Nature of alkalic volcanic rock series

Abstract

The alkalic rocks are here regarded as a category in a classification of igneous rock series (rock associations) and not as a class in petrographic systematics. The alkalic series as a whole are characterized by higher Na2O+K2O content than the subalkalic series in the alkali vs. SiO2 diagram. At least three different trends (types) of differentiation appear to exist in large-scale alkalic volcanic associations. One (here designated as the Kennedy trend) starts from weakly nepheline-normative basalt and shows increasing normative nepheline with advancing fractionation to reach a phonolitic composition. Another (here called the Coombs trend) starts from hypersthene-normative basalt and shows increasing normative hypersthene and then normative quartz with advancing fractionation to reach a comenditic composition. Besides these two trends, it seems that many alkalic associations exist which show a differentiation trend starting from nepheline-normative basaltic composition and leading to hypersthenenormative, and then to quartz-normative compositions (here designated as the straddle-B type). Alkalic rocks of these three trends are higher not only in Na2O+K2O but also in Rb, Ba, Sr and Zr than subalkalic rocks. The alkalic basalts as a whole are characterized by higher contents of such elements and not by any degree of silica undersaturation. It is widely believed that alkalic rocks are characterized by the presence of normative nepheline as well as by the absence of orthopyroxene and pigeonite. Indeed such a relationship holds for the Kennedy trend, but it is not always valid for other types of alkalic associations. Some alkalic rocks of the Coombs trend and straddle-B type have quartz (or other silica minerals) and orthopyroxene and pigeonite.