MAGMA SEPARATION DEPTHS AND LITHOSPHERIC STRUCTURE IN THE ANDEAN SOUTHERN VOLCANIC ZONE: IMPLICATIONS FOR BASALT MAGMA EVOLUTION ALONG A CONTINENTAL ARC

The Andean Southern Volcanic Zone (SVZ; 33-46˚S) has been the focus of research to understand the nature of continental arc magmatism, leading to diverse models for the sources of magma and its evolution in the mantle and crust. This study focuses on the conditions of primary basaltic magma separation in the SVZ. We report pressure and temperature conditions for primary magma separation for nine stratovolcanoes having mafic basaltic products from along the SVZ front: Osorno (41.1˚S), Casablanca (40.77 ˚S), Puyehue (40.59˚S), Villarrica (39.42˚S), Llaima (38.69˚S) and Antuco (37.4˚S) in the Central SVZ (37˚-42˚S) and Planchon (35.15˚S), San Pedro (35.98˚S) and Cerro Azul (35.65˚S) in the Transitional SVZ (34.4˚S-37˚S). Our calculated thermobarometric results indicate that separation conditions for primary magmas from both regions have similar minimum pressures between 1.30-1.61 GPa and hydrous temperatures of 1243-1333˚C. When compared with a recent model of lithospheric structure, this minimum depth is approximately 17-20 km below the Moho in the CSVZ, but within a few kilometers of the base of the crust in the TSVZ. The difference in crustal thickness between these two regions is well known and has been proposed as an important control on magma chemistry along the SVZ. Our results show that the depth of magma extraction along the SVZ is independent of crustal thickness, leading to an improved understanding of magmatic evolution. In the CSVZ magma separation occurs within the mantle, rather than at the Moho, and then ascends through the overlying mantle and lower crust with limited chemical interaction. Moreover, in both the TSVZ and CSVZ, magma separation depths are approximately 20 kilometers above the lithosphere-asthenosphere boundary, indicating that significant chemical and thermal interaction can take place between asthenospheric melt and a geochemically and petrologically diverse mantle lithosphere before interacting with the continental crust.