Petrogenesis of Andean andesites from combined O–Sr isotope relationships

The descent of the oceanic Nazca plate below the western margin of South America is responsible for the seismicity, tectonism and magmatism characteristic of Andean-type continental margins. Within this geological setting there is much debate concerning the petrogenesis of the calc-alkaline volcanic association. A thermal model for the descent of oceanic below continental lithosphere suggests that melting of subducted oceanic crust, the asthenospheric mantle wedge, and the lower crust might occur 1 , and hence calc-alkaline volcanic rocks erupted at the surface in such regions might be derived from one or more of these sources. In the Andes, for example, Pichler and Zeil 2 have invoked partial melting of lower continental crust, while others 3–5 envisage petrogenesis of andesites from sub-crustal mantle sources possibly in conjunction with crustal contamination 6–9 . We present here new O-isotope data for volcanic rocks from three contrasted volcanic settings in Ecuador, north Chile, and north-west Argentina. We then consider the significance of observed O–Sr isotope relationships, and attempt to evaluate the important factors in the petrogenesis of Andean andesitic lavas.