ORIGIN AND PERSISTENCE OF TOURMALINE OVERGROWTHS IN PALEOZOIC CLASTICS RANGING IN AGE FROM UPPER CAMBRIAN TO MIDDLE DEVONIAN IN THE APPALACHIAN BASIN

Tourmaline is a refractory component of sandstones and can persist through multiple cycles of reworking. P. D. Krynine and his students described recycling of tourmaline grains in the Appalachian Basin from Cambrian to Middle Ordovician strata. Tourmaline with overgrowths, described as authigenic, has received special attention by researchers. Krynine’s characterization of overgrowths as primary and recycled (abraded) in sandstones of the Basin promoted the idea of authigenic growth in unmetamorphosed clastics (a hypothesis we also once held); however, reassessment using electron microscopy and geochemical analysis does not support this view. Investigation of tourmaline from two units, the Upper Cambrian Gatesburg Formation and the Lower Devonian Oriskany Sandstone, previously reported to have primary authigenic overgrowths revealed that though pristine overgrowths are present, most overgrowths show evidence of abrasion. The extent of abrasion varies greatly even when grains are separated from extremely friable samples that crumble when touched or are disaggreated using only HCl to remove carbonate cement, showing that sample processing did not create the abrasion.

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 core’s surface. Where nucleation failed, elongate voids developed, mostly near the core’s 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.