Date File Created	Name of Creator																						
10/12/07	Taylor Brown																						
GeoRef Data	2006-058742																						
DN	GeoRef																						
TI	"(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"																						
AU	"Harlan, Stephen S"																						
SO	"Rocky Mountain Geology, vol.41, no.1, pp.45-55, Jun 2006"																						
DE	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																						
AB	"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."																						
Sample Data																							
Item	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	LatDD	LongDD	Loc_prec	Rock_Class	Sample comment	Collected_by	Sample_Description	Rock_Type	Location_Name	Material	State
Units	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	DD	DD									
Method	1	1	1	2	2	2	3	3	3	4	4	4	5	5									
Sample Name																							
82VP-2	50.61	50.75	50.47				48.5	53	44				46.2023	-110.4592	0.0001	Trachyte	Trachyte sill	Harlan	Sill	Plutonic	"Robinson anticline, Davey Butte"	mineral	Montana
202338				50.1	50.22	49.98				50.5	51.1	49.9	46.9943	-110.4479	0.0001	syenite	Mafic nepheline syenite 	Harlan	sill	Plutonic	Ibex Mountain Sill	mineral	Montana
83VP-75				50.1	50.29	49.91				50.3	52.2	48.4	46.1873	-110.4918	0.0001	syenite	Mafic nepheline syenite 	Harlan	sill	Plutonic	Robinson anticline	mineral	Montana
83VP-65				50.03	50.14	49.92				49.3	51.2	47.4	46.1875	-110.4637	0.0001	syenite	Mafic nepheline syenite	Harlan	 sill	Plutonic	Robinson anticline	mineral	Montana
83VPCC-1				50.06	50.23	49.89				52.3	54.3	50.3	46.2984	-110.4453	0.0001	syenite	Mafic nepheline syenite	Harlan	 dike	Plutonic	Comb Creek stock	mineral	Montana
83VPCC-12				50.22	50.37	50.07				48	49.9	46.1	46.307	-110.499	0.0001	syenite	Nepheline syenite 	Harlan	stock	Plutonic	Comb Creek stock	mineral	Montana
Method Data																							
1	AR-AR:HB	118																					
2	AR-AR:BIO	118																					
3	K-AR:HB	118																					
4	K-AR:BIO	118																					
5	TABLE	2006-058742