566 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



chemical features, although they commonly range from highly mafic to 

 highly felsic. Each stage is separated from the other by intervals during 

 which few or no eruptions occurred, but instead, extensive erosion. 



In each of the stages a rock near the mafic end is believed to represent the 

 primary magma. This rock ranges from an ordinary basalt to orthoclase basalt 

 to plagioclase shonkinite to shonkinite rich in potash and lacking plagioclase. 

 The gradational character of the eruptive stages and their close association in 

 time and space indicate a common origin. Two periods of magmatic differentia- 

 tion are required: first, a deep-seated differentiation of a basaltic magma 

 from which crystals of calcic plagioclase and hypersthene were removed and 

 second, a shallower differentiation to form the magmas of the individual erup- 

 tive stages. The relative flatness of the sedimentary rocks into which and 

 through which the magmas have moved indicates that the magmas have not 

 been disturbed by orogenic forces; therefore they could have differentiated 

 during the long quiet interval which seems necessary. The second period of 

 magmatic differentiation by crystal settiing was characterized, in most stages, 

 by assimilation of siliceous material. The amount of assimilated material 

 was especially large in the Crazy and Little Belt mountains, where syenites were 

 followed by granites (Larsen, 1940). 



The abundant flows and dikes of mafic phonolite, and flat laccoliths 

 and dikes of chemically equivalent shonkinite are derivatives of basic 

 potassic magmas. Syenite is undoubtedly a differentiate of a parent 

 shonkinite magma after intrusion as a sill or laccolith (Turner and Ver- 

 hoogen, 1951). Larsen (1940) believes essentially that all petrographic 

 and chemical variations within this region may be explained in terms of 

 magmatic differentiation from an olivine basalt. A long perod of un- 

 disturbed differentiation in depth is required in which settling of olivine 

 and diopsidic augite takes place to leave the melt enriched in IC.O. Turner 

 and Verhoogen ( 1951 ) would place more emphasis on reactive assimila- 

 tion with the granitic basement. 



Summary. The province of high alkalic rocks has the following charac- 

 teristics: 



1. The region is one of crustal stability for the most part. It was a shelf 

 to the west-lying miogeosyncline and part of the interior stable region in 

 Paleozoic time. Triassic and Jurassic deposition was thin but Cretaceous 

 sediments accumulated in several separate intermontane basins to a 

 thickness of about 5000 feet. The total section of nearly flat-lying sedi- 

 mentary rocks did not exceed 10,000 feet in any place, and in some areas, 



as in central Colorado, only a few hundred feet of sedimentary rocks 

 existed at the time of igneous activity. 



2. The relatively thin veneer of sedimentary rocks rest directly on 

 metamorphosed crystalline rocks, generally of a gneissic or schistose 

 character. In the region of high alkalic rocks no Beltian type rocks are 

 known, except in west-central Montana on the border of the alkalic 

 province. This feature correlates well with the common observation of 

 granitic, gneissic, and amphibolitic inclusions in rocks of a number of the 

 igneous centers, and also with the conclusion that such crystalline rocks 

 have been assimilated in various amounts by an olivine basalt magma. 

 The inference is warranted that olivine basalt underlies the "granitic" 

 crust directly, that the primary activity begins in the basaltic layer or sub- 

 crust, then proceeds to the granite crust where assimilation takes place. 

 With stable crustal conditions prevailing, the various alkalic rocks origi- 

 nate through fractional crystallization, intrusion, and further differentia- 

 tion. 



3. This is a region of high BaO and SrO and also of the most abundant 

 uranium ores so far discovered in the West. Such elements may have been 

 derived from the assimilated Precambrian crystalline rocks and later 

 concentrated by differentiation. The U s O s would be further concentrated 

 by meteoric or epithermal processes. 



4. No basalt is found in the laccolithic groups, but these igneous cen- 

 ters stand apart from the others in having only small volumes of intruded 

 magma and relatively stiff cold magmas at the time of intrusion. In the 

 other fields, in fact in most all volcanic fields of any size, basalt is erupted 

 generally either early or late in the history of the field, and therefore we 

 must think of a facility whereby some basalt from the subcrust makes its 

 way directly to the surface without an intermediate rest stage for assimila- 

 tion or differentiation. 



5. The Rockies of Montana, Wyoming, Colorado, New Mexico, and 

 Utah including the Colorado Plateau, are east of the fold belt of the cen- 

 tral Rockies and are the result primarily of large domal uplifts with 

 lateral gravity slide affects in places. See Fig. 25.12. The surficial igneous 

 centers in the trachyte-phonolite province occur in the basins, domes, and 

 across monoclinal flexures, and graben. If the domal uplifts are supported 



