Basic volcanic rocks and tectonic setting. A discussion of the Zr-Ti-Y discrimination diagram and its suitability for classification purposes

The Zr-Ti-Y diagram cannot discriminate between normal and anomalous oceanic ridge basalts: the latter tend to plot as within-plate basalts. This indicates that it is the mantle anomaly rather than the tectonic setting that characterizes the Zr-Ti-Y interelement ratios in such rocks. It is shown that rocks with SiO₂<56 of a tholeiitic series indicate a differentiation trend from ocean-floor via within-plate to calc-alkaline basalts. This feature shows that great care should be taken when data for altered or metamorphic rocks are selected for classification by this method.