Journal of Petrology Advance Access originally published online on June 3, 2005
Journal of Petrology 2005 46(11):2225-2252; doi:10.1093/petrology/egi053
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Temporal Evolution of Magmatism in the Northern Volcanic Zone of the Andes: The Geology and Petrology of Cayambe Volcanic Complex (Ecuador)
1 DEPARTAMENTO DE GEOFÍSICA, ESCUELA POLITÉCNICA NACIONAL, AP. 17-01-2759, QUITO, ECUADOR
2 UNIVERSITÉ BLAISE PASCAL, LABORATOIRE MAGMAS ET VOLCANS, UMR 6524, 5 RUE KESSLER, 63038 CLERMONT-FERRAND, FRANCE
3 IRD, UR 031, LABORATOIRE MAGMAS ET VOLCANS, 5 RUE KESSLER, 63038 CLERMONT-FERRAND, FRANCE
4 IRD, UMR GÉOSCIENCES AZUR, UNIVERSITÉ DE NICE-SOPHIA ANTHIPOLIS, PARC VALROSE, 06108 NICE CEDEX 2, FRANCE
5 UMR 6538, UNIVERSITÉ DE BRETAGNE OCCIDENTALE, BP 809, 29285 BREST, FRANCE
RECEIVED JANUARY 31, 2003; ACCEPTED APRIL 15, 2005
In the Northern Volcanic Zone of the Andes, the Cayambe Volcanic Complex consists of: (1) a basal, mostly effusive volcano, the Viejo Cayambe, whose lavas (andesites and subordinate dacites and rhyolites) are typically calc-alkaline; and (2) a younger, essentially dacitic, composite edifice, the Nevado Cayambe, characterized by lavas with adakitic signatures and explosive eruptive styles. The construction of Viejo Cayambe began >1·1 Myr ago and ended at 
1·0 Ma. The young and still active Nevado Cayambe grew after a period of quiescence of about 0·6 Myr, from 0·4 Ma to Holocene. Its complex history is divided into at least three large construction phases (Angureal cone, Main Summit cone and Secondary Summit cone) and comprises large pyroclastic events, debris avalanches, as well as periods of dome activity. Geochemical data indicate that fractional crystallization and crustal assimilation processes have a limited role in the genesis of each suite. On the contrary, field observations, and mineralogical and geochemical data show the increasing importance of magma mixing during the evolution of the volcanic complex. The adakitic signature of Nevado Cayambe magmas is related to partial melting of a basaltic source, which could be the lower crust or the subducted slab. However, reliable geophysical and geochemical evidence indicates that the source of adakitic component is the subducted slab. Thus, the Viejo Cayambe magmas are inferred to come from a mantle wedge source metasomatized by slab-derived melts (adakites), whereas the Nevado Cayambe magmas indicate a greater involvement of adakitic melts in their petrogenesis. This temporal evolution can be related to the presence of the subducted Carnegie Ridge, modifying the geothermal gradient along the Wadati–Benioff zone and favouring slab partial melting.
KEY WORDS: adakites; 40Ar/39Ar dating; Cayambe volcano; Ecuador; mantle metasomatism; Andes
* Corresponding author. Fax: +593 2 256 7847. E-mail: Pablo.Samaniego{at}ird.fr
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