Rapid Magma Generation or Shared Magmatic Reservoir? Petrology and Geochronology of the Rat Creek and Nelson Mountain Tuffs, CO, USA

Abstract

Large-volume silicic volcanism poses global hazards in the form of proximal pyroclastic density currents, distal ash fall and short-term climate perturbations, which altogether warrant the study of how silicic magma bodies evolve and assemble. The southern rocky mountain volcanic field (SRMVF) is home to some of the largest super-eruptions in the geological record, and has been studied to help address the debate over how quickly eruptible magma batches can be assembled-whether in decades to centuries, or more slowly over 100's of kyr. The present study focuses on the San Luis caldera complex within the SRMVF, and discusses the paradigms of rapid magma generation vs. rapid magma assembly. The caldera complex consists of three overlapping calderas that overlie the sources of three large-volume mid-Cenozoic ignimbrites: first, the Rat Creek Tuff (RCT; zoned dacite-rhyolite, 150 km 3), followed by the Cebolla Creek Tuff (mafic dacite, 250 km 3) and finally, the Nelson Mountain Tuff (NMT; zoned daciterhyolite, 500 km 3), which are all indistinguishable in age by 40 Ar/ 39 Ar dating. We argue for a shared magmatic history for the three units on the basis of (1) similar mineral trace element compositions in the first and last eruptions (plagioclase, sanidine, biotite, pyroxene, amphibole, titanite, and zircon), (2) overlapping zircon U-Pb ages in all three units, and (3) similar thermal rejuvenation signatures visible in biotite (low-Mn, high-Ba) and zircon (low-Hf, low-U) geochemistry within the RCT and NMT. It is postulated that the NMT was sourced from a pre-existing magma reservoir to the northeast, which is corroborated by the formation of the nearby Cochetopa Caldera during the eruption of the NMT. The inferred lateral magma transport has two important implications: (1) it demonstrates long-distance transport of highly viscosity magmas at volumes (100's of km 3) not previously recorded, and (2) the sourcing of magma from a nearby pre-existing magma reservoir greatly reduces the rate of magma generation necessary to explain the close coincidence of three overlapping, large-volume magma systems. Additionally, the concept of magmatic "flux" (km 3 kyr −1) is discussed in this context, and it is argued that an area-normalized flux (km 3 kyr −1 km −2) provides a more useful number for measuring magma production rates: it is expected that magmatic volumes will scale with footprint of the thermal anomaly, and not taking this into account may lead to errant volumetric

Timescales and thermal evolution of large silicic magma reservoirs during an ignimbrite flare-up: perspectives from zircon

“…2, 9). The hotter zircon temperatures in the dacitic Rat Creek Tuff are likely due to late-stage influx of hotter, more mafic magma, as hypothesized by previous studies (Sliwinski et al 2019;Curry et al 2021), but it could also be due in part to other factors such as a higher zircon saturation temperature (Fig. S6).…”

Long‐Distance Lateral Magma Propagation and Pamir Plateau Uplift

“…Although field exposures for magma migration are very rare, examples of such lateral migration of magmas within the crust have been documented in both volcanic and plutonic magmatic systems (e.g., M. T. Gudmundsson et al., 2016; Gutiérrez et al., 2018; Ishizuka et al., 2017; Johnson et al., 2003; Sliwinski et al., 2019). One example is the well‐exposed and well‐documented case of the middle Miocene (11.3–10.2 Ma) La Gloria Pluton in central Chile from the southern Andes, based on integrated studies including numerical simulations of magma fluid dynamics (Gutiérrez et al., 2013), magnetic and magmatic fabrics (Payacán et al., 2014) and zircon U‐Pb dating (Gutiérrez et al., 2018).…”

The Role of Crystal Accumulation and Cumulate Remobilization in the Formation of Large Zoned Ignimbrites: Insights From the Aso-4 Caldera-forming Eruption, Kyushu, Japan

“…Hence, distinguishing between effects of recharge and products of cumulate melting must rely on petrologic evidence (e.g., Wolff et al, 2020). Such evidence might be given by enrichment of elements (compatible in certain low temperature phases) like Ba, Ti, Eu and Sr in groundmass glasses and recrystallization rims revealing preceding recharge-induced cumulate rejuvenation (Hildreth and Wilson, 2007;Bachmann et al, 2014;Wolff et al, 2015;Forni et al, 2016;D'Oriano et al, 2017;Sliwinski et al, 2019). Hot recharge entering into the cumulate mush initiates (partial) melting of low temperature phases (e.g., alkali feldspar, biotite, low-An plagioclase) leading to local enrichment of the respective compatible elements in the surrounding melts.…”