THE MARBLE HILL HORNBLENDE SCHIST AMPHIBOLITE: THE KEY TO THE PALEOZOIC EVOLUTION OF THE WESTERN BLUE RIDGE

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) unconformably overlain by a turbiditic clastic sequence (Mineral Bluff Formation). At the base of this unconformity the Marble Hill Hornblende Schist (MHHS) amphibolite lies as a key marker unit in this central structure within the tectonic configuration of the WBR. Drill cores obtained within the Murphy Synclinorium present a thick sequence of the MHHS amphibolite that are inferred to have been basaltic flows.

The bulk-rock and zircon trace element systematics indicate that the source of the protolith was an alkaline basalt with Ocean-Island-Basalt characteristics, also supported by the zircon Lu-Hf isotopic compositions (ƐHf= +2.2-+3.7). The crystallization age of the basaltic protolith was determined through zircon U-Pb geochronology to have occurred at ca. 435 Ma. Hornblende ³⁹Ar-⁴⁰Ar geochronology of the MHHS amphibolite, including both multi- and single-crystal incremental heating results revealed plateau ages of ca. 340-385 Ma. Rutile U-Pb geochronology of the MHHS yielded a ca. 320 Ma age, both via discordant lower intercept and ²⁰⁷Pb corrected concordia calculations. Furthermore, the rutile grains were observed cross-cutting the hornblendes in thin-sections, thus interpreted to represent a younger phase.

The characterization of the MHHS amphibolite helps to better constrain the tectonic evolution and the timing of the amphibolite facies metamorphism of WBR. Results indicate a plume-related ocean floor magmatism potentially forming a seamount at the Laurentian passive margin during the Silurian. This is succeeded by its polymetamorphism through Devonian (Acadian Orogeny) and Carboniferous (Alleghanian Orogeny) events. The constraints for the MHHS amphibolite are consistent with a previously suggested widespread sub-Mineral Bluff regional unconformity formed on top of the Murphy Marble as a part of a successor basin development. The timing of these events requires that “isograds” mapped in the region of the Murphy Belt are polymetamorphic and represent combinations of effects from post-Silurian tectonic events.