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Lascar - Foto: Gerhard Prins | Wikimedia Commons

Andean magmatism from crystal to crustal scale or: how to make an andesite

Prof Gerhard Wörner | University of Göttingen

Abstract:  Magmatism in the Central Andes through time and space over more than 1500 km and from 35 Ma to present is compositionally and isotopically distinct. Episodes of voluminous plateau-forming ignimbrites follow intense phases of crustal shortening and uplift and document magmatic “flare-ups” are related to increased mantle and crustal melting which migrate from N to S from 25 to 10 Ma.  Compositional and isotopic signatures in andesites, that follow the ignimbrites, reflect distinct crustal domains and different lithologies that were assimilated at conditions of increasing pressure while the crust was further thickened to 70 km during Andean orogeny.

Isotopic compositions of magmas and Proterozoic basement constrains the proportion recycled crustal input at 20 to 50% with a juvenile magma production rate of 30 to 90 km3/km*Ma. The contribution of magmatic addition to crustal thickening during the past 35 Ma is less than 10%.

The evolution of individual magma batches, processes and endmembers of mixing in individual magma systems that feed stratovolcanoes of the Central Andes can be reconstructed from micro-meter zonation features in magmatic phenocrysts, high-resolution geochronology and U-series isotope analyses. On the other end of scales, these data distinguish “fast and deep” from “shallow and slow” endmember magma systems.