Dear RG members, IUGS is planning a new edition of the classical "Le Maitre" book devoted to the classification and nomenclature of igneous rocks. A group of 17 igneous petrologists (hereafter TGIR - Task Group on Igneous Rocks) is working for three years to update specific definitions or proposing entirely new sections.

As the Chair of the TGIR, I would like to start a discussion with all the interested people that want to give help concerning this task. I and the other members of the TGIR will start posting a series of arguments that will greatly benefit from your comments, so I hope to receive stimulating feedback.

The IUGS classification uses the QAPF diagram for volcanic rocks (depending on the crystallinity of the sample and the size of the minerals). What sounds strange is the very large field for basalts and andesites. Four are the main problems:

1) According to the existing QAPF double triangle, basalts can have a plagioclase/feldspars ratio down to 65, i.e., up to 35% of the feldspars in a basalt can be alkali feldspars (sanidine) and only 65% can be plagioclase. Every research that has investigated thin sections of basalts knows well that such a ratio is unrealistic. Feldspars in basalts are typically >90% (often >99%) plagioclase, with very minor, if any, alkali feldspar (sanidine or anorthoclase). The proposal is to increase the plagioclase/feldspars ratio to 90 in order to classify a rock as a basalt.

2) For andesites things are different, being these rocks more evolved and having the possibility to have higher alkali feldspar content compared to basalts, with lower plagioclase/feldspars ratios. This means that we should think to split the basalt/andesite field into at least two different fields.

3) According to the existing QAPF diagram, andesites can have also up to 10% modal foids (of course among the total QAPF minerals). I am wondering if any of you has ever found andesites with foids. Probably we should limit the andesite field to the QAP triangle only.

4) If the field of basalt is restricted we have to think how to name what was originally named "basalt/andesite". The possibility is to add terms existing in the TAS diagram (e.g., trachybasalts, basaltic trachyandesistes) not reported in the QAPF.

Attached you find a preliminary draft of the new QAPF diagram. Take it only as an exercise, not as a definitive proposal!

Comments are all welcome,

michele