POLYMETAMORPHIC EVOLUTION OF THE WESTERN BLUE RIDGE TERRANE THROUGH ACADIAN AND ALLEGHANIAN OROGENIES

The Western Blue Ridge (WBR) terrane in the Southern Appalachians has a basement of Mesoproterozoic and mid-Paleozoic rocks and a Neoproterozoic-Paleozoic rift-to-drift sequence. Within the WBR, the Murphy Synclinorium includes a shelf carbonate bank (Murphy Marble) stratigraphically above the Brasstown Formation, and these are unconformably overlain by the turbiditic clastic sequence of Mineral Bluff Formation. Drill cores obtained within the Murphy Synclinorium present a thick sequence of amphibolite that are inferred to have been basaltic flows and are locally named the Marble Hill Hornblende Schist (MHHS). Although it is presently in the overturned limb of a synform, the amphibolite is stratigraphically above the Dean Formation and below the Murphy Marble in a restored section.

The protolith of the MHHS has an OIB-type chemical signature and crystallized at ca. 435 Ma, as revealed through our new zircon U-Pb geochronology. Whole-rock and zircon trace element systematics indicate that the source of the protolith was likely basalt with enriched mantle chemical characteristics, potentially part of a seamount.

Hornblende ³⁹Ar-⁴⁰Ar geochronology of the MHHS, including both multi- and single-crystal step heating results revealed plateau ages of ca. 340-385 Ma. The Ar release spectra also display the effect of extraneous ⁴⁰Ar in hornblende, causing the apparently older ages for some of the samples analyzed. However, results are Devonian to early-Carboniferous. Furthermore, rutile grains were observed cross-cutting the hornblendes in thin-sections, representing a younger phase. This observation was supported by the ca. 320 Ma U-Pb ages obtained from the rutile grains. We interpret these ages to indicate a partial overprint during the late-Carboniferous.

These new results help to better constrain the tectonic evolution of the MHHS and the timing of the amphibolite facies metamorphism of WBR. Results are consistent with OIB-like ocean floor magmatism at the Laurentian margin during the Silurian, followed by previously proposed successor basin development supported by palaeontologic evidence from the Murphy Synclinorium. New U-Pb and Ar-Ar ages indicate a Devonian (Acadian Orogeny) tectonothermal event with a partial overprint in Carboniferous (Alleghanian Orogeny).