IGNEOUS PROVINCES IN WESTERN UNITED STATES 



579 



eruptives provided the material for the pyroclastics. The rocks are not 

 rich in the alkalies and hence stand apart from the nearby Adel volcanics. 

 The nearby Adel volcanics, according to Viele, are probably of St. Mary 

 River formation age (latest Cretaceous) and extend into the Paleocene. 

 The Hogan volcanics are slightly older and are correlated with the upper 

 part of the Two Medicine formation and the lower Horsethief formation. 

 The Adel field has been only slightly tilted; whereas the Hogan field 

 beds have been involved in the folding and thrusting of the Foothill 

 belt. 



Batholiths and Stocks. The region is noted for its large intrusives, 



particularly the largest, the Boulder batholith. This cluster of intrusives 



in west-central Montana and adjacent Idaho is easily the most voluminous 



! anywhere in the Laramide belts. The plutons consist dominantly of quartz 



| monzonites and granodiorites, although a number of variations in facies 



and separate intrusions have been noted which range from gabbro to 



granite. It has been pointed out by Emmons and Calkins (1912) that the 



j intrusives of the west-lying Philipsburg district are less alkalic than the 



i Boulder batholith and that the Boulder batholith has mild alkalic affinities 



to the high alkalic rocks of the central Montana petrographic province. 



\ This may mean that fusion of the crystalline basement is to be reckoned 



with, and that the fused rocks become less alkalic westward. 



The Boulder batholith has been studied by Profesor Knopf ( 1957 ) . He 

 I describes it as follows : 



On the basis of a recent potassium-argon age determination, which gave 

 87 million years as the most probable age of the granodiorite of the Boulder 

 bathylith, it is concluded that the bathylith was emplaced late in Cretaceous 

 time. 



The Boulder bathylith has hitherto been considered to be a one-magma 

 intrusion, but like other large plutonic masses it proves to be of composite 

 construction. The order of intrusion is ( 1 ) Unionville granodiorite, a basic 

 hypersthene-bearing granodiorite which itself has developed basic faces of 

 granogabbro; (2) Clancy granodiorite; (3) porphyritic granodiorite; (4) 

 biotite adamelite; and (5) muscovitic biotite granite. Alaskite and aplite 

 are abundant and were presumably (but not yet proved) developed most 

 abundandy during the final stages of bathylithic consolidation. The order of 

 emplacement of the successive intrusives is in the order of increasing silicity. 



The Boulder bathylith and its satellitic stocks, have exerted extensive 

 contact metamorphism, both purely thermal and pyrometasomatic. Most 



notably, the Helena dolomite has been transformed into aphanitic tremolite- 

 diopside hornfels to a maximum distance of 10,000 feet from the edge of the 

 bathylith. The highest rank of metamorphism attained is in sillimanite-cordi- 

 erite-microperthite hornfels, remarkable rocks that have formed at widely 

 separated localities. In places the magma has reacted with limestone xenoliths 

 with the result that the xenolith is surrounded by an aureole of augite granodio- 

 rite. In other places the evidence appears to demand that the magma in 

 depth had dissolved limestone. By this syntexis alkalic rocks were generated 

 that range from mildly alkalic, such as the Priests Pass leucomonzonite and 

 the syenodiorite of the large stock northwest of Helena, to strongly alkalic, as 

 represented by the nepheline shonkinite occurring east of Montana City. 



In order to be consistent with the epoch designations for the absolute 

 ages of the Sierra Nevada batholiths, we must assign a mid- or early Late 

 Cretaceous age to the 87 m.y. date by the potassium-argon method of the 

 Boulder batholith. Since the batholith is composite we wonder whether 

 an early or late pluton in the intrusion cycle there is dated. It should be 

 noted also that a date of about 103 m.y. by the lead-alpha activity ratio 

 method is assigned to the Idaho batholith, but again, it is not known what 

 part of the batholithic cycle is dated. See Chapter 21. The difference in 

 method used also leaves the comparison uncertain. For the time being, 

 however, we should presume that the Boulder batholith and associates in 

 western Montana are slightly younger than the Idaho batholith. 



Conclusions. The batholiths and stocks are fairly similar in composi- 

 tion to those of the Great Basin but have overtones of similarity in their 

 variations and size with the major batholiths of the Nevadan belt. The 

 volcanics are more andesitic than the dominantly latitic volcanics of the 

 Great Basin. It seems, therefore, that the western Montana and eastern 

 Idaho province displays characteristics transitional from the moderately 

 orogenic region of the Basin and Range province to the intensively oro- 

 genic region of the Pacific marginal regions. 



Late Precambrian (?) Sills and Flows. Sills, flows, and some dikes of 

 basic rock have been noted in a number of places in the region of the 

 Boulder batholith and northward through the Garnet Range to Glacier 

 National Park. The sills are all intrusive into the upper part of the Belt 

 series and appear as dark beds of remarkably uniform thickness for main- 

 miles. They also hold remarkably well to a single stratigraphic horizon 

 and range up to 300 feet thick. 



