DEFORMATION HISTORY AND EMPLACEMENT OF THE CROYDON DOME, SOUTHWESTERN NEW HAMPSHIRE

The Bronson Hill Anticlinorium is a lithotectonic belt that was multiply metamorphosed and deformed in the Paleozoic. The anticlinorium consists of a northeast trending series of gneiss domes surrounded by metasedimentary and metavolcanic rocks. Oliverian plutonic rocks commonly core the gneiss domes.

The Croydon Dome (CD) is an elongate gneiss dome within the Bronson Hill Anticlinorium in southwestern New Hampshire. The CD is cored by undeformed granodiorite with abundant pink feldspar phenocrysts. In the southern portion of the dome, geologic mapping was conducted along two transects perpendicular to the western and eastern margins of the dome. Along these transects, microstructural and metamorphic observations are used to assess the nature of the contact between the magmatic rocks and the overlying cover sequence. The western margin preserves an augen gneiss that grades into undeformed granodiorite towards the core of the dome. The cover sequence surrounding the CD consists of the strongly foliated Ordovician Ammonoosuc and Partridge Formations, Silurian Clough and Fitch Formations, and Devonian Littleton Formation. The dominant foliation in the augen gneiss and the cover sequence rocks strikes 005^° and dips ~85^°. Augen gneiss lineations are subhorizontal and trend N-S. Asymmetric feldspar + quartz aggregate augen and mica fish in the cover sequence schists indicate top-to-the-north shear sense. These fabrics and microstructures along the western margin display a ~250 m-wide shear zone, indicating that the dome and cover sequence contact is a tectonic contact and not intrusive. Within the schists an anastomosing foliation contains microlithons that preserve an older foliation. Mineral assemblages in the cover sequence indicate amphibolite facies metamorphism (Ms + Bt+ Qtz + Pl + Grt + Sil + St). The cover sequence schists contain pre- and syn-kinematic garnet. Pseudomorphs of sillimanite commonly occur after staurolite. Biotite and muscovite overgrow pre-kinematic garnet and wrap around syn-kinematic garnet.

The Grantham Fault, a steeply dipping Mesozoic normal fault transects the dome-cover sequence contact along the eastern margin of the Croydon dome. Distinct silicified zones, abundant m-scale kink folds, and m-scale fold cut-offs define the location of the Grantham fault.