Chemical signatures of magmas at times of frontal arc migration: Examples from the Central Andes and southern Central America

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Within the Central Andes (27?-28.5? S) and in southeastern Central America (7?-11? N), discrete episodes of late Miocene-Pliocene frontal arc migration were accompanied by backarc slab shallowing and increased rates of forearc subduction erosion. Arc lavas erupting during and following these short lived periods exhibit adakitic geochemical signatures indicative of high-pressure melting of mafic crust (i.e., steep REE patterns and elevated Sr concentrations). Geochemical and petrologic data from these lavas are used to address fundamental questions regarding the genesis of adakitic magmas spatially and temporally associated with arc migration.
Along both margins, enrichment in radiogenic isotopic ratios in late Miocene-Pliocene adakitic lavas compared to early-mid Miocene and Quaternary arc lavas cannot be attributed to melting of subducted oceanic crust or pelagic sediments. Instead, these trends are better explained by partial melting of mafic forearc crust transported to mantle depths during times of accelerated forearc subduction erosion. Mass balance calculations and isotopic modeling results agree with compositional estimates of the Central American and Chilean forearc and indicate that mantle contamination by eroded forearc crust is an inevitable and observable process. In arcs with thick crust such as the central Andes (> 60 km), partial melting of garnet-bearing lower crust contributes to the adakitic signature of lavas erupting during frontal arc migration. Strong HFSE-depletion characteristic of the Central Andean adakites is attributed to HFSE-bearing residual phases in equilibrium with the melt. Geochemical trends and near-chondritic Nb/Ta ratios support a change from a rutile-bearing eclogite to a garnet-bearing amphibolite residue consistent with cooler more-hydrous conditions that evolved over the shallowing slab during the late Miocene.
As magmatism diminished along the arc, rising adakitic magmas unreplenished by mantle-derived or lower crustal melts stalled within the Andean upper crust. Here, crustal assimilation and fractional crystallization subdued the primary adakitic signature and enriched the isotopic composition of the perched magma. After a 3-1 Ma period of effusive dome eruptions, rapid magma mixing caused the stalled system to explode at ~ 0.51 Ma. The resultant Incapillo Caldera and Ignimbrite mark the youngest volcanic event within the currently amagmatic flatslab segment of the Central Andes.

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NASA National Science Foundation

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American Geophysical Union


adakites; Andes; arc migration; Central America; andesite; caldera


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bibid: 6476457

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Goss, A.R. and Kay, S.M, 2006, Steep REE patterns and enriched Pb isotopes in southern Central American arc magmas: Evidence for forearc subduction erosion?: Geochemistry Geophysics Geosystems, v. 7, doi: 10.1029/2005GC001163

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