U-PB DATING OF DETRITAL ZIRCONS IN THE CRAM HILL FORMATION, VERMONT: IMPLICATIONS FOR PALEOGEOGRAPHY AND OROGENESIS

The tectonic history of the Vermont Appalachians can be further understood with the aid of geochronological data from detrital zircons. The Rowe-Hawley Belt in central Vermont is a critical tectonic zone because it contains a major suture formed during closure of the Early Paleozoic Iapetan Ocean. The Hawley slices within this belt in Vermont include the Moretown and Cram Hill formations. These formations have been interpreted as metamorphosed fore-arc sediments of a late Cambrian - early Ordovician volcanic arc (the Shelburne Falls Arc) in the Iapetan Ocean. Recent data have identified the provenance of metasediments within the Moretown Formation as Gondwanan, contrary to previous work which suggested a peri-Laurentian source. To follow up on this work, the current study focuses on the Cram Hill Formation, which lies immediately above the Moretown and just below Silurian-aged rocks. In particular, we use detrital zircon ages to provide new information on the provenance of the Cram Hill metasediments by comparing those ages to known age signatures of possible source regions. Zircons from samples of the Cram Hill near and north of Montpelier, Vermont have been separated and analyzed on the LA-ICP-MS at Rensselaer Polytechnic Institute. Preliminary results show a prominent age peak at 500-600Ma, which suggests a Gondwanan zircon source, and several less prominent peaks around 1,000-1,200Ma, which imply a Laurentian zircon source. Another peak at 1,500-1,600Ma is of uncertain origin. These data suggest that there were several available sources, consistent with deposition of the Cram Hill sediments as western parts of the Iapetan basin closed. Other Cram Hill samples will be analyzed to confirm these initial results. All results will be used to further refine our understanding of the northeast Appalachian orogenic model.