Evidence for polyphase deformation in the mylonitic zones bounding the Chester and Athens Domes, in southeastern Vermont, from 40Ar/39Ar geochronology Schnalzer, K., Webb, L., McCarthy, K., University of Vermont Department of Geology, Burlington Vermont, USA

CLM 40 39 Sample Mineral Assemblage Metamorphic Facies Abstract Microstructure and Ar/ Ar Geochronology 18CD08A Quartz, Muscovite, Biotite, Feldspar, Epidote Upper Greenschist to Lower Amphibolite The Chester and Athens Domes are a composite mantled gneiss QC Twelve samples were collected during the fall of 2018 from the shear zones bounding the Chester and Athens Domes for 18CD08B Quartz, Biotite, Feldspar, Amphibole Amphibolite Facies 18CD08C Quartz, Muscovite, Biotite, Feldspar, Epidote Upper Greenschist to Lower Amphibolite dome in southeast Vermont. While debate persists regarding Me 40 39 microstructural analysis and Ar/ Ar age dating. These samples were divided between two transects, one in the northeastern 18CD08D Quartz, Muscovite, Biotite, Feldspar, Garnet Upper Greenschist to Lower Amphibolite the mechanisms of dome formation, most workers consider the VT NH section of the Chester dome and the second in the southern section of the Athens dome. These samples were analyzed by X-ray 18CD08E Quartz, Muscovite Greenschist Facies domes to have formed during the Acadian Orogeny. This study di raction in the fall of 2018. Oriented, orthogonal thin sections were also prepared for each of the twelve samples. The thin sec- 18CD09A Quartz, Amphibole Amphib olite Facies 40 CVGT integrates the results of Ar/Ar step-heating of single mineral NY tions named with an “X” were cut parallel to the stretching lineation (X) and normal to the foliation (Z) whereas the thin sections 18CD09B Quartz, Biotite, Feldspar, Amphibole, Muscovite Amphibolite Facies grains, or small multigrain aliquots, with data from microstruc- 18CD09C Quartz, Amphibole, Feldspar Amphibolite Facies named with a “Y” have been cut perpendicular to the ‘X-Z’ thin section. 18CD10A Quartz, Muscovite, Biotite, Feldspar, Garnet Amphibolite Facies tural analyses from samples collected in multiple transects CD North Transect South Transect 18CD10B Quartz, Amphibole, Feldspar Amphibolite Facies across the dome-bounding shear zone(s) in order to understand GMM Post Ordovician Rocks 18CD10C Quartz, Feldspar, Muscovite, Graphite Granulite Facies MGC Mesozoic Rocks This is from the Barnard Gneiss from the shear zone. The thin section 18CD08B box heights are 1V 18CD10D Quartz, Feldspar, Graphite Granulite Facies the relationship between metamorphism and deformation. Re- Late Paleozoic shows an amphibolite porphyroclast with quartz strain shadows. The min- Forced plateau age= 344.4±2.3 Ma Forced plateau age= 406.2±2.2 Ma 08B Bt Qtz Intrusive Rocks 1V incl. J-err. of .3% 1V incl. J-err. of .29% This is from the Missisquoi Formation from the shear zone. The min- 550 MSWD = 0.52, prob. =0.60 MSWD = 0.113, prob. =0.89 08B Bt eralogy suggests amphibolite facies metamorphism. The quartz experi- Late Paleozoic incl. 28.8% of the 39Ar incl. 46% of the 39Ar 18CD10A sults from the sheared units along the north and south transects Plateau Bt o eralogy suggests amphibolite facies metamorphism. The thin sec- 08B Bt 08B Hbl enced GBM, indicating the deformation temperature is ~500 C. The horn- box heights are 1V Sedimentary Rocks 08B Hbl Ms Ma Forced plateau age= 406.3±2.4 Ma BM Connecticut 08B Hbl 10A Bt 10A Bt tion shows a porphyroclast with quartz wings. It also shows two foli- are presented from west to east. In the north, hornblende from blende has a closure temperature of ~500-600ºC, and the temperature of (1V) incl. J-err. of .53%) Bt 440 Plateau Amp MSWD = 1.5, prob. =0.21 Valley Gaspe 450 10A Bt CT incl. 16.5% of the 39Ar ations, S and S. S is the older overprinted