EMPLACEMENT OF THE CROYDON DOME IN SOUTHWESTERN NEW HAMPSHIRE USING MICROSTRUCTURES FROM THE NORTHERN DOME MARGIN

The Croydon Dome in southwestern New Hampshire is an elongate gneiss dome emplaced within the Bronson Hill Anticlinorium during the Neoacadian Orogeny. The Bronson Hill Anticlinorium is a major NE-trending lithotectonic belt comprised of Ordovician, Silurian, and Devonian metasedimentary and metavolcanic rocks. Along the axis of the anticlinorium, a series of NE-trending gneiss domes are commonly cored by Ordovician arc-related plutonic rocks that have been deformed and metamorphosed during the Paleozoic.

To assess emplacement mechanisms of the Croydon Dome, geologic mapping was conducted along a transect across the northern margin of the dome through foliated and folded Bronson Hill rocks and the Bethlehem Gneiss. Weakly foliated Ordovician Croydon Group Granodiorite forms the core of the dome and grades into gneissic fabric toward the dome margins. Near the dome margins, hornblende defines the foliation in Croydon Group rocks. The mantling cover sequences surrounding the dome include the Ordovician Ammonoosuc Volcanics and Silurian Clough and Fitch Formations. Along the northern margin of the dome these mantling units display foliations that are moderately to steeply NNE-dipping. NE-trending, shallowly plunging, open folds are preserved within the Clough Formation. Ptygmatically folded, cm-wide leucosomes are present within the Ammonoosuc Volcanics. Moderatly plunging, N-trending lineations ranging from 355° to 015° within the Ammonoosuc Volcanics and Clough Formation represent deformation related to the emplacement of the dome. These N-trending lineations are consistent with shallowly plunging N-trending lineations in the sheared western margin of the dome.

Mineral assemblages in the Ammonoosucs Volcanics indicate amphibolite facies metamorphism (Ms + Bt+ Qz + Pl ± Grt). Syntectonic garnet porphyroblasts in the Ammonoosuc Volcanics are strongly poikioblastic and contain folded quartz inclusion trails. Strong subgrain development in quartz is present in both the core and cover sequences rocks. Asymmetric porphyroclasts of plagioclase in the Ammonoosuc Volcanics indicate top-to-S shear sense. These asymmetric structures combined with sinistral shearing on the western margin suggest that the Croydon Dome moved northward relative to the mantling units.