The Tectonic Setting of Active Andean Volcanism

Abstract

The Andean plate margin of South America is characterized by the seismicity, tectonism and magmatism associated with the descent of the oceanic Nazca plate below the South American plate. The active volcanism occurs within segments in south Colombia—north Ecuador (Northern Volcanic Zone, NVZ: ca. 5°N–2°S), south Peru—north Chile (Central Volcanic Zone, CVZ: ca. 16–28°S) and in south Chile (Southern Volcanic Zone, SVZ: south of ca. 33°S). The active volcanic zones are underlain by more steeply-dipping oceanic lithosphere (ca. 30°) than that below the intervening non-volcanic zones (ca. 5–10°). This correlation indicates that the processes responsible for Andean magmatism are initiated within the mantle. The petrogenesis of Andean magmas involves formation of basaltic melts, followed by assimilation and fractional crystallization (AFC) during crustal ascent. Within this framework the basaltic-andesite-rhyolite association of the SVZ evolved largely by fractional crystallization of basaltic magma, while the basaltic andesites and andesites of the NVZ may have evolved by AFC of basaltic magma. The more evolved andesite-dacite lavas of the CVZ result from fractional crystallization of basaltic magma in association with assimilation (AFC) or crustal melting. Crustal growth within the central Andes has been accompanied by an increase in the role of crustal material as a result of AFC, and crustal melting processes relative to fractional crystallization of mantle-derived basaltic magmas. There are hence important links between lithospheric structure, tectonic history and magmatism within the Andes.