Deep crustal anatexis, magma mixing, and the generation of epizonal plutons in the Southern Rocky Mountains, Colorado

Jacob, K. H.; Farmer, G. Lang; Buchwaldt, Robert; Bowring, Samuel A.

doi:10.1007/s00410-014-1094-3

Cited by 17 publications

(39 citation statements)

References 61 publications

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“…As discussed above, we favor the interpretation that at least part of the Organ Mountains system formed from silicic lower crustal melts, based on the consistent ε Nd (t) ≈ −5 isotopic composition of the system through time, and similarities to the Never Summer magmatic system (Jacob et al 2015). In terms of magmatic fluxes, the overall dataset from the Organ Mountains could be interpreted to reflect a progressive shift from large-scale volcanic eruptions prior to eruption of the Squaw Mountain tuff at 36.215 ± 0.016 Ma, to a dominantly plutonic system after this eruption (Fig.…”

Section: Isotopic Constraints On Magma Evolution In the Organ Mountainssupporting

confidence: 55%

“…Jacob et al (2015) used whole-rock geochemistry and isotopic data to argue that small-volume silicic epizonal plutons, and associated volcanic rocks, in the Never Summer igneous complex, north-central Colorado, were generated by such a process. In the Never Summer system, evidence from crustal xenoliths entrained in Devonian kimberlites shows that Precambrian mafic lower continental crust beneath this portion of the southern Rocky Mountains has Nd and Sr isotopic compositions identical to those observed for silicic intrusive rocks and related, small-volume topaz rhyolites.…”

Section: Isotopic Constraints On Magma Evolution In the Organ Mountainsmentioning

confidence: 99%

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The link between volcanism and plutonism in epizonal magma systems; high-precision U–Pb zircon geochronology from the Organ Mountains caldera and batholith, New Mexico

Rioux

Farmer

Bowring

et al. 2016

Contrib Mineral Petrol

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of 36.441 ± 0.020 Ma (Cueva Tuff), 36.259 ± 0.016 Ma (Achenback Park tuff), and 36.215 ± 0.016 Ma (Squaw Mountain tuff). An alkali feldspar granite, which is chemically similar to the erupted tuffs, yielded a synchronous weighted mean 206 Pb/ 238 U date of 36.259 ± 0.021 Ma. Weighted mean 206 Pb/ 238 U dates from the larger volume syenitic phase of the underlying Organ Needle pluton range from 36.130 ± 0.031 to 36.071 ± 0.012 Ma, and the youngest sample is 144 ± 20 to 188 ± 20 ka younger than the Squaw Mountain and Achenback Park tuffs, respectively. Younger plutonism in the batholith continued through at least 34.051 ± 0.029 Ma. We propose that the Achenback Park tuff, Squaw Mountain tuff, alkali feldspar granite and Organ Needle pluton formed from a single, long-lived magma chamber/mush zone. Early silicic magmas generated by partial melting of the lower crust rose to form an epizonal magma chamber. Underplating of the resulting mush zone led to partial melting and generation of a highsilica alkali feldspar granite cap, which erupted to form the tuffs. The deeper parts of the chamber underwent continued recharge and crystallization for 144 ± 20 ka after the final eruption. Calculated magmatic fluxes for the Organ Needle pluton range from 0.0006 to 0.0030 km 3 /year, in agreement with estimates from other well-studied plutons. The petrogenetic evolution proposed here may be common to many small-volume silicic volcanic systems.

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Section: Isotopic Constraints On Magma Evolution In the Organ Mountainssupporting

confidence: 55%

Section: Isotopic Constraints On Magma Evolution In the Organ Mountainsmentioning

confidence: 99%

The link between volcanism and plutonism in epizonal magma systems; high-precision U–Pb zircon geochronology from the Organ Mountains caldera and batholith, New Mexico

Rioux

Farmer

Bowring

et al. 2016

Contrib Mineral Petrol

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“…Farther north in Colorado, the mid-Tertiary magmatism is marked by scattered shallow intrusions and sparse small erosional remnants of lava and tuff. A thick section of welded tuff associated with an intrusive complex in the Never Summer Mountains may record remnants of a small isolated caldera system active at 28-29 Ma (O'Neill, 1981;Jacob et al, 2011Jacob et al, , 2015, concurrent with peak activity in the San Juan Mountains to the south.…”

Section: Southern Rocky Mountain Volcanic Field and Its Batholithmentioning

confidence: 99%

“…Diverse petrologic and geochemical studies on processes of magma generation for the SRMVF and similar Cordilleran igneous provinces have led to broad consensus: The dominant andesitic to rhyolitic magmas were generated by rise of voluminous mantle-derived basalt that provided heat to assimilate variable amounts of lower crust, as the evolving magmas crystallized and fractionated (e.g., Lipman et al, 1978;DePaolo, 1981;DePaolo et al, 1992;Hildreth and Moorbath, 1988;Johnson, 1991;Riciputi et al, 1995;Farmer et al, 2008;Kay et al, 2010;Jacob et al, 2015). The much-discussed potential for mantle-generated mafic input to rejuvenate and prolong the life spans of upper-crustal magmatic systems (e.g., Smith, 1979;Hildreth, 1981;Mahood, 1990;Bachmann et al, 2007b;Cooper and Kent, 2014;de Silva and Gregg, 2014) has been explicitly proposed to explain complex petrography and mineral chemistry of large ignimbrites in the SRMVF and elsewhere (e.g., Lipman et al, 1997;Bachmann et al, 2002;Bachmann and Bergantz, 2004;de Silva and Gosnold, 2007;Huber et al, 2012).…”

Section: Srmvf Magma Generation and Volumementioning

confidence: 99%

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

2015

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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,...

