Recording the transition from flare-up to steady-state arc magmatism at the Purico–Chascon volcanic complex, northern Chile

Abstract The long-term evolution of continental magmatic arcs is episodic, where a few transient events of high magmatic flux or flare-ups punctuate the low-flux magmatism or “steady state” that makes up most of the arc history. How this duality manifests in terms of differences in crustal architecture, magma dynamics and chemistry, and the time scale over which transitions occur is poorly known. Herein we use multiscale geochemical and isotopic characteristics coupled with geothermobarometry at the Purico–Chascon Volcanic Complex (PCVC) in the Central Andes to identify a transition from flare-up to steady state arc magmatism over ∼800 kyr during which significant changes in upper crustal magmatic dynamics are recorded. The PCVC is one of the youngest volcanic centers related to a 10–1 Ma ignimbrite flare-up in the Altiplano–Puna Volcanic Complex of the Central Andes. Activity at the PCVC initiated 0.98 ± 0.03 Ma with the eruption of a large 80–100 km3 crystal-rich dacite ignimbrite. High, restricted 87Sr/86Sr isotope ratios between 0.7085 and 0.7090 in the bulk rock and plagioclase crystals from the Purico ignimbrite, combined with mineral chemistry and phase relationships indicate the dacite magma accumulated and evolved at relatively low temperatures around 800–850 °C in the upper crust at 4–8 km depth. Minor andesite pumice erupted late in the ignimbrite sequence records a second higher temperature (965 °C), higher pressure environment (17–20 km), but with similar restricted radiogenic bulk rock 87Sr/86Sr = 0.7089–0.7091 to the dacites. The compositional and isotopic characteristics of the Purico ignimbrite implicate an extensive zone of upper crustal mixing, assimilation, storage and homogenization (MASH) between ∼30 and 4 km beneath the PCVC ∼1 Ma. The final eruptions at the PCVC 0.18 ± 0.02 Ma suggest a change in the magmatic architecture beneath the PCVC. These eruptions produced three small The textural and compositional characteristics of the PCVC attest to two distinct stages in its history. At ∼1 Ma the system was broadly homogeneous and dominantly dacitic recording extensive upper crustal magmatism. By ∼0.2 Ma the PCVC had transitioned to a more compositionally heterogeneous, smaller volume, mixed dacite to basaltic-andesite system, coinciding with the appearance of less-enriched “baseline” compositions. The evolution of PCVC is a microcosm of the Central Andean arc in this region where, from 10 to 1 Ma, upper crustal MASH processes resulted in the production and eruption of large volumes of homogeneous crystal-rich dacite during a regional ignimbrite flare-up. Since ∼1 Ma, decreasing explosivity, smaller eruptive volumes, increasing heterogeneity, and the emergence of less isotopically enriched basaltic-andesite to dacite composite volcanoes signal a return to steady-state arc volcanism. We posit that the transition from flare-up to steady state captured at the PCVC tracks the waning of the arc scale “thermal engine”. High magmatic fluxes during the flare-up would lead to elevated geothermal gradients and efficient crustal processing leading to a dominantly “crustal” magmatism feeding the large volume Purico ignimbrite. This upper crustal MASH zone would act as an efficient filter to any parental compositions precluding them from the eruption record. As magmatic flux and thermal energy wanes, crustal isotherms would relax leading to greater thermal contrast between parental magmas, upper crust, and remnant felsic magmas stored in the upper crust. These changes are manifested in the preservation of textural and compositional heterogeneity and the survival of less isotopically enriched magmas in the upper crust. The chemical imprint of these arc-scale changes in magma dynamics is recorded at all scales from bulk rock to intra-crystalline. The distinct magma dynamics and chemical signatures of the two modes of arc magmatism detailed here should provide a model for investigations of mature continental arc evolution through time and space.