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Reference Number: 2016 GeoREF Number: 2006-058742 Publication Year: 2006 Abstract: "Intrusive rocks in the Crazy Mountains, Montana, consist of numerousstocks, dike swarms, laccoliths and/or sills of strongly alkaline tosubalkaline affinity that have intruded and metamorphosed Tertiarysedimentary strata of the Crazy Mountains Basin. The subalkaline rocksform stocks, sills, and radiating dikes and are located primarily inthe southern Crazy Mountains (e.g., Big Timber stock, Loco Mountainstock). With the exception of the Ibex Mountain sill (?), the alkalinerocks are restricted to the northern Crazy Mountains. New (super 40)Ar/ (super 39) Ar dates are reported for alkaline rocks of thenorthern Crazy Mountains, with results ranging between 50.61 Ma and50.03 Ma. Five dates from the strongly alkaline nepheline and maficnepheline syenites of the Ibex Mountain sill (?), Robinson anticlineintrusive complex, and Comb Creek stock (?) and dike swarm givetightly clustered dates suggesting that they were emplaced during arestricted time interval at approximately 50.1 Ma. The dates from thealkaline rocks of the northern Crazy Mountains are slightly older thanthose previously reported from the subalkaline Big Timber stock in thesouthern Crazy Mountains (i.e., 49.3-49.2 Ma, biotite (super 40) Ar/(super 39) Ar) (du Bray and Harlan, 1996). However, the limited spanof dates (i.e., 50.6-49.2 Ma) and the geographic proximity between thealkaline and subalkaline rocks indicate that the magmas represented bythese different geochemical groups were closely associated in bothtime and space. Furthermore, all the igneous rocks in the CrazyMountains were emplaced in a narrow time interval of 1-2 m.y. On aregional scale, the 51-49-Ma age span from the Crazy Mountains issimilar to that of most of the igneous centers of the central Montanaalkalic province and is coeval with the peak of widespread volcanismin the Absaroka-Gallatin volcanic field immediately to the south ofthe Crazy Mountains Basin." Descriptors: Absaroka-Gallatin volcanic field; absolute age; alkalic composition;amphibole group; Ar/Ar; basement tectonics; biotite; Cenozoic; centralMontana; chain silicates; clinoamphibole; Cordilleran Orogeny; CrazyMountains; dates; decollement; dike swarms; Disturbed Belt;emplacement; faults; folds; geometry; hornblende; igneous rocks;intrusions; laccoliths; magmas; magmatism; mica group; Montana; NorthAmerica; Paleogene; partitioning; Rocky Mountains; sheet silicates;silicates; sills; tectonics; Tertiary; thin-skinned tectonics; thrustfaults; trachytes; United States; volcanic rocks; west-centralMontana Classification: 05A Igneous and metamorphic petrology; 03 Geochronology Source: "Rocky Mountain Geology, vol.41, no.1, pp.45-55, Jun 2006"
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Reference Number: 2046 GeoREF Number: 2007-028096 Publication Year: 2006 Abstract: "New, high-precision U-Pb titanite (sphene) and zircon dates from fivesamples of the Bear Mountain intrusive complex establish the timingand duration of magmatism. The oldest, magmatic date (150.5+ or -0.6Ma) comes from dark-colored titanite from a biotite-hornblendetonalite that is part of a composite pluton that intrudes the BlueRidge ultramafic-gabbroic intrusion. Pale titanite and zircon fromthis sample yielded a distinctly younger date of 149.3+ or -0.3 Ma. Asimilar pattern of mineral dates is also apparent in two samples ofthe areally extensive Punchbowl unit of the Bear Mountain pluton.Dark-colored titanite in one of these samples yielded a date of 149.5+or -0.6 Ma, whereas the dates of pale titanite and zircon are 147.4+or -0.3 Ma. The second sample of the Punchbowl unit only contained asingle morphology of pale titanite, which yielded the same date aszircon (148.2+ or -0.3 Ma). The U-Pb zircon date of the Buck Lake unitof the Bear Mountain pluton, 148.2+ or -0.2 Ma, supports