Andean Adakites: Products of Slab Melting, Magma Evolution in Thickened Crust and Crustal Recycling by Forearc Subduction Erosion
Abstract
Adakites in the southern and central Andes show a residual garnet signature that can variously be related to local slab melting associated with subduction of hot oceanic crust at the Chile Triple Junction, widespread interaction of mafic magmas in regions of thickened crust, and episodic melting of crust removed by forearc subduction erosion, particularly at times of frontal arc migration. Among the most convincing slab-melt adakites on Earth are the late Miocene Cerro Pampa type dacitic adakites east of the Chile Triple Junction whose low 87Sr/86Sr ratios (0.7028-0.7033) and high Sr contents (up to 2300 ppm) are difficult to explain by any other mechanism. Elsewhere the appearance of transient extreme adakitic signatures at times of frontal arc migration can be explained by forearc subduction erosion and the evolution of magmas at deep crustal levels in a contractional regime provided the crust is thick. Transient steep adakitic-like REE patterns at times of arc migration fit with forearc crust being transported down the subduction channel, entering the tip of the asthenospheric wedge and being incorporated into the arc mantle source. Evidence for a genetic link for transient adakite signatures, arc migration and forearc subduction erosion comes from changing isotopic ratios in mafic magmas erupted before and after arc migration on the edges of the Chilean flat-slab near 27°S and 34°S (Kay et al 2005) where the arc front has migrated up to 50 km eastward in the last 8 Ma. The chemistry of these mafic magmas cannot be explained by enriched mantle or incorporation of subducted sediments or in situ crust. Sharp increases in 87Sr/86Sr ratios and transient steep REE patterns in Andean arc rocks erupted in the final stages of Cretaceous to early Tertiary magmatic cycles at 21°S to 26°S (see Haschke et al. 2002) can also be attributed to forearc subduction erosion. Forearc subduction erosion provides a better explanation for the formation of Aleutian and Central American adakitic magmas erupted through crust that is less than 40 km thick than slab melting or crystallization of garnet from hydrous magmas.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V34A..01K
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes;
- 1037 GEOCHEMISTRY / Magma genesis and partial melting;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 3619 MINERALOGY AND PETROLOGY / Magma genesis and partial melting