Role of the subducted slab, mantle wedge and continental crust in the generation of adakites from the Andean Austral volcanic zone

All six Holocene volcanic centers of the Andean Austral Volcanic Zone (AVZ; 49-54 degrees S) have erupted exclusively adakitic andesites and dacites characterized by low Yb and Y concentrations and high Sr/Y ratios, suggesting a source with residual garnet, amphibole and pyroxene, but little or no olivine and plagioclase. Melting of mafic lower crust may be the source for adakites in some arcs, but such a source is inconsistent with the high Mg# of AVZ adakites. Also, the AVZ occurs in a region of relatively thin crust (<35 km) within which plagioclase rather than garnet is stable. The source for AVZ adakites is more likely to be subducted oceanic basalt, recrystallized to garnet-amphibolite or eclogite. Geothermal models indicate that partial melting of the subducted oceanic crust is probable below the Austral Andes due to the slow subduction rate (2 cm/year) and the young age (<24 Ma) of the subducted oceanic lithosphere. Geochemical models for AVZ adakites are also consistent with a large material contribution from subducted oceanic crust (35-90% slab-derived mass), including sediment (up to 4% sediment-derived mass, representing approximately 15% of all sediment subducted). Variable isotopic and trace-element ratios observed for AVZ adakites, which span the range reported for adakites world-wide, require multistage models involving melting of different proportions of subducted basalt and sediment, as well as an important material contribution from both the overlying mantle wedge (10-50% mass contribution) and continental crust (0-30% mass contribution). Andesites from Cook Island volcano, located in the southernmost AVZ (54 degrees S) where subduction is more oblique, have MORE-like Sr, Nd, Pb and O isotopic composition and trace-element ratios. These can be modeled by small degrees (2-4%) of partial melting of eclogitic MORE, yielding a tonalitic parent (intermediate SiO2, CaO/Na2O>1), followed by limited interaction of this melt with the overlying mantle (greater than or equal to 90% MORE melt, less than or equal to 10% mantle), but only very little (less than or equal to 1%) or no participation of either subducted sediment or crust. In contrast, models for the magmatic evolution of Burney (52 degrees S), Reclus (51 degrees S) and northernmost AVZ (49-50 degrees S) andesites and dacites require melting of a mixture of MORE and subducted sediment, followed by interaction of this melt not only with the overlying mantle, but the crust as well. Crustal assimilation and fractional crystallization (AFC) processes and the mass contribution from the crust become more significant northwards in the AVZ as the angle of convergence becomes more orthogonal.