534 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



andesites and basalts, but a clear relation to either of the parent basalt 

 magma types has generally not been agreed upon or established. Great 

 volumes of andesite are erupted in some orogenic belts with little or no 

 accompanying olivine basalt, and this gives rise to the belief that the 

 roots of the thickened "granitic" crust in orogenic belts may be melted 

 and in part mixed with basalt magma to form andesitic magma directly 

 and even rhyolitic magma at times (Waters, 1955). 



Kuno (1954) reports on a volcanic zone on the Izu peninsula southwest 

 of Tokyo, Japan, which is a small part, but perhaps typical, of the Circum- 

 Pacific igneous association. Most of the lavas (basalts and andesites) are 

 characterized by a low MgO:FeO + Fe 2 3 ratio and low alkalies, and 

 also by low normative feldspar rich in An and high normative quartz. 

 However, the lavas of the Omuro-yama group, a small field in the zone, 

 are high in the MgO:FeO + Fe 2 3 ratio and in alkalies. Some of them 

 have a considerable amount of normative olivine, and most of them con- 

 tain resorbed xenoliths captured from a granitic rock. He concludes that 

 the main rocks of the zone represent various stages of fractionation of a 

 tholeiitic magma, but that the Omuro-yama rocks represent products of 

 contamination by granitic rock. The xenoliths were taken from the wall 

 of the magma reservoir which supplied extrusive lavas, and not from the 

 walls of the conduits, because in order to effect assimilation, the magma 

 must have been in contact with the salic plutonic rock for a considerable 

 time, otherwise only mechanical mixing would have taken place. 



The spilites of the orogenic zones are soda-rich olivine-poor basalts, 

 with albite or oligoclase the sole or principal feldspar. Some of the albite 

 in certain flows is secondary. A keratophyre is a sodic trachyte with albite 

 as the principal constituent. Many spilites are pillow lavas and are inter- 

 bedded with marine sediments; hence probably erupted on the sea floor 

 as submarine flows. The spilites and keratophyres are commonly associ- 

 ated with normal basalts and andesites, and are typical volcanic rocks of 

 the eugeosyncline. Recause of this position they are particularly subject 

 to low-grade metamorphism and become the greenschists of the orogenic 

 belts. Waters (1955) regards the spilite-keratophyre association in the 

 Coast Ranges of Washington and Oregon as a tholeiitic province, but 

 Turner and Verhoogen ( 1951 ) think the chemical data yet insufficient to 



establish a clear-cut relation to the tholeiitic or the olivine basalt magma 

 types: 



The spilitic association, whatever its relation to the basaltic kindreds, is one 

 of striking individuality maintained in widely scattered provinces of all ages 

 and recognized wherever the rocks of geosynclinaal terranes have been petro- 

 graphically investigated (Turner and Verhoogen, 1951, p. 205). 



The olivine basalt-trachyte-phonolite association is displayed in places 

 in the Rocky Mountains, particularly in moderately deformed belts of 

 Laramide orogeny. It is an extensive differentiation series ranging from 

 olivine basalt to basanites to trachybasalts and trachyandesites to phono- 

 lites. The members generally have alkaline affinities. Within a single vol- 

 canic episode hundreds of flows together with much pyroclastic material 

 may be erupted from numerous centers to form a continuous field 50 to 

 75 miles across. Intrusive sills, laccoliths, plugs, and dikes are a minor 

 part of the field. Xenoliths are commonly conspicuous in the flows and 

 several authors believe the original olivine basalt magma was contam- 

 inated by reaction solution (fusion) of the wall rock. The type of wall 

 rock and the amount assimilated determines to a large extent the course 

 of differentiation of the magma. This general association is represented 

 by the San Juan volcanic field (Larsen and Cross, 1956) and probably 

 other fields in Colorado and New Mexico. 



The leucite basalt-trachybasalt and trachyte association in the western 

 United States is represented by the feldspathoid, alkali-rich rocks of the 

 Colorado Plateau, Leucite Hills and Rlack Hills in Wyoming, and the 

 well-studied region of central Montana (Larsen, 1940). 



The association called tholeiitic basalts and equivalent quartz diabases 

 are the flood basalts of such volcanic fields as the Columbia River Plateau. 

 The most striking characters are the enormous volume, wide extent, and 

 uniform composition of the basalt sheets. 



Tyrrell's Tectono-lgneous Cycle 



Emphasizing the tectonic and time aspect of petrographic provinces 

 Tyrrell ( 1955 ) has proposed the following tectono-igneous cycle." It 

 applies to the complicated region of northwestern Europe consisting of 





