COMPOSITIONAL ZONING IN AMPHIBOLES: EVIDENCE FOR SYNTECTONIC GROWTH AND DISSOLUTION CREEP IN THE ASHE METAMORPHIC SUITE, NC

Detailed electron microprobe analyses along the length of single amphibole grains in lineated amphibolites of the Neoproterozoic Ashe Metamorphic Suite in the Mount Rogers area, VA/NC, exhibit compositional zoning. Amphiboles are calcic with compositions ranging from edenite to paragasite. Some zoning profiles display arcuate patterns, while other profiles tend to yield oscillating patterns along their length. Regardless of internal zoning patterns, boudin-necks and rims of crystals consistently trend towards lower Na, Ca, Ti, and octahedral Al cation concentrations while the opposite trend is observed in Si concentrations.

Thermobarometric calculations (Berman, 1991) using compositions of garnet, amphibole, and plagioclase in the presence of quartz yield temperatures of ~600˚C and pressures of ~10 kbars. Temperature estimates are consistent with previous work (McSween et. al., 1989), however pressure estimates are ~4 kbars higher. Application of the amphibole-plagioclase thermometer (Blundy and Holland, 1994) yields a range in temperatures consistent with those above, but also predicts higher temperatures in the cores, and lower temperatures in the boudin-necks and rims of single amphibole grains. We thus interpret this compositional zoning to reflect syntectonic growth during exhumation of the Ashe Metamorphic Suite. This suggests crystals grew during waning metamorphic temperatures with a range in temperature conditions from as much as 700˚C in the cores to 625˚C in the rims. Furthermore, this chemical zoning is identified parallel to the length of crystals defining a strong lineation suggesting a dissolution-creep mechanism during exhumation of the Ashe Metamorphic Suite.