IGNEOUS PROVINCES IN WESTERN UNITED STATES 



573 



Taylor, but most of them were erupted after Mount Taylor had become 

 quiescent and they overlap the outer edges of the Mount Taylor cone (Hunt, 

 1938, p. 58). 



The Mount Taylor central vent volcanics are slightly more alkalic than 

 the rocks of the laccolithic groups of the Colorado Plateau, and Hunt, 

 therefore, points out a close tie of the two. 



The San Francisco volcanic field is much larger than the Mt. Taylor, 

 and the initial activity consisted of the eruption of about 30 cubic miles 

 of sheet basalt over a broad structural dome, the Coconino Plateau. 

 Several large vent-type volcanoes broke out; San Francisco Mountain 

 being built of almost 40 cubic miles of volcanic ejecta, Kendrick Peak 

 of more than 6 cubic miles, Rill Williams Mountain of 3 cubic miles, and 

 O'Leary Peak of 2 cubic miles. The five stages of eruption of San Fran- 



' cisco Mountain volcano were as follows: 21 cubic miles of latitic lava, 

 tuff, and breccia, 13 cubic miles of pyroxene dacite lava, 0.5 cubic mile of 

 hornblende dacite, 0.5 cubic mile of rhyolite, and 3 cubic miles of andesite. 

 On a succession of erosion surfaces four separate basalt flows occurred, 



I and basalt lavas and pyroclastics were extruded from about 200 small 

 vents. This was the last phase of activity and about 20 cubic miles of lava 

 was extruded. One of the cinder cones, Sunset Peak, was active 800 years 

 ago (Robinson, 1913). 



Datil field of eastern Arizona and adjacent New Mexico, is largely 

 andesite, with subordinate amounts of rhyolite, rhyolite tuff, quartz latite, 

 and various pyroclastics consisting mostly of basalt (Sabins, 1957). The 

 Mogollon Mining District is within this large field, and there Ferguson 

 (1927) describes 8000 feet of andesite, rhyolite, rhyolite tuff, and quartz 

 latite. This assemblage savors of the Great Rasin latite-monzonite prov- 

 ince, and perhaps has some welded tuffs. Variations from mostly basalt 

 o mostly latite and rhyolite would appear to be dependent upon the 

 imount of silicic crust effected by partial melting. Refer to theory 

 oresented under next heading, Rasin and Range latite-monzonite province. 



kisin and Range Latite-Monzonite Province 



Extent and General Characteristics. Rutler ( 1920 ) summarized the 

 Volcanic rocks of western Utah and adjacent parts of the Rasin and Range 



physiographic province as follows. They range in composition from 

 rhyolite to basalt, but the great bulk of the material is of intermediate 

 composition, including rather basic rhyolites, quartz latites, dacitcs, and 

 andesites. Rasalt is very subordinate in amount when compared with 

 the series as a whole though present in many localities and usually con- 

 spicuous as representing the latest volcanic outflows. 



A large region in Nevada and western Utah consists dominantly of 

 welded tuffs of approximate quartz-latite composition. The alkalic types 

 of rock, such as the leucite and nepheline-bearing lavas are to the writer's 

 knowledge, very scarce, and have only been noted in East Fork Canyon 

 of the High Plateaus where Dutton described an isolated occurrence 

 of phonolite and in the Traverse volcanics of the Oquirrh Mountains 

 (Gilluly, 1932). A brief review of the Tertiary volcanic rocks in southern 

 Arizona indicates that they are essentially the same as in Nevada and 

 western Utah, and fit Rutler's general description. The intrusive rocks are 

 principally in the form of stocks. They have a dioritic to granitic compo- 

 sition, with monzonitic the most common. Like the extrusives the intru- 

 sives are preponderantly intermediate to acidic in composition. 



Intrusive Rocks. The following is a tabulation of the intrusive rocks of 

 ninety-five mineral districts which Stringham has made in the course of a 

 systematic study of the mineralized and barren stocks of western Utah, 

 Nevada, Arizona, California, and New Mexico (personal communication). 

 About 300 plutons, mostly stocks, are shown on various maps of areas in 

 these states or are known from personal field work, according to Dr. 

 Stringham. When a district is mapped, more intrusive bodies are usually 

 found, so he estimates that possibly 1000 intrusions may exist. In western 

 Nevada most of these are probably satellites of the Sierra Nevada batho- 

 lith and not Laramide or later in age as elsewhere in the Great Rasin. 



The intrusive bodies shown in the table on page 574 are charted on 

 Fig. 36.3, where it is seen that the three classifications of the tabulation 

 have little significance geographically. It might be concluded that western 

 Utah and eastern Nevada are free of intrusions as basic as diorite, but 

 elsewhere in the Great Rasin the three divisions are fairly well scattered. 



The Tertiary intrusives of central New Mexico range from diorite to 

 granite, with a preponderance of monzonite and quartz monzonite ( Lind- 



