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 actual classification of charnockites as igneous rocks (Le Maitre, 2002) is far from being clear. According to the sub-commission on igneous rocks, the classification of charnockitic rocks "should be used only if the rock is considered to belong to the charnockitic suite of rocks, which is characterized by the presence of orthopyroxene (or fayalite plus quartz)". This definition has several weak points:

1) It is a sort of circular reasoning (i.e., the classification of charnockitic rocks can be used only for charnockitic rocks);

2) It takes into consideration only two very common minerals (orthopyroxene and fayalite);

3) It does not consider the grain size (i.e., if it is related to phaneritic or to fine-grained rocks);

4) It uses a specific olivine composition (fayalite) but a generic orthopyroxene definition (i.e., without specifying if the Mg/Fe ratio is relevant or not).

Another major problem related to the definition of "charnockite" is (Le Maitre, 2002): "Although many show signs of metamorphic overprinting, such as deformation and recrystallization, they conform to the group of “igneous and igneous-looking rocks” and have, therefore, been included in the classification scheme."

According to this definition, metamorphic rocks (rocks characterized by deformation and recrystallization) should be classified as igneous rocks, which is contrary to the recommendation of the IUGS sub-commission on metamorphic rocks, which published a book five years later (Desmond and Fettes, 2007).

The third problem of the actual classification of "charnockites" is that Le Maitre (2002) proposes to use the QAP diagram, despite these rocks are commonly characterized by one feldspar (perthitic to antiperthitic). To solve this problem, Le Maitre (2002) propose to allocate perthites to alkali feldspars only and antiperthites to plagioclase only. We note that this has not been proposed for "normal" phaneritic rocks with granitoid compositions.

We are open to modify this interpretation, using a simple approach:

1) If the orthopyroxene±fayalite-bearing QAP rock has metamorphic textures it has to be classified among metamorphic rocks and the term "charnockite" should be used (following the recommendations of Desmond and Fettes, 2007);

2) If the orthopyroxene±fayalite-bearing QAP phaneritic rock has igneous textures it should be classified among igneous rocks following the classical indications for all the phaneritic rocks, with the addition of prefix orthopyroxene±fayalite-bearing.

There is no petrogenetic nor petrographic reason to ask for a specific name for a orthopyroxene ± fayalite-bearing phaneritic rock, considering that the volcanic rock equivalents have "normal" rock names (e.g., "orthopyroxene-bearing andesite" or "fayalite-bearing rhyolite".

Comments welcome,

Michele