Petrology of the Taconian-Acadian Overlap Zone, Hartland Belt, Western Connecticut
Welch, Peter W.
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ABSTRACT Cameron's Line in western Connecticut and its equivalents in western Massachusetts and Vermont mark a major tectonic boundary in the New England Appalachians and are considered by many to delineate the trace of the Taconian suture zone. The Cambro-Ordovician Ratlum Mountain and Rowe Formations lie to the east of Cameron's Line in western Connecticut. Mineral equilibria and compositional zoning studies for pelitic units of the Ratlum Mountain and Rowe Formations indicate that garnet porphyroblast rims equilibrated with matrix minerals at P-T conditions that are consistent with an Acadian (Devonian) metamorphic field gradient for this area. P-T conditions were calculated by simultaneously solving for a geothermometer and a geothermobarometer in low-variance assemblages. All of the possible equilibria for each sample were then calculated using an internally consistent thermodynamic database with the software TWEEQU (thermobarometry with estimations of equilibria). Both of these methods produced consistent results with peak P-T conditions of 575-650 Â° C and 6-9 kbars. Wavelength dispersive X-ray compositional images (WDS images) along with quantitative traverses for major (Fe, Mg, Mn and Ca) elements were collected for garnet porphyroblasts in each of the samples. Trace (Y, P, Sc, Ti and Cr) element WDS images and quantitative traverses were then collected for representative samples. Petrographic observations coupled with WDS compositional imaging show that early garnet porphyroblasts have been modified either by overgrowths of biotite and chlorite or by a second phase of garnet growth. For those garnets that show two phases of growth, compositional images reveal patchy Ca content in rounded overgrowths surrounding more homogeneous euhedral cores. This is consistent with a second phase of growth of garnet at higher pressure accompanied by anorthite breakdown. Yttrium compositional images for these sample show a very narrow band of Y enrichment that lies just inside these high-Ca overgrowths. Garnets that have been overgrown by biotite have compositional images and quantitative traverses indicative of continuous prograde growth with minor resetting of chemistry along garnet rims. These overgrowths are thought to be the result of the compositionally complex continuous reaction, Grt + Ms -> An + Bt. Although Y images for these samples show a similar narrow band of Y enrichment, in these samples this band of enrichment lies within a few microns of the garnet rim. Biotite overgrowths truncate compositional zoning for both major and trace elements and therefore postdates porphyroblast growth. Compositional images for these samples generally show euhedral zoning patterns suggesting that diffusion was very limited both during and after growth. Peak P-T conditions are consistent with previously documented conditions for this region in that they suggest a predominantly Acadian metamorphic signature. There is strong evidence that garnet porphyroblast cores predate these overgrowth textures and thus may represent a resetting of Taconian garnet cores with matrix minerals during the Acadian. Maximum P-T conditions (600 Â° C and 9 kbars) were obtained from a sample collected in close proximity to Cameron's Line. If these P-T conditions represent the thermal maximum accompanying Acadian metamorphism then there is likely also some resetting of mineral assemblages to the west of Cameron's Line.
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