Date File Created	Name of Creator
2/8/06	Rich Gaschnig
GeoRef Data	2005-062998
DN	GeoRef
MT	"super 40 Ar/ super 39 Ar and K-Ar geochronology and tectonicsignificance of the Upper Cretaceous Adel Mountain Volcanics andspatially associated Tertiary igneous rocks, northwestern Montana"
AU	"Harlan, Stephen S; Snee, Lawrence W; Reynolds, Mitchell W; Mehnert,Harald H; Schmidt, R G; Sheriff, Steve D; Irving, Anthony J"
AF	"George Mason University, Fairfax, VA, United States USA"
SP	"USGS, Publications of the U. S. Geological Survey"
SO	"U. S. Geological Survey Professional Paper, Report: P 1696, 29 pp.,2005"
IS	1044-9612
RP	P 1696
DE	absolute age; Adel Mountain Volcanics; anticlinoria; Ar/Ar; Cenozoic;Cretaceous; dates; deformation; dikes; extension; feldspar group; foldand thrust belts; folds; framework silicates; igneous rocks;intrusions; K/Ar; lithostratigraphy; Mesozoic; Montana; monzonites;northwestern Montana; paleomagnetism; plagioclase; plutonic rocks;silicates; sills; tectonics; Tertiary; United States; UpperCretaceous; USGS
AB	"We report new super 40 Ar/ super 39 Ar and K-Ar dates from theUpper Cretaceous Adel Mountain Volcanics of northwestern Montana andspatially related Tertiary igneous rocks. The Adel Mountain volcanicfield consists of about 900 square kilometers of lavas, associatedvolcaniclastic strata, and intrusions that lie astride the easternmostfolds of the Montana disturbed belt of the Cordilleran fold and thrustbelt. The Adel Mountain volcanic rocks have been intensely deformed byfolds and thrust faults along their southwestern margin but areessentially undeformed to the east. Prior to isotopic dating, the ageof the Adel Mountain Volcanics was the subject of debate, with ageassignments ranging from Late Cretaceous to early Tertiary. Isotopicdates reported here demonstrate that the Adel Mountain Volcanics areclearly Late Cretaceous and that the volcanic rocks were probablyemplaced during an approximately 2- to 3-million-year interval betweenabout 76 to 73 mega-annum Ma. The new dates from the Adel MountainVolcanics are significant in that they provide a more refined andreliable age for the Late Cretaceous cratonic paleomagnetic referencepole for North America. The dates from the Adel Mountain Volcanics, aswell as those from spatially related younger intrusions, also provideimportant constraints on the age of fold and thrust-belt deformationalong the eastern margin of the Montana disturbed belt. Syntectonicdeformation of the Adel Mountain Volcanics, as well as apparentfolding and faulting of Tertiary quartz monzonite sills, indicatesthat contractional deformation clearly spanned the Late Cretaceous andmay have extended to as young as the Paleocene/Eocene boundary atabout 55.5 Ma. Elsewhere, posttectonic field relationships indicatethat deformation may have ended prior to 60 Ma. Complexities in fieldrelationships with respect to folds and faults shown by the earlyTertiary intrusions as well as complications in the argon systematicsindicate that these interpretations must be considered preliminary.Further field work and structural studies and additionalhigh-precision geochronology are needed in order to place limits onthe cessation of contractional deformation in this part of Montana.Unambiguously posttectonic dikes 47.5 Ma that cut all deformed rocksand structures in the area indicate that disturbed belt deformationhad clearly ceased by the early middle Eocene prior to the onset ofwidespread crustal extension in this part of the northern Cordillera."
