Seismological and Geochemical Evidence for Forearc Crust and Mantle Removed by late Neogene Andean Forearc Subduction Erosion Entering the Mantle Wedge and the Andean Arc Magma Source Suzanne Mahlburg Kay*. Adam R. Goss, Patrick Mulcahy Dept. Earth Atm. Sci., INSTOC, Cornell Univ. Ithaca, NY, 14853, USA *Contact email: [email protected] Abstract. Forearc subduction erosion has been suggested Geochemical evidence for recycling of forearc crust in to be a major process in shaping the Andean margin from Neogene arc magmas has been suggested in regions where Colombia to Patagonia, yet questions remain as to how the frontal arc has migrated into the foreland, in particular much removed forearc lithosphere reaches the arc magma in the northern part of the Southern Volcanic Zone on the source and the backarc. Geochemical evidence for forearc southern margin of the Chilean-Pampean flat-slab where crust in Neogene magmas comes from the northern part of the SVZ where the arc front migrated ~ 35 km eastward at the arc front migrated ~ 35 km eastward at 19-16 Ma and 19-16 Ma and ~ 50 km at 7-4 Ma and the southernmost another ~ 50 km at 7-4 Ma (Kay et al., 2005). A similar CVZ (27°S to 28°S) where the frontal arc migrated ~ 40-50 case for a pulse of forearc subduction erosion has been km from 8 to 3 Ma over the developing bend in the made in the southernmost Central Volcanic Zone on the subducting Nazca plate on the northern boundary of the northern margin of the flat-slab at 27°S to 28°S where the flat-slab region. Here, the average calculated forearc loss frontal arc migrated ~ 40-50 km from 8 to 3 Ma (Kay and 3 is 164 km /my/km over 6 million years. A similar amount of Mpodozis, 2002; Goss and Kay, 2009; Kay et al., 2011). forearc subduction erosion can be inferred in the flat-slab During this time, mafic adakitic andesitic magmas (Pircas region where low Vp/Vs ratios (1.65-1.72) in the mantle Negras lavas) were erupted between the ~26 to 8 Ma wedge could be due to forearc lithosphere entering the Maricunga and < ~ 3 Ma Central Volcanic Zone arc fronts. wedge at 8-3 Ma at the time of most rapid shallowing of the flat-slab. A case for a current accelerated pulse of The region is over the subducting Nazca slab where forearc subduction erosion can also be made in the revised contours to the Wadati-Benioff zone from the southern NVZ where the Carnegie ridge is subducting and southern PUNA seismic deployment show the slab bending erupting magmas can have adakitic signatures. eastward to form the northern boundary of the modern Chilean (Pampean) flat-slab region (Mulcahy et al., 2010; Keywords: Forearc subduction erosion, flat-slab, Central Mulcahy, 2012). A restoration, assuming a constant 300 Andes, arc magmatism. km frontal arc to trench gap since the early Miocene, suggests an average forearc loss of 164 km 3/my/km over 6 million years (based on Goss et al., 2012) in this region. 1 Introduction North-south Vp tomographic profiles from the southern Forearc subduction erosion has been suggested to be a Puna seismic experiment (Bianchi et al., 2012) along the major process shaping the Andean margin from Colombia southern part of the Central Volcanic Zone volcanic arc to Patagonia (e.g., Clift and Hartley, 2007). Evidence for reveal a low velocity anomaly beneath the Ojos del Salado this process has largely come from the forearc, and volcano and a high velocity anomaly just to the north, questions remain as to how much of the removed forearc which is most likely related to the effects of the subducting crust and mantle lithosphere reaches the arc magma source slab. One explanation of the high velocity region is that it region and the wedge under the backarc, and ultimately reflects forearc crustal and mantle lithosphere built up how much is recycled into the mantle. This crustal and under the arc as a result of subduction erosion associated mantle lithosphere can potentially be tracked using both with the frontal arc migration in this region (Kay and geochemical and geophysical methods as shown below in Mpodozis, 2002; Goss et al., 2012). examples from the northern part of the Southern Volcanic Zone, the southernmost part of the Central Volcanic Zone, Geochemical evidence for subducted eroded forearc crust the Chilean (Pampean) flat-slab region and the southern entering the arc magma source in this region comes from: part of the Northern Volcanic Zone. (1) transient steep REE patterns in adakitic arc magmas, which are not attributable to slab melting nor to any simple model with in situ garnet-bearing lower crustal residues, 2 Late Neogene Subduction Erosion on the (2) elevated Mg, Cr and Ni contents consistent with partial Northern and Southern Margins of the melts