IGNEOUS AND TECTONIC PROVINCES OF THE WESTERN CORDILLERA 



535 



'three ancient orogens welded onto the Scandinavian-Raltic shield," and 

 particularly to the Scottish Paleozoic. 



Diastrophism 



Kindreds 



Locus 



I. Geosynclinal phase 1. Ophiolitic kindred 



II. Orogenic phase (with 2. Granodiorite-andesite 



two or three subphases) kindred 



III. Post-orogenic phase 3. Trachybasaltic kindred 



(with two subphases) 4. Quartz dolerite kindred 



In orogen 



In kratogen 



Proposed Classification of Provinces 



In the western United States, certain igneous rock associations stand 

 clearly apart from others. Discussions generally center about such strik- 

 ing igneous provinces as the Cascade Mountains, the San Juan Moun- 

 tains, or central Montana, yet no one has published a map of the entire 

 western United States on which are grouped the many volcanic fields 

 and plutons into igneous provinces. Several emphasize the transitional 

 and elusive nature of boundaries, and this is certainly realized when one 

 attempts to draw them. The main goal of this chapter is thwarted, how- 

 ever, if the petrographic provinces are not mapped and compared with 

 the tectonic provinces. 



In struggling with the problem, difficulties in two categories arise. 



First, in the provinces of extensive basalt outpourings a distinction be- 



jtween rocks of the olivine basalt kindred and the tholeiitic kindred is 



Commonly obscure. The problem is met with specifically in classifying 



\he Malheur and Snake River basalt fields. Second, in the alkalic and 



^alc-alkalic "provinces" of the Rocky Mountain states, the boundaries 



)f the numerous subdivisions suggested in the literature are generally 



mpossible to fix or map. Second, the main kind or kinds of rock present 



s generally a characteristic feature which can be mapped objectively, 



vhereas the kindred represented may be controversial. 



A classification believed better suited for tectonic studies is as follows, 

 t will serve as a guide in the discussion of the igneous rock provinces of 



the western Cordillera of the Americas, and is especially adapted to the 

 western United States. 



A. Rasalt provinces 



1. Oceanic (mostly olivine basalts) 



2. Continental flood and cinder cone fields (both olivine and tholeiitic 

 basalts) 



R. Andesite provinces 



1. Eugeosynclinal (mostly tholeiitic basalts and andesites-spilites and 

 keratophyres characteristic ) 



2. Volcanic arcs 



3. Orogenic belt ( post-batholithic volcanic fields) 



4. Stratovolcanos of continental margin 



C. Trachyte and phonolite provinces 



1. Alkalic (leucite basalt-trachyte-phonolite group) 



2. Calc-alkalic ( olivine basalt-phonolite association, also andesite and 

 rhyolite ) 



D. Latite-monzonite provinces 



E. Rasalt-rhyolite provinces 



F. Grandiorite-granite batholithic provinces 



1. First cycle 



2. Second cycle 



The petrologic terms basalt, andesite, latite, trachyte, and phonolite 

 are used to denote the main type of rock of the province. In the andesite 

 provinces especially, differentiation products are common as well as 

 olivine and tholeiitic basalts. The basalt-rhyolite provinces specify those 

 in which the intermediate to subacid differentiates are dominant. 



Evident Tectonic and Igneous Cycle 



In the orogenic belts that form the margins of the continents, such as 

 exemplified by the Sierra Nevada of California and the Acadian belt of 

 New England, the main events follow a fairly consistent pattern or cycle. 

 The one given below is modeled after Turner and Verhoogen, (1951), 

 but with additions and modifications as seen necessary from a study of 

 the Cordillera of South and North America. 



