THE PROBLEM OF THE DIVERSITY OF IGNEOUS ROCKS

G. MALCOLM BROWN

Department of Geological Sciences, University of Durham, Durham, England

The science of igneous petrogenesis is a study of the origin and evolution of rocks that have formed through the generation of magmas by melting processes, and the cooling of those magmas from a liquid, or liquid and crystalline state, into a glassy to crystalline state. Field and textural observations do not constitute sufficient evidence for defining a rock as igneous in origin (Barth, 1962, p. 51) but when allied with experimental studies on silicate melts similar in composition to the rock in question, the origin of the rock can be verified. Experimental studies, primarily by N. L. Bowen, formed the only firm basis from which the science of igneous petrogenesis could develop. If there were no diversity of igneous rocks, there would be no science of igneous petrogenesis. Large-volume flows of apparently homogeneous basalt are problematical, but only when viewed in relation to the more common type of diverse flow. The "problem" of the diversity of igneous rocks is more a matter of "problems" that will not be solved until all the complex processes of igneous petrogenesis have been understood. Bowen was as aware of that, in 1928, as one is now, and selected the topic of diversity for the opening chapter of his book. He stressed the importance of igneous rock associations and the need to explain differences in mineral assemblages and rock compositions, both between and within those associations, according to definite physicochemical processes. One should first consider, briefly, what type of diversity led Bowen to make the investigations that were described in the rest of his book, and then view the subject in the light of contemporary knowledge.

Bowen endorsed the concept of "petrographic provinces" (Judd, 1886), but pointed to the importance of time as well as place in such groupings and introduced "rock associations" as a preferable name. Such an association of rocks, sharing certain properties and connected by a "community of origin," was said to have been derived from a single original magma. He favored a basaltic magma in the parental role but did not think it was requisite in all cases. The question of whether more than one type of parental magma was involved did not tempt Bowen to provide an opinion. A division into alkaline and subalkaline types he thought useful, yet he did not quibble with R. A. Daly's view that there were no essential differences between the basaltic magmas of the various associations. At that time, Bowen was much more concerned with diversity produced within, rather than between rock associations. He stressed the continuous nature of variations in composition within a rock association, such that the division of such a series into specific members could only be arbitrary. Then followed his major discussion on the processes likely to be responsible for diversity within series, i.e., differentiation processes. He discounted compositional gradients in the liquid phase (temperature or gravity controlled), except insofar as pressure gradients could affect the concentration of volatile components. His support was entirely for processes involving separation of distinct phases either by gaseous transfer, liquid immiscibility, or crystallization. He went on to demonstrate the dominant role of fractional crystallization (associated sometimes with the additional effects of assimilation).

Petrologists would agree that there have been few major changes in emphasis since Bowen's exposition, regarding the general concept of igneous rock associations and the main processes responsible for variation within the associations. They would emphasize that their main concern is still with the diversity of what are now called primary, parental, and derivative magmas and their rock products, but particularly with the definition of what constitutes a primary magma. Since 1928 the subject has been expanded enormously through access to information on ocean-floor igneous rocks, and the theory of global plate tectonics has required a reappraisal of what are now called petrogenic provinces. The vast amounts of experimental data on multicomponent systems, referred to in subsequent chapters, have led to an expansion of knowledge about the deep regions of primary magma generation in the Earth's upper mantle, and about the processes of crystal fractionation, liquid immiscibility, and the role of volatile components in further promoting diversity in the patterns of magmatic evolution. Further advances relate to the application of thermodynamics to an understanding of the liquid state, rates of crystallization and nucleation, solid-liquid-gas equilibrium relations, and the effects of temperature and pressure on mineral equilibria. Extensive studies on the distribution of chemical elements and certain isotopes, and on element partitioning between crystalline and liquid phases, have been accompanied by consideration of the relative importance of fractional crystallization, fractional melting, and equilibrium partial melting in the derivation of particular natural silicate liquids (e.g., Gast, 1968; Shaw, 1970).

In order to recognize diversity in any population it is necessary first to define the criteria for distinctions and the limits of tolerance beyond which uniformity becomes diversity. Igneous rocks as a whole are uniform according to the criteria and broad limitations defined in the opening sentence of this chapter, and thus diverge from metamorphic and sedimentary rocks. The introduction of sets of criteria such as liquidus temperature, silica...