ORIGIN AND PERSISTENCE OF TOURMALINE OVERGROWTHS IN PALEOZOIC CLASTICS RANGING IN AGE FROM UPPER CAMBRIAN TO MIDDLE DEVONIAN IN THE APPALACHIAN BASIN
Though the term authigenic for the overgrowths is in doubt, this unique varietal type still has significance in interpreting the provenance and recycling of sand grains in the Basin. The colorless overgrowths consistently display sharply demarcated growth zoning, grow only on the antilogus (+) end of the detrital core, and are formed by multiple coalescing crystallites that nucleated on the cores surface. Where nucleation failed, elongate voids developed, mostly near the cores surface, creating the zone of roots. Overgrowths from New York to West Virginia are all schorl-dravite and fall within 3 petrogenetic fields on a tourmaline Al-Fe-Mg ternary diagram: 1) metapelites and metapsammites coexisting with an Al-saturating phase and 2) without an Al-saturating phase; and 3) low-Ca metaultramafics and Cr- and V-rich metasediments. The great morphological and chemical similarities of the overgrowths strongly suggest that they may all have originated during a single metamorphic event prior to the Upper Cambrian and then were reworked across the Basin, remaining a prominent detrital component until the Mid-Devonian.