Oxygen Isotope Evidence for the Role of Crustal Contamination in the Evolution of the Chalupas Caldera System, Northern Andes, Ecuador.

Of particular interest to the study of the generation and eruption of large volumes of silicic magma are the relative roles of crustal assimilation, fractional crystallization, magma supply and magma storage. The Chalupas caldera, located in the eastern Cordillera of the Ecuadorian Andes is a 12km diameter caldera which formed approximately 200 kya with the eruption of ~200km³ of rhyolitic pyroclastic material. Continental crust beneath the northern volcanic zone (NVZ) of the Andes is not as thick as that of the Central Volcanic Zone (CVZ) and consequently, NVZ lavas do not show strong chemical signatures of crustal contamination. In addition, systems that generate large volumes of rhyolite are relatively rare. However, isotopic and trace element data from many NVZ volcanic centers, including the Chalupas caldera system, require the addition of some crustal material. Here we present new oxygen isotope data from the Chalupas system and some other Ecuadorian systems that are consistent with the involvement of crustal material during the early evolution of the magmas. Lavas from the Chalupas system have δ¹⁸O = +7.0 to +7.8, a range of values significantly elevated above the average value for island arc lavas (+6.0 +/- 0.3) and at the upper end of the range reported for lavas from the NVZ (+6 to +7.7). The high δ¹⁸O of the Chalupas lavas can only be explained by assimilation of crustal material or by incorporation of subducted sedimentary material. Trace element abundances are not consistent with the incorporation of large amounts of sedimentary material into the mantle source. Crustal rocks from the region have δ¹⁸O of around +9 to +16. A degree of contamination of 15 to 20% by this type of crustal material can account for the oxygen ratios of the Chalupas lavas. This value is consistent with Sr and Nd isotopic values and trace element abundances from the Chalupas system. δ¹⁸O correlates well with ⁸⁷Sr/⁸⁶Sr and does not vary greatly between the Chalupas lavas. This is consistent with a two-stage model of magmatic evolution whereby crustal contamination occurs while the magma is traversing the lower crust. Once the magma evolves to andesite it is transported to the upper crust where it is either erupted or evolves by fractional crystallization alone to more silica rich magma types. The continental crust beneath the Eastern Cordillera in Ecuador is relatively thick (~50km) and of continental affinity. Beneath the Western Cordillera, the crust is thinner and of oceanic affinity. Although Sr isotope ratios do not vary across the arc, Nd isotope values of lavas from Western Cordillera volcanoes are significantly higher. Oxygen isotope ratios do not vary greatly between the Western and Eastern Cordillera but lavas from the Eastern Cordillera appear to reach higher values (δ¹⁸O > +7 ) suggesting that variations in the thickness and composition of the continental crust do have an effect on the composition of Ecuadorian lavas.