TEMPERATURE AND DEPTH CONSTRAINTS ON BASALTIC MAGMA GENESIS IN THE SVZ, AND GEOCHEMICAL EVIDENCE FOR SUBDUCTION-RELATED PYROXENITE IN THE SUBARC LITHOSPHERE

The Andean Southern Volcanic Zone (SVZ; 33-46˚S) has been the focus of research on continental arc volcanism, in part owing to its petrologically and geochemically segmented nature. Volcanoes in the Northern SVZ are mainly comprised of hornblende andesite, whereas to the south in the Transitional (TSVZ), Central (CSVZ) and Austral (ASVZ) volcanic zones basaltic rocks become more common. The Azufre-Planchon-Peteroa volcanic complex (34.5˚S; TSVZ) is the northernmost volcanic center in the SVZ erupting basaltic magma. In this study, we compare the geochemistry of basalts from Planchon to those of Villarrica, a well-studied center in the CSVZ, to interpret geochemical differences between the TSVZ and CSVZ. We used the approach of Lee et al. (2009) to calculate primary magma compositions and pressure and temperature conditions for magma separation for most mafic basalts from these centers. Our results indicate that primary magmas from both Villarrica and Planchon have minimum pressures of ~ 1.3 GPa and temperatures ranging from 1240˚C to 1330˚C. Using the gravity-based structure for upper and lower crust, mantle lithosphere, asthenosphere and slab proposed by Tassara et al. (2006), the minimum depth of magma separation for Villarrica is approximately 10 to 15 km below the crust, but at Planchon coincides with the base of the continental crust. Primary magmatic separation depths of Planchon basalts are confined to depths for the mantle lithosphere, suggesting it is thermally abraded to lower crustal depths. Depths of primary magma separation at Villarrica are within the lithosphere - asthenosphere boundary.

Many of the chemical features of Planchon basalts are consistent with a greater contribution from ancient mafic lower crust or mantle lithosphere compared with Villarrica - for example, ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf are lower, La/Yb is higher at Planchon. Modeling of REE data, using Villarrica basalt as a proxy for the subduction-zone magma endmember, shows that Planchon basalts could form the same source by lower extents of melting with a clinopyroxene-rich residue, but a more plausible model is that subduction zone magmas rising from the asthenosphere are interacting with ancient underplated subduction-zone basalt now residing in the mantle-lithosphere and lowermost crust as pyroxenite +/- garnet.