Ignimbrites to batholiths: Integrating perspectives from geological, geophysical, and geochronological data

Abstract

Multistage histories of incremental accumulation, fractionation, and solidification during construction of large subvolcanic magma bodies that remained sufficiently liquid to erupt are recorded by Tertiary ignimbrites, source calderas, and granitoid intrusions associated with large gravity lows at the Southern Rocky Mountain volcanic field (SRMVF). Geophysical data combined with geological constraints and comparisons with tilted plutons and magmatic-arc sections elsewhere are consistent with the presence of vertically extensive (>20 km) intermediate to silicic batholiths (with intrusive:extrusive ratios of 10:1 or greater) beneath the major SRMVF volcanic loci (Sawatch, San Juan, Questa-Latir). Isotopic data require involvement of voluminous mantle-derived mafic magmas on a scale equal to or greater than that of the intermediate to silicic volcanic and plutonic rocks. Early waxing-stage intrusions (35-30 Ma) that fed intermediate-composition central volcanoes of the San Juan locus are more widespread than the geophysically defined batholith; these likely heated and processed the crust, preparatory for ignimbrite volcanism (32-27 Ma) and largescale upper-crustal batholith growth. Age and compositional similarities indicate that SRMVF ignimbrites and granitic intrusions are closely related, but the extent to which the plutons record remnants of former magma reservoirs that lost melt to volcanic eruptions has been controversial. Published Ar/Arfeldspar and U-Pb-zircon ages for plutons spatially associated with ignimbrite calderas document final crystallization of granitoid intrusions at times indistinguishable from the tuff to ages several million years younger. These ages also show that SRMVF caldera-related intrusions cooled and solidified soon after zircon crystallization, as magma supply waned. Some researchers interpret these results as recording pluton assembly in small increments that crystallized rapidly, leading to temporal disconnects between ignimbrite eruption and intrusion growth. Alternatively, crystallization ages of the granitic rocks are here inferred to record late solidification, after protracted open-system evolution involving voluminous mantle input, lengthy residence (10 5 -10 6 yr) as near-solidus crystal mush, and intermittent separation of liquid to supply volcanic eruptions. The compositions of the least-evolved ignimbrite magmas tend to merge with those of caldera-related plutons, suggesting that the plutons record nonerupted parts of long-lived cogenetic magmatic systems, variably modified prior to final solidification. Precambrian-source zircons are scarce in caldera plutons, in contrast to their abundance in some peripheral waning-stage intrusions of the SRMVF, implying dissolution of inherited crustal zircon during lengthy magma assembly for the ignimbrite eruptions and construction of a subvolcanic batholith. Broad age spans of zircons (to several million years) from individual samples of some ignimbrites and intrusions, commonly averaged and interpreted as "intrusion-emplacement age,...

Geology and evolution of the McDermitt caldera, northern Nevada and southeastern Oregon, western USA

“…Intrinsic variables such as peralkalinity and inferred high temperature strongly encouraged the intense rheomorphism shown by the McDermitt Tuff and most strongly peralkaline tuffs, but seem to have had little influence on caldera character. Note: From this study, Mahood (1984), Swanson and McDowell (1984), McIntosh et al (1992), Christiansen (2001), Christiansen (2005), Lipman (2007), Christiansen and McCurry (2008), Lipman and McIntosh (2008), Bonnichsen et al (2008), John et al (2008), McDowell and McIntosh (2012), Henry and John (2013), Ellis et al (2013), Best et al (2013aBest et al ( , 2013bBest et al ( , 2016, and Lipman and Bachmann (2015).…”

“…The ignimbrite flareup formed a semicontinuous belt of ignimbrites and source calderas from the Great Basin of the United States to the southern end of the Sierra Madre Occidental in Mexico (Swanson and McDowell, 1984;Ferrari et al, 2007;McDowell and McIntosh, 2012;Henry et al, 2012b;Best et al, 2013aBest et al, , 2016. Overall understanding of the geometry, evolution, and geotectonic setting of calderas is based heavily on study of the ignimbrite flareup, particularly in the United States (Table 7; Lipman, 1984Lipman, , 2007Best et al, 2013aBest et al, , 2013bBest et al, , 2016Henry and John, 2013;Lipman and Bachmann, 2015).…”

Seismic evidence for significant melt beneath the Long Valley Caldera, California, USA

“…Studies on the generation of ignimbrite eruptions and silicic batholiths provide constraints on the plutonic/volcanic ratios necessary to generate mid-to-upper crustal rhyolitic magmas (Lipman and Bachmann, 2015). By increasing the minimum V S contrast between the reservoir and WLC, we can calculate a range of reservoir and melt volumes across these geologically reasonable plutonic/volcanic ratios.…”

Eruptive history, geochronology, and post-eruption structural evolution of the late Eocene Hall Creek Caldera, Toiyabe Range, Nevada

“…Although no Eocene plutonic rocks are exposed in the study area, the tuff of Hall Creek was most likely sourced from a large silicic magma chamber that began forming at least several hundred k.y. prior to the main eruption, as was the case for the nearby Caetano caldera (John and others, 2008;Watts and others, 2016), and is typical for large silicic calderas (for example, Lipman and Bachmann, 2015). The eruption was fed by tuff venting along the ring-fracture faults that accommodated caldera collapse ( fig.…”