IGNEOUS PROVINCES IN WESTERN UNITED STATES 



581 



numerous stocks and widespread volcanism. The magma has generally 

 risen through a thick sedimentary veneer, and little basalt has emerged 

 at the surface. However, similar intrusions and extrusions occur in south- 

 ern Arizona where the sedimentary rocks are thin, so the sedimentary 

 veneer is not important, it seems. The latite magma is considered to be a 

 primary one, and its origin will be taken up on later pages. 



The Laramide Rockies of central Montana, Wyoming, and Colorado, 

 as well as the Colorado Plateau constitute a large calc-alkalic and alkalic 

 province where assimilation of calcium-, sodium-, and potassium-rich 

 rocks in the crystalline "granitic" crust has been a prominent process. The 

 belt of Rockies through the shelf region seems to have affected the igneous 

 suites very little — they are approximately the same in the Colorado Pla- 

 teau, in the Wyoming and Colorado Rockies, and in the fairly stable area 

 east of the Rockies in Montana. Their prolonged and complicated differ- 

 entiation history bespeaks rather stable crustal conditions. 

 | The Columbian River tholeiitic flood basalts are principally of Miocene 

 age, center approximately in the great batholithic bulge, and have been 

 jfed upward through the batholithic complex (see Fig. 36.5). 



The vent basalt field of the Snake River Valley and Malheur Plateau, is 

 ■principally one of late Pliocene and Quaternary activity and occupies a 

 downwarp around the south side of the Idaho batholith and, very approxi- 

 mately, along the south side of the great bulge which seems to be con- 

 tinued eastward into western Montana by the large Laramide plutons 

 ,there. It is suggested that since the basalt came directly from the subcrust, 

 lind the downwarp is across the Laramide trends of the central Rockies, 

 diat the folding and thrusting is shallow and that the downwarp is due 

 :o movements in the subcrust. It must be noted, however, that a large part 

 )f the field lies on the batholithic belt and has been fed by basalt from 

 he subcrust through the batholithic complex to the surface. 



DISTRIBUTION OF PRIMARY MAGMAS 



The map of Fig. 36.7 has been prepared to show the distribution of the 

 Afferent types of primary magma postulated to have given rise to the 

 gneous rocks now displayed at the surface. 



Two principal types of primary magma are postulated, namely, the 



GRANODIORITE GRANODIORITE ANDESITE 

 l$t CYCLE 2nd CYCLE 



SPILITE- THOLEIITIC 



KERATOPHYRE BASALT 



— LATITE -MONZONITE 



OLIVINE 

 BASALT 



Fig. 36.7. Distribution of primary magmas. Batholiths of the second cycle occur in the Cas- 

 cade andesite province. The spilite-keratophyre magmas of the pre-Nevadan batholithic time 

 had about the same distribution as the batholiths. Andesite magma refers to the basalt- 

 andesite association of the orogenic belts (post-batholithic). 



