Abstract
Four voluminous ignimbrites (150–500 km3) erupted in rapid succession at 27 Ma in the central San Juan caldera cluster, Colorado. To reconstruct the timescales and thermal evolution of these magma reservoirs, we used zircon ID-TIMS U–Pb geochronology, zircon LA-ICP-MS geochemistry, thermal modeling, and zircon age and crystallization modeling. Zircon geochronology reveals dispersed zircon age spectra in all ignimbrites, with decreasing age dispersion through time that we term a ‘chimney sweeping’ event. Zircon whole-grain age modeling suggests that 2σ zircon age spans represent approximately one-quarter of total zircon crystallization timescales due to the averaging effect of whole-grain, individual zircon ages, resulting in zircon crystallization timescales of 0.8–2.7 m.y. Thermal and zircon crystallization modeling combined with Ti-in-zircon temperatures indicates that magma reservoirs were built over millions of years at relatively low magmatic vertical accretion rates (VARs) of 2–5 × 10–3 m y−1 (2–5 × 10–6 km3 y−1 km−2), and we suggest that such low VARs were characteristic of the assembly of the greater San Juan magmatic body. Though we cannot unequivocally discern between dispersed zircon age spectra caused by inheritance (xenocrystic or antecrystic) versus prolonged crystallization from the same magma reservoir (autocrystic), our findings suggest that long-term magma input at relatively low VARs produced thermally mature upper crustal magma reservoirs resulting in protracted zircon crystallization timescales. Compiling all U–Pb ID-TIMS zircon ages of large ignimbrites, we interpret the longer timescales of subduction-related ignimbrites as a result of longer term, lower flux magmatism, and the shorter timescales of Snake River Plain ignimbrites as a result of shorter term, higher flux magmatism.
Highlights
Large caldera-forming eruptions in close temporal and spatial proximity, known as ignimbrite flare-ups, are one of the most spectacular expressions of silicic magmatism on Earth
New zircon ID-TIMS geochronology and LA-ICP-MS geochemistry for the last four ignimbrites in the central San Juan caldera cluster combined with thermal modeling, zircon crystallization modeling, and synthetic age modeling provide evidence for a new model for magmatism related to the construction of one of the world’s most voluminous and rapid ignimbrite flare-ups
We estimate the dominant vertical accretion rates (VARs) that built the magma reservoirs responsible for these four ignimbrites, and possibly the overall magmatic system below the central San Juan caldera cluster, to be 2–5 × 10–3 m y−1. This protracted, low magma input model contrasts with other models of high magmatic fluxes over shorter timescales responsible for ignimbrites
Summary
Large caldera-forming eruptions in close temporal and spatial proximity, known as ignimbrite flare-ups, are one of the most spectacular expressions of silicic magmatism on Earth. Ignimbrite flare-ups occur around the world mainly at subduction-related settings, and in non-subduction settings such as the Snake River Plain and the Afro-Arabian volcanic field (e.g., Christiansen 2001; Peate et al 2005). They consist of temporally and spatially associated, largevolume ignimbrites (> 10 km3) with total erupted volumes in the 1 03 s to 104 s of km (e.g., Gravley et al 2016). These events are typically considered to be the result of dramatic, transient increases in magmatic input and associated with major tectonic changes
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