Overlapping Plutonism, Volcanism, and Tectonism in the Boulder Batholith Region, Western Montana

Robinson, G. D; Klepper, M.R.; Obradovich, John D.

doi:10.1130/mem116-p557

Cited by 54 publications

(21 citation statements)

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“…without deep root of more-mafi c granitoid or any mantle input Lake and Farmer (2015) San Juan locus; mantle-magma input estimate from isotope data on May 30, 2015 geosphere.gsapubs.org Downloaded from Keller, 1994), the Marysvale volcanic field in Utah (Steven et al, 1984), Elkhorn Mountains volcanic field and underlying Boulder batholith in Montana (Robinson et al, 1968;Hamilton and Myers, 1974;Biehler and Bonini, 1969;Vejmelek and Smithson, 1995), and the Yellowstone caldera cluster (Eaton et al, 1975;Christiansen, 2001).…”

Section: Proposed For San Juan Locus (But Volcanic Volume Is Too Large)mentioning

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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Section: Proposed For San Juan Locus (But Volcanic Volume Is Too Large)mentioning

confidence: 99%

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

2015

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“…During Early Cretaceous, the eastern edge of this highland may have extended into or near the present Lewis and Clark Range (Mudge and Sheppard, 1968). Plutonism, volcanism, and tectonism very likely began in Late Cretaceous time in the southern part of the area inasmuch as similar events occurred in the Boulder batholith region to the south as described by Robinson, Klepper, and Obradovich (1968). At the close of the Mesozoic, regional uplift resulted in doming and broad folding (Erdmann, 1944).…”

Section: General Geologymentioning

confidence: 86%

Aeromagnetic, Bouguer gravity, and generalized geologic studies of the Great Falls-Mission Range area, northwestern Montana

Kleinkopf¹,

Mudge²

1972

Professional Paper

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“…100-55 Ma; Armstrong et al, 1977;Lageson et al, 2001), the Boulder batholith and its satellites (81-73 and 64-61 Ma;Tilling et al, 1968;Hamilton and Meyers, 1974;Brumbaugh and Hendrix, 1981;Lund et al, 2002;Wooden et al, 2008), the Pioneer batholith (77-65 Ma; Snee, 1982;Marvin et al, 1983;Arth et al, 1986), and the Tobacco Root batholith (77-71 Ma; Mueller et al, 1996). Extrusion of the Cretaceous Elkhorn Mountains volcanics was coeval with early stages of Boulder batholith intrusion (80-83 Ma; Robinson et al, 1968;Tilling, 1974;Lageson et al, 2001), resulting in a thick volcanic carapace (26,000 km 2 and 4.6 km thick) that blanketed the area (Klepper and Smedes, 1959;Robinson et al, 1968). Extrusion of the Lowland Creek, Dillon, Challis, Absaroka, and Helena volcanic fi elds occurred during the early Eocene (ca.…”

Section: Geologic Settingmentioning

confidence: 95%

“…The paucity of Elkhorn Mountains volcanics detritus relative to the abundance of plutonic detritus in the oldest exposed fl uvial units of the Renova Formation is indicative of long-term uplift and erosion prior to Renova deposition. By mid-Eocene time, much of the Elkhorn Mountains volcanics mass (originally 26,000 km 2 and 4.6 km thick; Klepper and Smedes, 1959;Smedes, 1966;Robinson et al, 1968) had to have been stripped from the Boulder batholith region and was likely transported eastward (e.g., Gill and Cobban, 1973;Rice and Shurr, 1983). Dissection of this carapace exposed the batholiths, as well as other metamorphic and sedimentary rocks in the Sevier and Laramide structural culminations.…”

Section: Some New Constraints For a Basin Evolution Modelmentioning

confidence: 98%

Paleogene postcompressional intermontane basin evolution along the frontal Cordilleran fold-and-thrust belt of southwestern Montana

Schwartz

2013

Geological Society of America Bulletin

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The Paleogene Renova Formation is the earliest record of postcompressional sedimentation within and adjacent to the Helena Salient of the Cordilleran fold-and-thrust belt in southwestern Montana. Paleocurrent and compositional data from basin-margin facies document radiating paleodispersal away from high-relief (≥2 km) highlands coincident with modern mountainous areas. Source rocks within the paleohighlands included the same Archean metamorphic; Proterozoic, Paleozoic, and Mesozoic sedimentary; and Mesozoic plutonic and volcanic rocks as exposed in modern uplifts. Paleocurrent and compositional data from trunk fl uvial conglomerates and sandstones document the existence of an interbasinal drainage system that connected the Three Forks, western Gallatin, and Townsend Basins with headwaters farther to the west and southwest near the present-day Montana-Idaho border. Overall, the distribution of Paleogene mountainous areas and basins closely resembled modern geography, and the Paleogene drainage network was strikingly similar to the modern Missouri River headwater system. The Renova Formation records the early stages of decay of the Cordilleran orogenic belt, including the evolution of a complex intermontane basin network in southwestern Montana. High-energy Late Cretaceous to early Eocene fl uvial systems carved deep, large-scale paleovalleys into the orogenic wedge along zones of structural and stratigraphic weakness. At least a 5 km thickness of overburden was removed during this time. Incision was temporally correlative with early Cenozoic regional uplift and subtropical climatic conditions. Subsequent deposition of the Renova Formation was temporally correlative with the cessation of uplift, the initiation of crustal extension, and climatic cooling. However, extension is not interpreted to have played a major role in earliest basin development.

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Overlapping Plutonism, Volcanism, and Tectonism in the Boulder Batholith Region, Western Montana

Cited by 54 publications

References 0 publications

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

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

Aeromagnetic, Bouguer gravity, and generalized geologic studies of the Great Falls-Mission Range area, northwestern Montana

Paleogene postcompressional intermontane basin evolution along the frontal Cordilleran fold-and-thrust belt of southwestern Montana

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