foliation that is trapped RI Trough deformation when the shear zone was active was at least 500ºC. This indi- Plateau the Barnard Gneiss yielded a weighted mean age of 406 Ma 10A Wm S 1 in the microlithons that are dened by S which is the younger Age Age (Ma) cates the hornblende likely constrains the timing of the high-T shearing of 400 10A Wm Plateau from a plateau -like segment and biotite yielded a weighted Kilometers S the sample at amphibolite facies conditions. The biotite has a closure Grt more dominant foliation dened by the planar orientation of the 350 2 S 0 50 100 150 2 micas and compositional layering. This indicates that there was two temperature of ~300-400ºC. The biotite contains a complex spectrum 360 mean age of 344 Ma. Muscovite from a second sample of the suggestive of resetting. Microstructural analysis suggests two generations Age (Ma) phases of deformation. The biotites older age might be explained Laurentian Margin Peri-Gondwanan Terranes Qtz 10A Wm Ordovician Foreland Bronson Hill Ganderian Arc 250 of biotite, where the older was reset by later deformation. There is one by the possible older generation of biotite in the sample. The white Barnard gneiss yielded a weighted mean age of 388 Ma for a 0 20 40 60 80 100 320 Plateau age= 365.6±2.0 Ma 39 Cambrian to Ordovician Arc Rocks Cumulative Ar Percent (1V) incl. J-err. of .48%) mica was interpreted to constrain the timing of formation of the foliation found. MSWD = 1.6, prob. =0.072 plateau -like segment and biotite yielded a plateau age of 334 Platform Shelburne Ganderia incl. 68.4% of the 39Ar dominant foliation S. Conversely, the older biotite may place con- This is from the Missisquoi Formation from the shear zone. The thin 280 Rowe Schist Falls Arc 18CD08D box heights are 1V Avalon 450 0 20 40 60 80 100 39 Slope-Rise Moretown Plateau age= 387.8 ± 1.4 Ma Cumulative Ar Percent straints on the minimum age of the formation of S. Ma. One analysis of biotite from the Devonian Waits River For- Plateau age= 334.3 ± 1.3 Ma Ms section shows group 2 muscovite mica sh. The mineralogy sug- (1V incl. J-err. of .28%) (1V incl. J-err. of .27%) 08D Bt Deposits Terrane MSWD = 2.4, prob. = 0.014 MSWD = 1.6, prob. = 0.12 08D Bt Grenvillian Basement incl. 57.3% of the 39Ar Plateau gests upper greenschist to lower amphibolite facies metamor- mation yielded a plateau age of 403 Ma, and muscovite yielded 08D Ms 08D Bt 08D Ms box heights are 1V This is from the Missisquoi Formation from within the shear Figure 1: Tectonic map of New England modied after phism. The quartz experienced GBM, and some SGR, indicating the 18CD10B800 400 08D Ms Hbl #1 Hbl #1 Plateau Qtz Forced Plateau age= 623±51 Ma zone. The mineralogical composition suggests amphibolite a plateau age of 362 Ma, consistent with microstructural evi- Karabinos and others (2017) and Hibbard and others (2006). Abbre- deformation temperature is >500ºC and the sample possibly (95% conf.) incl. J-err. of .55%) Hbl #1 MSWD = 34, prob. =0.000 Plateau viations include: BM – Berkshire Massif, CLM – Chain Lakes Massif, incl. 37.5% of the 39Ar facies metamorphism. The thin section shows only one folia- Ms cooled during deformation from 500-450ºC. The white mica has a 700 Hbl #2 S dence of muscovite growing at the expense of biotite. In this GMM – Green Mountain Massif, MGC – Massabesic Gneiss Complex, Age (Ma) 2 Hbl #2 tion. The temperature of deformation is >500C. Hornblende closure temperature of ~450-550ºC. Since the deformation tem- Plateau CVGT – Connecticut Valley Gaspe Trough and CD – Chester and 350 S #1 was much older than the other samples from both the perature is >500ºC, this indicates the white mica constrains the 600 2 transect, the deformation ages inferred for the samples include Athens Dome. Black box represents Figure 2. minimum age for the