New ⁴⁰Ar/³⁹Ar, major and trace element, and Sr, Nd and Pb isotopic data for the c. 24-7 Ma volcanic rocks from the Andean back-arc (35°S – 38°S) in the Mendoza and Neuquén (Argentina) regions shed light on the Miocene evolution of the back-arc of the Southern Volcanic Zone. Incipient shallowing of the subducting slab at ca. 20 Ma is inferred. The eruption of 24-20 Ma alkali olivine basalt up to 500 km east of the trench marks the beginning of a long-lasting magmatic episode with widespread volcanism north of the Cortaderas lineament following a regional magmatic hiatus lasting from 39 Ma to 26 Ma. Geochemical characteristics indicate a back-arc mantle devoid of arc-like components and with a strong OIB-like signature. They erupted at a time of extension along the Andean margin. After 20 Ma, basaltic – trachyandesitic lavas with a more depleted isotopic and incompatible trace element signature and showing evidence of a weak, but temporally increasing, arc component in the mantle source were erupted in a contractional regime. The increasing arc-influence in lavas erupted up to 500 km east of the trench indicates a shallowing of the subducting zone. A long period of volcanic quiescence followed lasting from ~17 to ~9 Ma. The reoccurrence of extensive magmatism in the Sierra de Palaoco provides evidence for a retreat of the shallow subduction zone towards the west during the Late Miocene. Evidence for the ending of the time of flat subduction comes from major- and trace element chemistry and Nd-Sr-Pb isotopic compositions. The arc-like component that dominates the geochemistry of the Palaoco rocks is absent in both the Early Miocene and the Pliocene-Pleistocene in the same area.
Young volcanic Provinces in the main arc, retro-arc and back-arc are further investigated by U-series analyses which confirm the fluid-enriched nature of arc-related rocks (U-excess are found in most rocks) and the more OIB-like nature of the Payún Matrú complex (Th-exsess is observed in all rocks). The fluid addition to the mantle source is modeled revealing timescales of 10 – 100 ka for the fluid enrichment. For the back-arc lavas model ages of differentiation from basalt to trachybasalt are on the order of 100 ka.