of subducted eroded crust reacting with the mantle Chilean-Pampean Flat-slab Region wedge prior to interacting with the overlying crust, (3) a marked step in isotopic enrichment in similar age mafic to 215 silicic magmas at the time of arc migration, and (4) plate being too low to release a sufficient volatile flux to temporal isotopic changes in primitive basaltic magmas hydrate the overlying mantle wedge. with near mantle δ18 O ratios (Goss and Kay, 2009; Kay et al. 2011; Goss et al., 2012). These geochemical and 4 Active Forearc Subduction Erosion in the isotopic features contrast with those in magmatic rocks that Southern Part of the Northern Volcanic were erupted in the nearly stationary Neogene central Zone Andean arc front north of 25°S, where only slow subduction erosion rates are inferred (Clift and Hartley, 2007). A case for a current accelerated pulse of forearc subduction erosion can be made along the southern Columbian and northern Ecuadorian margin where the Carnegie ridge is 3 Late Neogene Subduction Erosion in the being subducted beneath the Andean margin, and erupting Chilean-Pampean Flat-slab Region Northern Volcanic Zone magmas can have adakitic signatures. In this region, a broadened volcanic arc is An amount of forearc subduction erosion similar to that present with centers located from 260 to 380 km east of the inferred for the regions just north of 28°S and south of trench over a shallowed subduction zone (e.g., Bourdon et 33°S where the frontal arc was translated some 40 to 50 al., 2003). Heated debate goes on over the role of slab km to the east can also be inferred for the intervening melting versus contamination of mantle derived magmas in amagmatic Chilean-Pampean flat-slab region, in which the thickened garnet-bearing lower crust in producing the late Oligocene to Miocene arc front is currently some 260 adakitic volcanic rocks of the Northern Volcanic Zone km east of the Chile trench. If magmatism was still active (e.g., Bryant et al., 2006; Samaniego et al. 2010; Chiaradia in the region of the current flat-slab, the volcanic arc et al. 2011) The debate centers around the questions as to joining the Central and Southern Volcanic zones would whether or not substantial slab melting can occur in this most likely extend through the prominent Calingasta- region, how crustal contamination of thickened lower crust Uspallata valley in Argentina, which is some 300 km from can be compatible with a restricted range of relatively the trench. Thus the amount of frontal arc migration would primitive isotopic compositions in lavas erupted through be ~ 40 km, essentially equivalent to that in the adjoining different types of crust, and why some adakitic lavas occur Central and Southern Volcanic Zones and the removed where the crust seems too thin to stabilize residual garnet amount of forearc crust and mantle would be similar. In (see discussion in Bryant et al., 2006; Samaniego et al., analogy with the forearc subduction erosion models to the 2005, 2010). north (e.g., Goss et al, 2012) and south (Kay et al. 2005), much of this material would have entered the mantle A little discussed alternative is that these adakitic wedge under the flat-slab in a major pulse of forearc signatures could, at least in part, be due to crust that has subduction erosion between 8-3 Ma, which coincides with been removed by forearc subduction erosion, entering the the inferred period of most rapid shallowing of the subduction channel and participating in the arc magma Chilean-Pampean flat-slab (e.g., Kay and Mpodozis, production process. Notably, the Cono La Virgin andesite 2002). The flat-slab geometry could play a role in and some ignimbrites of the Cayambe Volcanic Complex concentrating eroded forearc material in the mantle wedge in the main Ecuadorian arc in the Eastern Cordillera at ~0° under the flattest part of the subducting Nazca plate. latitude (Samaniego et al., 2005) have geochemical similarities to the Pircas Negras lavas and some of the An intriguing and related question in this region is the Jotabeche/Incapillo ignimbrites (Goss et al., 2011) that origin of the low Vp/Vs ratios (1.65-1.72) in the mantle have been associated with the pulse of forearc subduction wedge above the flat-slab (Wagner et al., 2006), which are erosion components in magmatic rocks in the attributed to a combination of low Vp and high Vs (~4.75) southernmost Central Volcanic Zone (Goss and Kay, 2009, seismic velocities. Wagner et al (2006) argue that these Goss et al., 2012). Forearc subduction erosion components low Vp/Vs ratio are best explained by the presence of in the magma source could likewise provide an explanation orthopyroxene in the mantle wedge, and Wagner et al.