HR: 1340h
AN: V23B-1434    [Abstracts]
TI: Interpretation of ⁴⁰Ar/³⁹Ar Age-spectra in Low-grade Polymetamorphic Rocks: The Importance of Petrologic constraints
AU: * Kunk, M J
EM: mkunk@usgs.gov
AF: U. S. Geological Survey, MS 926A National Center, Reston, VA 20192, United States
AB: ⁴⁰Ar/³⁹Ar age-spectra of white micas from low-grade polymetamorphic terranes can be difficult to interpret, because these low-grade rocks frequently have multiple generations of cleavage-forming white mica that grew at temperatures below their closure for diffusion of argon (~350°C). Under such conditions, each generation of cleavage-forming white mica will retain its original growth age. In addition, rocks from metamorphic terranes can also contain populations of detrital and diagenetic white micas which also have distinct ages associated with them. A final complication, frequently found in these low-grade polymetamorphic samples is the presence of inseparable, intergrown chlorite and the problems with associated ³⁹Ar recoil during irradiation of the samples, prior to sample analysis. The shapes of ⁴⁰Ar/³⁹Ar age spectra of white micas separated from such samples are frequently complex and are quite variable in shape. Some age spectra show a steady increase in age with increase in the temperature of release, while others have sigmoidal or saddle-shaped patterns. The age of most of the steps in such age spectra are mixtures of the various age populations of white mica in the sample, and as such are geologically meaningless. Nonetheless, useful constraints on the ages of at least some events recorded in these complex low-grade polymetamorphic rocks can be discerned with the help of petrographic observations, and by placing the results in a spatial context to test for reproducibility and/or the occurrence of meaningful patterns. If no chlorite is present and the sample contains no detrital white mica, an estimate of the maximum age of the youngest component and the minimum age of the oldest cleavage forming white mica populations can frequently be discerned. If the sample contains detrital white mica, a minimum age for its cooling or crystallization can be estimated. The presence of a small amount of intimately intergrown chlorite masks the age of the youngest white mica population, but the minimum age of the oldest generation of white mica may still be estimated. Samples from the Blue Ridge anticlinorium in northern VA indicate cleavage formation during both the Devonian and the Pennsylvanian. Similar, but somewhat more complex samples from the Potomac terrane in the eastern Piedmont of northern VA and MD indicate discrete tectonothermal events in the Ordovician, Silurian, and Pennsylvanian, while samples from the Westminster terrane in the westerm Piedmont of MD suggest cleavage formation in the middle Silurian, the Devonian and the Pennsylvanian. While the results are not of high precision, they are very useful at the orogenic level.
DE: 1100 GEOCHRONOLOGY
DE: 1115 Radioisotope geochronology
DE: 1140 Thermochronology
DE: 1194 Instruments and techniques
DE: 3600 MINERALOGY AND PETROLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting