16-38 Assessing the Impact of High Temperature Metamorphism and Partial Melting on the Viscosity of the Tatnic Hill Formation

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

The Earth’s crust primarily deforms through elastic or brittle mechanisms. However, ductile flow within the crust is proposed for several major mountain belts. Research on this phenomenon has been limited as extensive exposures of the deep crust are rarely present in active orogenic systems. In this study, we targeted the Tatnic Hill Formation in eastern Connecticut—a potential locality for orogenic ductile channel flow during the late Paleozoic Acadian-Neoacadian Orogeny. To characterize the behavior of the lithology during orogenesis, we have calculated metamorphic conditions (pressure and temperature) and bulk strengths for the Tatnic Hill Formation using thermodynamic modeling and a mineral aggregate mixing law, respectively. Additionally, we have analyzed U-Pb in zircon to constrain the age of metamorphism and garnet growth within two samples. Samples yielded peak metamorphic temperatures of 750–800 ℃, pressures of 10 ± 1 kb for garnet cores, and 6 ± 1 kb for garnet rims. The change in metamorphic pressures suggests a clockwise P-T-t path, with isothermal decompression from 10 to 6 kb at peak temperature. Calculated metamorphic zircon ages span ~400–320 Ma, with a peak in zircon crystallization ages of melanocratic layers around ~380 Ma. In contrast, a sample of leucocratic gneiss yielded a distinct zircon peak date of ~360 Ma, consistent with a younger, igneous origin for the leucocratic unit. The zircon ages and textures imply that partial melting began in the Middle Devonian and crystallization by the early Carboniferous. Finally, we calculated the effects of variable melt volume on the rock’s strength. Our calculations suggest that the rock remained significantly weaker than a comparable solid lithology for at least ~20 Myrs. Ultimately, the constraints on the magnitude and duration of peak metamorphism imply a long-lived and regionally significant partially molten weak layer in the Acadian-Neoacadian Orogen.

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