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In the Central Southern Volcanic Zone (CSVZ) of the Andes, the location of stratovolcanoes and monogenetic small eruptive centers (SEC) is controlled by the Liquiñe-Ofqui Fault Zone (LOFZ), a trench-parallel strike-slip feature of over 1000 km length. The geochemistry of basalts from SEC is different from those of stratovolcanoes, and are termed Type 2 and Type 1 basalts, respectively. In the region of Villarrica stratovolcano, contemporaneous SEC are more MgO-rich, and have greater light rare earth element (LREE) enrichment, lower 87 Sr/ 86 Sr and 143 Nd/ 144 Nd, and lower ratios of large ion lithophile elements (LILE) to LREE and high field strength elements (HFSE). A unique finding in this region is that basalts from one SEC, San Jorge, has Type 1 character, similar to basalts from Villarrica stratovolcano. Type 1 basalts from Villarrica and San Jorge SEC have strong signals from time-sensitive tracers of subduction input, such as high 10 Be/ 9 Be and high (238 U/ 230 Th), while Type 2 SEC have low 10 Be/ 9 Be and (238 U/ 230 Th) near secular equilibrium. Based on new trace element, radiogenic isotope and mineral analyses, we propose that Type 1 basaltic magma erupted at San Jorge SEC and Villarrica stratovolcano forms by melting of the ambient actively subductionmodified asthenosphere, while Type 2 SEC incorporate melts of pyroxenite residing in the supra-subduction zone mantle lithosphere. This scenario is consistent with the close proximity of the volcanic features and their inferred depths of magma separation. The pyroxenite forms from arc magma produced during earlier episodes of subduction modification and magmatism, which extend back >300 Ma along this segment of the western South American margin. Type 2 basaltic magmas may reach the surface during LOFZ-related decompression events, and they may also be a normal but episodic part of the magma supply to large stratovolcanoes, resulting in cryptic geochemical variations over time. The presence and mobilization of stored and aged subduction-related pyroxenite in the mantle lithosphere complicates the identification of subducted, mantle and crustal inputs to magma erupted in continental arcs.

Sun, Murong, "Geochemical variation among small eruptive centers in the central SVZ of the Andes : an evaluation of subduction, mantle and crustal influences" (2001 We have read this dissertation and recommend that it be approyd. 209xxi Chapter 1 Introduction Geochemical processes in subduction zonesSubduction zones are regions where relatively colder, denser and mechanically stronger lithospheric plates sink into the hotter and weaker asthenospheric mantle. It is well accepted that the subducted slabs play a very important role in driving the motion of plates at trenches due to their negative buoyancy, generally described as the "slab-pull force" (Forsyth and Uyeda, 1975; Carlson, 1981; Carlson et al., 1983). Subduction zones usually comprise the trench, the forearc region including accretionary complexes, the volcanic arc where subduction-related volcanism takes place, and the back arc region (usually associated with extension, faulting, rifting and magmatism).Magmas generated at subduction zones are geochemically distinct from those produced at mid ocean ridges or oceanic islands by their significant enrichment of LILEs (large ion lithophile elements) relative to HFSEs (high field strength elements). There is a useful link between the chemistry of arc volcanics and the processes of subduction, which continuously recycle crustal materials back into the interior of the Earth.Many important advances have been made over the past decades in the understanding of subduction zone magmatism. These advances address how elements are transported from their mantle source into the continental crust, how they are recycled by sediment subduction and addition, and how they are incorporated into arc magmas through hydrous fluids or melts. At present, most of the previous work concludes that arc magmas are generated by a three -component (mantle wedge, subducted oceanic crust and sediments) model, i.e., melting of mantle peridotite metasomatized with fluids or 1 melts contributed from subducted oceanic slab and/or sediments. At continental arcs such as the Andes, some arc samples may also have been modified by crustal processes such as intra -crustal fractionation and assimilation (Tilton and Barreiro, 1980;James, 1984), MASH (melting, assimilation, storage and homogenization) (Hildreth and Moorbath, 1988) or crustal foundering (Arndt and Goldstein, 1989). However, debate continues because there remain many uncertainties regarding the fundamentals of arc magma genesis, such as: 1) What is the composition of the pre-subduction mantle source of arc magmas?The mantle wedge overlying the subducted slab is usually considered to be like the mantle source of MORB or OIB, and its compositional variability along or across arc strikes has been extensively investigated using trace element or isotope tracers (Varne, 1985; Hickey et al., 1986;Hickey -Vargas et al., 1989; Hochstaedter et al., 1990; Pearce and Parkinson, 1993). Previous studies pointed out that mantle source heterogeneity is observed in many arc locations, e.g., evide...

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