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“…Evidence for crustal interaction between mafic magmas and continental crust occurs in andesitic Cenozoic volcanic rocks in western North America (Christiansen & McCurry, 2008; Streck et al, 2007), but such interaction does not necessarily result in rocks with intermediate Ta/Th values. For example, in the Never Summer Mountains of southern Colorado, mixing between silicic, anatectic melts of mafic Precambrian crust (>70 wt% SiO 2 ) and ascending mafic melts was likely responsible for the generation of intermediate composition, Oligocene plutonic rocks (Jacob et al, 2015). Both mafic and felsic composition end‐members involved in mixing had similar Nd isotopic compositions, ε Nd (T) ~−5, and only small differences in Ta/Th (0.3 vs. 0.2).…”

Section: Discussionmentioning

confidence: 99%

Identifying Metasomatized Continental Lithospheric Mantle Involvement in Cenozoic Magmatism From Ta/Th Values, Southwestern North America

Farmer

Fritz

Glazner

2020

Geochem Geophys Geosyst

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The accessory minerals rutile and apatite are rare or absent in the convecting upper mantle but occur in shallow, cooler, metasomatized continental lithospheric mantle (CLM) where they serve as carrier phases for the trace elements Ta (in rutile) and Th (in apatite). Because both minerals crystallize near-solidus and are eliminated early during partial mantle melting, the relative abundances of rutile and apatite should control the Ta and Th abundances of mantle melts and provide a means of identifying the involvement of rutile-and/or apatite-bearing metasomatized CLM in mafic continental magmatism. As a test, we investigated published Ta and Th abundances data from~2,000 whole-rock samples of mafic to intermediate composition, Cenozoic volcanic rocks in southwestern North America. Roughly half of the samples have Ta/Th values similar to those of island arc volcanic rocks (<0.2) or ocean island and mid-ocean ridge basalts (>0.6). The remaining samples have intermediate and variable Ta/Th values between 0.2 and 0.6, independent of specific indices of crustal interaction (e.g., wt% P 2 O 5 /wt% K 2 O). We interpret the intermediate Ta/Th rocks as the products of direct melting of, or of extensive melt-rock interaction with, rutile-and/or apatite-bearing CLM. Intermediate Ta/Th rocks also have uniformly high 87 Sr/ 86 Sr (0.706 to 0.708) compared to oceanic basalts that, unlike their Nd isotopic compositions, do not covary with lithospheric age. These observations are consistent with widespread metasomatism of the CLM by Sr-rich, Nd-poor, aqueous fluids generated by dehydration of oceanic lithosphere, and its overlying tectonic mélange during early Cenozoic subduction beneath southwestern North America.Plain Language Summary Volcanism occurred throughout southwestern North America (SWNA) over the past 40 million years, but exactly what melted to produce the resulting volcanic rocks is unclear. We address this issue by investigating the relative amounts of the elements tantalum (Ta) and thorium (Th) in these volcanic rocks. In oceanic settings, volcanic rocks have well-defined Ta/Th values either from~0.6 to~1 for oceanic islands and ocean floor basalts or <0.2 for island-arc volcanoes. But in SWNA, approximately 40% of the~2,000 volcanic rocks for which Ta and Th abundances are available have variable and intermediate Ta/Th values (0.2 to 0.6) that occur independently of chemical indicators used to identify interaction between continental crust and ascending magmas. We suggest that the intermediate Ta/Th values reflect melting of mantle directly beneath the continent that contains minerals that concentrate Ta and Th, such as rutile (a titanium oxide) and apatite (a calcium phosphate). Both minerals can precipitate from water-rich fluids that cool when infiltrating and altering the continental mantle but readily melt upon subsequent heating. We conclude that hydrated, rutile-and/or apatite-bearing continental mantle was widespread beneath SWNA and was an important contributor to the magmas from which the intermediate...

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Deep crustal anatexis, magma mixing, and the generation of epizonal plutons in the Southern Rocky Mountains, Colorado

Cited by 17 publications

References 61 publications

The link between volcanism and plutonism in epizonal magma systems; high-precision U–Pb zircon geochronology from the Organ Mountains caldera and batholith, New Mexico

The link between volcanism and plutonism in epizonal magma systems; high-precision U–Pb zircon geochronology from the Organ Mountains caldera and batholith, New Mexico

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

Identifying Metasomatized Continental Lithospheric Mantle Involvement in Cenozoic Magmatism From Ta/Th Values, Southwestern North America

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