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Title:
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(super 40) Ar/ (super 39) Ar dates from alkaline intrusions in thenorthern Crazy Mountains, Montana; implications for the timing andduration of alkaline magmatism in the central Montana alkalicprovince
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Reference Number:
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2016
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Publication Year:
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2006
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GeoREF Number:
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2006-058742 (View Original Data File)
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Authors:
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Harlan, Stephen S
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Descriptors:
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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
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Source:
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Rocky Mountain Geology, vol.41, no.1, pp.45-55, Jun 2006
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Abstract:
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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.
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Copyright:
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GeoRef, Copyright 2007, American Geological Institute. Abstract,copyright, University of Wyoming | Reference includes data fromGeoScienceWorld, Alexandria, VA, United States
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