A SHORT SUPERCONTINENT CYCLE: RELICTS OF EARLY TO MIDDLE ORDOVICIAN CLOSURE OF THE IAPETUS OCEAN IN THE NEW ENGLAND APPALACHIANS

The tenure of the Iapetus Ocean is bracketed by the break-up of the supercontinent Rodinia and the Ordovician docking of Gondwanan terranes on the Laurentian margin. New U-Pb zircon geochronology demonstrates that the Shelburne Falls arc was built on a Gondwanan-derived microcontinent that is preserved in part in the Moretown Fm. of Vermont and Massachusetts. The Moretown Fm. was intruded by rocks as old as 502 Ma, and by the 475 Ma Hallockville Pond Gneiss, which is coeval with peak activity in the Shelburne Falls arc. Importantly, the structurally higher Hawley Fm. contains distinctly Grenvillian detrital zircon and was intruded by the 475 Ma Dell Trondhjemite. We suggest that metasediments in the Hawley Fm. captured detritus, shed from Laurentia onto the Gondwanan Moretown terrane during the closure of the Iapetus at or just prior to 475 Ma.

The closure of the Iapetus in New England has traditionally been dated by the appearance of foreland detritus in the ca. 457-453 Ma Mount Merino-Pawlet succession of the Taconic Allochthon. We suggest that the Taconic Allochthon restores between the Berkshire massif para-authochthon and the more distal Laurentian-derived Rowe belt, and that Early to Middle Ordovician peripheral foreland deposits are present in the Poultney Formation of the Taconic Allochthon and the Chazy Group of the Berkshire massif. The onset of Early to Middle Ordovician accretion is recorded at the base of the Chazy Group as a sandstone-capped unconformity. Late Ordovician foreland deposits preserved in rocks of the Taconic Allochthon formed after closure of the Iapetus Ocean, slab reversal, and subduction under North America during shortening above a west-dipping slab in a Cordilleran-style orogeny. These data indicate that the 70 Myr duration of continental rifting from ca. 610-540 Ma was followed by a brief 65 Myr history for the Laurentian passive margin and an even shorter 40 Myr interval of sea-floor spreading before the initiation of subduction in Iapetus. The Laurentian margin was active for the next 200 m.y. during amalgamation of Gondwanan terranes that culminated with the formation of Pangea.