Retroarc Volcanism as Evidence of Changing Subduction Zone Dynamics: New Insights into the Late Cenozoic Magmatic Evolution of the Southern Central Andes (34°S–38°S)

Abstract The late Cenozoic retroarc volcanism in the Southern Central Andes (34°S–38°S) exhibits pronounced geochemical variations, which provide critical insights into subduction-related mantle processes and magmatic systems. This volcanism is divided into three stages: an early Neogene stage (24–17 Ma), characterized by alkaline and transitional alkaline-subalkaline mafic magmas; a late Neogene stage (17–4 Ma), exhibiting andesitic arc-related magmas; and a Quaternary stage (4–1 Ma), comprising intraplate mafic magmas. This study presents new geochemical and geochronological data, along with petrogenetic modeling for the early Neogene stage, represented by the La Matancilla, Filo Morado, and Huantraico volcanic sequences. Combined with an extensive geochemical compilation, these data support a comprehensive tectono-magmatic model for the late Cenozoic retroarc volcanism. The La Matancilla and Filo Morado lavas are alkaline basalts derived from low-degree (1–5%) partial melting of an enriched, OIB-like, garnet-bearing mantle source without subduction signatures. Contrarily, the Huantraico lavas range from basalts to trachydacites with transitional alkaline-subalkaline compositions, reflecting a progressive shift from low-degree (<5%) partial melting of a deep garnet-bearing mantle source to shallower spinel-garnet-bearing mantle, influenced by subduction-related components. Based on geochemical signatures and regional tectonic reconstructions, we propose that the La Matancilla and Filo Morado lavas originated from subduction-driven asthenospheric upwelling and enhanced mantle convection promoted by the Nazca plate rollback during the late Oligocene. Subsequently, the Huantraico lavas record deep dehydration processes as the Nazca slab reached the lower boundary of the mantle transition zone during the early Miocene. This process generated a hydrous plume that ascended and triggered partial melting at shallower depths, imparting weak arc-like signatures. This model underscores the role of retroarc magmatism as a tracer of mantle dynamics during the evolution of the Southern Central Andes.