fieldevidence that the Buck Lake unit was emplaced synchronously with thePunchbowl unit. A lower age limit on magmatism in the Bear Mountainintrusive complex comes from a 145.4+ or -0.4-Ma zircon date from alate crosscutting mafic dike. All samples exhibit slight inheritancein the zircon data, with 152- to 150-Ma minimum ages. The mafic dikecontains inherited components that are at least 264 Ma and possiblyPaleo-proterozoic in age. The new dates constrain magmatism in theBear Mountain intrusive complex to the period from 151 to 147 Ma, witha minimum duration of 1.5 m.y. and a maximum of 6 m.y. The datesestablish that the emplacement and crystallization of the BearMountain intrusive complex post-dated regional thrust faulting(Orleans fault) associated with the Nevadan orogeny, including theSouth Siskiyou Fork fault, which is interpreted as an oblique-sliptear fault associated with the Orleans (thrust) fault system. Thepattern of mineral dates from the composite pluton intruded into theBlue Ridge intrusion as well as the areally more extensive Punchbowlunit indicate that crystallization of these bodies occurred over 1.5-2m.y., due to either insulating effects of the intrusive complex and/ormagma recharge." Descriptors: absolute age; Bear Mountain Complex; Bear Mountain Pluton; California;crystallization; dates; dikes; diorites; faults; igneous rocks;intrusions; Jurassic; Klamath Mountains; magmas; magmatism; Mesozoic;nesosilicates; Nevadan Orogeny; oblique-slip faults; Orleans Fault;orthosilicates; Paleoproterozoic; patterns; plutonic rocks; plutons;Precambrian; precision; Proterozoic; silicates; South Siskiyou ForkFault; tear faults; thrust faults; titanite; titanite group; tonalite;U/Pb; United States; upper Precambrian; variations; zircon; zircongroup Classification: "03, Geochronology" Source: "Special Paper - Geological Society of America, vol.410, pp.317-332,2006" map_coordinates: "Latitude:N410000,N430000 Longitude:W1230000,W1243000"
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Reference Number: 7476 GeoREF Number: 2007-026458 Publication Year: 2006 Classification: "27A Economic geology, geology of ore deposits; 05A Igneous and metamorphic petrology; 16 Structural geology" Source: "U. S. Geological Survey Professional Paper, Report: P 1733, 46 pp., 1 Sheet, 2006"
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Reference Number: 2017 GeoREF Number: 2006-060821 Publication Year: 2006 Abstract: "The Crazy Mountains dike swarm is a radial array associated with theEocene (approximately 49 Ma) Big Timber stock in the Crazy Mountainsof south-central Montana. Dikes are hosted by the stock and byPaleocene Fort Union Formation rocks that host the stock. The spatialcoincidence, field relations, and similarity of petrographic andgeochemical features suggest that the dikes and the stock arecogenetic and coeval. The dikes depict a broad range of compositions,from rhyolite to basalt; dike composition is only weakly related toposition relative to the Big Timber stock. The dikes range from 51 to71 weight percent SiO2, a slightly more restricted range than thatexhibited by the stock. Compositional variation among the dikes isless systematic than is characteristic of the stock; dike compositionsform more broadly diffuse arrays and tend to be slightly morealkaline. Both of these factors could result from late-stagemagmatic-hydrothermal processes that seem to have weakly altered manyof the dikes. Major-oxide and trace-element characteristics of thedikes and the associated stock are consistent with a mantle-dominatedgenesis in a subduction-related volcanic arc setting. In addition topetrologic and geochemical data, new (super 40) Ar/ (super 39) Argeochronologic data for strongly alkaline sills in the northern CrazyMountains are also presented. Dates for the sills, which areperipheral to the Crazy Mountains dike swarm, indicate that alkalinemagmatism was temporally