ADAKITIC VOLCANISM WITHIN AN UNSTABLE ANDEAN ARC: THE PIRCAS NEGRAS AND DOS HERMANOS ANDESITES, CHILE/ARGENTINA (~27º S, ~68º W)

The adakite-like 7 to 4 Ma Pircas Negras (SiO₂ =57-62 %; La/Yb= 20-60; Sr = 600-900 ppm, Cr to 200 ppm) and ~8 Ma Dos Hermanos (SiO₂ =56-59 %; La/Yb= 57-61; Sr = 1200-1500 ppm, Cr to 60 ppm) andesites are located at the transition between the northern margin of the Chilean flat-slab and the southernmost Andean Central Volcanic Zone (CVZ). These lavas erupted in an unstable arc as the volcanic front broadened and migrated ~50 km eastward during the peak of Nazca plate shallowing (8-4 Ma). Extreme HFSE depletion (La/Ta= 90-100), chondritic Nb/Ta ratios (19-21), and low Zr/Sm ratios (25-30) in the older Dos Hermanos lavas contrast with less HFSE depletion (La/Ta = 40-80), sub-chondritic Nb/Ta (11-18), and higher Zr/Sm (30-45) in the younger Pircas Negras lavas. These data fit a progressive change during the course of arc migration from melts derived from a less-hydrous rutile-bearing eclogitic source to a garnet-bearing amphibolitic source. This pattern is consistent with hydration of the mantle wedge as the subducting slab shallowed and cooled. High ⁸⁷Sr/⁸⁶Sr (> 0.7055) and melting models for rutile-bearing eclogite and amphibolite from a MORB protolith show that the source of these adakitic lavas is not the subducting Nazca plate. The high-pressure source of these lavas is mafic arc-derived crust from in situ Andean lower crust and forearc rocks like the Jurassic La Negra basalts that were transported into the melt zone via forearc subduction erosion. Support for this conclusion comes from: (1) steep REE patterns and high La/Ta in the Dos Hermanos and Pircas Negras lavas that are a transient feature confined to the peak of arc migration; they contrast with lower La/Yb (< 20) and La/Ta (< 40) in <2 Ma CVZ lavas erupted through a similar thickness crust; (2) higher ⁸⁷Sr/⁸⁶Sr ratios in both the <2 Ma Incahuasi basalts and the Dos Hermanos and Pircas Negras andesites contrast with lower ratios in late Oligocene to early Miocene mafic lavas. These data can be modeled by a transient pulse of increased subduction erosion during frontal arc migration in which forearc lithosphere was transported into a progressively wetter mantle wedge and partially melted.