Petrogenesis of the large-volume cardones ignimbrite, Chile; development and destabilization of a complex magma-mush system

The 21.9 Ma Cardones ignimbrite, a member of the early Miocene Oxaya Formation in northernmost Chile, is a crystal-rich (c. 40 vol. %) rhyolite with an extra-caldera thickness up to 1000 m. Access to core from eight c. 1 km drill holes allowed sampling and petrological characterization of pumice throughout the entire thickness of the ignimbrite at an unprecedented range of spatial scales. Mineral chemistry and modes reveal the presence of sanidine-poor and sanidine-rich pumice types, representing two petrologically distinct magmas with the former being hotter (850-750°C) than the latter (770-670°C). The sanidine-poor magma is amphibole-bearing and has a Ba-rich melt phase (400- 1000 ppm), whereas the sanidine-rich magma has a Ba-poor melt phase (<200 ppm) and lacks amphibole. Thermobarometry of antecrystal remnants in the sanidine-poor pumice clasts indicates derivation of rhyodacitic melts from a middle crustal hot zone (950-850°C) where wet, intermediate to mafic magmas stalled and fractionated. Textures and zoning in zircon and plagioclase are consistent with the interpretation that rhyodacitic melts were episodically emplaced into the shallow crust at 6.0 to 8±7 6 2.0 km depths over a period of c. 200 kyr. Sanidine-rich magma compositions are consistent with the melt phase in the sanidine-poor magma, hence the former could be derived by residual melt segregation from the latter. High-Ba and high-Sr zones in sanidine have compositions up to 28 000 ppm and 400 ppm respectively. Such high concentrations require input of more intermediate magmas and/or partial melting of feldspar-rich cumulates. The sanidine-rich magmas probably formed above the sanidine-poor magmas. However, there is no systematic relationship between the different pumice types and the ignimbrite stratigraphy, suggesting that major pre-eruptive destabilization of the magmatic system led to the amalgamation and mixing of different, melt-dominated lenses and surrounding crystalmush to form a large heterogeneous eruptible magma body.