Devonian to Early Carboniferous Retreating-Advancing Subduction Switch in the Northwestern Patagonia Accretionary Orogen: U-Pb and Lu-Hf Isotopic Insights

In this contribution, we present new early middle Devonian igneous and metaigneous units with a major juvenile magmatic source input in the North Patagonian Massif, which were discovered through U-Pb and Lu-Hf zircon analyses. Afterward, we assessed their tectonic implications for northwestern Patagonia and then for southern South America, combining our results with available database information consisting of igneous crystallization ages and isotopic data of the Devonian to early Carboniferous magmatic units, tectonic-metamorphic analyses, and thermochronologic record. This study allows for distinguishing retreating and advancing subduction switching in northwestern Patagonia (38 degrees 30 ' to 44 degrees S) and a contrasting coetaneous evolution for basement outcrops exposed further north (27 degrees 30 ' and 37 degrees 30 ' S). The early middle Devonian (400-380 Ma) northwestern Patagonian magmatism is characterized by widespread magmatism and positive epsilon Hf-epsilon Nd linked to forearc and backarc magmatism that evolved within a retreating subduction stage. A tectonic switching toward advancing orogeny stage began in the late Devonian, evidenced by a lull in magmatic activity with a negative epsilon Hf-epsilon Nd trend, possibly contemporaneous with the first tectonic-metamorphic event in western Patagonia. An early Carboniferous magmatic gap, followed by the subsequent development of the main foliation in the basement during the Carboniferous-Permian period, denotes the acme of this contractional stage. In contrast, the Devonian period in the northern segment is characterized by mostly negative epsilon Hf-epsilon Nd values, reverse shear zone activity in the foreland, and an inboard magmatism migration, evidencing a compressive tectonic setting that changed to an extensional configuration in the early Carboniferous with widespread arc magmatism development.