FABRIC ANALYSIS AND OPTICAL COMPARISON OF WESTERN AND EASTERN OUTCROPPINGS OF THE ORONOQUE MEMBER OF THE DERBY HILL SCHIST, CONNECTICUT, USA

The Ordovician Oronoque member of the Derby Hill Schist is mapped in two northeast striking, oppositely dipping bands on the Ansonia bedrock geologic map, and is interpreted to be a single lithologic unit correlated across a regional syncline [Fritts, 1965]. In the west, the Oronoque member lies between two bands of the Derby Hill Schist which juxtapose high grade orthogneisses obliquely against the barrovian metamorphic isograds within the metasedimentary Wepawaug Schist. In the west, the Oronoque member is intercalated with the Allingtown and Maltby Lake Metavolcanics. The different tectonic and lithologic settings suggest that these two bands of the Oronoque member are individual lithologic units.

The Oronoque member is described as a thinly laminated, medium to fine-grained paragneiss with schistose to phyllitic partings containing abundant muscovite and chlorite [Fritts, 1965]. Observations of different mineral assemblages in thin sections from the eastern and western Oronoque members suggest diverse and most likely unrelated protoliths. Along with mineralogical observations, microtextures suggest that the western rocks are pelitic, while the eastern rocks have a volcanogenic origin. The kinematics of the Ansonia area can be seen in the microstructures of both the eastern and western thin sections. The structures, including asymmetric mica fish, porphyoblasts, and S-C fabrics indicate dextral shearing.

This research will undertake a thorough mineralogical, microtextural and microchemical analysis of the western and eastern bands of the Oronoque member and will show that these are two separate lithologic units with dissimilar protoliths.

Fritts, C. E. (1965), Bedrock geologic map of the Ansonia Quadrangle, Fairfield and New Haven counties, Connecticut, USGS, 1965.