timing of formation for the sample. The bio- Hbl #2 Qtz Hbl north and south transects. Hornblende #2 contained younger Forced plateau age= 392±13 Ma 388 Ma in association with upper greenschist to lower amphibolite-facies metamorphism, and 362 Ma Age (Ma) 500 (95% conf.) incl. J-err. of .55%) ages c. 360 Ma. This corresponds to two southern samples tite has a closure temperature of ~300-400ºC, so it constrains the MSWD = 7.7, prob. =0.000 300 incl. 85.2% of the 39Ar 0 20 40 60 80 100 which also contained ages c. 360 Ma. This 392 Ma could be in association with greenschist-facies metamorphism. In the south, muscovite from the basement Cumulative 39Ar Percent cooling history. There is one foliation found. 400 the minimum age for the high-temperature deformation. cover contact yielded a weighted mean age of 365 Ma. Biotite from this sample yielded a weighted 18CD08E box heights are 1V 580 This is from the Waits River Formation outside of the shear 08E Wm 300 0 20 40 60 80 100 mean age of 358 Ma. A hornblende analysis from the Missisquoi Formation yielded a weighted mean 08E Wm Qtz zone. The thin sections shows quartz experiencing GBM. The Cumulative 39Ar Percent 540 Plateau 08E Bt1 08E Bt #1 Forced plateau age= 557±11 Ma mineralogy suggests greenschist facies metamorphism. Bio- age of 392 Ma. Muscovite from another sample of the Missisquoi Formation yielded a weighted mean (95% conf.) incl. J-err. of .27%) 08E Bt #1 500 MSWD = 3.8, prob. =0.000 Plateau tite #1 is 577 Ma and is interpreted to be a detrital grain. Bio- box heights are 1V This is a sample of basement rock from the basement cover con- incl. 71.3% of the 39Ar 18CD10C 380 08E Bt2 08E Bt #2 10C Bt age of 365 Ma and biotite yielded a weighted mean age of 406 Ma. Along this southern transect, two 460 Plateau age= 362.0±1.9 Ma 08E Bt #2 tite #2 is 403 Ma and may record the timing of deformation tact within the dome. The thin section shows one foliation dened Plateau age= 403.0±1.8 Ma 10C Bt (1V) incl. J-err. of .36%) (1V) incl. J-err. of .4%) Plateau Plateau Age Age (Ma) MSWD = 2.4, prob. =0.038 MSWD = 1.5, prob. =0.12 08E Wm incl. 62.1% of the 39Ar associated with greenschist facies metamorphism. The white 10C Wm by compositional layering and preferred orientation of platy min- incl. 73.7% of the 39Ar sets of foliations were observed in thin section; an older S is preserved in the microlithons of a young- 420 360 10C Wm Ms mica is 364 Ma. There is textural evidence for the white mica Plateau erals. The mineralogical composition suggests granulite facies er, more dominant foliation. The dominant age signals in the integrated data from both transects are 380 growing a the expense of biotite, so it may record a later de- metamorphism. The age of the biotite is very close to the age of 340

Age Age (Ma) the muscovite, with the biotite being slightly younger. This older 340 formation related event that occurred below the closure tem- Qtz c. 406 Ma, 388 Ma, 365 Ma,and 344 Ma. The geochronology, along with the local preservation of an 0 20 40 60 80 100 39 Cumulative Ar Percent perature of both minerals. There is one foliation. 10C Bt 10C Wm younger relationship could be consistent with white mica record- 320 Forced plateau age= 359.7±2.2 Ma (1V) incl. J-err. of .53%) Forced plateau age= 365.7±7.1 Ma Bt older S foliation in thin section, indicates that the samples experienced multiple phases of deforma- Plateau steps are magenta, rejected steps are cyan box heights are 1V (95% conf.) incl. J-err. of .51%) ing the deformation age of the sample whereas the biotite records 450 MSWD = 1.8, prob. =0.12 S2 This thin section shows quartz containing GBM and incl. 33% of the 39Ar MSWD = 4.3, prob. =0.001 19CD02A Ms incl. 81.3% of the 39Ar a cooling age of the sample. tion. We also analyzed muscovite from a leucocratic Acadian