NEW CONSTRAINTS ON THE NATURE AND HISTORY OF THE MODOC ZONE FROM U-PB GEOCHRONOLOGY

The southern Appalachian mountain belt is one of Earth’s classic examples of continental collision. However, a polyphase history, subsequent extension and sedimentary overlap complicate the resolution of progressive Pangean amalgamation. Here we present new LA-ICPMS zircon U-Pb data from in and around the Modoc Zone that shed new light on accretion history. The Modoc Zone is a steeply dipping package of sheared metamorphic rocks in the eastern Piedmont of South Carolina and Georgia that separates migmatitic sillimanite-bearing rocks of the southern Kiokee Belt from the greenschist facies Carolina Slate Belt. Our detrital-zircon U-Pb age distributions from samples on opposite sides of the Modoc Zone are dominated by late Neoproterozoic-Cambrian grains, consistent with peri-Gondwanan origins akin to other parts of the Carolina Zone. Although there is some variation between samples in the relative abundance of subpopulations within the broad late Neoproterozoic-Cambrian distributions, these differences neither require nor preclude the Modoc Zone from being a boundary between terranes. Three detrital samples from the Modoc Zone have similarly dominant Neoproterozoic-Cambrian zircon ages, and two other samples therein also contain these ages as subordinate populations. Our weighted mean zircon U-Pb age of 423.9±4.0 Ma from the intrusive(?) Lake Murray Gneiss in the Modoc Zone overlaps a prior TIMS age for the same unit, supporting a late Silurian maximum age for ductile deformation, consistent with conventional interpretations of an ‘Alleghanian’ (Carboniferous-Permian) origin for the Modoc Zone. However, the presence in three Modoc Zone schist samples of abundant zircons with overlapping or younger ages than the Lake Murray Gneiss, but older than ‘Alleghanian’, complicates simple interpretation. Slightly (>5) to hyper-elevated (>100,000) U/Th in these zircons suggest the growth of new zircon during metamorphism. A mean weighted age of 387.8±2.1 Ma from zircons from a single Modoc Zone pelitic sample with high (>100) U/Th suggest near-peak metamorphic conditions in the Modoc Zone during the early ‘Acadian’ phase of the Appalachian orogeny. Together these results suggest an earlier onset of Modoc Zone development than usually considered, likely followed by an ‘Alleghanian’ phase.