IGNEOUS PROVINCES IN WESTERN UNITED STATES 



577 



cally and texturally to the monzonite porphyry of the intrusive laccolith 

 there. 



The mode of origin of the extrusive rocks of the Pine Valley Mountain 

 appears to be related to their chemical composition. The basalts, andesites, 

 and latites are flows; the dacites are found both as ignimbrites and 

 as flows; and the rhyolitic rocks are all ignimbrites. Apparently the more 

 acidic magma effervesced into nuees ardentes, although some of 

 dacitic composition merely foamed into frothy flows; the intermediate and 

 basic lavas, on the other hand, welled up without violent loss of gas to 

 form finely vesicular flows. (Cook, 1957, p. 49). 



A study by Van Houten (1956) of the Cenozoic sedimentary and re- 

 lated volcanic rocks of Nevada and western Utah indicates that a good 

 I datum for correlation is a tuffaceous unit of late Miocene and early and 

 mid-Pliocene age. A vitric tuff in this general bentonitic and tuffaceous 

 unit is prominent and widespread. It rests on somewhat older Oligocene 

 and Miocene (?) volcanic rocks in southern, central, and western Nevada, 

 as well as locally in the northeastern part of the state. 



The lower volcanic units were tilted by fault block rotation and eroded 

 j before the widespread, late Miocene-Pliocene tuffaceous unit began to 

 I accumulate. During this late Miocene to mid-Pliocene interval the south- 

 ern Cascade andesites were accumulating as well as the younger basic 

 i lavas of the Sierra Nevada. The inference is natural that the volcanism 

 I and faulting are related, but this subject will be left until later for dis- 

 I cussion. 



The study by Van Houten emphasizes the existence of extensive fluvi- 

 atile and lacustrine deposits derived largely from the eruptive centers. 

 The sedimentary derivatives fill the numerous intermontane valleys in 

 places to the depth of over 5000 feet and have been tilted in the Basin and 

 Range faulting to be exposed on the backs of the tilted blocks. In some 

 places as much as 10,000 feet of volcanic rock, including derived strati- 

 fied outwash and lacustrine deposits have been measured (personal com- 

 munication, various petroleum geologists ) . The volcanic fields, other than 

 the avalanche subprovince, have not been determined and circumscribed. 

 Those shown on the map of Fig. 33.7 are taken from the Geologic Map 

 -of the United States (1932), and it is presumed they are the most con- 



tinuous and thus indicate the major centers of volcanism. It is evident 

 that this representation is likely to be changed considerably by future 

 work. 



As a result of the extensive exploratory work for oil in Nevada, nearly 

 every intermontane valley is regarded as a downfaulted block, but only in 

 a few places have the faults been shown on maps either published or 

 available to the writer. The best recourse, it seems, is to show each valley 

 by a single fault, and this has been done in the absence of better informa- 

 tion. The Basin and Range fault system is undoubtedly more complicated 

 than shown. 



Origin of the Latite Magmas. In 1932, Gilluly observed from a study 

 that focused in the Oquirrh Mountains that a close relation of all the 

 extrusions is evident, and although several of the volcanic masses have 

 been described as andesites, these when analyzed have the alkali ratios 

 characteristic of latites. Whether they contain orthoclase or not, their 

 chemistry justifies the inference that they all belong to the latite and 

 quartz latite group. 



Plotting the CaO, K s O, and Na 2 as ordinates and the silica as abscissas 

 reveals the interesting fact that the soda shows almost linear decrease with 

 increase of silica. The average slope of the curve of soda is almost precisely 

 that which would appear from the mere addition of silica to the monzonite 

 magma. However, the lime decreases at a rate altogether disproportionate to 

 the silica content, and the potash remains very nearly constant or decreases at 

 a much lower rate than the silica increases. This relation is close to that which 

 would be expected as a result of differentiation of a monzonitic magma by 

 fractional crystallization. 



Both intrusive and effusive rocks of western Utah have CaO, Xa.O. and 

 K,0 proportions close to the average quartz monzonite, and hence are believed 

 to be of the monzonite kindred. Wherever chemical analyses are available, 

 it is seen that the so-called andesites are without exception so high in potassa 

 that they are properly classed as latites. Similarly, several so-called dacites 

 resemble quartz latite closely (Gilluly, 1932, pp. 66, 67). 



Gilluly concluded and reaffirms the conclusion in recent correspondence 

 that there is no evidence here of any more basic rocks that could be 

 considered parental to the latite-monzonite magmas. 



In Bowen's scheme of magma evolution it would be necessary to have 

 basaltic and andesitic rocks prior to the quartz monzonite. All of the volcanics 