dike that crosscuts the dominant pene- 300 Graphite GBAR. Only one deformation event. Evidence of 0 20 40 60 80 100 350 Cumulative 39Ar Percent trative foliation along the W margin of the Dome. Muscovite from this sample yielded a plateau age of Acadian Dike new mineral growth during static crystallization in 250 Qtz the quartz. Intrusion formed after the shearing 344 Ma. While all samples within the attenuated mantling units appeared to exhibit a single dominant Age (Ma) event occurred and is a continuation of the existing 02A Ms 150 Plateau age= 344.3 ± 1.7 Ma deformation, representing the timing of the con- (1V incl. J-err. of .25%) Conclusion foliation in the eld, the results of this study suggest a complex history of deformation based on the MSWD = 0.99, prob. = 0.45 S incl. 60.4% of the 39Ar clusion of deformation in the Dome 50 0 20 40 60 80 100 microstructural analysis, the variety of plateau/weighted mean ages obtained, and the complexity of Cumulative 39Ar Percent Cumulative Graph Conclusions: The Domes experienced two phases of deforma- 08B Bt tion, which were interpreted to have occurred during the Salinic age gradients in the individual age spectra. box heights are 1V 08B Bt 460 Plateau Orogeny, due to deformation ages of 406 Ma, and during the Acadi- 08B Hbl 440 an Orogeny, due to deformation ages of 365 Ma and 344 Ma. The Discussion Taconic Orogeny 08B Hbl Geologic Overview 420 Plateau 08D Bt samples that were collected within the attenuated mantling units Salinic Orogeny 400 08D Bt North Transect South Transect Plateau appeared to exhibit a single dominant foliation in the field. Upon These domes are cored by Middle Proterozoic Grenvillian 08D Ms 380 72º 45’ 43’’ 30 72o 30’ 43’’ 30 08D Ms closer examination in thin section, it could be seen that the samples box heights are 1V box heights are 1VV Acadian Orogeny Plateau basement and are separated from the metamorphic Silurian to 460 460 360 08B Bt 10A Bt 08E Bt from the Missisquoi Formation in the southern transect contained

Age Age (Ma) 08E Bt 08B Bt 10A Bt 340 Devonian-aged rocks of the Connecticut Valley Trough by 440 440 Plateau the dominant foliation overprinting an older weaker S foliation. Taconic Orogeny Plateau Taconic Orogeny Plateau 320 08E Ms 420 08B Hbl 420 10A Wm 08E Ms 08B Hbl This supports the interpretation that the shear zone experienced mylonitic shear zones (Karabinos, 1999; Karabinos et al., 2010; 10A Wm 300 Plateau Salinic Orogeny 400 Plateau Salinic Orogeny 400 10A Bt multiple phases of deformation, where the S formed during the 1 Plateau 10A Bt Doll et al., 1961). Legend 08D Bt 10B Hbl2 280 380 08D Bt 380 Plateau 10B Hbl2 10A Wm first deformation event during the Salinic Orogeny, and it was then Plateau Acadian Orogeny 260 K Alkaline Intrusion Acadian Orogeny 360 Plateau 0 20 40 60 80 100 10A Wm The faults surrounding the domes are now interpreted to be 08D Ms 39 later overprinted by a second foliation during the second deforma- (Cretaceous) 360 10C Bt Cumulative Ar Percent Plateau 08D Ms 10C Bt 43º 22’ 30’’ Age (Ma)

43º 22’ 30’’ Age (Ma) 340 10C Bt tion event from the Acadian Orogeny, with a more minor overprint low-angle normal faults (Karabinos et al., 2010) that formed 340 Plateau 10C Bt Ultramac Rocks (age Plateau Plateau U 08E Bt 320 10C Wm again c. 344 Ma. The results support the interpretation that the uncertain) 320 08E Bt 10C Wm during the Acadian orogeny (Vance and Holland, 1993). 