CONSTRAINTS ON THE ARRIVAL OF THE MORETOWN TERRANE AND THE TACONIC OROGENY IN THE NEW ENGLAND APPALACHIANS: DETRITAL ZIRCONS FROM THE CRAM HILL FORMATION, VERMONT

In the New England Appalachians, the Ordovician Taconic orogeny resulted from the collision of the Gondwanan-derived Moretown terrane with the Laurentian margin. Considerable debate surrounds the timing of the collision, and the subsequent course of the orogeny. The Indian River Fm was deposited on the Laurentian margin, and quarry exposures in the Giddings Brook thrust sheet in Granville, NY, contain 466 Ma volcanic ash beds erupted from the Shelburne Falls arc, which formed on the Moretown terrane above an east-dipping subduction zone. The ash beds clearly indicate that the Moretown terrane and the Shelburne Falls arc were proximal to Laurentia by 466 Ma. Detrital zircon age spectra from meta-sedimentary rocks of the 475 Ma Hawley Fm in MA, an integral part of the Shelburne Falls arc, indicate that its detritus was derived from syn-depositional ~475 Ma eruptions in the arc, 550 to 750 Ma rocks typical of Gondwanan-derived terranes, and 950 to 1250 Ma rocks characteristic of the Grenville of Laurentia (Karabinos et al., 2017). These age spectra from the Hawley Fm suggest that the Moretown terrane and the developing Shelburne Falls arc were receiving sediment from the Laurentian margin by ~475 Ma, thereby pushing back their arrival by another 10 million years.

To test this hypothesis further, we analyzed detrital zircon populations from seven samples of rare quartzite and quartz-rich schist from the Cram Hill Formation in VT, a unit that is contiguous and correlative with the Hawley Fm; both are dominated by mafic rocks and graphitic schist. One sample contains four prominent Proterozoic peaks typical of the Moretown terrane. Another sample with only 28 grains has a single peak at ~470 Ma suggesting that the sediment was volcaniclastic and deposited during arc volcanism. One sample contains peaks typical of both Gondwanan and Grenville sources, but no Ordovician grains. Finally, four samples contain peaks consistent with Ordovician arc, Gondwanan, and Grenville sources. Thus, our new data are consistent with an Early Ordovician arrival of the Moretown terrane off the Laurentian margin. Subduction of distal Laurentian margin rocks (Rowe Schist and equivalents) must have already been in progress or begun soon after ~475 Ma, consistent with 460 to 470 Ma ⁴⁰Ar/³⁹Ar cooling ages of Laurentian rocks in northern VT (Laird et al., 1984).