596 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Hawaii, Kuno et al. (1957) conclude that neither basalt magma may be 

 derived from the other by fractional crystallization, or in other words, 

 that neither is the parent of the other. This emphasizes again the origin 

 by partial melting of the peridotitic mantle. 



Two possible reasons may be presented for the origin first of one magma 

 and then of the other, or of one magma in one place, and the other in 

 another place. The first is based on the assumption that the mantle is 

 slightly heterogeneous in composition and that by partial melting of one 

 part an olivine-rich basalt will be produced and by partial melting of an- 

 other part with a slight difference in composition a tholeiitic basalt will 

 result. Since these variations in the mantle are not tied to the tectonic 

 divisions of the continental or oceanic crust in any way at present recog- 

 nizable, it would thus be apparent why either variety of basalt rises in 

 most any tectonic setting. 



Kuno proposes a second possibility, namely that the mantle is of uni- 

 form composition and that different pressures will cause slightly different 

 melting of the peridotite. In the Japanese Archipelago he suggests that 

 the parental tholeiite magma is produced by partial melting of the peri- 

 dotite layer (mantle) at depths shallower than 200 kilometers, and that 

 the parental alkali olivine basalt magma is produced by partial melting 

 at depths greater than 200 kilometers (Kuno, 1959). 



Basalt-Andesite Assemblages of the Eugeosyncline and Orogenic Belts 



On previous pages we have seen that the igneous rocks, so abundant 

 in the stratified sequences of eugeosynclines are principally andesite and 

 tholeiitic basalt. Spilites are common, and according to Waters (1955) 

 they are tholeiitic basalt altered mostly by rising hydrothermal solutions 

 but in part by sea water in connection with submarine flows. The albite 

 may also be added in subsequent metamorphism, but in any case, they do 

 not therefore add to the problem of the origin of the association of 

 tholeiitic basalt and andesite. Keratophyres bear much the same relation 

 to andesite as the spilites do to basalt and, hence, likewise do not pose an 

 additional problem in the nature and origin of the primary magma. Olivine 

 basalt is reported in places in the eugeosyncilinal assemblage but infor- 

 mation on its relative volume and distribution is not well at hand; never- 



theless it seems that provision should be made for its presence in the 

 eugeosyncline in any theory of origin devised of the igneous complex 

 there. Certain acid varieties are present in small amounts and are un- 

 doubtedly derivatives of the others. 



The association of basalt and andesite in the volcanic fields of post- 

 batholithic age has been elaborated on in previous pages. Reference to 

 the cross sections of Fig. 34.5 and the map of Fig. 36.5 indicates that the 

 site of most extensive occurrence is on the deformed belt immediately 

 inside the batholithic belt toward the continent, which embraces parts 

 of the older eugeosyncline not metamorphosed and invaded by the batho- 

 liths, and most or all of the miogeosyncline. In the United States where a 

 belt of Laramide deformation is beyond the miogeosyncline in the shelf 

 region, andesites also occur. About half the bulk of the San Juan field in 

 Colorado is andesite, the other rhyolite, with basalt subordinate, so it is 

 evident that somewhat different conditions apply there. The broad Great 

 Rasin region of the western United States is also unusual in relation to the 

 general composition of the great western Cordillera of the Americas and 

 will need special consideration. 



The Cascade volcanic complex of Oregon and Washington should be 

 mentioned in regard to post-batholithic activity because of significant 

 associations there. It, however, is not a parallel with apparent normal 

 conditions in the Cordillera, because it is a local field on the batholithic 

 belt and also probably on the oceanward side of the batholithic belt built 

 as part of a new continental margin. The older Cascade complex is more 

 variable than that of the younger stratovolcanoes and according to 

 Waters is a tholeiitic-andesite assemblage with some olivine basalt present 

 whereas the younger is an olivine basalt-basaltic andesite assemblage. 



Andesite is also found on Hawaii, in an olivine basalt, ocean basin 

 assemblage. In connection with this occurrence and with the transitional 

 nature of basalt and andesite the following quotation from Williams et al. 

 ( 1954 ) is significant. 



Olivine-bearing andesites. These are widespread on oceanic volcanoes, 

 like those of the Hawaiian Islands, and in orogenic belts of the continents. 

 Indeed they probably predominate among the Tertiary and Quaternary lavas 

 of the Circum-Pacific belt. Many of them lie so close to the boundary between 



