IGNEOUS PROVINCES IN WESTERN UNITED STATES 



567 



by downward protuberances of the granitic crust or of the basaltic sub- 

 crust, and if these are melted and responsible for the location of the 

 igneous centers, such as is generally held to be the case in the Nevadan 

 orogenic belt, then the upward coursing magma must have worked later- 

 ally considerable distances to have found outlet in the interuplift sedi- 

 mentary basins. 



The domal uplifts are structures caused by vertical forces, and hence it 

 is believed that roots could not have developed; roots are the result of 

 horizontal compression or crustal shortening. The conclusion seems evi- 

 dent that the domes are themselves the result of igneous activity; they 

 are great blisters above giant laccoliths or thick megasills in the "granitic" 

 layer. The original magma in the megasills is postulated to be olivine 

 basalt, which while still molten, assimilated variable amounts of the 

 crystalline basement, and then as a secondary magma intruded through 

 the overlying crystalline basement and the sedimentary veneer to the 

 surface. In certain places like the Henry Mountains, minor amounts 

 worked somewhat laterally to emerge in the adjacent basin. The position 

 of some of the igneous rocks which have penetrated the sedimentaiy 

 veneer poses a problem, it must be admitted, but then, to the writer's 

 knowledge no attempt has been made to explain their distribution by any 

 other hypothesis. 



The blister concept is illustrated in Fig. 36.4. 



West Texas and Mexican Coastal Plain. The principal volcanic field 

 in the west Texas province is the Davis Mountains which extend from the 

 southern flank of the Delaware basin to and across the Rio Grande into 

 Mexico, a distance of 125 miles ( Tectonic Map of the United States, 1944). 

 The Chisos Mountains and the Terlingua-Solitario region to the south- 

 east in the Rig Rend Country, have many igneous bodies. A number of 

 intrusives are known in adjacent Mexico in the Sierra Madre Oriental 

 and Serrania del Rurro uplift. Northwest of the Davis Mountains are the 

 Eagle Mountains and Quitman Mountains which contain intrusive and 

 extrusive bodies, and north of these and east of El Paso are a group of 

 >mall intrusives that make up the Cornudus field. The Marathon basin 

 also contains a number of plugs and dikes. 



An alkalic composition has been noted in many of the igneous rocks of 



S "4p f 



MES 



ZESE 



SILICIC LAYER 



.BASALTIC LAYER. 



3/ 



""«« .«w yii 

 1LLLLUJ 



5 £ 



2 



to g 



IS Ml 



20 



BASALTIC LAYER. 



Fig. 36.4. Concept of blister structure and gravity mass movements of oval-shaped uplifts in 

 shelf province. 



west Texas but some are calc-alkalic. The overall province, however, is 

 generally referred to as alkalic and related to the Spanish Peaks and 

 central Montana provinces. 



The extrusive rocks of the Davis Mountains are trachytes, phonolites 

 and some rhyolites. Intrusive rocks are syenite and sodic syenite porphy- 

 ries. Olivine basalt occurs in minor amounts. All these igneous rocks are 

 early Tertiary in age, but one Recent vent has been observed (King, 1937). 



The igneous rocks of the Cornudus field are augite syenites and analcime 

 nepheline syenite. 



A volcanic area in the Quitman Mountains has a ring-dike and stock 

 of quartz monzonite as a central feature. This locally cuts a volcanic 

 series which consists of lower rhyolites, intermediate trachytes, rhyolites, 

 latites, and andesites, and upper trachytes. The total thickness is about 

 3500 feet, and rhyolite appears to occur in largest amounts. Indirect 

 fossil evidence suggests an early Tertiary age. According to the alkali- 

 lime index of Peacock, the volcanics of the Quitman Mountains fall near 

 the boundary of the two intermediate series, alkalic-calcic and calc- 

 alkalic (Huffington, 1943). 



