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Rhyolite ignimbrite generation in the northern andes: The chalupas caldera, Ecuador

Abstract:

This study evaluates how magma supply, silicate liquid properties, and crustal structure control the composition and volume of lava and pyroclastic materials in the Chalupas caldera system, a late Quaternary center in the Ecuadorian Andes that produced a 60 to 80 km3 caldera-forming rhyolite ignimbrite at 200 ka. The Sr, Nd, and O isotopic compositions of andesites (55 to 57 wt% SiO2) indicate they are products of assimilation and fractional crystallization of mantle-derived basalt, with 13 to 25% by mass derived from crust. The rhyolite ignimbrite (72% SiO2) was generated mainly by fractional crystallization of andesite; the overall fraction of crustal melting involved is small. Crustal assimilation is inferred to occur mainly at ca. 45 km depth, at a boundary between crustal rocks that are more and less dense than hydrous basalt. The absence of lavas with SiO2 < 55% is attributed to this density filter. Geochronology, petrogenetic models, and eruption volumes indicate the ignimbrite was produced during a 200-to 250,000-year period when magma supply was 5 times higher than in the periods before and after when mainly andesite was erupted. The higher magma flux meets the thermal requirements to grow a silicic magma chamber in the upper crust large enough to produce the Chalupas ignimbrite, but not a larger volume explosive eruption. Ignimbrite eruption volumes greater than 100 km3, as in the central Andes, require larger amounts of crustal melting and crustal thickness greater than 60 km.