Online

Schnalzer, Kristin

Geology

2020

M.S.

The Chester and Athens Domes are a composite mantled gneiss dome in southeast Vermont. While debate persists regarding the mechanisms of dome formation, most workers consider the domes to have formed during the Acadian Orogeny. This study integrates the results of 40Ar/39Ar step-heating of single mineral grains, or small multigrain aliquots, with data from microstructural analyses from samples collected in multiple transects across the dome-bounding shear zone(s) in order to understand the relationship between metamorphism and deformation. Results from the sheared units along the north and south transects are presented from west to east. In the north, hornblende from the Barnard Gneiss yielded a weighted mean age of 406 Ma from a plateau-like segment and biotite yielded a weighted mean age of 344 Ma. The hornblende is interpreted to constrain an earlier phase of deformation and the formation of the dominant foliation whereas biotite constrains the timing of the later deformation event. Muscovite from a second sample of the Barnard Gneiss yielded a weighted mean age of 388 Ma for a plateau-like segment and biotite yielded a plateau age of 334 Ma. The muscovite constrains the timing of deformation whereas the biotite likely constrains the timing of the later phase of deformation. One analysis of biotite from the Devonian Waits River Formation yielded a plateau age of 403 Ma, and muscovite yielded a plateau age of 362 Ma, consistent with microstructural evidence of muscovite growing at the expense of biotite. The white mica is interpreted to represent the deformation age of the sample whereas the biotite may relate to the early stages of metamorphism. In this transect, the deformation ages inferred for the samples include 388 Ma in association with upper greenschist to lower amphibolite-facies metamorphism and 362 Ma in association with greenschist-facies metamorphism. In the south, muscovite from the basement cover contact yielded a weighted mean age of 365 Ma. Biotite from this sample yielded a weighted mean age of 358 Ma. The white mica is interpreted to constrain the timing of deformation, whereas the biotite reflects a cooling age. A hornblende analysis from the Missisquoi Formation yielded a weighted mean age of 392 Ma. The spectrum contained younger steps showing resetting around 356 Ma, which may provide an estimate for the timing of deformation for this sample. Muscovite from another sample of the Missisquoi Formation yielded a weighted mean age of 365 Ma and biotite yielded a weighted mean age of 406 Ma. This sample showed two foliations in thin section. The white mica was interpreted to be the age of a younger deformation event and the formation of the dominant foliation whereas the biotite age constrains an earlier phase of metamorphism. Along this southern transect, two foliations were observed in thin section; an older S1 is preserved in the microlithons of a younger, more dominant S2 foliation. The dominant age signals in the integrated data from both transects are c. 406 Ma, 388 Ma, 365 Ma, and 344 Ma. While all samples within the attenuated mantling units appeared to exhibit a single dominant foliation in the field (S2), the local preservation a crenulated S1 foliation within S2 microlithons implies that S1 was overprinted and largely transposed by the development of S2 during the Acadian Orogeny. The multiple age signals coupled with more subtle metamorphic textures and microstructures within S2 cleavage domains suggest that S2 may be a composite foliation and that the shear zone may have been reactivated multiple times during the Acadian Orogeny.