Provenance of Devonian-Carboniferous sedimentary rocks of the Tarija Basin, southern Bolivia: Implications for the geodynamic evolution of the southwestern margin of Gondwana

The western margin of SW Gondwana was a place of active convergence between the Gondwana shield and the oceanic lithosphere of the Panthalassa Ocean during most of the Paleozoic. However, several studies have indicated that especially to the north of latitude 22(degrees)S, the Devonian-early Carboniferous was a time of relative quiescence without magmatic activity, metamorphism, or deformation. This interval has been termed the "Devonian Problem." As the Devonian-Carboniferous is extensively well represented by over 4000 -m -thick sedimentary sequences in the Tarija Basin in the Southern Bolivian Subandean Zone, provenance analyses-U-Pb isotope analysis on detrital zircon, Sr-Nd whole -rock isotope analysis, and X-ray diffraction (XRD)-were conducted on stratigraphically controlled sedimentary units to attempt to constrain the tectonic setting of the basin during that time. U-Pb on zircon provenance analysis indicates that only the Carboniferous units show input from comparatively young sources (<14%), with ages between 420 Ma and 320 Ma, which can be correlated with a Devonian magmatic arc. The dominant source areas for Devonian-Carboniferous sediment were the Sierras Pampeanas to the southwest and the Arequipa-Antofalla Massif and Famatinian Arc to the west, besides a few zircon grains (<18% of dates) that were possibly derived from pre -Andean inliers with ages typically >1800 Ma. The combined Sr-Nd isotope and XRD results for the pelites and the multidimensional scaling (MDS) analysis indicate that the Carboniferous units most likely represent reworked material from older units, with a minor contribution from the western part of the basin. All currently available data lead us to propose that the Tarija Basin developed during the Devonian-Carboniferous in a foreland basin setting related to the convergent margin, with important glacial sedimentary input from the continent during the Carboniferous. The absence of a contribution from a concomitant magmatic arc for the Devonian units of the Tarija Basin and the scarce magmatic zircon input (<14%) into the Carboniferous units indicate a shift from flat -slab subduction during the Devonian to normal subduction during the Carboniferous following complete delamination of the flat slab. Our comparison of the detrital record for the Tarija Basin with that of the Parana, Chaco-Parana, Sauce Grande, Paganzo, Navidad Arizaro, Madre de Dios, and Karoo basins supports an active margin setting for the SW margin of Gondwana during this time interval.