associated with subalkaline magmatismrepresented by the Big Timber stock and related dikes. Characteristicsof the subalkaline magmatism are consistent with renewed volcanic arcmagmatism during the Eocene westward hingeline retreat and subsequentsouthwestward migration of the magmatic front in response tosteepening subduction along the western edge of North America." Descriptors: absolute age; Ar/Ar; Cenozoic; Crazy Mountains; dates; dike swarms;dikes; Eocene; Fort Union Formation; geochemistry; igneous rocks;intrusions; magmatism; mineral composition; Montana; Paleogene;plutonic rocks; sills; south-central Montana; Tertiary; United States;USGS; whole rock Classification: 05A Igneous and metamorphic petrology; 03 Geochronology Source: "U. S. Geological Survey Professional Paper, Report: P 1715, 21 pp.,2006" map_coordinates: "Latitude:N450000,N463000 Longitude:W1090000,W1110000"
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Reference Number: 2027 GeoREF Number: 2006-071548 Publication Year: 2006 Abstract: "Late Miocene-Pliocene (8-3 Ma) olivine basalt lavas, dated in thisstudy by the (super 40) Ar/ (super 39) Ar method, have been faultedand tilted on both the east and west sides of the Warner Range of NECalifornia, which is itself a tilted block rising to 2960 m at itscrest that is composed of Miocene-Oligocene lavas and volcaniclasticrocks. The late Miocene-Pliocene lavas, distinctively poor in K (sub2) O and rich in MgO, are called low-K olivine tholeiites and have adifferent mantle source region than that of the oldersubduction-related lavas of the main Warner Range. Hays Canyon Range(max. elev. 2400 m) lies to the east of the Warner Range, and thebroad Surprise Valley separates the two fault-bounded ranges. MiddleMiocene (ca. 15 Ma) basic lavas, with a small easterly dip, cap theHays Canyon Range and overlie Oligocene silicic ash-flow deposits anda basaltic andesite spatter volcano. Middle Miocene basic lavas alsoform the crest of the Warner Range and its westerly dip slope (approximately 15 degrees ). Nearly horizontal basic lavas of the sameage are also found on both sides of the Warner Range, and it is aplausible conclusion that these middle Miocene basalts were acontiguous group before faulting and uplift of the Warner Range.Derived estimates of uplift rates ( approximately 1 mm/yr) of theWarner Range indicate that uplift could have been initiated at ca. 4Ma, a period of the most voluminous eruption of low-K olivinetholeiite lavas. If the slower Cretaceous exhumation rate of theSierra Nevada (0.5-1.0 mm/yr) is applied to the total offset of theWarner Range (4270 m), and it did not vary with time, then the upliftof the Warner Range was initiated at ca. 8 Ma, which coincides withthe age of the oldest low-K olivine tholeiite lava (8 Ma). Low-Kolivine tholeiites require a hot shallow asthenospheric source, and itis the rise of this hot mantle that is presumed to have caused theuplift of the Warner Range. Whether or not the widespread eruption ofsmall volumes of Pliocene low-K olivine tholeiites in central andeastern Oregon is associated with crustal uplift is unknown." Descriptors: absolute age; Alturas California; Ar/Ar; basalts; Basin and RangeProvince; block structures; California; Cenozoic; dates; faults;histograms; igneous rocks; lava; Modoc County California; Neogene;nesosilicates; North America; Northern California; olivine; olivinegroup; orthosilicates; Pliocene; pumice; pyroclastics; silicates;statistical analysis; Surprise Valley; systems; Tertiary; tholeiite;tilt; United States; uplifts; volcanic rocks; volcanism; volcanoes;Warner Range Classification: 16 Structural geology; 05A Igneous and metamorphic petrology Source: "Geological Society of America Bulletin, vol.118, no.9-10,pp.1196-1211, Oct 2006" map_coordinates: "Latitude:N411500,N420000 Longitude:W1194500,W1204000"
