NEW INSIGHT INTO INNER PIEDMONT (AND BLUE RIDGE) GEOLOGY FROM DETAILED GEOLOGIC MAPPING AND MODERN GEOCHRONOLOGY

Properly made geologic maps are reproducible data, although interpretation of the nature of contacts may change. Detailed geologic mapping of small areas that grow into larger islands of knowledge provide critical ground truth related to the nature and geometry of rock bodies and surficial deposits, kinematics of fault zones and terranes, and even the relationships between rock units and topography. We have gained considerable insight into the geology of the Inner Piedmont (and Blue Ridge) during the past several decades using the techniques of field geology, today augmented by modern digital technology. Moreover, detrital and igneous zircon, zircon metamorphic rim, and monazite TIMS and ion microprobe ages now provide insight into the ages of plutons and basement, timing of metamorphism, the provenance of rock units and terranes, and even led to the discovery of new terranes. The Cat Square terrane (CSt) was recognized through a combination of careful geologic mapping and detrital zircon geochronology in the Carolinas, and its continuity into west-central Georgia has been verified by these techniques. Demonstration of the lack of variability of the age of Grenville basement (1.15-1.0 Ga) in the southern Appalachians was facilitated by ion microprobe geochronology. Extension of the Acadian-Neoacadian orogeny into the southern and central Appalachians is the product of combined high quality mapping and modern geochronology. Timing of metamorphism has been better defined with ion microprobe dating of zircon rims and chemical dating of monazite, demonstrating that Taconian metamorphism occurred in the western and central Blue Ridge ~460 Ma, that metamorphism in the eastern Blue Ridge is Taconian and Neoacadian (360-350 Ma), and that metamorphism in the Inner Piedmont is Neoacadian and Alleghanian (325-300 Ma). Plutons in the western Inner Piedmont and eastern Blue Ridge are arc derived and range from 480-450 Ma, to Carboniferous and Permian, while CSt plutons range from 424 to 360 Ma, with many being anatectic.