Sunday, 3 March 2019

Plotting TAS diagram of Le Bas et al. (1986)

By far the most common bug report that we are getting concerns the implementation of the total alkali-silica (TAS) diagram of Le Bas et al. (1986) in GCDkit. But in reality, it is  not a bug, but a feature. One should be aware of the fact that the analyses plotted have been recalculated on 100% anhydrous (volatile-free) basis.  This is in accordance with the recommendation of the IUGS  Subcommission on the Systematics of Igneous Rocks (Le Bas et al. 1986; Le Maitre ed. 2002).

On the other hand, such a recalculation is not carried out for subsequent modifications of the TAS diagram (Cox et al. 2002; Middlemost 1994), where rough data are plotted.

In any case, using analyses with too high volatile contents is to be avoided, as they are suspicious from being affected by alteration and attendant mobility of alkalis.   


Cox KG, Bell JD & Pankhurst (1979) The Interpretation of Igneous Rocks.
Allen & Unwin, London

Le Bas MJ, Le Maitre RW, Streckeisen A & Zanettin B (1986)
A chemical classification of volcanic rocks based on the total alkali-silica
diagram. J Petrology 27: 745-750

Le Maitre RW (ed) (2002) Igneous rocks: a classification and glossary of terms: recommendations
of the International Union of Geological Sciences, Subcommission on the Systematics of
Igneous Rocks, 2nd edition. Cambridge University Press


Middlemost EAK (1994) Naming materials in the magma/igneous rock system.
Earth Sci Rev 37: 215-224

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