Three tectonothermal pulses recorded in eclogite and amphibolite of the Eastern BlueRidge, Southern Appalachians

Large bodies of eclogite in the Eastern Blue Ridge Province of western North Carolina crop out immediately southeast of the Burnsville fault zone, an Acadian dextral strike-slip fault that separates Laurentian Mesoproterozoic basement and Neoproterozoic to earlyPaleozoic units (Western Blue Ridge) from an inferred accretionary wedge complex (EasternBlue Ridge). The peak metamorphic assemblage in eclogite is omphacite (Jd ₂₇ - ₃₅ ) + garnet (Alm ₄₈ Prp ₃₀ Grs ₂₂ ) + quartz + rutile ± zoisite ± zircon ± apatite ± sulfides ± Fe-Ti oxides; evidence of an amphibolite-facies overprint is regionally widespread but variably developed. Geochemical and isotopic characteristics of the eclogites and some surrounding amphibolites are consistent with their derivation from mid-ocean-ridge basalt protoliths. Zircon from the least-altered eclogite yielded a U-Pb, isotope dilution-thermal ionizationmass spectrometry age of 459.0 +1.5/-0.6 Ma. Multimineral plus whole-rock Sm-Nd isotopicdata indicate that Sm-Nd mineral systematics were disturbed, likely during amphibolite-facies metamorphism. Partly amphibolitized eclogite contains titanite with a U-Pb age of 394 ± 4 Ma; titanite from another sample shows disturbed U-Pb systematics with apparent ages between 448 Ma and 417 Ma. Both eclogite and partly amphibolitized eclogite contain rutile with a U-Pb age of ca. 334-340 Ma. These ages correspond broadly to the time of the Taconian, Acadian, and Alleghanian orogenesis, respectively, and match the timing of metamorphic events and pluton emplacement in the Eastern Blue Ridge Province. The Ordovician geodynamic setting in which the eclogite formed was possibly a complex plate arrangement of island arcs, accretionary complexes, rift basins, and rifted microcontinental blocks, perhaps similar to the Australia-Pacific.