PROVENANCE OF EASTERN BLUE RIDGE LITHOSTRATIGRAPHIC UNITS, NC–VA: IMPLICATIONS FOR A PERI-GONDWANAN SOURCE

The eastern Blue Ridge (eBR) in the southern Appalachians, part of the Jefferson or Tugaloo terrane, records the closing of the Iapetus ocean. Collectively, the eBR consists of metamorphosed clastic sedimentary rocks with lesser mafic and ultramafic rocks intruded by Paleozoic granitoids. In northwestern NC and southwestern VA, the eBR consists of different lithostratigraphic sequences that have been interpreted with different ages, provenances and tectonic affinities. In northwestern NC, the Ashe and overlying Alligator Back Metamorphic Suites comprise the eBR, whereas correlative units in southwestern VA include the Wills Ridge and Lynchburg Formations and Ashe and Alligator Back Metamorphic Suites.

This study presents results from 22 new U-Pb detrital zircon geochronology samples from two strike-perpendicular traverses across the eBR from the Wills Ridge and Lynchburg Formations and Ashe and Alligator Back Metamorphic Suites. Detrital zircons were analyzed by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). Twenty of the samples have similar detrital zircon age spectra with the greatest population of ages between 1300–900 Ma, with either a single peak at ~1030 Ma or peaks at ~1140 and ~1030Ma. Neoproterozoic zircon at ~700 Ma occur in several samples, with a sample from the Alligator Back having a major peak at ~715 Ma. Other minor or single grain analyses are 2550, 1650, 1600, 1400, and 800 Ma. Minor Paleozoic ages of 460, 400, and 340 Ma occur in a few samples and may represent metamorphic rims. Two samples from the Alligator Back Metamorphic Suite in NC contain a distinctly peri-Gondwanan provenance, dominated by Neoproterozoic zircon at 700­–500 Ma, peaks at 623, 606, 530 Ma, and variable minor Paleoproterozoic to Mesoproterozoic ages and rare Archean ages. The dominance of 1300–1000 Ma coupled with 700 Ma detrital zircon from eBR lithostratigraphic units suggest a Laurentian provenance, and that the closing of the Iapetus ocean in the Blue Ridge mostly involved the interaction of different peri-Laurentian terranes. The recognition of rocks with a peri-Gondwanan signature in the Alligator Back Metamorphic Suite suggests either the previously unrecognized interaction of a peri-Gondwanan terrane during a later stage in the Taconic orogeny or during an entirely different tectonic event.