Magma sources and tectonic setting of Central Andean andesites (25.5–28°S) related to crustal thickening, forearc subduction erosion and delamination

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 K 2 O andesites erupted from 25.5°S to 28.2°S exhibit a large variation in trace element and isotopic ratios, reflecting formation over an evolving slab, a crust thickening to 65–75 km and a frontal arc that migrated c. 45 km eastward at 8–3 Ma. Andesites at 28–26.8°S have the most variable and extreme heavy rare earth element (REE), high field strength element (HFSE) and Ba/La ratios and wt% Na 2 O, 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°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.