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⁴⁰Ar/³⁹Ar dates from alkaline intrusions in the northern Crazy Mountains, Montana

Implications for the timing and duration of alkaline magmatism in the central Montana alkalic province

Department of Environmental Sciences and Policy, George Mason University, Fairfax, VA 22030-4444, U.S.A.

e-mail: sharlan{at}gmu.edu

Intrusive rocks in the Crazy Mountains, Montana, consist of numerous stocks, dike swarms, laccoliths and/or sills of strongly alkaline to subalkaline affinity that have intruded and metamorphosed Tertiary sedimentary strata of the Crazy Mountains Basin. The subalkaline rocks form stocks, sills, and radiating dikes and are located primarily in the southern Crazy Mountains (e.g., Big Timber stock, Loco Mountain stock). With the exception of the Ibex Mountain sill (?), the alkaline rocks are restricted to the northern Crazy Mountains. New ⁴⁰Ar/³⁹Ar dates are reported for alkaline rocks of the northern Crazy Mountains, with results ranging between 50.61 Ma and 50.03 Ma. Five dates from the strongly alkaline nepheline and mafic nepheline syenites of the Ibex Mountain sill (?), Robinson anticline intrusive complex, and Comb Creek stock (?) and dike swarm give tightly clustered dates suggesting that they were emplaced during a restricted time interval at 50.1 Ma. The dates from the alkaline rocks of the northern Crazy Mountains are slightly older than those previously reported from the subalkaline Big Timber stock in the southern Crazy Mountains (i.e., 49.3–49.2 Ma, biotite ⁴⁰Ar/³⁹Ar) (du Bray and Harlan, 1996). However, the limited span of dates (i.e., 50.6–49.2 Ma) and the geographic proximity between the alkaline and subalkaline rocks indicate that the magmas represented by these different geochemical groups were closely associated in both time and space. Furthermore, all the igneous rocks in the Crazy Mountains were emplaced in a narrow time interval of 1–2 m.y. On a regional scale, the 51–49-Ma age span from the Crazy Mountains is similar to that of most of the igneous centers of the central Montana alkalic province and is coeval with the peak of widespread volcanism in the Absaroka-Gallatin volcanic field immediately to the south of the Crazy Mountains Basin.

Key Words: Absaroka-Gallatin volcanic field • alkaline magmatism • Cenozoic • central Montana alkalic province • Crazy Mountains • Cordilleran orogenic belt • disturbed belt • geochronology • intrusive structures • magmatism • Montana • tectonics

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