GEOCHRONOLOGY AND OROGENIC CONTEXT OF NORTHERN APPALACHIAN LITHIUM-CESIUM-TANTALUM PEGMATITES

Lithium-cesium-tantalum (LCT) pegmatites crop out in nine clusters in the Appalachians from Connecticut to Maine. New U-Pb, ⁴⁰Ar/³⁹Ar, and fission track results are reported from the Anderson (CT), Clark Ledge (MA), Beryl Mtn., Parker Mtn., and Palermo (NH), and Emmons, Black Mtn., and Mt. Mica (ME) pegmatites. Precise dates are essential for understanding causes of pegmatite-related magmatism, which could include ordinary arc processes, overthickening during collision or subduction, slab breakoff or delamination during or after collision, or late collisional extensional collapse with decompression melting. However, concordant U-Pb ages are not easily obtained from the inclusion-ridden, metamict, and/or inherited zircons that are typical of LCT pegmatites. By using Chemical Abrasion -TIMS zircon geochronology we were able to obtain crystallization ages for six pegmatites. The results provide a new means of interpreting these LCT deposits in the context of the polyphase orogenic and exhumation history of the Appalachians. The oldest pegmatite, at 371 Ma, was part of the biggest magmatic pulse in the Northern Appalachians, the Neoacadian. The extent and magnitude of Neoacadian magmatism are evident in a new detrital zircon dataset from New England river and beach sands (10 samples, 1200 zircons). The youngest pegmatites (274 and 264 Ma) were intruded during the final (minor?) increments of Laurentia-Africa collision—the Alleghanian. Curiously, no LCT pegmatites can be tied to the Taconic or Acadian phases, both times of significant granitoid magmatism and major plate convergence. Instead, all LCT pegmatites in the New England Appalachians were emplaced during the latter half of the polyphase orogenic history. ⁴⁰Ar/³⁹Ar muscovite ages range from 339 to 251 Ma; all of these cooling ages predate Triassic-Jurassic continental breakup. Apatite fission-track pooled ages range from 133 to 38 Ma, all from the drift phase of Atlantic opening.