STRUCTURAL EVOLUTION OF THE SOAPSTONE MOUNTAIN METAGABBRO COMPLEX, SOMERS, CONNECTICUT

Soapstone Mountain and several surrounding hills near Somers, Connecticut consist of metagabbro of unknown tectonic affinity that contrasts with the surrounding felsic orthogneisses of the Bronson Hill Terrane. Detailed structural mapping using a 1m LiDAR bare earth model was carried out over summer, 2013 to define the nature of the metagabbro contacts with the surrounding Glastonbury Gneiss and document internal petrological variations and bedrock structures. The major objective was to determine whether the metagabbro bodies are allochthonous relative to the surrounding felsic orthogneisses and whether they contain lithological characteristics of ophiolite affinity.

We demonstrate that the metagabbro occurrences are variably amphibolitized and tectonized. Massive less foliated zones comprise summits and ridge lines, whereas strongly tectonized amphibolites typically crop out on steep S, and SE-facing hillslopes. The base of the metagabbro is a mixed zone of gabbro and felsic orthogneiss (Glastonbury Gneiss). Where observed, the contact is everywhere intrusive with the felsic orthogneiss protolith intrusive into the metagabbro protolith. There is no evidence that the metagabbro occurrences comprise their own allochthonous sheet. Rather, they appear to represent a mafic intrusive complex intruded by a granitic complex with associated pegmatite dikes and apophyses.

At the time of writing, the kinematics of deformation are not fully resolved, but several major shear zones with S or SE directed displacements occur within the metagabbro. They are variably talc and serpentinite altered. The major soapstone occurrences which were previously quarried are thus structurally controlled. In addition, the main fabric that occurs in the amphibolitized gabbro is locally tightly folded and transposed by a second foliation. Further fieldwork and thin-section petrography are being carried out to better understand both major structural events and how they link with the polyphase Acadian-Alleghenian metamorphic history of the Bronson Hill terrane reported elsewhere in northern Connecticut.