Relative Roles of Source Composition, Fractional Crystallization and Crustal Contamination in the Petrogenesis of Andean Volcanic Rocks

There are well established differences in the chemical and isotopic characteristics of the calc-alkaline basalt--andesite--decite--rhyolite association of the northern (n.v.z.), central (c.v.z.) and southern volcanic zones (s.v.z.) of the South American Andes. Volcanic rocks of the alkaline basalt--trachyte association occur within and to the east of these active volcanic zones. The chemical and isotopic characteristics of the n.v.z. basaltic andesites and andesites and the s.v.z. basalts, basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere. Conversely, the alkaline lavas are derived from basaltic parent magmas formed from mantle of `within-plate' character. Recent basaltic andesites from the Cerro Galan volcanic centre to the SE of the c.v.z. are derived from mantle containing both subduction zone and within-plate components, and have experienced assimilation and fractional crystallization (a.f.c.) during uprise through the continental crust. The c.v.z. basaltic andesites are derived from mantle containing subduction-zone components, probably accompanied by a.f.c. within the continental crust. Some c.v.z. lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing. The petrogenesis of the c.v.z. lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation, fractional crystallization, and magma mixing during uprise through the continental crust. Active Andean volcanoes of the calc-alkaline basalt--andesite--dacite rhyolite association occur within a northern (n.v.z.), central (c.v.z.) and southern volcanic zone (s.v.z.) (figure 9). Alkaline volcanic rocks occur within and to the east of these zones. The n.v.z. and s.v.z. lavas have chemical and isotope characteristics consistent with an origin by fractional crystallization of magmas derived from mantle containing subduction zone-derived components. By contrast, the alkaline lavas result from partial melting and fractional crystallization of magmas derived from mantle enriched in within-plate components. The c.v.z. lavas contain either a distinctive mantle-derived or a continental crustal component. The relative roles of mantle heterogeneity and crustal contamination have been evaluated by study of recent basaltic andesite lavas from the Cerro Galan area of NW Argentina (southeast of the c.v.z.). The chemical characteristics of these lavas indicate derivation from mantle with chemical characteristics intermediate between the source of the n.v.z.--s.v.z. and alkaline lavas. The basaltic andesite lavas show abundant petrologic, chemical and isotopic evidence for occurrence of crustal contamination (assimilation) combined with fractional crystallization (a.f.c.). We propose that these lavas evolved from parental olivine tholeiite magma which experienced contamination by varied crustal anatectic melts allied with fractional crystallization of olivine and pyroxene during uprise--storage within the continental crust. The combined effects of contamination and fractional crystallization upon basaltic magma rising through continental crust may obscure the chemical and isotope characteristics of the parental magmas of c.v.z. and Cerro Galan basaltic andesites. However, noting that none of the analysed lavas is unfractionated and the most basic Cerro Galan lavas show evidence of crustal contamination, we tentatively suggest that the parental magmas are silica-saturated basaltic magmas with isotopic characteristics intermediate between those of the most primitive c.v.z.--Cerro Galan basaltic andesites and the n.v.z.--s.v.z. alkaline lavas. The more evolved c.v.z.--Cerro Galan andesites--decites and rhyolites (ignimbrites) result from complex a.f.c. and magma mixing processes within the continental crust. Fieldwork in the Andes has been generously funded by the N.E.R.C. by grants over several years. Fieldwork in Argentina was financed by the Royal Society, the Servicio Minero Nacional and Servicio Geologico Nacional, the Royal Geographic Society, the Mount Everest Foundation and other bodies. We thank C. J. Hawkesworth and A. G. Gledhill for making new Sr isotope determinations on Cerro Galan basaltic andesites, and S. Moorbath for allowing us to quote further unpublished work. We are grateful to many colleagues in Chile and Argentina for help and logistic support during fieldwork. We thank T. Faulkner, A. G. Tindle, J. S. Watson and O. Williams Thorpe for skilled technical assistance, R. S. Harmon, C. J. Hawkesworth and M. A. Menzies for discussion and comment, Neil Mather for drawing the diagrams and Marilyn Leggett for typing the manuscript.