THE GRENVILLE SUPEROROGENY REVEALED BY DETRITAL ZIRCONS IN APPALACHIAN RIVERS

Detrital zircons in sands from the east-flowing Susquehanna, Potomac, James and Savannah rivers, the west-flowing New River and Ohio River below its confluence with the Tennessee River and a 3 Ma heavy mineral sand in Virginia were dated in situ by U-Th-Pb isotopes using ELA-ICP-MS. Of the 905 grains analyzed, 825 (91%) were more than 95% concordant and over 70% of these yield ages between 950 and 1250 Ma, defining a continuous, long-lived magmatic episode that is skewed towards younger ages. These data contrast with previous geochronological studies of Grenville-age crystalline rocks in eastern Canada, the Adirondacks and the Appalachians that have identified discrete events rather than a continuum of ages. This apparent discrepancy can best be explained in terms of recycling of zircons from Paleozoic sedimentary rocks in the Valley and Ridge Province. Detrital zircons provide a broader geographic and lithologic sample base and therefore a more complete sampling of the Grenville orogeny than is possible from outcrop-based studies.

The overwhelming dominance of Grenville-age zircons in rivers draining the Appalachian Mountains, as well as the presence of significant numbers of Grenville-age zircons in the Missouri River and upper Mississippi River, implies the existence of a massive mountain belt that has dominated the sedimentary mass in eastern North America and beyond for the past 1.0 billion years. In the zircon age spectra, which is a measure of the intensity of crustal melting associated with an orogen, the Grenville dwarfs the collective Paleozoic orogenies in the Appalachians by a factor of five and therefore can be called a "Superorogeny".