10C Wm Plateau 300 10C Wm Plateau Chester and Athens Dome experienced a complex history of defor- 300 08E Ms Plateau 40 39 DSwn Waits River and Previous work involving Ar/ Ar and K-Ar geochronology 08E Ms 280 mation. Northeld Forma- 280 tions, undivided Plateau Comparisons between the data indicate that both transects were subjected to Acadian de- 260 has been completed in New England in an e ort to determine (Devonian and Acknowledgments 260 0 20 40 60 80 100 formation, and in some places the samples may also record evidence of Salinic deforma- Silurian) 0 20 40 60 80 100 Cumulative 39Ar Percent Cumulative 39Ar Percent I would like to thank the Ver- and understand the timing of deformation. Chester Dome tion. Hornblende and biotite samples were collected from the Green Mountain Massif Om Moretown Member of mont Geological Society and Missiquoi (or from Proterozoic gneisses for Ar/Ar geochronology, and interpretations determined However, despite all of the geochronological data in the Moretown) Formation the American Federation of that the Massif experienced Taconic deformation whereas the areas to the east of the region, there are very limited data that exist for the Chester Mineralogical Societies Omc Rusty Schist member Data for this transect were sampled from the Missisquoi For- Data for this transect were sampled from the basement cover Massif (like the Chester and Athens Dome) experienced Acadian deformation (Sutter et. al., of Missiquoi Forma- Scholarship Foundation for and Athens Domes. Work completed by McWilliams (2008) has mation and the Connecticut Valley Gaspe Trough. In the rst contact in the west and from the Missisquoi Formation in the 1985). The Acadian deformation ages recorded by the white micas does agree with the in- tion (Ordovician) their generous grants that sample, the hornblende age is interpreted to constrain an ear- east. In the rst sample, 10C, the white mica plateau was inter- terpretation that areas on the east side of the Green Mountain Massif experienced Acadian suggested the domes experienced a polyphase tectonic Omb Barnard Volcanic helped pay for the irradiation Member of Missiquoi lier phase of deformation and to constrain the formation of preted to constrain the timing of deformation that the sample deformation. Monazite grains were dated in situ in thin section to date foliations of rocks Formation of my samples. That allowed history rather than experiencing solely Acadian deformation. (Ordovician) the dominant foliation at c. 406 Ma, whereas the biotite data experienced whereas the slightly younger biotite may reect a from near the Chester and Athens Dome from the Spring Hill Synform (Bell and Welch, 43º 07’ 30’’ 43º 07’ 30’’ me to complete the Ar/Ar For example, deformation may have involved two folding OCu Hoosac, Pinney Hollow, constrains the timing of the later deformation event at tem- cooling age. Moving eastward to sample 10B, the rst old horn- 2002). These grains contain ages of 405, 386, and 366 Ma (Bell and Welch, 2002), which Ottauquechee and geochronology and further Stowe Formations, peratures close to its closure temperature c. 344 Ma. In the blende was interpreted to possibly be a detrital grain that sur- correlates to the ages obtained from both the north and south transects. Electron micro- stages including recumbent folds developing due to a nappe 2 undivided (Ordovician to Cambrian) second sample, the white mica plateau age of c. 387 Ma can vived metamorphism. For the second hornblende, the age gra- probe dating of monazites was completed in order to constrain the age of deformation in my research. stage which involved shortening, followed by a doming phase Athens Dome Y Gneiss and Granite, be interpreted as constraining the timing of deformation at dients show some resetting around 356 Ma that may provide the high-strain zone, which lead to the suggestion that the grains were recrystallized undi erentiated (Hepburn et. al., 1984). 