SQUEEZING BLOOD (OR ZIRCONS) FROM A STONE: A SILURIAN U-PB ZIRCON AGE FOR MAFIC METAVOLCANIC ROCKS CONSTRAINS TIMING OF SUPRASUBDUCTION ZONE TECTONICS, SUCCESSOR BASIN FORMATION, AND METAMORPHISM IN THE SOUTHERN APPALACHIAN BLUE RIDGE

The Marble Hill Hornblende Schist (MHHS) is a stratabound unit found at the base of the youngest cover sequence in the polydeformed and metamorphosed southern Appalachian western Blue Ridge. It lies unconformably above the Laurentian margin Cambrian drift sequence (Murphy Marble and Nantahala-Brasstown Formations) at the base of the ~2.5 km thick Mineral Bluff Group (MBG) successor basin where it is in sharp, pre-metamorphic contact. The MHHS is up to 100 m thick, laminated in places with alternating plagioclase and hornblende-rich layers and in massive, less schistose portions, contains igneous textures including relict clinopyroxene.

The MHHS protolith is an alkaline basalt that has geochemical characteristics of ocean island basalt. Trace element data is similar to that of alkaline back-arc basalt fields (e.g. East China Sea/Sea of Japan)—enriched in large ion lithophile elements and elevated high field strength element concentrations.

Novel mineral separation methods were used to identify hundreds of zircon grains and grain fragments that have anhedral/irregular to subhedral morphology, but also contained rounded grains, the majority of which are interpreted as xenocrysts.

Over 150 individual zircon U-Pb analyses were conducted on a sample of MHHS collected 30-40 m above the sub-MBG unconformity. Two peak age modes are present in this sample at ~1015 Ma and ~424 Ma, the younger of which we interpret as its crystallization age. Thirty U-Pb analyses each of subhedral zircon collected from two feeder dikes/sills that intrude the Murphy Marble yielded similar results. One feeder dike/sill yielded only zircon with an age ~424 Ma, whereas a second contained only xenocrystic zircon > ca. 1.0 Ga. The ~424 Ma zircon show typical igneous zoning and U/Th <10. Thus, we interpret the MHHS to have a Silurian crystallization age and the MBG successor basin to be Silurian or younger.

Regional Barrovian metamorphic isograds overprint these successor basin units and postdate the Taconic orogeny. We argue that the ~ 424 Ma MHHS and younger MBG rocks are genetically related to other suprasubduction units in the structurally higher eastern Blue Ridge and Piedmont terranes, with the MHHS intruding and erupting atop early Paleozoic, Laurentian margin shelf rocks at the distal edge of the Wedowee-Emuckfaw-Dahlonega back-arc basin.