The complementariness of orogenic andesite and alpine peridotite

Current concepts on the origin of the calc-alkaline andesites of orogenic belts range widely with respect to the mechanism of magma generation, the place of origin and the nature of the source material. The hypothesis which the author believes best fits the evidence is that of the derivation of andesite through fractional crystallization of a body of primary olivine basalt which rises from the mantle into geosynclinal rocks, and which during the fractional crystallization experiences an exchange with the surrounding country rock of water flowing into the magma to a total amount of probably 2 or 3 per cent, and hydrogen flowing out. The small amount of oxygen being added to the magma during fractional crystallization is necessary to develop and maintain the calc-alkaline trend of the liquid and crystal compositions. Complementary bodies of settled crystals remaining at depth in the geosynclinal rocks after the andesite liquid is driven off are alpine peridotites. Research bearing on this hypothesis includes: (1) experimental work on the system, CaO-MgO-iron oxide-plagioclase-SiO ₂, showing that the process is possible;

(2) experiments on basalts and andesites at controlled pO ₂, and on a basalt at 1 kbar water pressure as well as at controlled pO ₂, showing the effect of water pressure and pO ₂ on temperature of crystallization of the phases and providing data on the equilibrium pO ₂ of these rocks and the solubility of water in them;

(3) extensive chemical analyses of andesites and other geosynclinal rocks providing data on Sr isotopes, trace elements and alkali-silica ratios, all of which argue against formation of andesite by partial melting of crustal rocks or by assimilation of crustal rocks by basaltic magma;

(4) field and laboratory studies of alpine peridotite complexes leading to the conclusion that they represent cumulate bodies formed by crystal settling during crystallization of a magma;

(5) composition data on alpine peridotite complexes demonstrating their general calc-alkaline trend characteristics as opposed to the higher iron oxide trend of cumulates of the layered intrusions which do not develop calc-alkaline andesites.