IGNEOUS PROVINCES IN WESTERN UNITED STATES 



575 



come up through Pennsylvanian and Permian strata which are nearly 

 horizontal. 



Another fairly large pluton is one in the Organ Mountains, about 30 

 miles north of El Paso. It extends 18 miles north-south and 9 miles east- 

 west. 



Lindgren et al. (1910) describe the large plutons mentioned above as 

 monzonite and quartz monzonite porphyries. They observe that the in- 

 trusive monzonites and effusive latites and andesites in general in the 

 central north-south belt of New Mexico have a fairly uniform composition 

 and suggest that all were derived from an intermediate magma. The last 

 differentiates were basalt and rhyolite, which were the last ejections. 



The general problem of the nature of the primary magmas will be dis- 

 cussed presently, but it should be said that the fusion of a gneissic, 

 schistose, and granitic basement such as would produce a monzonitic 

 magma, would not have enough magnesium and iron in it to yield a 

 basaltic differentiate, especially an olivine basalt. Also the volume of basalt 

 in some fields is too much to have been derived from the postulated parent 

 primary monzonitic magma. 



Extrusive Rocks. As for the extrusive rocks only limited information 

 is at hand. Those of central and eastern Nevada and southwestern Utah 

 consist of a thick older assemblage and a thinner younger group which is 

 approximately equal in age to the younger volcanics of western Nevada, 

 southern California, Arizona, and southwestern New Mexico. The older 

 assemblage is dominantly of the quartz latite type, and more conspicu- 

 ously, it consists chiefly of a great series of avalanche or welded tuff 

 deposits, whereas those in peripheral location are more of the basalt- 

 andesite-dacite-rhyolite suite. 



Rrief descriptions of selected fields outside of the avalanche sub- 

 province follow. 



In the Ajo District of south-central Arizona the lavas are basaltic, 

 [ andesitic, and latitic. In southern Nevada in the Goodspring's quadrangle 

 the extrusive rocks range from latite through andesite to basalt. At Gold- 

 field, Nevada, the eruptive sequence is rhyolite, latite, rhyolite, olivine 

 basalt, andesite, da cite, andesite, rhyolite, andesite, olivine basalt, rhyolite, 

 and olivine basalt. At Gold Hill on the Utah-Nevada line a normal series 



of basalts, andesites, and rhyolites occurs. They are all rich in potash. 

 Latite Welded Tuffs Subprovince. The welded tuff subprovince is 

 shown on the maps of Fig. 36.1 and 36.3, and its existence has only re- 

 cently become clear. Mackin and Cook and associates in southwestern 

 Utah and several petroleum and U.S. Geological Survey geologists have 

 recognized the welded tuffs (ignimbrites) and something of their magni- 

 tude. However, the writer is especially indebted to Dr. Howel Williams 

 for the following resume. He was among the first to gain the conception of 

 the unique field and the almost unbelievable magnitude and awesomeness 

 of the eruptions. 



Welded tuffs are formed during eruption bv distention of magma in which 

 the vapor tension is low. Instead of explosive eruption of vitric ashes, the 

 discharge is in the form of a glowing avalanche that sweeps rapidly down- 

 slope. The most widespread of these result from escape of foaming magma 

 through swarms of fissures as a mixture of incandescent spray, droplets, and 

 larger clots enveloped in hot, expanding gas. So mobile are these mixtures that 

 they spread over vast areas, down even the gentlest gradients. Other glowing 

 avalanches issue from the flanks of volcanic domes of Pelean type; still 

 others originate when foaming magma is upheaved en masse until it spills 

 over a crater rim and then, aided by gravity, races downward. Because these 

 avalanche deposits accumulate rapidly and usually to great thickness, many 

 remain hot for a long time, especially in their central parts. As a result, the 

 shards of glass, while still hot and under heavy overburden, are squeezed and 

 flattened, and some are buckled between phenocrysts. At the same time 

 pumiceous lapilli and bombs are deformed to disks, some of them paper-thin, 

 and all the constituents become firmly annealed. The rocks thus formed are 

 called welded tuffs. They have a delicate, streaky lamination deceptive!) like 

 the fluidal banding seen in many lavas. Besides, some of them develop 

 columnar and spherulitic structures as they cool, so that their resemblance 

 to lavas is increased. Little wonder, therefore, that welded tuffs have often been 

 wrongly identified. The fact is that they are now known to be of truly colossal 

 extent in the circum-Pacific volcanic regions, and they are undoubtedly wide- 

 spread elsewhere (Williams et al., 1954). 



The welded tuffs according to Williams constitute 95 percent of the 

 older and more voluminous volcanics of the avalanche subprovince. They 

 probably average over 2000 feet thick, and south of Eureka, Nevada, 

 they are 8000 feet thick. This general area is the part of the field of maxi- 

 mum thickness. Since immense amounts of these easily weathered tuffs 

 have been removed, the original volume was undoubtedly larger than 



