582 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



basaltic and the granitic. The basaltic magma is of two classes, olivine and 

 tholeiitic, with transitional varieties recognized. The granitic type ranges 

 from tonolite to alaskite, and is considered to have originated in two 

 slightly different ways. The origin of the primary magmas will be con- 

 sidered under later headings. 



It is evident that both olivine basalt and tholeiitic basalt magmas have 

 been conducted up through the granitic batholitic complex, and hence 

 both varieties in large amounts can supersede the granodioritic magma in 

 certain places. Smaller amounts of basaltic magma, probably all of the 

 olivine variety, have made their way up through the crust in the province 

 of the miogeosyncline or the latite-monzonite igneous province generally 

 as a prelude or as a closing note to the main magnetic activity. 



Olivine basalt is considered the primary magma of the alkalic and 

 calc-alkalic provinces, although appreciable assimiliation and contamina- 

 tion of the magma has occurred. In a few places, considerable melting of 

 short-lived roots may have occurred, and here, by definition, the primary 

 magma would be granodioritic, quartz dioritic, or augite dioritic as locally 

 identified. Even here, some basalt may have been mixed in. 



The andesite and spilite-keratophyre provinces probably do not mark 

 primary magma types. It is concluded on a later page too that they are 

 fractional differentiates of primary basalt, probably of tholeiitic basalt, 

 but because there is doubt about this conclusion they are shown separately 

 (Fig. 36.7). The latite-monzonite province is concluded to be a primary 

 magma province, although perhaps an unusal one. 



