A simple total iron (as FeO) versus TiO2 diagram appears to be highly effective in the chemical distinction between ortho- and para-amphibolites. The distinction is particularly sharp for iron, with para-amphibolites having FeOt values less than 8%. The distinction is less clear when the para-amphibolites include considerable amounts of basic detritus or tuffs and, therefore, have a composition similar to orthoamphibolites. Ortho-amphibolites derived by metamorphism of basic cumulitic rocks (e.g., cumulitic clinopyroxene and plagioclase) may have low Fe and Ti contents but should be distinguishable from para-amphibolites on the basis of other chemical elements in conjunction with field and pétrographie evidence.
