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Ignimbrites to Batholiths Ignimbrites to Batholiths: Integrating Perspectives from Geological, Geophysical, and Geochronological Data

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Ignimbrites to Batholiths Ignimbrites to Batholiths: Integrating Perspectives from Geological, Geophysical, and Geochronological Data Ignimbrites to batholiths Ignimbrites to batholiths: Integrating perspectives from geological, geophysical, and geochronological data Peter W. Lipman1,* and Olivier Bachmann2 1U.S. Geological Survey, Mail Stop 910, Menlo Park, California 94028, USA 2Institute of Geochemistry and Petrology, ETH Zurich, CH-8092 Zürich, Switzerland ABSTRACT related intrusions cooled and solidified soon shorter. Magma-supply estimates (from ages after zircon crystallization, as magma sup- and volcano-plutonic volumes) yield focused Multistage histories of incremental accu- ply waned. Some researchers interpret these intrusion-assembly rates sufficient to gener- mulation, fractionation, and solidification results as recording pluton assembly in small ate ignimbrite-scale volumes of eruptible during construction of large subvolcanic increments that crystallized rapidly, leading magma, based on published thermal models. magma bodies that remained sufficiently to temporal disconnects between ignimbrite Mid-Tertiary processes of batholith assembly liquid to erupt are recorded by Tertiary eruption and intrusion growth. Alternatively, associated with the SRMVF caused drastic ignimbrites, source calderas, and granitoid crystallization ages of the granitic rocks chemical and physical reconstruction of the intrusions associated with large gravity lows are here inferred to record late solidifica- entire lithosphere, probably accompanied by at the Southern Rocky Mountain volcanic tion, after protracted open-system evolution asthenospheric input. field (SRMVF). Geophysical data combined involving voluminous mantle input, lengthy with geological constraints and comparisons residence (105–106 yr) as near-solidus crystal INTRODUCTION with tilted plutons and magmatic-arc sections mush, and intermittent separation of liquid elsewhere are consistent with the presence of to supply volcanic eruptions. The composi- Recent geochronologic and petrologic stud- vertically extensive (>20 km) intermediate tions of the least-evolved ignimbrite magmas ies have convincingly demonstrated that large to silicic batholiths (with intrusive:extrusive tend to merge with those of caldera-related magma bodies, which form granitoid crustal ratios of 10:1 or greater) beneath the major plutons, suggesting that the plutons record plutons in Cordilleran-arc settings, were assem- SRMVF volcanic loci (Sawatch, San Juan, nonerupted parts of long-lived cogenetic bled incrementally and crystallize over 105–106 Questa-Latir). Isotopic data require involve- magmatic systems, variably modified prior yr intervals (Coleman et al., 2004; Matzel et al., ment of voluminous mantle-derived mafic to final solidification. Precambrian-source 2006a; Memeti et al., 2010; Frazer et al., 2014). magmas on a scale equal to or greater than zircons are scarce in caldera plutons, in Building on twentieth-century discussions that of the intermediate to silicic volcanic and contrast to their abundance in some periph- (such as Daly, 1914; Kennedy and Anderson, plutonic rocks. Early waxing-stage intrusions eral waning-stage intrusions of the SRMVF, 1938; Buddington, 1959; Smith, 1979; Lip- (35–30 Ma) that fed intermediate-composi- implying dissolution of inherited crustal man, 1984; Macdonald and Smith, 1988), these tion central volcanoes of the San Juan locus zircon during lengthy magma assembly for results have promoted renewed controversy are more widespread than the geophysi- the ignimbrite eruptions and construction concerning connections between volcanic and cally defined batholith; these likely heated of a subvolcanic batholith. Broad age spans intrusive processes, especially how magma res- and processed the crust, preparatory for of zircons (to several million years) from ervoirs for large ignimbrite eruptions are related ignimbrite volcanism (32–27 Ma) and large- individual samples of some ignimbrites and to the emplacement of granitic plutons and the scale upper-crustal batholith growth. Age intrusions, commonly averaged and inter- crustal depths at which silicic compositions and compositional similarities indicate that preted as “intrusion-emplacement age,” are generated (Glazner et al., 2004; Metcalf, SRMVF ignimbrites and granitic intrusions alternatively provide an incomplete record 2005; Bachmann et al., 2007b; Lipman, 2007; are closely related, but the extent to which the of intermittent crystallization during pro- Miller et al., 2011; Davis et al., 2012; de Silva plutons record remnants of former magma tracted incremental magma-body assembly, and Gregg, 2014; Frazer et al., 2014; among reservoirs that lost