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



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andesite and basalt that only chemical analyses serve adequately to classify 

 them; in default of analyses, these borderline lavas are sometimes spoken of 

 as "basaltic andesites." Olivine and labradorite may be their principal minerals, 

 yet their silica content and the presence of normative quartz relate them to 

 the andesite family. 



Another variety is pyroxene andesite which according to the above 

 authors is especially common on large composite volcanoes in the orogenic 

 belts. Still others are hornblende and biotite andesites. These generally 

 form thick short flows, steep-sided domical protrusions, or intrusive plugs 

 and dikes, and are generally more siliceous and alkaline, and graded into 

 dacites and trachyandesites. 



With the above observations about the tectonic distribution and petro- 

 logic relations of andesite in mind, we must recognize four possibilities 

 of origin: (1) a rock of andesitic composition melting completely and 

 furnishing a primary andesitic magma; (2) a more basic rock melting and 

 partially freeing a liquid of andesitic composition; (3) a granodiorite- 

 granite primary magma mixing with a primary basaltic magma to form 

 an andesitic magma; or (4) formation of an andesite by some variation 

 of fractional crystallization. 



The fourth category has two variations according to Turner and Ver- 

 hoogen (1951). Ry fractional crystallization tholeiitic basalt may yield 

 andesite; and a primary granodiorite-granite magma may yield andesite as 

 a basic differentiate. The fractional crystallization of an alkali olivine 

 basalt, according to Kuno (1959), could not result in an andesite, but 

 instead various rocks like trachybasalt, nepheline basalt, trachyte, phono- 

 lite, or syenite would form. A calc-alkalic olivine basalt, however, can 

 differentiate to an andesite (Kuno, 1959). The andesites in the San Juan 

 field are regarded by Larson and Cross, with various mixings and con- 

 taminants, to have come from an olivine basalt. It is concluded that both 

 tholeiitic and olivine basalt can give rise by differentiation to one varietv 

 or another of andesite. 



For the small quantities of oceanic andesite the process of fractional 

 crystallization from a tholeiitic basalt seems the most likely origin. This 

 theory necessitates the presence of tholeiitic basalt in an olivine basalt 

 province, but fortunately tholeiitic basalt is present in some oceanic 

 islands. 



As to the mixing of primary granodioritic magma with primary basalt 

 magma to yield andesite magma, the process conceivably could occur in 

 the root region of the batholithic belt and would involve the rise of pri- 

 mary basalt from below, according to the theory proposed on previous 

 pages for the origin of either primary olivine or tholeiitic basalt. This 

 could probably produce the necessary large volumes of andesitic magma 

 necessary and also the transitional varieties from the two primary types. 

 Mixing is not possible if the batholiths form by granitization. 



In connection with the concept of roots of the batholithic belt it does 

 not seem logical to think of them at one time melting to form a magma 

 of granodioritic and granitic composition and then later melting to form 

 one of andesitic composition. This is probably a good argument to the 

 effect that the batholiths formed in place, and have nothing to do with 

 roots. If it is accepted that the batholiths form by granitization, then it 

 seems possible that roots, if they exist, could melt to form the andesites. It 

 has been suggested, also, that the eugeosynclinal sequence of graywacke, 

 argillite, and basic volcanics, if melted in bulk, would form a magma of 

 andesitic composition. Inspection of the maps of South America, Figs. 

 34.1 and 34.2, will reveal that the large basalt-andesite complexes spread 

 about equally over the eugeosyncline and miogeosyncline, so that the 

 rocks in the eugeosyncline do not seem to have a direct bearing on the 

 origin of the andesitic magma. 



Ry elimination then, and with a bias for the magmatists, we arrive at 

 the conclusion that the andesites are differentiation products of basaltic 

 magmas, which vary in composition themselves between olivine and 

 tholeiitic. The andesites in the alkalic and calc-alkalic provinces ( techni- 

 cally the shelf, partly deformed in the Laramide orogeny) are probably a 

 different breed from those of the deformed eugeosyncline and miogeo- 

 syncline, and have come about through an eventful history of mixing of 

 differentiating magmas, and by appreciable assimilation of high calcic and 

 alkalic rocks of the silicic crust. The andesites of the eugeosyncline and 

 post-batholithic orogenic belts are only a step away from the basalts, the 

 more acidic differentiates are centainly in the small minority, and the 

 volumes of andesites and basalts are great, and the succession of flows 

 and repetition in space monotonous. 



