U-PB AGES OF METARHYOLITES OF THE CATOCTIN AND MOUNT-ROGERS FORMATIONS, CENTRAL AND SOUTHERN APPALACHIANS - EVIDENCE FOR 2 PULSES OF IAPETAN RIFTING

U-Pb ages of zircon from rhyolites of the Catoctin and Mount Rogers Fo rmations demonstrate that rifting of the Laurentian continent to form the Iapetus Ocean was a prolonged event spanning 200 my involving two important pulses of extrusive igneous activity. Rhyolitic flows of the non-fossiliferous Catoctin and Mount Rogers Formations, long correlat ed with one another on the basis of similar stratigraphic constraints, are dated at 564 +/- 9 Ma and 758 +/- 12 Ma, respectively. A hypabyss al felsic dike, intruding Middle Proterozoic (Grenville) granitic gnei ss basement and presumed to feed the Catoctin flows, is dated at 572 /- 5 Ma. These new data invalidate previous geochronology that combine d U-Pb data from both units to derive an upper intercept age of about 810 Ma. Ages of anorogenic granitoids of the Crossnore Complex (760-74 0 Ma), Robertson River Igneous Suite (730-700 Ma), and the Bakersville mafic dike swarm (734 Ma) are bracketed by the new ages presented her ein, but all are closer to the age of the Mount Rogers than the Catoct in. All these data suggest a history of rifting in the central and sou thern Appalachians spanning 200 my near the end of the Late Proterozoi c. The earliest pulses, represented by the Mount Rogers Formation and by granitoids, did not proceed to continental separation and are not r ecorded north of the Potomac River. The later pulse or pulses, which p roduced the areally more extensive Catoctin Formation, affected the ar ea from Newfoundland (ages of 617-590 Ma) to North Carolina and result ed in the opening of the Iapetus Ocean.