melt to volcanic eruptions with final solidification only when the system others). For example, geochronologic data and has been controversial. Published Ar/Ar- began to wane. Analyses of whole zircons chemical patterns of volcanic and shallow plu- feldspar and U-Pb-zircon ages for plutons cannot resolve late stages of crystal growth, tonic rocks have been interpreted by some as spatially associated with ignimbrite calderas and early growth in a long-lived magmatic indicating melt generation in the deep crust with document final crystallization of granitoid system may be poorly recorded due to peri- minimal differentiation at shallow crustal levels, intrusions at times indistinguishable from ods of zircon dissolution. Overall, construc- leading to pluton assembly in small increments the tuff to ages several million years younger. tion of a batholith can take longer than that crystallized rapidly, with temporal and geo- These ages also show that SRMVF caldera- recorded by zircon-crystallization ages, while metric disconnects between ignimbrite eruption the time interval for separation and shallow and intrusion growth (Glazner et al., 2004; Bart- *[email protected] assembly of eruptible magma may be much ley et al., 2005; Annen, 2009; Coleman et al., Geosphere; June 2015; v. 11; no. 3; p. 705–743; doi:10.1130/GES01091.1; 14 figures; 7 tables. Received 18 June 2014 ♦ Revision received 30 December 2014 ♦ Accepted 19 February 2015 ♦ Published online 2 April 2015 For permission to copy, contact [email protected] Geosphere, June 2015 705 © 2015 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/3/705/3334629/705.pdf by guest on 26 September 2021 Lipman and Bachmann 2012; Zimmerer and McIntosh, 2012a; Mills associated SRMVF volcanic and granitoid rocks Tertiary rocks in the northern San Juan Basin and Coleman, 2013). Alternatively, others have are interpreted to record late solidification after (Gonzales et al., 2010; Lake and Farmer, 2015; concluded that rhyolitic magmas are commonly prolonged histories of open-system evolution Gonzales and Pecha, 2015), but such composi- generated at low pressure (e.g., Tuttle and in the shallow crust, involving recurrent mag- tions have not been identified centrally within Bowen, 1958; Lipman, 1966; Fowler and Spera, matic recharge, lengthy (105–106 yr) residence loci of large-volume San Juan volcanism. Vol- 2010; Gualda and Ghiorso, 2013) by crystal- as near-solidus crystal mush, and intermittent canic centers tended to migrate from north to liquid separation in the upper crust, comple- upper-crustal separation of liquid to supply south in the SRMVF, both intermediate-compo- mented by voluminous underlying cumulates eruptions. Ignimbrite eruptions in the SRMVF sition lava eruptions and ignimbrites (Fig. 1B, (e.g., Hildreth, 1981, 2004; Bacon and Druitt, are inferred to record relatively brief episodes Table 1; Lipman, 2007); the general southward 1988; Vazquez and Reid, 2002; Bachmann and of increased mantle-magma recharge and con- migration is parallel to that long documented Bergantz, 2004; Deering et al., 2011). These and current upper-crustal pluton construction at for eruptive centers in the Basin-Range region, other recent studies (e.g., Wilson and Charlier, focused sites within the broader areal extent of probably related to disruption of the subducted 2009; Allan et al., 2013; Pamukcu et al., 2013; the volcanic field. The eruptible magmas that Farallon plate (Stewart et al., 1977; Lipman, Wotzlaw et al., 2014; Cashman and Giordano, sourced SRMVF ignimbrites are considered to 1980; Henry and John, 2013). 2014) have inferred broadly lenticular shapes, be short-lived and volumetrically minor, shal- Structural unroofing, associated with later rapid assembly rates, and brief life spans for low portions of much longer-lived and verti- extension along the Rio Grande rift and deep the shallow magma bodies that erupt as large cally extensive underlying plutonic systems erosion of this high-standing region, has ignimbrites. From complementary perspectives, dominated by near-solidus mushy magma. Such exposed broadly synvolcanic batholithic intru- this paper evaluates the vertical extent and tem- temporal and volumetric features of volcanic- sions associated with the ignimbrite centers. The poral construction of the overall crust-mantle plutonic evolution at the SRMVF are suggested earliest well-documented regional ignimbrite, magmatic system inferred to have developed to be representative of continental-arc magma- erupted from a caldera source in the SRMVF, concurrently with ignimbrite eruptions. tism worldwide. was the far-traveled Wall Mountain Tuff at 37 Although available analytical methods and Ma (Chapin and Lowell, 1979; Zimmerer and resulting data have thus far been only partly SOUTHERN ROCKY MOUNTAIN McIntosh, 2012a), erupted from the Princeton successful in addressing such issues, recent vol- VOLCANIC
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