NW-plunging folds. Tight to isoclinal NW-plunging S- and Z-folds occur in the VHF near the contact with the RHC.

Mafic rocks like those in the RHC do not exist in the Merrimack belt or Nashoba terrane, and their formation may have to do with their location between splays of the CNF. The RHC has been predominantly deformed by asymmetric NW-side up folds and NW-plunging asymmetric folds, which may all be consistent with motion along the CNF. The final orientation of the complex and the distribution of and close folds in the VHF may have been a partial result of intrusion of the Rocky Pond Granite south of the complex.