578 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



and plutonics, with the trivial exception of some alkaline bodies, are either 

 quartz latite or quartz monzonite. In other words there is no evidence of any 

 magma antecedent to these. In that sense the monzonite is primary magmatic 

 rock. 



I know of no discussions of this problem in the literature. I would be inclined 

 to feel that the magma is perhaps a regenerated one, produced by melting 

 of the deeper part of the crust (James Gilluly, personal communication, 1957). 



In a detailed study of the Ringham stock Stringham ( 1953 ) finds types 

 other than monzonite according to mineral content, such as granite, 

 monzonite, diorite, syenite, and syenodiorite, with granite the dominant 

 variety. Chemically, however, the granite is not much different from 

 quartz latite or quartz monzonite. The granite and actinolite syenite ap- 

 pear to have originated by granitization, perhaps as fringe effects of a 

 central or more deep-seated magma, according to Stringham. 



Western Montana and Eastern Idaho Andesite-Granodiorite Province 



Peculiar Nature of Province. The term, andesite-granodiorite province, 

 is used for want of a better one for the assembly of igneous rocks in south- 

 western Montana and adjacent Idaho. The province does not fall readily 

 into any of the other categories outlined at the beginning of this chapter. 

 It seems to be a hybrid of the Nevadan batholithic province and the mon- 

 zonite-latite province of the Great Rasin with certain aspects of the an- 

 desite province added. 



Several volcanic fields of Late Cretaceous age are known in the general 

 province of western Montana as shown on Fig. 36.1. They may be grouped 

 into the Livingston, the Elkhorn Mountains, and Adel Mountain fields 

 and the Hogan formation. The Livingston and Adel Mountain fields are 

 classed under the Yellowstone subprovince of calc-alkalic rocks, whereas 

 the Elkhorn Mountains and Hogan fields are andesites and latites, and 

 therefore less calc-alkalic. They belong more properly to the orogenic belt. 



Elkhorn Mountains Field. 



Remnants of a thick plateaulike accumulation of calc-alkaline volcanic rocks 

 of probable Late Cretaceous age — the Elkhorn Mountains volcanic rocks — are 

 exposed in an area of about 3700 square miles around the Boulder batholith 

 in the Elkhorn Mountains and Boulder Mountains, western Montana. The 

 presence of similar rocks across the Jefferson River to the south, and near Wolf 



Creek to the north, suggests that the volcanic pile once covered more than 

 10,000 square miles. 



In places these rocks rest unconformably on Paleozoic and perhaps older 

 rocks. Elsewhere they are gradational into underlying tuffaceous sedimentary 

 rocks of Late Cretaceous age, and the contact is arbitrarily placed at the base 

 of die lowest volcanic conglomerate, breccia, or flow. 



The volcanic pile comprises three major units; maximum thickness of each 

 exceeds 5000 feet. The lower unit consists predominandy of dacitic, andesitic, 

 and basaltic fragmental rocks and autobrecciated lava flows. The middle 

 unit is about half quartz latite in welded tuff sheets as much as 300 feet 

 thick, interlayered with more calcic bedded pyroclastic rocks and autobrecciated 

 lava flows; it is locally unconformable on the lower unit. The upper unit 

 consists dominandy of reworked volcanic rocks and subordinately of fine- 

 grained pyroclastic rocks. A thick succession of basalt flows near Elliston, 

 Montana, may be equivalent to the upper part of this unit or may be younger. 



The volcanic rocks were altered by and locally foundered in penecon- 

 temporaneous shallow-magma reservoirs. They were folded and faulted and 

 later invaded and thermally metamorphosed by the Boulder batholith (Klepper 

 and Smedes, 1959). 



Regarding the intrusive rocks, Klepper et al. (1957) say: 



The intrusive igneous rocks, except for a few felsite dikes of uncertain age, 

 are divisible into two groups, primarily on the basis of structural relations 

 and secondarily on the basis of composition and fabric. The older group of 

 dioritic and andesitic rocks were intruded in part, if not wholly, prior to the 

 main folding and are similar in chemical and mineralogical composition to the 

 Elkhorn Mountains volcanics. They were probably emplaced throughout the 

 period of volcanism that commenced in late Niobrara time and continued 

 until late Cretaceous time. The younger group consists chiefly of quartz- 

 bearing phanerites but includes rocks ranging from gabbro to alaskitic granite 

 and aplite. These rocks were emplaced after the main episode of folding and 

 faulting. The Boulder batholith, composed dominantly of quartz monzonite, 

 is the principal body of this younger group (Klepper et al., 1957). 



Although Klepper and Smedes class the Elkhorn volcanics as calc- 

 alkalic, the rocks are less alkaline than the Livingston and Adel Mountain 

 volcanics and, with the associated intrusive rocks, are more closely re- 

 lated to the igneous rocks of the Great Rasin than to those of central 

 Montana. 



Hogan Field. The volcanic rocks of the Hogan formation, according 

 to George Viele (Ph.D. thesis, University of Utah, 1960), are nearly 2500 

 feet thick and consist of interbedded breccia, welded tuff, volcanic-rich 

 graywackes, shale, black sandstone, and arkose. Andesitic and more acidic 



