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Journal of Petrology Volume 43 Number 2 Pages 199-217 2002
© Oxford University Press 2002
Adakite-like Lavas from Antisana Volcano (Ecuador): Evidence for Slab Melt Metasomatism Beneath Andean Northern Volcanic Zone
1IRD CENTRE DE BRETAGNE, BP 70, 29280 PLOUZANÉ CEDEX, FRANCE
2IRD ET UMR 6524, 5 RUE KESSLER, 63038 CLERMONT-FERRAND CEDEX, FRANCE
3UMR 6538, UNIVERSITÉ DE BRETAGNE OCCIDENTALE, BP 809, 29285 BREST CEDEX, FRANCE
4INSTITUTO GEOFÌSICO, EPN, APARTADO POSTAL 17-01 2759, QUITO, ECUADOR
Extensive sampling of the Antisana volcano in Ecuador (Northern Volcanic Zone of the Andes) has revealed the presence of adakite-like rocks throughout the edifice, i.e. rocks with geochemical characteristics close, but not identical, to those of slab melts. Two main volcanic groups have been distinguished, characterized by two distinct evolutionary trends. The AND group, mostly composed of andesites, shows the clearest adakitic characteristics such as high La/Yb and Sr/Y ratios and low heavy rare earth element (HREE) contents. The CAK group, composed of high-K andesites and dacites, displays less pronounced adakitic-like characteristics. Although the more basic rocks of each group are difficult to distinguish on many geochemical diagrams, a geochemical study shows that the evolution of the AND and CAK groups is dominated by different petrogenetic processes. The isotopic characteristics of the CAK rocks suggest that evolution of this group is dominated by a limited assimilation–fractional crystallization process within the granitic continental basement of the cordillera. In the AND group, the abundances of incompatible elements, such as Nb or HREE, suggest that the series was produced by a partial melting process in a mantle rich in garnet, amphibole and/or clinopyroxene. Such a mantle source has been demonstrated (experimentally and by exhumed mantle xenoliths) to be produced in subduction zones where slab melts react with and metasomatize the mantle wedge. In Ecuador, magmas erupted in the Western Cordillera (trenchward relative to Antisana volcano) are true adakites, suggesting that slab melts can be responsible for the metasomatism of the mantle wedge beneath the NVZ in Ecuador. If mantle convection can drag down this modified mantle beneath Antisana volcano, destabilization of metasomatic amphibole at appropriate pressures in this modified garnetiferous mantle can adequately explain the formation and the geochemical features of Antisana lavas.
KEY WORDS: subduction; adakite; metasomatism; Ecuador; AFC; Sr and Nd isotopes
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