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Reference Number: 2037 GeoREF Number: 2006-000020 Publication Year: 2006 Source: "Journal of Petrology, vol. 47, no. 12, pp.2463-2489"
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Reference Number: 7437 GeoREF Number: 2006-091383 Publication Year: 2006 Abstract: "Deciphering the magmatic history of continental magmatic arcs, ingeneral, and the growth history of individual intrusions, inparticular, is key to understanding the complex history of magmageneration, segregation, and transport that define the dynamics ofcrustal growth. We utilize high precision U-Pb geochronology toresolve a detailed magmatic history from two composite intrusions, the2-4 kbar Mount Stuart Batholith and the 7-10 kbar Tenpeak Pluton,emplaced in the Cretaceous North Cascades Arc. This temporal frameworkprovides a way to evaluate models of pluton growth that explain commonfeatures of intrusions such as concentric compositional zoning andinternal magmatic contacts. U-Pb zircon crystallization ages wereobtained from 12 samples of the Mount Stuart Batholith and 8 samplesof the Tenpeak Intrusion, representing the range of compositionaldiversity and geographical extent. These dates indicate that the MountStuart Batholith was constructed over a approximately 5.5 m.y. timeperiod that was punctuated by four intervals of high magma flux. Thedurations of the high-flux periods are short (a few hundred thousandyears) relative to the duration of the batholith. The consistentpattern of magmatic fabrics and the lack of distinct contacts in thebatholith may be explained by the juxtaposition of melt-rich and mushzones with subtle contacts between mineralogically and texturallysimilar tonalite and time-transgressive magma fabrics.In contrast, theTenpeak Intrusion was constructed over a approximately 2.6 m.y. timeperiod, with magma influx distributed throughout the intrusive historyand texturally distinct magma bodies. The Tenpeak Intrusion lacksdistinct age domains, which suggests that any magma reservoir wassmaller in size and potentially more ephemeral. Although the distinctage domains and discrete compositional and textural phases indicatethat pluton growth occurred incrementally, neither pluton bearsresemblance to a purely end-member incremental growth model whereby apluton is constructed from hundreds to thousands of discrete magmapulses that have little, if any, interaction. In particular, ages fromthe youngest domain of the Mount Stuart Batholith indicate that amelt-rich magma reservoir of > or =520 km (super 3) existed over a170 + or - 90 k.y. time span." Descriptors: absolute age; batholiths; Cascade Range; Cenozoic; Chiwaukum Schist;continental crust; crust; dates; emplacement; granodiorites; igneousrocks; Ingalls Complex; intrusions; island arcs; Jurassic;lithostratigraphy; magma chambers; magmas; magmatism; Mesozoic;models; Mount Stuart; Napeequa Complex; Nason Ridge Gneiss; Neogene;nesosilicates; orthosilicates; P-T conditions; Pliocene; plutonicrocks; processes; silicates; Swakane Gneiss; Tenpeak Pluton; Tertiary;U/Pb; United States; Washington; zircon; zircon group Classification: 05A Igneous and metamorphic petrology; 03 Geochronology Source: "Geological Society of America Bulletin, vol.118, no.11-12,pp.1412-1430, Dec 2006" map_coordinates: "Latitude:N474500,N481500 Longitude:W1205500,W1211000"
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Reference Number: 1997 GeoREF Number: 2006-000009 Publication Year: 2006 Source: Idaho Geological Survey Digital Analytical Data 2; part 1
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Reference Number: 2038 GeoREF Number: 2006-900050 Publication Year: 2006
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Reference Number: 2858 GeoREF Number: 2006-900001 Publication Year: 2006 Descriptors: isotopes; lamproites; metasomatism; Leucite Hills; Wyoming Source: Journal of Petrology volume 47 number 12 pages 2463-2489 2005
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