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Poster Final 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 diraction 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.
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