Origin of andesite and its bearing on the Island arc structure

Abstract

The hypothesis that andesite magmas originate from basalt magmas through fractionation is supported for the following reasons: 1) A close association of andesite and dacite with basalt in many volcanoes and a complete gradation in chemistry and mineralogy throughout this suite. 2) Formation of andesite magmas from basalt magmas by differentiation in situ of some intrusive and extrusive bodies. 3) Agreement between the calculated compositions of solid materials to be subtracted from basalt magmas to yield andesite magmas and the observed mineralogy of phenocrysts in these rocks. 4) Higher alkali contents in andesite and dacite associated with high-alumina basalt than in those associated with tholeiite. 5) A complete gradation from the high iron concentration trend of basalt magma fractionation (Skaergaard) to the low or noniron concentration trend (the calc-alkali series) which can be ascribed to the difference of the stage of magnetite crystallization. 6) Similarity between the orogenic rock suite and plateau basalts in the preferential eruption of magmas of middle fractionation stage, givin rise to the great volume of andesite in the orogenic belts and iron-rich basalt in the plateau lavas. Petrological and seismic refraction studies suggest that a great volume of gabbroic materials are present in the lower crust underneath the volcanic belts as a complementary material for the andesite lavas. The island arc structure would develop by repeated eruption of andesite on the surface and by thickening of the oceanic crust underneath the arc due to the addition of gabbroic materials. The suitable portion of the lower crust may be subjected to partial melting to produce granitic magma in the later stage of development of the arc, successively changing it to a part of the adjacent continent.