The nature of the lithospheric mantle beneath orogenic belts is incompletely understood due to the paucity of mantle xenolith-bearing basaltic magmas in such regions. One such place where we are afforded the opportunity to study the deep lithosphere beneath an orogenic belt is in central Texas, United States. Mantle xenoliths occur in Late Cretaceous alkali magmas erupted through the remnants of the Appalachian-Ouachita structural belt. Here, we show that geochemical signatures in the form of enrichments in fluid-mobile trace elements (e.g., La) relative to fluid-immobile trace elements (e.g., Nb) are preserved in these xenoliths. We interpret these signatures to represent metasomatism by subduction-related fluids, which implies that the mantle xenoliths represent fragments of continental lithospheric mantle that served as the upper plate during a convergent episode. These observations suggest that some of the original continental lithosphere was preserved beneath the orogenic belt during collision and did not undergo wholesale delamination.