SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS ELEMENTS AND THE ORIGIN OF MAGMAS 



599 



the Silver City stock of the East Tintic Mountains, mid-Eocene or 38 to 

 46.5 m.y. ( Morris, 1957 ) . No one has proposed that the stocks may have 

 fed the welded tuff flows because the stocks have generally been con- 

 sidered older than the volcanics, yet the new age determinations indicate 

 some may be younger. The welded tuffs are undoubtedly fissure eruptions. 



It appears logical, therefore, to conclude that the ignimbrite magma 

 was similar to that of the stocks, at least the porphyry stocks. Elsewhere 

 in the general province a flow and a pyroclastic sequence of greater vari- 

 ability without the preponderance of welded tuffs and generally with a 

 greater amount of basalt ( olivine ) occur. 



Only one area within the ignimbrite subprovince of which the writer is 

 aware has appreciable basalt. In a 6000-foot sequence of volcanics in one 

 of the canyons of the Pioche, Nevada, district, is an olivine basalt unit 

 120 feet thick. The rest of the rocks are described as rhyolite, dacite and 

 andesite, with the last two predominating. These are taken to be the 

 welded tuff sequence, but some of them could be the younger Miocene- 

 Pliocene volcanics. 



Relation of Flows to Miogeosyncline . The latite avalanche subprovince 

 is entirely within the miogeosyncline, with the exception of a very small 

 overlap on the eugeosyncline southeast of Winnemucca. The western 

 limit of the avalanche flows is close to the boundary between the eugeo- 

 syncline and miogeosyncline. The writer is not inclined to take this dis- 

 tributional relation to the geosynclinal divisions as very significant, be- 

 cause welded tuffs occur in the eugeosyncline of western Nevada and 

 eastern California as part of the younger volcanics, and hence magma 

 of avalanche composition and propensity can form in the crust under the 

 eugeosynclinal strata. 



Relation of Flows to Gravity Faults. The Wasatch and Cache Valley 

 faults extend the Basin and Range system into the trench faults of south- 

 eastern Idaho and westernmost Wyoming from where they continue north- 

 ward through Montana to the trenches of British Columbia with very 

 little volcanism evident. The faults of the High Plateaus of Utah extend 

 southward beyond the welded tuff subprovince. 



Basin and Range faulting is believed to have started in about early 

 Oligocene time, at least in southern Nevada, and to have continued from 







CUVMAIUNC B»SE»CNT 



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Fig. 38.2. Postulated origin of the monzonite-latite magmas. Section A is of the Oquirrh and 

 Wasatch Mountains, Utah, and is factual at the surface but interpretive at depth. Section B 

 represents the Laramide folding before the quartz monzonite and quartz diorite intrusions. 

 Section C is section B restored to pre-folding time. 



place to place to the present. Since early Oligocene is the time of the major 

 avalanches a tie may be imagined. The flows are widely broken and tilted 

 by the faults. After considerable erosion they were covered by the later 

 volcanics and associated sediments, which in turn have been broken in 

 places by further faulting. The association implies a genetic relation of 

 the volcanics and gravity faults, but close scrutiny leaves one with the 

 thought that the association is not as ubiquitous as desired. 



Relation of Laramide Structures to Crustal Velocity Layers. In order 

 to approach the problem of the origin of the latite magma, the rela- 

 tion of the Laramide structures to seismic velocitv layers must be 



