EVIDENCE FOR POLYPHASE DEFORMATION IN THE MYLONITIC ZONES BOUNDING THE CHESTER AND ATHENS DOMES, SOUTHEASTERN VERMONT, FROM 40AR/39AR GEOCHRONOLOGY

The Chester and Athens Domes are a composite mantled gneiss dome in SE Vermont. The oldest rocks are Proterozoic gneisses found in the cores of the domes and the youngest rocks in the attenuated mantling units are Devonian metasedimentary rocks of the Connecticut Valley Basin. While debate persists regarding the mechanisms of dome formation, most workers consider the domes to be Acadian. This study integrates ⁴⁰Ar/³⁹Ar step-heating of small aliquots (one to two grains) and microstructural analyses from samples collected in multiple transects across the dome-bounding shear zone(s). Preliminary results from the sheared units along the E margin are presented from west to east. 1) Hornblende from the Barnard gneiss yielded an age gradient with a minimum age c. 286 Ma and a weighted mean age of 406 Ma from a plateau-like segment. Biotite from this sample yielded a minimum age c. 307 Ma and a weighted mean age of 344 Ma. Microstructural analysis suggests there may be two generations of biotite, in which possibly older biotite was reset by later deformation. 2) Muscovite from a second sample of the Barnard gneiss yielded an age gradient with a minimum age c. 335 and weighted mean age of 388 Ma for a plateau-like segment. Biotite yielded a plateau age of 334 Ma. In thin section, the muscovite mica fish are locally partially replaced by biotite. 3) Muscovite from the Devonian Waits River Formation yielded a minimum age of 345 Ma and a plateau age of 362 Ma. One analysis of biotite is consistent with microstructural evidence for inherited detrital grains (i.e. ages older than the depositional age), and a second analyses yielded an age gradient from ~367–414 Ma. We also analyzed muscovite from a leucocratic Acadian dike that crosscuts the dominant penetrative foliation along the W margin of the Chester Dome but locally exhibits a concordant mylonitic foliation. Muscovite from this sample yielded a plateau age of 344 Ma. While all samples collected within the attenuated mantling units appeared to exhibit a single dominant foliation in the field, our preliminary ⁴⁰Ar/³⁹Ar results suggest a complex and protracted history of deformation based on both the variety of plateau/weighted mean ages obtained, as well as the complexity of age gradients in the individual age spectra.