Abstract

The Central Andean margin, where the name andesite originated, is the type locality for arc andesites erupted through thick continental crust. The < 25 Ma mid to high K2O andesites erupted from 25.5 degrees S to 28.2 degrees S exhibit a large variation in trace element and isotopic ratios, reflecting formation over an evolving slab, a crust thickening to 65-75 kmand a frontal arc that migrated c. 45 km eastward at 8-3 Ma. Andesites at 28-26.8 degrees S have the most variable and extreme heavy rare earth element (REE), high field strength element (HFSE) and Ba/La ratios and wt% Na2O, with the highest values in those erupted as the frontal arc migrated and the slab shallowed to the south. The required garnet-bearing, feldspar-free residue is generated in both the thick crust and the mantle wedge, into which crust was injected in a peak of forearc subduction erosion as the arc migrated. Andesites at 25.5-26.8 degrees S, east of the Puna plateau under which the slab shallowed at 18-7 Ma and then steepened as lithospheric delamination occurred, generally lack extreme REE and HFSE ratios. Their upper crust-like features reflect eruption in a mixed stress regime and incorporation of westward-flowing radiogenic crust from a region of extensive deep crustal melting to the east.