LA	English
FE	"References: 44; illus. incl. 1 table, geol. sketch maps"
PY	2005
PT	Serial; Report; Monographic																																						
CP	United States USA																																						
LL	"Latitude:N470000,N471500 Longitude:W1114500,W1121500"																																						
CL	05A Igneous and metamorphic petrology; 16 Structural geology																																						
CY	"GeoRef, Copyright 2005, American Geological Institute."																																						
UD	200521																																						
AN	2005-062998																																						
Sample Data																																							
Item	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Volcanic_Field	Material	sample comment	Rock_Class	Rock_Type	LatDD	LongDD	Loc_prec	Location_Quad	Location_Name	Sample_Description	State
Units	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	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	5	6	6	6	7	7	7	8	8	8	9	9	9						11	11					
Sample Name																																							
WC91-14b																						82.4	83.1	81.7				"Two Medicine Formation, Western Volcanic Member, Unit C"	whole rock	crystal-vitric ash-flow tuff	ash-flow tuff	volcanic	46.9943	-112.0731	0.001	Sheep Creek 7.5	"exposed in Burlington Northern ballast quarry along U.S. 287, approximately 1 mile south of Wolf Creek"	devitrified; contains abundant pumice and small lithic fragments	MT
WC69-3	73.2	75.3	71.1	75.7	76.2	75.2																			75.7	76.2	75.2	Adel Mountain Volcanics	whole rock	augite-biotite trachybasalt breccia	trachybasalt	volcanic	47.1306	-111.8833	0.001	Mid Canon 7.5	"east wall of valley of the Missouri River, 3 to 6 m east of Frontage Road formerly US Hwy 91"	"porphyritic with large phenos of augite in groundmass of labradorite, sanidine, augite, biotite, hornblende, quartz, magnetite, and apatite; sample is from a large angular block of trachybasalt in a thick layer of coarse volcanic breccia in lower part of the Adel Mountain Volcanics"	MT
88AV-12													81.1	88.1	74.1	74.8	75.3	74.3										Adel Mountain Volcanics	whole rock	aphyric shonkinite flow	shonkinite	volcanic	47.1339	-111.861	0.001	Mid Canon 7.5			MT
A-29a-89										74.74	75.16	74.32													74.74	75.16	74.32	Adel Mountain Volcanics dike	whole rock	hornblendite xenolith in trachyandesite dike	trachyandesite	volcanic	47.1722	-111.6833	0.001	Hardy 7.5	exposed 0.5 km north of Hardy Bridge		MT
A-31-89										73.17	73.59	72.75													73.17	73.59	72.75	Adel Mountain Volcanics dike	whole rock	sanidine-phyric microsyenite dike	syenite	plutonic	47.1889	-111.7958	0.01	Hardy 7.5	"exposed along Chestnut Valley road, 1 km NE of Halfbreed Rapids"	"dike contains large, flow-aligned, altered sanidine phenocrysts in a groundmass of hb, augite, feldspar, and ox"	MT
WC70-2	72.9	75	70.8	73.9	74.6	73.2	75	77.3	72.7	73.9	74.3	73.5													73.9	74.3	73.5	Adel Mountain Volcanics dike	whole rock	hornblende-biotite monzonite dike	monzonite	plutonic	47.1883	-111.7958	0.001	Hardy 7.5	dike exposed along south side of Sheep Creek road	dike is considered to be the youngest or one of the youngest rock types in the Adel Mountain volcanic field	MT
AMV90-3				74.2	74.8	73.6																						Adel Mountain Volcanic dike	whole rock	biotite trachybasalt dike	trachybasalt	plutonic	46.9361	-111.6833	0.001	Middle Creek Lake 7.5	NW edge of Middle Creek Lake		MT
WC70-8	73.7	75.8	71.6	73.72	74.34	73.1																			73.72	74.34	73.1	Adel Mountain Volcanics dike	whole rock	biotite trachybasalt dike	trachybasalt	plutonic	47.0783	-111.8117	0.001	The Sawteeth 7.5	about 49 m above and south of Little Stickney Creek on nose of ridge		MT
WC70-6	74	76.1	71.9	74.67	75.09	74.25																			74.67	75.09	74.25	intrusion cutting Adel Mountain Volcanics	whole rock	biotite monzonite	monzonite	plutonic	47.0828	-111.7908	0.001	The Sawteeth 7.5	near crest of ridge between Little Stickney Creek and North Fork of Stickney Creek	probably part of Three Sisters stock	MT
