KEYNOTE: THE ROLE OF SUBDUCTION EROSION IN PRODUCING TEMPORAL AND SPATIAL VARIATIONS IN ANDEAN MAGMA CHEMISTRY

Subduction erosion occurs at all convergent plate boundaries. Frontal subduction erosion results from a combination of erosion and structural collapse of the forearc wedge into the trench, and basal subduction erosion by abrasion and hydro-fracturing above the subduction channel. Subducted sediments and crust removed from the over-riding forearc wedge may be carried into the subarc mantle and incorporated into arc magmas by either dehydration of the subducted slab and the transport of their soluble components into the overlying mantle wedge source of arc basalts, and/or bulk melting of the subducted crust to produce adakites (Stern 2011 Gondwana Research 20: 284-308). In selected locations such as in Chile, Costa Rica, Japan, the Aleutian Islands and SW USA, strong cases can be made for the temporal and spatial correlation of distinctive crustal isotopic characteristics of arc magmas and episodes or areas of enhanced subduction erosion. For Andean volcanoes in southern compared to northern Chile, increasing ⁸⁷Sr/⁸⁶Sr from 0.7035 to >0.7055 and decreasing ε_Nd from +6 to -3 can be explained by increasing the subducted continental component in the mantle source region from 1 to 10% due to increasing rates of subduction erosion as aridity increases and sediment supply to the trench decreases northwards (Stern 1991 Geology 19: 78-81). For mafic mantle-derived magmas erupted at 34°S in central Chile between the early Miocene and present, which varied from initially tholeiitic basalts to ol-lamprophyres derived from progressively more hydrated mantle, similar temporal isotopic variations can be explained by a similar increase in the proportions of subducted crust added into the subarc mantle source region resulting from increasing rates of subduction erosion as the locus of subduction of the Juan Fernandez Ridge migrated southwards towards this latitude (Stern et al 2011 Andean Geology 38: 1-22). Intra-crustal assimilation (MASH processes) cannot explain these variations.