IGNEOUS AND TECTONIC PROVINCES OF THE WESTERN CORDILLERA 



533 



oceanic volcanic outpourings and in some of the large basalt fields of the 

 continents. In the second, olivine is generally absent or if present, is sub- 

 ordinate. Pyroxene ( hypersthene ) is prominent. This is the primary basalt 

 of the majority of plateau or flood basalts, such as in the Columbia 

 River basalt field, generally in the eugeosynclinal assemblages, and to 

 some extent in the andesite complexes of the orogenic belts. The scheme 

 of magmatic descent as he gave it is as follows: 



Olivine basalt 

 (Alkalic) 



I 



Trachyandesite 



I 



Trachyte 



I 



Phonolite 



Tholeiitic basalt 

 (Calc-alkalic) 



Andesite 



Rhyolite 



Kennedy also recognized a third magma association which he called 



the plutonic. This igneous kindred appears to be limited to the cores of 



orogenic belts, and includes all discordant and concordant batholiths, 



stocks, and sheet complexes there. It also includes the minor associated 



aplitic, pegmatitic, and lamprophyric intrusions. The plutonic associations 



consist almost entirely of granodiorite and granite together with the small 



amounts of hornblendic, basic, and ultrabasic types. The granodioritic and 



granitic plutons are generally emplaced after an episode of intense com- 



pressional orogeny, but some in places are known to have accompanied 



:the orogeny. 

 i 



Many of the rock types possess no effusive equivalents nor has any true 

 subjacent plutonic mass been found within a nonorogenic area. This latter 

 feature alone is sufficient evidence of some fundamental genetical distinction 

 between rocks of the volcanic and plutonic associations. 



We know that a granitic liquid can be produced by the fractional crystalliza- 

 tion of basaltic magma and, within the volcanic associations, the relative pro- 

 portion of acid to basic rock types and the chemical composition of the former 

 is consistent with the view that the rhvolites, granophyres and granites of the 



non-orogenic suites have been formed by high-level differentiation and frac- 

 tionation of a primary basaltic liquid. This mode of origin applies also to the 

 volcanic associations of the orogenic zones where subordinate quantities of acid 

 lavas are associated with the predominantly basic extrusives. 



The acid rocks of the true plutonic associations, however, represent such an 

 enormous bulk of granitic and granodioritic material that it is impossible to 

 conceive of their derivation from a basaltic parent and we are forced to con- 

 clude that they must have formed from some primary acid magma . . . 

 (Kennedy, 1933). 



Whereas many volcanic associations are believed to have been derived 

 from a basaltic magma which originates by remelting of a universal sub- 

 crustal basaltic layer, or by partial melting of the outer mantle, plutonic 

 associations are believed to originate by melting of a downfolded or 

 thickened part of the overlying "granitic" layer. It is commonly stated 

 that such thickening seems possible only where compressional orogeny 

 has caused the base of the silicic crust to extend down into the range of 

 melting. 



Turner and Verhoogen's Associations 



Following Kennedy, Turner and Verhoogen ( 1951 ) define a volcanic 

 association or kindred as one including all igneous rocks, intrusive as well 

 as strictly volcanic, that are genetically related to a cycle of volcanic 

 activity. They emphasize a classification based on oceanic and continental 

 distribution which is as follows: 



1. Oceanic associations (for the Pacific) 



a. Olivine basalt-trachyte (Intra-Pacific) 



b. Andesite dacite-rhyolite of marginal island arcs ( Circum-Pacific ) 



2. Volcanic associations of nonorogenic continental regions 



a. Olivine basalt-trachyte-phonolite association 



b. Leucite basalt-potash trachybasalt-trachyte association 



c. Tholeiitic basalts and equivalent quartz diabases 



3. Volcanic associations of orogenic zones 



a. Spilite-keratophyre association 



b. Basalt-andesite-dacite-rhyolite association 



The Circum-Pacific oceanic association is similar to the continental 

 orogenic basalt-andesite-dacite-rhyolite association. Both are dominantly 



