BASALTIC MAGMA GENESIS IN THE ANDEAN SVZ: INFLUENCE OF P, T, WATER CONTENT AND CRUSTAL THICKNESS

The Andean Southern Volcanic Zone (SVZ, 33˚S-46˚S) is well established as a natural laboratory for the understanding of subduction, mantle and crustal influences on magma chemistry. We compared basaltic rocks from the Azufre-Planchon-Peteroa (APP) volcanic complex, in the Transitional SVZ (TSVZ), basalts from Llaima and Villarrica stratovolcanoes in the Central SVZ and small eruptive centers (SEC) near Villarrica. APP is the northernmost volcanic center in the SVZ that produces basalts, and sits on crust ~45 km thick, while 30 km of crust underlies Llaima, Villarrica and SEC. Using the approach of Lee et al. (2008) we calculate primary magma compositions and pressure and temperature conditions for magma separation at varying water contents. Our results show that all calculated primary magmas have minimum pressure of ~ 1.3 GPa, and temperatures ranging from 1240 to 1380˚ C with decreasing H₂O content. In the TSVZ, this minimum depth coincides with the base of the crust, but is 10-20 km beneath the crust in the CSVZ. Depths of primary magma separation of TSVZ basalts are thus confined to depths for the mantle lithosphere between the boundary with the asthenosphere and the base of the crust, which suggests that the lithosphere is thermally abraded to lower crustal depths. Depths of primary magma separation in the CSVZ are all within depths appropriate for the lithosphere-asthenosphere boundary and do not reach crustal depths. Primary magma compositions for Villarrica and Llaima plot along common P-T-H₂O lines with magma separation for Llaima extending to greater depths. Assuming magma separation occurs at the base of the mantle lithosphere, this could indicate a thicker, less abraded lithosphere beneath Llaima. Basalts from the SEC’s plot at lower temperatures for a given pressure when compared to basalts from Villarrica. This shift implies that SEC basaltic magmas have lower water contents, assuming all these magmas arise from similar depth and temperature within the mantle. Lower primary water contents are consistent with geochemical features of the SEC’s, such as low fluid mobile element contents when compared to Villarrica and evidence for lower extents of melting. Primary magma separation at the base of the crust in the TSVZ is also consistent with many geochemical differences between basalts from the CSVZ and TSVZ.