DISPARATE U-PB AND SM-ND AGES FROM THE LICK RIDGE ECLOGITE, EASTERN BLUE RIDGE PROVINCE, NORTH CAROLINA

The largest eclogite bodies in North America are in the Eastern Blue Ridge Province of western North Carolina. The eclogite forms lenses 200-300 m wide by up to 1 km long within foliated amphibolites and metapelites of the Ashe Metamorphic Suite. Our previous work has shown that zircons yielding an age of 459 +1.5/-0.6 Ma contain inclusions of eclogite-facies minerals, which indicates that zircon crystallized at near-peak eclogite conditions. Partly amphibolitized eclogite contains titanite that yielded a U-Pb age of 394 ± 4. Both eclogite and amphibolitized eclogite contain ca. 335 Ma rutile. These ages correspond broadly to the timing of the Taconian, Acadian, and Alleghanian orogenies, respectively.

In an attempt to define better the age of eclogite-facies metamorphism we conducted Sm-Nd analyses on whole rock, garnet, omphacite, diopside, hornblende, apatite, rutile, zircon, and ilmenite. Mineral fractions were small (4-30 mg), carefully handpicked to minimize inclusions, and analyzed as the oxide using the O2 leak valve technique for more precise analyses. As a test of the technique, the whole rock sample was analyzed as both the metal and oxide; showing only a 7 ppm discrepancy, which compares to the 5 ppm internal measurement uncertainty and is much less than the 20-40 ppm external reproducibility of three different standards analyzed as both the metal and oxide. The whole rock plus first six minerals lie on a reasonable isochron with a slope corresponding to 438 ± 9 Ma and an initial 143Nd/144Nd ratio of 0.51241. Ilmenite and zircon fall off of this isochron on a line with a slope suggestive of ca. 460 Ma and a much higher initial ratio of 0.51267.

These data are consistent with other studies that suggest zircon may be absent from the basalt protolith, crystallizing from Zr released by ilmenite during eclogite metamorphism. Neither zircon nor ilmenite were involved in later overprinting reactions and, thus, preserve their Sm-Nd compositions from peak metamorphism. The other minerals and whole-rock, however, were partially re-equilibrated during retrograde or overprinting events accompanied by a metasomatic REE influx, most likely from fluids derived from adjacent metapelites of the AMS, their migmatitic melts, and Devonian Spruce Pine granitoids.