Temporal Evolution of Magmatism in the Northern Volcanic Zone of the Andes: The Geology and Petrology of Cayambe Volcanic Complex (Ecuador)

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 similar to 1.0 Ma. The young and still active Nevado Cayambe grew after a period of quiescence of about 0.6 Myr, from similar to 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.