HR: 12:05h
AN: T42A-05    [Abstracts]
TI: Frontal Arc Migration, Forearc Subduction Erosion and Adakitic-like Magmas: Examples From the Andean Margin and Implications for Other Arcs
AU: * Kay, S M
EM: smk16@cornell.edu
AF: Cornell University, INSTOC Snee Hall, Ithaca, NY 14853, United States
AU: Goss, A
EM: arg32@cornell.edu
AF: Cornell University, INSTOC Snee Hall, Ithaca, NY 14853, United States
AU: Kay, R
EM: rwk6@cornell.edu
AF: Cornell University, INSTOC Snee Hall, Ithaca, NY 14853, United States
AU: Mpodozis, C
EM: cmpodozis@sipetrol.cl
AF: SiPetrol SA, Avenida Vitacura 2736, Santiago, Chile
AB: Geochemical evidence for forearc subduction erosion linked to crustal modification of the mantle wedge is most evident in arc magmas erupted at times of frontal arc migration. The most convincing signals are from changes in isotopic ratios in mafic magmas erupted in the same place before and after arc migration, and from transient steep REE patterns and strong HFSE depletions in magmas erupted near times of arc migration. Such patterns are well seen on the margins of the Chilean flat-slab around 27S and 34S lat. where the arc front migrated about 50 km to the east in the last 8 Ma. The best evidence comes from increasing ⁸⁷Sr/⁸⁶Sr in mafic lavas that cannot be explained by enriched mantle, subducted sediments or in situ crust. Transient steep adakitic-like REE patterns at times of arc migration fit with a model in which forearc crust is transported down the subduction channel, enters the tip of the asthenospheric wedge and is incorporated into the arc mantle source. Sharp increases in ⁸⁷Sr/⁸⁶Sr ratios and transient steep REE patterns in Andean arc rocks between 21S and 26S lat. described by Haschke et al. (2002) and Mathews and Cornejo (2004) also occur in the final stages of magmatic cycles near times of frontal arc migration at about 130 (?), 85, 65-60 and 40-35 Ma. These patterns are interpreted by the authors as reflecting contractional crustal thickening at the end of the magmatic cycles. The sharp changes near the end of these cycles could additionally reflect crust introduced into the mantle source by subduction erosion at the time of arc migration. Transient adakitic-like arc magmas with steep REE patterns erupted elsewhere at times of arc instability and front migration could also reflect crustal contamination of the mantle source by forearc subduction erosion. Forearc subduction erosion often provides a better explanation for adakitic magmas in arcs with thin crust than slab melting as has been argued for Aleutian and Central America arc rocks.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Tectonophysics [T]
MN: 2007 Joint Assembly