Fluid-metasomatized mantle beneath the Ouachita belt of southern Laurentia: Fate of lithospheric mantle in a continental orogenic belt

The nature and history of the cratonic lithosphere beneath the southern margin of Laurentia are poorly understood due to the paucity of basement exposure in the Gulf Coast region. One place where we are afforded the opportunity to study the deep lithosphere of southern Laurentia is within the Balcones igneous province of Texas. Mantle xenoliths occur in Late Cretaceous alkali basaltic magmas erupted through the remnants of the Appalachian-Ouachita structural belt. 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 mantle xenoliths. We interpret these signatures to represent metasomatism by subduction-related fluids, suggesting that the mantle xenoliths represent fragments of continental lithospheric mantle that served as the upper plate during a convergent episode. These xenoliths may thus represent fragments of Laurentian lithospheric mantle that were stabilized by collisional processes during the continent-continent collisions associated with the Grenville orogeny and subsequent Paleozoic orogenies. These observations suggest that some of the original continental lithosphere was preserved beneath the orogenic belt during continent-continent collision and did not undergo wholesale removal or delamination. Alternatively, the fluid signatures may have been caused by Farallon flat-slab hydration of the overriding lithosphere, but such a scenario seems unlikely because flat subduction is thought to have culminated in the early Cenozoic and hence after the eruption of these xenoliths.