Geochronology and Tectonic Context of Lithium-Cesium-Tantalum Pegmatites In the Appalachians

Abstract

Abstract Lithium-cesium-tantalum (LCT) and niobium-yttrium-fluorine (NYF) pegmatites occur at intervals along the Appalachian orogen from Alabama to Nova Scotia. We report new U-Pb zircon (CA-TIMS), 40 Ar/ 39 Ar muscovite, U-Pb apatite (LA-ICP-MS), and apatite fission track ages for 14 pegmatites. Based mostly on these new results, we recognize a minimum of four ages of LCT pegmatites in the Appalachians. From oldest to youngest, these are: (1) Brazil Lake, Nova Scotia, dated at ca . 395 (U-Pb columbite) by Kontak et al . (2005); (2) Clark Ledge, Massachusetts, at ca . 371–339 Ma; (3) Palermo, New Hampshire, at ca . 326–294 Ma; and (4) Mount Mica, Maine, at ca . 264–260 Ma. The NYF pegmatites in the Appalachians must be of at least three ages: Topsham, Maine, dated at ca . 273 Ma (U-Pb monazite) by Tomascak et al . (1996); Morefield, Virginia, at ca . 253 Ma; and Hurricane Mountain, New Hampshire, which cross-cutting relations prove can be no older than ca . 183 Ma. The CA-TIMS method yielded high-resolution, concordant to near-concordant ages from most zircons, but at least 25 of the 41 zircons dated are interpreted to be xenocrysts or to have an inherited component. Thus a set of zircon ages from an LCT pegmatite must be treated as analogous to detrital zircon ages from a metasedimentary rock, which can be no younger than the depositional age of the protolith. All of the dated LCT pegmatites of the New England Appalachians are significantly younger than the two main orogenies in this sector of the orogen, the Taconic (Ordovician) and Acadian (Late Silurian to Middle Devonian). The pegmatites broadly correlate with the two younger orogenies, namely the Late Devonian to Mississippian Neoacadian orogeny and the Pennsylvanian to Permian Alleghanian orogeny. Thus the New England LCT pegmatites were emplaced during the latter stages of the assembly of Pangea. 40 Ar/ 39 Ar muscovite ages from 11 LCT and one NYF pegmatites range from ca . 339 to ca . 243 Ma and are interpreted to record cooling of the various pegmatites through ca . 425 °C. These ages are broadly consistent with the pattern of muscovite argon cooling ages from metasedimentary and granitic rocks across the region. Apatite fission-track ages for the pegmatites range from ca . 139 to ca . 68 Ma and are broadly consistent with regional results from plutonic and metamorphic rocks.