88AV01	71.2	76.6	65.8	73.4	73.8	73																			73.4	73.8	73	Adel Mountain Volcanics dike	whole rock	biotite shonkinite	shonkinite	volcanic	47.2885	-111.9398	0.001	Telegraph Mountain 7.5	outcrop obvious where the dike crosses the St. Peter Mission road	"phenos of plagioclase, cpx, very fresh biotite, and magnetite"	MT
WC91-23				73.68	74.08	73.28																						Shaw Butte laccolith	whole rock	biotite monzonite	monzonite	plutonic	47.4742	-111.8345	0.001	Cascade Colony 7.5	"taken from freshly blasted leucocratic vein exposed in quarry, north side of Shaw Butte"	"prominent euhedral cpx similar to those in Square Butte intrusion; no obvious K-bearing phases in main mass of shonkinite but diversely oriented leucocratic veins, possibly representing late-stage melt segregation, contain euhedral platy minerals appearing to be biotite"	MT
WC69-5							83.3	85.8	80.8	81.2	81.7	80.7														75.9	0	intrusion that may cut Adel Mountain Volcanics ?	whole rock	hornblende monzonite	monzonite	plutonic	47.2278	-111.7972	0.001	Hardy 7.5	south flank of Finigan Mountain	"porphyritic with phenos of hornblende and andesine in groundmass of andesine, sanidine, hornblende, biotite, quartz, magnetite, and apatite; poor exposure makes it difficult to determine the true relation of this unit to the volcanics; it is believed to be younger than the Adel Mountain complex"	MT
WC69-1	59.6	61.2	58																									sill apparently cut by Steinbach thrust	whole rock	biotite quartz monzonite porphyry	monzonite	plutonic	47.1593	-112.3158	0.001	Johnson Mountain 7.5	"3 to 6 m thick sill exposed in south wall of valley of Green Creek, approximately 15 m E of Montana State Hwy 200"	"phenos of sanidine in groundmass of sanidine, oligoclase, quartz, biotite, hornblende, magnetite, and apatite"	MT
WC70-3	55.8	57.2	54.4	56.8	57.1	56.5																			56.8	57.1	56.5	sill intruding lower part of western facies of Two Medicine Formation	whole rock	biotite monzonite porphyry	monzonite	plutonic	47.1572	-112.1467	0.001	Comb Rock 7.5	exposed in roadcut on Deadman Coulee Road		MT
WC91-11				56.2	56.5	55.9																						sill intruding lower part of western facies of Two Medicine Formation	whole rock	biotite monzonite porphyry	monzonite	plutonic	47.1573	-112.1463	0.001	Comb Rock 7.5	probably same or similar exposure that WC70-3 was collected from		MT
WC70-4	54.9	56.5	53.3	52.1	52.7	51.5	64.8	66.4	63.2	60.7	61.1	60.3																sill intruding western facies of Two Medicine Formation and cut by thrust fault	whole rock	biotite hornblende quartz monzonite porphyry	monzonite	plutonic	47.1806	-112.1625	0.001	Comb Rock 7.5	exposed in upper part of a steep ravine on E side of prominent ridge The Reef about 2.7 km N of Deadman Coulee Road		MT
WC70-5							50.4	51.6	49.2	47.48	47.78	47.18													47.21	47.51	46.91	dike intruding volcanic member of western facies of Two Medicine Formation	whole rock	hornblende monzonite	monzonite	plutonic	47.0778	-112.1417	0.001	Roberts Mountain 7.5	dike is exposed in roadcut on secondary Hwy 434 through Rattlesnake Reef	47.21 +/- 0.3 is preferred age	MT
WC69-2							47.5	48.8	46.2	47.55	48.03	47.07													47.55	48.03	47.07	dike intruding volcanic member of western facies of Two Medicine Formation	whole rock	hornblende monzonite	monzonite	plutonic	47.0778	-112.1417	0.001	Roberts Mountains 7.5	dike is exposed in roadcut on E side of Stearns Wolf Creek Road	"rock is fine-grained and composed of small crystals of sanidine, oligoclase, hornblende, augite, biotite, quartz, apatite, and magnetite; dike extends NW, crosses trace of Eldorado thrust, and extends into Middle Proterozoic strata of the Belt Supergroup in the hanging wall of the Eldorado thrust plate"	MT
Method Data																																							
1	K-AR:BIO	106																																					
2	AR-AR:BIO	106	
3	K-AR:HB	106	
4	AR-AR:HB	106	
5	K-AR:WR	106	
6	AR-AR:WR	106	
7	K-AR:PLAG	106	
8	AR-AR:PLAG	106	
9	REG_COR	106	"Indicates the preferred age of the sample, if both K-Ar and 40Ar/39Ar dates are given or if field relations contradict the dates"
10	TABLE	2005-062998	
11	NN	2005-062998