NEW APPROACHES TO DISCERN THE TIMING OF THE METAMORPHIC HISTORY OF THE HORSE CREEK SCHIST, NORTH CAROLINA EASTERN PIEDMONT

The Horse Creek schist from the Crabtree terrane in the NC eastern Piedmont is medium crystalline and well foliated. It has a conspicuous kyanite stretching lineation and microstructurally complex porphyroblasts of garnet. This metasedimentary schist lies within the Nutbush Creek fault zone of the Eastern Piedmont fault system. The Crabtree Creek pluton intrudes Crabtree terrane metasedimentary and meta-igneous rocks and yields discordant zircon ages from ca. 566-542 Ma, indicating the Horse Creek schist is Neoproterozoic in age. However, geochronologic constraints on the timing of Crabtree terrane metamorphism and schist exhumation are not well documented.

In this study, we collected Horse Creek schist from near Wake Forest, NC. We combined in-situ LA-ICP-MS U-Pb dating of monazite and rutile with petrological and microfabric data from upper-amphibolite facies pelitic schist, integrated with spatially resolved major and trace element garnet zoning to demonstrate their potential for elucidating reaction sequences during porphyroblast growth. In thin section, monazite grains were resolved using backscatter electron (BSE) imaging on a scanning electron microscope while rutile grains were resolved under the petrographic microscope. Phase equilibria calculations were performed with the software Theriac Domino in the model chemical system MnNCKFMASHTO, using the latest internally-consistent thermodynamic dataset. Pseudosection phase equilibria modeling combined with conventional geothermobarometry constrains a polyphase P–T history in which garnet crystallized after incipient core growth at 650˚C and 8-10 kbar. Multiple in situ monazite U-Pb ages from single grains indicate a unimodal population with a concordia age ~ 300 Ma while rutile ages indicate two populations ~ 335 Ma and a younger population ~ 263 (lower intercept isochron ages). Data are consistent with monazite growth during prograde (perhaps peak) and rutile with prograde (inclusions in garnet and kyanite) and retrograde portions of the PT path. U–Pb data on rutile and monazite as well as a single Ar–Ar-age (251 ± 1.16 Ma) on biotite (Blake et al. 2021) encompass a time span around 50-80 Ma. Combined, these approaches provide constraints on the framework of Crabtree terrane metamorphism and its timing in the southern Appalachian orogen.