598 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



If the andesites of the basalt-andesite complexes of the eugeosynclines 

 and orogenic belts are differentiates of basaltic magma, then large volumes 

 of the rising basalt from the upper mantle are trapped or arrested in the 

 basaltic subcrust, where partial fractional crystallization and the develop- 

 ment of andesitic and basalt-andesitic magmas takes place. Then as fissures 

 in the overlying crust come into existence various magma pools are tapped, 

 which may be basaltic through transitional phases to andesitic or even 

 in rare instances, dacitic, and the surficial basalt-andesite complexes are 

 extruded. In the voluminous outpourings andesite is commonly the most 

 acidic rock produced, and so it would appear that the arrested bodies or 

 reservoirs of magma in the subcrust are sill-like and not very thick, other- 

 wise if in large bodies of several kilometers in vertical dimensions more 

 varied and more silicic magmas might result. 



Why the restriction to the orogenic belts and the eugeosynclines? The 

 eugeosynclines are essentially orogenic belts themselves, but probably 

 without appreciable roots until involved in the climactic batholithic orog- 

 eny. The blanketing sediments of the geosyncline result in the rise of 

 temperature in the underlying crust and upper mantle, and hence may be 

 thought of as bringing on the subcrustal igneous cycle. However, basalt 

 has risen under the shelf of the stable region in considerable amounts 

 without a thick, widespread, sedimentary blanket. The miogeosyncline 

 developed irregularly with basins and arches, and these from previous 

 discussions would have resulted from expanding and contracting columns 

 in the mantle below without appreciable exclusion of magma. The eugeo- 

 syncline, on the other hand, has been built partly by volcanic activity. 

 This is part of the unrest of the continental margin, and for some reason 

 the mantle there has been, since Ordovician time at least, the site of exces- 

 sive heat evolution causing magmatism and surficial orogeny. 



Magmas of the Latite Ignimbrite Subprovince 



Petrology. The first requirement in consideration of magmas of the 

 latite-ignimbrite subprovince is a voluminous supply of a fairly uniform 

 quartz latite magma. The volume is comparable to that of the Columbia 

 River basalt field. The composition appears fairly uniform within the 

 province; according to Howcl Williams a number of rocks called andesites 



and dacites are only such by certain systems of nomenclature, and are 

 really close to the quartz latite welded tuffs. Certain stocks are as basic 

 as diorite or quartz diorite, and provision for them must be made in any 

 theory of origin of the magmas. 



Relation of Welded Tuffs to Stocks. The commonness of monzonite 

 and quartz monzonite stocks and their similar composition to the 

 quartz latites is striking. Nothing can be added to Gilluly's discussion 

 of the close relation of the two as reviewed on previous pages, in which 

 he postulated a reservoir of primary magma of quartz latite composition 

 from which both the intrusives and extrusives were derived without 

 further differentiation. Stringham's survey of the stocks of western Utah, 

 Nevada, southern California, Nevada, and New Mexico indicates that 

 some are as basic as diorite, but these are few. The quartz diorite of the 

 Cottonwood stock of the central Wasatch ( Fig. 38.2 ) lies close geographi- 

 cally to the Bingham quartz latite or granite stock and indicates the 

 variation in composition that can exist within a few miles. As to the dis- 

 tribution, the stocks are abundant in the ignimbrite subprovince but 

 equally abundant, evidently, outside the subprovince but within the Basin 

 and Range province. It is computed that one stock occurs in about every 

 100 square miles, on the average, and each stock has an exposed area of 

 about 5 square miles. From data at hand no difference in composition can 

 be noted in the intrusive rocks inside the subprovince from those outside, 

 but possibly there is a small difference which has not been detected. 



Stringham (1958) has classed the stocks in two divisions, the aphanitic 

 matrix porphyry and granitoid. The first he regards as mobile intrusions, 

 but the second he believes formed by granitization. The Cottonwood 

 stock of Fig. 38.2, for instance, formed by granitization, and the Bingham 

 stock was probably intruded. 



The age of the stocks is approximately the same as the welded tuffs. 

 Some stocks are as old as Eocene, and others as young as Miocene accord- 

 ing to zircon and potassium-argon age determinations, so they seem to 

 predate, possibly accompany, and postdate the great avalanche eruptions. 

 For instance, the Cottonwood (Alta) stock of the Wasatch Mountains 

 is late Eocene ( Crittenden et al., 1952 ) , the Sheeprock stock of the Sheep- 

 rock Range is middle Miocene or 15 to 17 m.y. (Cohenour, 1957), and 



