High pressure acadian metamorphism of the Straits Schist, Western Connecticut

Abstract

A detailed petrologic study of the maximum assemblage, minimum variance pelitic units contained within The Straits Schist in western Connecticut was undertaken to provide data on the Acadian Orogeny in the northern half of the western highlands of Connecticut. The Straits Schist, contained within the Connecticut Valley synclinorium, is structurally involved in portions of both the Hartland and Gneiss Dome belts, outlining isoclinally refolded nappes plunging off the Waterbury Dome. The Straits Schist is believed to be of Silurian—Devonian age, based on proposed correlations with the Goshen and the Waits River Formations of Massachusetts and Vermont, respectively. Furthermore, the age of metamorphism is interpreted as being Acadian based on the proposed age and stratigraphy, simple metamorphic textures, and uniform grain sizes. Quantitative estimates of the pressure and temperature conditions under which the mineral assemblages of The Straits Schist equilibrated were calculated using calibrated mineral reactions and ion exchange equilibria. Temperatures range from 607° — 747°C, and pressures range from 6.4 — 8.9 kbar across the study area. Estimates of PH₂0/Ptotal were obtained and range between 0.21 and 0.57. Although exchange mechanisms have not (or could not have) been determined exactly, biotite and muscovite do show deviation from ideal tri- and di-octahedral stoichiometry, up to 13.7% di-octahedral character and 2.2% tri-octahedral character, respectively. Compositional X-ray maps indicate that The Straits Schist underwent rapid cooling. Therefore, the Acadian Orogeny in western Connecticut is characterized as a high-pressure event that underwent rapid postmetamorphic uplift and cooling.