STRUCTURAL ANALYSIS OF A DUCTILE SHEAR ZONE AT WELLESLEY ISLAND, THOUSAND ISLANDS, NEW YORK

Wellesley Island occurs in the St. Lawrence R. on the Frontenac Arch, which is the exposed connection between the Adirondacks, NY, and the Grenville Provence of Ontario. Detailed geologic maps were produced on Wellesley Isl. to study the relationships between metaplutonic and metasedimentary rocks that occur there. Medium- to coarse-grained pink granitic gneiss underlies most of the study area. Steeply dipping, NE striking, foliation is defined by planar aggregates of recrystallized K-spar, quartz and biotite. Quartz grains define a mineral lineation that is subhorizontal. Ellipsoidal lenses of amphibolite and quartzite occur in the granitic gneiss, and are interpreted to be deformed xenoliths. When viewed on surfaces perpendicular to foliation and parallel to lineation, these lenses have aspect ratios of about 7:1. In places, the granitic gneiss is interlayered with m-thick layers of quartzite and amphibolite, and they are sometimes partially dismembered forming large boudins with subhorizontal bulk extension. The SW part of the study area contains large blocks and layers of quartzite that occur in the granitic gneiss. All contacts are parallel to the penetrative foliation. In the quartzite, the foliation is defined by dynamically recrystallized aggregates of quartz (5-10 mm thick) that alternate with mica-rich layers (mm thick). Mineral lineations are defined by quartz aggregates and mica streaks in the quartzite, and this lineation is parallel to the lineation in the granitic gneiss. Kinematic analysis was performed on subhorizontal outcrop surfaces of the granitic gneiss. In places, Type I S-C foliation suggests right-lateral shear. This is also consistent with kinematic indicators such as sigma- and delta-porphyroclasts, and broken grains. Reconnaissance in areas south and NE of Wellesley Island suggests that this ductile shear zone is several kilometers wide, but more research is planned. Granitic rocks in the Adirondack Lowlands have been associated with the suite of plutonic rocks that comprise the AMCG suite, and the granitic rocks on Wellesley Island would fit this category. Therefore, it is conclude that the ductile deformation is post-AMCG, and possibly associated with the Ottawan orogeny. This NE striking ductile shear zone may be a conjugate to the sinistral shear zones that occur in the central Adirondacks (Moose River Plain and Piseco Lake shear zones).