Single-crystal 40Ar/39Ar age variation in muscovite of the Gassetts Schist and associated gneiss, Vermont Appalachians
-
Willis E. Hames
,
John T. Cheney
Abstract
An exposure near Gassetts, Vermont, contains lithologies varying from staurolite-kyanite grade aluminous schists with paragonitic muscovite to potassic gneiss with phengitic muscovite. Singlecrystal laser fusion 40Ar/39Ar ages for paragonitic and phengitic muscovite yield similar distributions with ranges between 366 ± 4 and 326 ± 4 Ma. Intracrystalline ages vary from ca. 394 ± 4 to 330 ± 4 Ma. Thus, we find that the intracrystalline (core-rim) age distribution of relatively large, single crystals essentially encompasses the range of ages obtained through total fusion of smaller crystals, consistent with models for development of diffusion profiles and 40Ar-closure during cooling with a diffusion dimension controlled by the physical grain size. However, some of the larger crystals studied, particularly those with prominent microscopic defects (features readily evident such as internal grain boundaries and twin planes), yield relatively young ages and lack significant core-rim age discordance. Furthermore, the overall distribution of single-crystal ages in the two samples is bimodal, and we suggest that this age distribution reflects metamorphic deformation and recrystallization event(s) superimposed on early generation muscovite. Thus, the mean age of muscovite in these samples (typical of K/Ar and 40Ar/39Ar incremental heating analysis of bulk mineral separates) has little relationship to any single, hypothetical closure temperature. In view of the similar results we obtain for muscovite of contrasting composition, the net effects of variations in grain size, deformational character, and growth history are interpreted to be more important in forming the observed variations in age than are the chemical substitutions in these samples.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Why is amazonitic K-feldspar an earmark of NYF-type granitic pegmatites? Clues from hybrid pegmatites in Madagascar
- Metamorphosed Ordovician Fe- and Mn-rich rocks in south-central Maine: From peri-Gondwanan deposition through Acadian metamorphism
- Boralsilite, Al16B6Si2O37, and “boron-mullite:” Compositional variations and associated phases in experiment and nature
- Prograde muscovite-rich pseudomorphs as indicators of conditions during metamorphism: An example from NW Maine
- Wagnerite in a cordierite-gedrite gneiss: Witness of long-term fluid-rock interaction in the continental crust (Ile d’Yeu, Armorican Massif, France)
- Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions
- Multiple titanium substitutions in biotites from high-grade metapelitic xenoliths (Euganean Hills, Italy): Complete crystal chemistry and appraisal of petrologic control
- Proto-polymorphs of jimthompsonite and chesterite in contact-metamorphosed serpentinites from Japan
- Silicate garnet: A micro to macroscopic (re)view
- Monazite occurrence, chemistry, and chronology in the granitoid rocks of the Lachlan Fold Belt, Australia: An electron microprobe study
- Single-crystal 40Ar/39Ar age variation in muscovite of the Gassetts Schist and associated gneiss, Vermont Appalachians
- Single-crystal X-ray studies of trioctahedral micas coexisting with dioctahedral micas in metamorphic sequences from western Maine
- Tourmaline chemistry and the IIIB site
- The octahedral sheet of metamorphic 2M1-phengites: A combined EMPA and AXANES study
- Crystal chemistry of phlogopite from Vulture-S. Michele Subsynthem volcanic rocks (Mt. Vulture, Italy) and volcanological implications
- Beneath the Stillwater Complex: Petrology and geochemistry of quartz-plagioclasecordierite (or garnet)-orthopyroxene-biotite ± spinel hornfels, Mountain View area, Montana
- Correlation between crystallization pressure and structural parameters of phengites
- The modular structure of dovyrenite, Ca6Zr[Si2O7]2(OH)4: Alternate stacking of tobermorite and rosenbuschite-like units
- Environmental parameters affect the physical properties of fast-growing magnetosomes
- In situ Raman spectroscopy measurements of MgAl2O4 spinel up to 1400 °C
- Vibrational properties of δ-AlOOH under pressure
- Mössbauer spectroscopic study of synthetic leucophosphite, KFe2(PO4)2(OH)·2H2O
- Rhönite in Luna 24 pyroxenes: First find from the Moon, and implications for volatiles in planetary magmas
- New high-pressure B2 phase of FeS above 180 GPa
- Magnesium K-edge EXAFS study of bond-length behavior in synthetic pyrope-grossular garnet solid solutions
