A CHEMICAL CLASSIFICATION OF VOLCANIC ROCKS BASED ON THE TOTAL ALKALI-SILICA DIAGRAM

Abstract

This contribution summarizes and brings up to date the recommendations made by the IUGS Subcommission on the Systematics of Igneous Rocks for the classification of volcanic rocks when modal analyses are lacking. The classification is on a non-genetic basis using the total alkali-silica (TAS) diagram, and is as nearly consistent as possible with the QAPF modal classification. The diagram is divided into 15 fields, two of which contain two root names which are separated according to other chemical criteria, giving the following 17 root names: basalt, basaltic andesite, andesite, dacite, rhyolite, trachybasalt, basaltic trachyandesite, trachyandesite, trachyte, trachydacite, picrobasalt, basanite, tephrite, phonotephrite, tephriphonolite, phonolite and foidite. Using Na-K criteria, trachybasalt may be further divided into the sub-root names hawaiite and potassic trachybasalt, basaltic trachyandesite into the sub-root names mugearite and shoshonite, and trachyandesite into the sub-root names benmoreite and latite.