Multi-element isotopic evolution of magmatic rocks from Caviahue-Copahue Volcanic Complex (Chile-Argentina): Involvement of mature slab recycled materials

In the Southern Volcanic Zone (Chile-Argentina), the active Caviahue-Copahue Volcanic Complex (CCVC) is approximately 30km east of the main N-S trending volcanic front, where the Nazca Plate subducts under the South American Plate. CCVC activity includes three major stages: the 5–4Ma old Ante-Caviahue series, the 2.6Ma old Caviahue series, and Copahue volcano that has been active since 1.2Ma ago. CCVC volcanism results from extension and slab steepening since 2.6Ma ago that produces the asthenospheric influx under Copahue. Here we investigate the link between the geochemical and multi-element (He, Li, N, Sr, Nd, Hf, and Pb) isotopic composition of CCVC magmas and evolution of the subduction regime since 5Ma ago. The CCVC magmatic source is characterized by a mid-ocean ridge basalt-like mantle signature (high 3He/4He=8 Ra) and a high δ15N (+5.8‰) related to subducted sediments. These data suggest a significant degree of N recycling, but low 4He recycling over time. Trace element and isotope modeling indicates that the influence of sediments is strongest in Copahue magmatism, whereas the mantle wedge contribution was strongest in Caviahue and Ante-Caviahue magmatism. The light rare earth element-enriched nature of Copahue rocks (compared to Caviahue and Ante-Caviahue rocks) is more likely due to incorporation of sediments from the slab rather than a very low degree of partial melting. The low δ7Li (0.44±0.31‰ to 1.42±0.17‰) of Copahue and Caviahue magmas indicates the contribution of a mature slab depleted in 7Li during previous dehydration events. Conversely, Ante-Caviahue rocks have a moderate δ7Li value (2.63‰), suggesting input from a less-dehydrated slab. This is consistent with trace element data, showing that the Copahue (and Caviahue) source is less enriched in fluid-mobile elements (low Ba/Th and U/Th ratios). We propose that the geochemical and isotopic evolution of CCVC magmas records the evolution of the subduction regime under the CCVC. Ante-Caviahue magmas were produced in an arc front extensional regime in which the shallow, extensively dehydrating slab delivered large amounts of fluids to the melting zone. Later slab steepening caused the volcanic front to migrate west of the CCVC, probably due to the beginning of subduction in the Mocha fault zone (south of CCVC) 2.5–5Ma ago. Crustal attenuation and asthenospheric influx under the CCVC produced Caviahue magmatism. Further progressive slab steepening and successive dehydration events produced a mature slab and dry mantle wedge that explain the particular signature of Copahue magmas.