INTERPLAY BETWEEN STRAIN AND METAMORPHISM IN AMPHIBOLITES OF THE BRONSON HILL TERRANE, CT

Amphibolites of the Middletown formation in the Bronson Hill terrane, Vernon, Connecticut preserve high-grade syntectonic fabrics. ⁴⁰Ar/³⁹Ar geochronology (Wintsch et al., 2003) indicates peak metamorphic conditions during the Alleghanian Orogeny. Some rocks preserve a coarse-grained, undeformed, granofelsic texture while others contain a distinct NNW-plunging amphibole lineation. The rocks studied are composed of amphibole + plagioclase + epidote + quartz + ilmenite with and without garnet. There are two pargasitic amphibole populations: (1) elongate, lineated pargasite showing prograde chemical zoning with Si decreasing and Al, Ti, and K increasing from core to rim; (2) blocky unlineated pargasites showing higher average Si cations p.f.u. and retrograde zonation with Si increasing and Al and Ti decreasing from core to rim; We conclude that a period of strain was synchronous with early amphibole crystallization, but pargasitic amphibole crystallization continued post-strain. Edenite-richterite thermometry (Holland & Blundy 1994) yields pargasite crystallization temperatures between 650 and 690 °C. A third population of low temperature well lineated cummingtonite amphibole with ~7.8 Si cations p.f.u. exists in some samples. Cummingtonite occurs with plagioclase (An30) and exhibits retrograde zonation with Si increasing Al decreasing from core to rim. We propose the reaction pargasite + garnet yields cummingtonite + plagioclase for cummingtonite appearance. With a positive ΔVr, this reaction is consistent with growth during decompression. Garnets record a variety of P-T conditions with monotonic zoning with increasing Ca, Mg, and Mn suggesting prograde garnet growth before peak pressure, as well as before, during, and after peak pressure. Using the amphibole-garnet-plagioclase-quartz thermobarometer (Berman, 1991) we calculated core pressures and temperatures of 2.6 kbars and ~450 °C to rise to 6.9 and ~685 °C in the rims. These results constrain and generally support the CW PTt path modeled by (Wintsch et al., 2003) indicating upper amphibolite facies Alleghanian metamorphism as far north as northern Connecticut.