Geochemical signatures from the Atlantic coast of Tierra del Fuego and their provenance implications for Magallanes basin sediments

- Fuegian Andes mudstones show geochemical REE patterns typical to post-Archean upper crust.
- Older Cretaceous-middle Eocene samples suggest a Patagonian Batholith and mafic volcanic rock provenance.
- Younger middle-upper Eocene and lower Oligocene samples indicate Eastern Andean metamorphic complex provenance.
- Nd isotope data also show a coeval shift in εNd values from less negative to more negative at middle Eocene time.
- Data suggest an increased Eocene-Oligocene tectonic exhumation of the southern Andes.

The Magallanes foreland basin, located along the northern periphery of the Scotia Arc and extending up to 51°S to its northern terminus, contains a near-complete sedimentary record of Fuegian Andes tectonics since the Early Cretaceous. Herein we report trace- and rare-earth-element (REE) geochemistry and Nd isotope ratios from Upper Cretaceous to lower Oligocene mudstones of the eastern Magallanes basin, Argentina. The REE patterns of all of these samples are typical of average post-Archean upper continental crust. The older Upper Cretaceous to middle Eocene samples contain an overall lower concentration of light REE (smaller LaN:SmN ratio) than do the younger, middle-upper Eocene and lower Oligocene samples, although a strict temporal change is not observed. These REE patterns suggest a Patagonian Batholith and mafic volcanic provenance for the older successions and an Eastern Andean metamorphic complex provenance for the younger sediments. The lower Th:Sc ratio of the older samples suggests a mafic origin, whereas the higher Th:Sc ratio of younger samples indicates a felsic origin. Nd isotope data show a broadly coeval shift in εNd values from less negative to more negative. These data, combined with previously reported geochronological and mineral composition provenance analysis, suggest an increased Eocene-Oligocene tectonic exhumation of the southern Andes, concurrent with independent evidence of the opening of the adjacent Drake Passage.