DETRITAL ZIRCON CONSTRAINTS ON THE AGE OF THE POPLAR MOUNTAIN GNEISS IN THE PELHAM DOME: IMPLICATIONS FOR THE TECTONIC SETTING OF THE BRONSON HILL ARC

Several controversies surround the tectonic significance of the Bronson Hill arc (BHA) and the Pelham Dome (PD) in the New England Appalachians including 1) whether the BHA is a peri-Laurentian or peri-Gondwanan arc, 2) the nature of the contact between the Ordovician arc rocks and the Neoproterozoic core rocks of the PD, and 3) whether the PD is a fragment of Ganderia or Avalon. We confirmed and refined the Neoproterozoic age of the Dry Hill Gneiss in the PD with an upper intercept U/Pb CA-ID-TIMS zircon age of ca. 607 Ma. Diatexite of the Poplar Mtn. Gneiss, which was also assigned a Neoproterozoic age, was sampled from a structural window in the PD. It contains detrital zircon grains with a Gondwanan provenance. However, cores of nine detrital grains gave LA-ICPMS ages between ca. 450 to 320 Ma. Metamorphic rims gave ages of ca. 280 Ma. The youngest detrital ages constrain the Poplar Mtn. Gneiss to a Carboniferous age, and require an Alleghanian tectonic contact between the arc rocks and the core rocks of the PD. The 607 Ma age of the Dry Hill Gneiss is indistinguishable from Avalonian basement, but also overlaps in age with Ganderian granites. A sample of the Partridge Fm from the BHA yielded a small (25 dated grains) detrital zircon population that includes Ordovician grains, presumably derived from arc volcanism, and Neoproterozoic grains suggesting a Gondwanan source. New evidence indicates that the SFA was built on a Gondwanan fragment preserved in the Moretown Fm, which also contains abundant ca. 607 Ma detrital grains. After collision of the SFA with Laurentia in the Early Ordovician, a west-dipping subduction zone developed under the newly accreted SFA and Moretown terrane. At least that portion of the BHA located in MA and southern NH. formed above this new subduction zone. Thus, although the BHA preserves evidence for a Gondwanan basement it may have formed along the Laurentian margin in the Late Ordovician. The Dry Hill Gneiss in the Pelham dome has been previously proposed to represent a Ganderian or Avalonian sliver thrust beneath the BHA. However, another possibility is that the Dry Hill Gneiss represents basement to the Moretown terrane. Viable models must include formation of a Carboniferous extensional or transtensional basin on a formerly accreted terrane, and thrusting of the Dry Hill Gneiss and BHA over these young rocks.