Some aspects of magmatic evolution

Abstract

Summary The address is concerned with some aspects of magmatic evolution in volcanic successions, and includes particularly a discussion of the nature and genetic relations of primary and derivative basic and intermediate magmas. Among the volcanic associations of the oceanic areas, those of the Hawaiian archipelago provide data of critical importance in the study of basaltic evolution. The established chronological succession of magmas in these islands makes it probable that the alkali-poor tholeiites to be recognized in the lavas of the primitive shield volcanoes form the closest approach to the primary magma of the ocean basins, and from them derive by fractional crystallization processes the Hawaiian alkali magma series of alkali olivine-basalt, mugearite (so-called andesite) and trachyte. The course of volcanic variation there exhibited provides a new orientation to the study of the genetic evolution of the Tertiary igneous Brito-Icelandic province, and leads to the conception that the Hebridean alkali magma series derives through tholeiites as an alternative line of descent. The course of fractional crystallization of tholeiite magma in the great layered intrusion with its final product of granophyre is illustrative of the great flexibility in line of descent which fractionation processes permit. Derivation by crystallization differentiation in a convecting system comparable to that demonstrated in layered intrusions seems, indeed, the most likely mechanism for the genesis of the primitive sialic layer of the crust itself. The problem of intermediate magma in the volcanic successions of the orogenic belts is critical in petrogenic theory. Though fractionation processes appear most competent to give a liquid line of descent through andesite to rhyolite, normal andesite magma does not reflect in its chemistry the course of basaltic (tholeiitic) differentiation. Consideration is devoted to the conception that andesites, though ultimately of basaltic parentage, have derived in part by sialic contamination. The testing of the genetic relations of assemblages within the basalt-andesite range of the volcanic suites of the orogens remains one of the most important problems of volcanic petrogenesis. Systematic attack through intimate phase analysis and geochemical study may provide the data which will go far to solve, not only the status of the voluminous andesite, but of intermediate magma in general in igneous rock evolution.