Ediacaran bimodal volcanism in the southernmost Dom Feliciano Belt, Uruguay: Implications for the evolution of SW Gondwana

New zircon U-Pb-Hf-O, whole rock geochemical and Sr-Nd-Pb isotope data of Ediacaran volcanic rocks from the Cuchilla Dionisio (CDT) and Nico Pérez (NPT) terranes in Uruguay are presented. High-K calc-alkaline rhyolite from the Cerros de Aguirre Formation in the southern CDT formed at an active continental margin at 569 ± 5 Ma. A metasomatized mantle source is invoked for the formation of the rhyolitic melts. Cogenetic andesite and basaltic andesite have Sr-Nd isotope compositions that are close to that of enriched mantle 1, which is consistent with melt formation in the subcontinental mantle in the presence of crustal material. Coeval rhyolite in the northern CDT is inferred to have formed distally to the active margin. Rhyolite and trachyte of the Sierra de Ánimas Formation in the NPT formed at 589 ± 3 Ma and 586 ± 3 Ma, respectively. The NPT basalts originated in the mantle and may be related to a mantle plume. During ascent they were variably contaminated by continental crust. Zircon in NPT rhyolite have very low δ18O values that are ascribed to hydrothermal alteration and melting at shallow crustal levels. We propose that the late Ediacaran igneous activity in the NPT was related to anorogenic, extensional processes in an intraplate setting but the c. 20 million years younger volcanic activity in the southern CDT occurred in a supra-subduction setting above the subducting Brazilides ocean. We ascribe the bimodal intraplate volcanism in the NPT to far-field forces caused by slab-pull in response to eastward-directed subduction of the Brazilides ocean underneath Arachania, which is a rifted-off segment of the Kalahari Craton. Thus, late Ediacaran subduction-induced lithospheric stretching and horizontal extension of the eastern margin of the Río de la Plata Craton is proposed as the main cause for bimodal volcanism and the deposition of coeval volcano-sedimentary successions at the end of the Neoproterozoic.