APPLICATIONS OF LASER 40AR/39AR DATING TO STUDIES OF APPALACHIAN METAMORPHIC EVOLUTION, EXAMPLES FROM NEW ENGLAND AND THE SOUTHWESTERN BLUE RIDGE

Laser 40Ar/39Ar techniques for determining the age of single crystals, and even areas within crystals, provide a powerful means for constraining the crystallization and cooling history of metamorphic rocks. Several recent case studies for laser 40Ar/39Ar dating have utilized muscovite and focused on resolving complex, polymetamorphic evolutions or slow-cooling histories in the Grenvillian, Taconian, and Acadian belts of the northern Appalachians. In one spectacular example, muscovite from the summit of Mt. Washington, New Hampshire (elevation 6288'), yields ages ranging from ~410 Ma (in the cores of crystals) to ~300 Ma (along crystal edges), which is interpreted to document the mid-crustal 'residence' of this terrane from Early Devonian to Carboniferous time. Minerals from the tops of the highest mountains hold the potential to record the oldest cooling ages in a given metamorphic terrane, assuming slow cooling and a simple geotherm. We therefore have undertaken a study of muscovite from Mt. Mitchell, North Carolina (elevation 6684'), and vicinity in order to more fully evaluate the regional relationships between Taconian and Acadian metamorphism. Samples of aluminous gneisses at the summit of Mt. Mitchell contain early-generation deformed pegmatites with coarse muscovite, coarse fabric-forming muscovite, and late-generation, crosscutting pegmatites. Laser 40Ar/39Ar analyses of muscovite phenocrysts from the early-generation pegmatites yields ages that vary from ca. 385 Ma to 325 Ma. These ages generally decrease from core to rim, in a manner consistent with volume diffusion and loss of radiogenic 40Ar, however the crystals studied thus far also appear to have lost 40Ar through deformation and introduction of crystal defects. Collectively, the data obtained thus far from a few samples at the summit of Mt. Mitchell is consistent with an Acadian history of pegmatite generation, high temperature metamorphism, and subsequent protracted Devonian-Carboniferous cooling as previously outlined for this region. Our continuing studies in the western Blue Ridge will be designed to further resolve the duration of Taconian slow cooling and the extent of Acadian metamorphic overprint.