3 (Middle Proterozoic) mid greenschist-facies conditions and the biotite plateau age an estimate for the timing of deformation. The last sample had during deformation in the high-strain zone and that the shear zone was active c. 380 Ma of 334 Ma likely constrains the timing of the later phase of de- a weak overprinted foliation and evidence for two generations (Karabinos et al., 2010). The 08D sample correlates with the monazites, indicating that the In contrast, Karabinos et al. (2010) suggested extension + 43º 00’ 43º 00’ 72’’ 45 Sampling Locations of biotite. The white mica was interpreted to be the age of the occurred along the shear zone in between the nappe stage 72º 30‘ formation. The third sample contained a c. 362 Ma white mica shear zone was active c. 380 Ma and experienced Acadian deformation. 0 365 Ma deformation event resulting in the formation of the Bell, T.H., and Welch, P.W., 2002, Prolonged Acadian Orogenesis: Revelations from foliation intersection axis (FIA) controlled monazite dating of foliations in porphyroblasts and matrix: American Journal of Science, v. 302, p. 5 10KM Shear Zone interpreted to represent the deformation age whereas the References 549-581 and the doming stage. dominant foliation, S. The biotite is interpreted to be part of Doll, C.G., Cady, W.M., Thompson, J.B., Jr., and Billings, M.P., 1961, Centennial geologic map of Vermont: Vermont Geological Survey, scale 1:250,000. second biotite containing an age of c. 403 Ma may relate to Hepburn, J.C., Trask, N.J., Rosenfeld, J.L., and Thompson, J.B., Jr., 1984, Bedrock geology of the Brattleboro quadrangle, Vermont-New Hampshire: Vermont Geological Survey Bulletin, v. no. 32, p. 162. the older population associated with S, for which the 406 Ma Hibbard, J.P., Van Staal, C.R., Rankin, D.W., and Williams, H., 2006, Lithotectonic map of the Appalachian Orogen: Canada-United States of America: Geological Survey of Canada, “A” Series Map, n. 2096A, p. 2 Resolving between the various hypotheses and constraining Figure 2: Geologic map modied after Ratcli e and Armstrong (1995) show- burial metamorphism. Karabinos, P., 1999, Evidence for low-angle normal faulting in the Acadian Orogen: is the Chester dome a core complex in Vermont?: Geological Society of America,, Abstracts with Programs, v.31, p. A-370. ing the Chester and Athens Domes. The three white stars, one located in the age constrains the timing of formation. Therefore, this sample Karabinos, P., E. S. Mygatt, S. M. Cook, and M. Student, 2010, “Evidence for an Orogen-Parallel, Normal-Sense Shear Zone around the Chester Dome, Vermont: A Possible Template for Gneiss Dome Formation in the New England Appalachians, USA.” the timing of the deformation, particularly the formation of northeast of the Chester Dome, one in the southeast of the Athens Dome, and The Geological Society of America Memoir 206, pp. 183–203. experienced metamorphism during a c. 406 Ma deformation Karabinos, P., Macdonald, F.A., and Crowley, J.L., 2017, Bridging the gap between the foreland and hinterland I: Geochronology and plate tectonic geometry of Ordovician magmatism and terrane accretion the dome bounding shear zones, has yet to be completed. one in the southwest of the Athens Dome, show the locations of the samples event and then experienced deformation again c. 365 Ma. on the Laurentian margin of New England: American Journal of Science, v. 317, p. 515-554 that were taken. McWilliams, C. K. 2008. The tectonic evolution of the Connecticut Valley Synclinorium: Constraints from argon/argon thermochronology, uranium-lead geochronology, thermobarometry and thermal modeling (Doctoral dissertation, Indiana Universi- ty). Sutter, J.F., Ratcli e, N.M., and Mukasa, S.B., 1985, 40Ar/39Ar and K-Ar data bearing on the metamorphic and tectonic history of western New England, Geological Society of America Bulletin, v. 96, p. 123-136 Vance, D., and Holland, T., 1993, A detailed isotopic and petrological study of a single garnet from the Gassetts Schist, Vermont: Contributions to Mineralogy and Petrology, v. 114, p. 101-118