GEOCHEMICAL AND SEISMIC EVIDENCE FOR CRUST REMOVED BY FOREARC SUBDUCTION EROSION ENTERING THE MANTLE WEDGE AND ARC MAGMA SOURCE

Forearc subduction erosion is generally considered to be a major process in shaping the Andean margin with most evidence coming from the forearc. Geochemical evidence for forearc crust entering the arc magma source has been inferred for northern Southern Volcanic Zone (SVZ) and southern Central Volcanic Zone (CVZ) magmas erupted on the margins of the Chilean-Pampean flat-slab where the frontal arc migrated ~ 40-50 km into the foreland at ~8 to 3 Ma. This process could play a role in the timing and localization of mineralization between 26° and 34°S on the margins and within the flat-slab. Geochemical evidence for partially melted subducted forearc crust entering the arc magma source comes from transient steep REE patterns, elevated Mg, Cr and Ni contents and marked isotopic steps in the full range of basaltic to rhyodacitic magmas. The case for forearc crust in the magma source is particularly strong for the glassy adakitic plagioclase-phenocryst free andesitic Pircas Negras magmas erupted between the ~26-6 Ma Maricunga and <3 Ma CVZ arc fronts, over a region where revised Wadati-Benioff contours show the slab bending on the northern boundary of the flat-slab. A well constrained restoration based on a constant 300 km arc-trench gap by Goss et al. (2012) suggests an average forearc loss of 164 km³/my/km from 8 to 3 Ma. A similar rate can be inferred in the intervening amagmatic flat-slab region (28°-33°S), where the ~27-6 Ma arc front is now ~260 km east of the trench and traces of the active CVZ and SVZ arcs join through the amagmatic Calingasta valley, ~40 km to the east and ~300 km from the trench. A related consequence could be low Vp/Vs seismic ratios in the mantle wedge above the flat-slab, which are attributed to orthopyroxene in the mantle wedge (Wagner et al., 2010). A possible way to generate this orthopyroxene is contamination of the mantle wedge by subducted forearc crust during the rapid shallowing of the flatslab at the time of frontal arc migration in a major pulse of forearc subduction erosion at 8-3 Ma that extended from at least 26° to 34° S.