Discrimination of the Milton belt and the Carolina terrane in the southern Appalachians: A Nd isotopic approach

The hypothesis that the Milton belt is a high-grade infrastructural equivalent of the immediately adjacent suprastructure of the Carolina terrane, the Carolina slate belt, was tested by comparing the Nd isotopic compositions of the two belts. Milton belt samples have ∈_Nd (600 Ma) values ranging from approximately -6.5 to +1, and depleted-mantle model (T_DM) ages ranging from 930 to 1690 Ma. Samples of the Carolina slate belt, collected from near the ductile shear zone that separates the Milton and Carolina slate belts, have ∈_Nd (600 Ma) values from +1 to +6, and T_DM ages from 745 to 1040 Ma. The Milton belt is therefore composed of isotopically more evolved crust than the northwestern portion of the Carolina slate belt. These results are not compatible with the simple infrastructure-suprastructure relationship envisioned for the Milton belt and the Carolina terrane. A more likely interpretation is that the belts are distinct crustal blocks that had unrelated histories prior to their assembly.