GEOCHRONOLOGY AND OROGENIC CONTEXT OF LITHIUM-CESIUM-TANTALUM PEGMATITES IN THE APPALACHIANS

Seventeen clusters of lithium-cesium-tantalum (LCT) pegmatites are spaced along the Appalachians. Reliable ages, which are needed to relate pegmatites to tectonic events, have proven elusive. We report new ages based on U-Pb CA-TIMS for zircon and columbite, ⁴⁰Ar/³⁹Ar for muscovite, and U-Pb LA-ICPMS and fission track for apatite. Although zircon ages typically have better resolution, many zircons in our LCT pegmatites are xenocrystic, making columbite the most robust geochronometer. Available data now show that LCT pegmatites formed in at least five episodes: 395±2 (Brazil Lake, Nova Scotia, published); 349 Ma (Foote, NC); 326-295 Ma (Palermo, NH); 274 Ma (Anderson, CT); and 264-260 Ma (Irish Pit, ME). These episodes are broadly contemporaneous with Devonian, Carboniferous, and Permian orogenic phases in the Appalachians; no LCT pegmatites are known from Ordovician or Silurian orogenies. On a Pangea reconstruction, Appalachian-Caledonian pegmatites stretch ~4500 km along strike. A 900-km-long LCT gap in the central Appalachians is puzzling because the metasedimentary and plutonic rocks here are similar to rocks directly north or south that host LCT pegmatites of many ages. The ages show no pattern that might reveal tectonic causes. We explore four explanations: (1) The presence or absence of LCT pegmatites could have been caused by along-strike segmentation of the Cambrian Laurentian margin into lower- and upper-plate sectors. (2) The Central Appalachian gap lies opposite the Reguibat promontory of West Africa, a configuration that perhaps could have thwarted LCT genesis during collision. (3) Laurentian-margin strata that would later be partly melted might have varied in trace-element fertility owing to inhertited, paleoclimate-related differences in sediment composition. (4) Synorogenic conditions may have varied along strike with latitude, and thus aridity, such that parts of the orogen featured high-altitude salars perched above magma chambers.