Andesite in oceanic regions

Abstract

Among the chemically intermediate rocks of the central ocean basins, three genetic series-tholeiitic, low-K alkalic, and high-K alkalic-are recognizable on the basis of relations between the differentiation index (DI) and total alkalies and between DI and the alkali index [AI, here defined as 100 × K2O/(K2O + Na2O)]. The present nomenclature for these rocks, using terms based on locality names (hawaiite, tristanite, icelandite, and so forth), is inconsistent. Names based on chemical differences do not cover the range in composition, and names based on mineralogical differences do not reflect existing differences in chemistry. Alternatives to this “locality” nomenclature are considered: First, andesite is redefined, regardless of its geographic occurrence, as a volcanic rock in which more than 2/3 of the feldspar is plagioclase, the groundmass plagioclase is oligoclase or andesine, normative quartz is less than 15 percent, and the color index is less than 40. By prefixing this generalized andesite with appropriate terms indicating the genetic series, geographic location (for example, “oceanic”), and distinctive mineralogy, a consistent and informative nomenclature is possible. A second alternative is to define in a more consistent fashion the present locality names of the intermediate rocks by grouping them into two sets, one based on modal composition (mugearite, hawaiite), the other on chemistry (trachyandesite, trachybasalt).