33. 



IGNEOUS AND TECTONIC 



PROVINCES OF 



THE WESTERN CORDILLERA 



OBJECTIVES 



Volcanic rocks cover large parts of the surface of the western United 

 States and, by forming appreciable segments of certain sedimentary 

 sequences, underlie other extensive areas. The Nevadan batholiths are 

 possibly the most voluminous of all rock units. At least three hundred 

 stocks and small batholiths occur in Nevada, Utah, Arizona, Colorado, 

 Montana, and Idaho, and numerous laccoliths, sills, and dikes have been 

 described in the Colorado Plateau, Wyoming, Montana, and Colorado. 

 So much of our attention is focused on the sedimentary rocks that the 

 extensive array of igneous rocks is generally passed by with only in- 



cidental reference. It is the object here first to summarize the kinds and 

 distribution of the igneous rocks in the western Cordillera of South and 

 North America, and then second, to find a relation, if any, to the tectonic 

 divisions. 



We are always seeking an answer to the deep-seated cause of mountain 

 building, and since the primary magmas are generally thought to have 

 developed in the base of the silicic crust, in the basaltic subcrust, or in 

 the outer mantle shell, it is possible that a careful analysis of the distri- 

 bution patterns of igneous rocks and their parentage may help us under- 

 stand the nature of orogeny. This will be the final objective. 



CONCEPT OF IGNEOUS PROVINCES 



Kennedy's Associations 



It has long been recognized that certain regions are characterized by a 

 related assemblage of extrusive and intrusive rocks, and that this assem- 

 blage differs from an adjacent one in dominant petrologic types, chemical 

 composition, and nature of extrusion or intrusion. Such a region will here 

 be called an igneous province. The rocks of one province may be relatively 

 uniform in composition such as the basaltic rocks of the Columbia River 

 Plateau, or they may be varied both in mineralogy and chemical composi- 

 tion, such as the olivine basalt-nepheline basalt-melilite basalt-trachy- 

 andesite-trachyte-phonolite differentiation series of the San Juan 

 Mountains. 



In spite of the striking variation in mineral and chemical composition 

 in these series, it is evident that certain primary magmas are indicated from 

 which the series have evolved either directly by magmatic differentiation 

 or by differentiation along with the assimilation of certain kinds and 

 amounts of country rock. (See Turner and Verhoogen, 1951, for a system- 

 atic discussion of the process and problems.) 



Professor W. Q. Kennedy of the Scottish Geological Survey postulated 

 in 1933 that the differentiation series and the great basalt fields come from 

 two basic kinds of primary magmas, namely, olivine basalt and tholeiitic 

 basalt. The first is characterized by appreciable olivine and augite, and 

 is commonly alkalic. Kennedy recognized it as the type present in the 



532 



