ELECTRON MICROPROBE AGE MAPPING AND DATING OF MONAZITE: DECONVOLUTING POLYPHASE PALEOZOIC TECTONISM IN THE CENTRAL APPALACHIANS

Polydeformed and metamorphosed gneiss and schist of the Wissahickon formation in the Central Appalachian Piedmont in southeastern Pennsylvania are characterized by mineral assemblages that define a low-pressure, andalusite-sillimanite facies series metamorphic field gradient (400-500 MPa) which are variably overprinted by staurolite- and kyanite-bearing medium-pressure (~800 MPa) assemblages. East of the andalusite zone, where there is little evidence for early low-pressure metamorphism, the medium-pressure assemblages define a classic Barrovian sequence. Monazite was examined from rocks that exhibit a range of metamorphic histories including samples with little or no evidence of overprinting; partial to complete overprinting of early assemblages; and no evidence for early low-P metamorphism. Electron microprobe analysis shows that monazite of different ages is present in individual thin sections; that most monazite crystals exhibit compositional zoning; and that some but not all compositional domains represent distinct age domains. In situ analysis places textural constraints on the significance of monazite ages which facilitates assignment of absolute ages to specific metamorphic and deformational stages. Our results suggest that (1) the oldest monazite growth occurred during the Ordovician, 480 - 475 Ma; (2) considerable tectonism in the region is Silurian, with high-temperature low-pressure metamorphism spanning 442 - 431 Ma followed by deformation at 420 - 416 Ma; (3) medium-pressure metamorphism and deformation in terrain-bounding shear zones is Devonian, 380 - 375 Ma; and (4) late stage deformation and/or retrograde metamorphism is Mississippian, based on 365 - 360 Ma overgrowths. Our study demonstrates the value of the microprobe as a geochronological tool in analyzing multiply metamorphosed terrains.