Classification of igneous rocks revisited

Petrology, I must admit, was never my favourite subject even though I had to do quite a bit of it during my time as a student. Even my Honours project had a petrological component. An important tool for the classification of igneous rocks is the QAPF diagram which forms the basis of the IUGS classification of igneous rocks (Le Bas and Streckeisen, 1991). It was interesting to go back to this original publication to read about the considerations that led to the adoption of the classification schema. The general principles are solid science. In some of the finer points, however, it becomes apparent that this classification schema comes from the pre-digital age.

"One of the merits of the TAS system is that the boundaries are definitive although they could be criticized as over simplistic. Definitive boundaries remove the ambiguity in naming a rock which plots near a boundary between two adjacent rock types. The simple boundaries of the TAS system also enable the classification to be constructed in a few minutes by pencil and ruler"

Furthermore, the TAS classification system only takes into account the sum of Na₂O + K₂O over SiO₂ and does not account for any other major elements even though such classification systems had already been proposed (e.g. De La Roche et al. 1980).

"[It] was not adopted, partly because the two cation per mil parameters were difficult to calculate without electronic assistance"

Certainly, in the age before widespread use of spreadsheet calculation software, this might have been an argument. A bit more difficult to follow is the following argument:

"The most common interpretations made by geologists are petrogenetical, but there could also be aesthetic considerations."

Robert Huber, Jess Robertson and myself (JK) have started to look more closely at the classification of igneous rocks in the dawning age of data intensive research in the geosciences. Watch this blog for more on this subject.

Ackonwledgements

Thank you to Jess Robertson for pointing out this gem.

References

De la Roche, H., J. Leterrier, P. Grandclaude, and M. Marchal (1980), A classification of volcanic and plutonic rocks using R1R2-diagram and major-element analyses — Its relationships with current nomenclature, Chemical Geology, 29(1–4), 183–210, doi:10.1016/0009-2541(80)90020-0.

Le Bas, M. J., and A. L. Streckeisen (1991), The IUGS systematics of igneous rocks, J Geol Soc, 148(5), 825–833, doi:10.1144/gsjgs.148.5.0825. Online also available at https://www.utdallas.edu/~aiken/SHAKEBAKE/rockclassification.pdf