564 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



parentage. The writer has not found it possible to chart these three 

 kindreds in separate provinces in the Rocky Mountains, and therefore 

 does not try to distinguish them. They will be referred to collectively as 

 the trachyte and phonolite province, (see map, Fig. 36.3). Igneous rocks 

 adjacent to the region of alkali-rich igneous rocks in the Rocky Mountains 

 are generally more calcic or do not display an excess of alkaline elements 

 such as to yield die feldspathoid minerals, and are grouped in calc- 

 alkalic subprovinces. 



Colorado Plateau. The chief igneous centers in the Colorado Plateau 

 which belong to the high alkalic subprovince are the laccolithic groups 

 (Henry, La Sal, Abajo, Ute, and Carrizo Mountains), the Navajo and 

 Hopi Ruttes volcanic fields of northeastern Arizona; and the San Rafael 

 Swell. The Elkhead Mountains, White River, Grand Mesa, and Rattle- 

 mount Mesa fields are also of alkalic affinities and are grouped in the 

 Colorado Plateau for convenience sake. 



In the laccolithic groups (Hunt, 1954, 1956) the first intrusions are 

 diorite porphyry which constitutes about 60 percent of the total volume 

 of igneous rock. Intrusions of monzonite porphyry follow to the extent 

 of about 25 percent, and then last a syenite porphyry to the extent of 

 about 13 percent. The last intrusion is noted only in the La Sal Moun- 

 tains. The rocks are high in Na 2 0, but the ratio of K..O to CaO -f Na 2 

 increases eastward. The earliest intrusions in each group contain about 

 5 to 6 cubic miles of rock. These were stiff and relatively low-temperature 

 magmas. The central stocks probably breached the surface and erupted 

 more potassic rock than contained in the intrusions. 



The magmas were intruded in basins, broad domes, and benches of 

 the Colorado Plateau. Olivine basalt is regarded as the primary magma 

 which assimilated amphibolite and hornblende gneiss to yield a potash- 

 rich magma which then differentiated (Waters, 1955). 



The Hopi Ruttes (Williams, 1936) is a volcanic field of lava-capped 

 mesas and many necks. Ejecta consists of limburgite (dark, glass-rich and 

 usually minus feldspar) and monchiquite (nepheline basalt) in sedi- 



Fig. 36.3. Igneous provinces of the western United States. The numbers relate to in- 

 trusions listed in the table on page 574. 



