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.
It may look strange, but IUGS never gave a rigorous definition of what distinguishes rocks belonging to alkaline series to those belonging to subalkaline series.
SUBALKALINE.
In the Le Bas (2002) book, the word "subalkaline" simply does not exist. You can find in the glossary of terms a short definition of the word "Subalkali", which is far from being satisfying for me. Indeed, you can read: "Subalkali: A term used for rocks that are not alkaline in character". Absolutely insufficient, according to me. It is not correct to define a rock saying what it is not. Above all, a lherzolite, which is not an alkaline rock, should be classified as subalkaline, according to this definition, which is hard to understand for me. In addition, not clear why IUGS has accepted "alkaline" and not "subalkaline", preferring the term "subalkali".
ALKALINE.
What sounds strange, is that in the glossary of terms of Le Maitre (2002) there is no definition for "alkaline" either. The reader asking for what "subalkali" is, is invited to check the term "alkaline" that, however, does not exist in the glossary. We urgently need to fill this gap of information. In case the TGIR does not reach an agreement, with the help of RG members, we have to report the difficulty to define these two terms.
ALKALI.
Le Maitre (2002) only report the definition for the term "Alkali", which is: "A prefix given to a rock which contains either: (1) modal foids and/or alkali amphiboles or pyroxenes or (2) normative foids or acmite.". Of course not all alkaline series rocks fall in this definition. A trachybasalt does not need to have foids nor alkali amphiboles or pyroxenes. The same holds for trachyandesites or similar rock types. Note, in addition, that there are "alkali" rocks that have also quartz (e.g., alkali granites and alkali rhyolites. This means that also the term "Alkali" necessitates a strong revision.
No comment is reported on the concept of alkaline and subalkaline series. In the TAS diagram, Le Maitre (2002) reports that the straight line dividing the fields S (for silica Saturated) and O (for silica Oversaturated) divides "alkaline rock series" from "calc-alkaline rock series". What is surprising, is that IUGS does not provide any definition for both "alkaline" and "calc-alkaline".
The TGIR proposes that the fields O and S reported in the TAS diagram delimit the "subalkaline" (not a part of them, i.e., "calc-alkaline") from "alkaline" (better: "mildly alkaline" rock series. This is what we can say (adding a correct and more precise definition in the glossary of terms. Nothing can be said when entering the rhyolite field, considering that there could be alkaline and subalkaline rhyolites, but no straight line to separate the two compositional types. The same holds for the low-SiO2 side, i.e., in the basalt field. There is no official way to chemically distinguish an alkaline basalt from a subalkaline basalt. In order to do that we should focus on groundmass mineralogy, to see if olivine or quartz or foids (nepheline) are present. However this is only a part of the story. We do not want to focus on the distinction between alkaline and subalkaline basalts, but on the distinction between alkaline and subalkaline rock series.
Once having accepted this definition (i.e., above or below the line separating "trachy-" rocks from "non-trachy-" rocks, we can definitively write a comment saying that it is better to definitively cancel the MacDonald and Katsura (1964) and the Irvine and Baragar (1971) division lines, commonly used in scientific articles.
A proposal for the glossary of terms would be:
SUBALKALINE SERIES: A genetically linked series of rocks plotting below the straight line in the TAS diagram with coordinates: Na2O+K2O = 5 wt%; SiO2 = 52 wt% and Na2O+K2O = 10 wt%; SiO2 = 82 wt%. Commonly with CIPW normative quartz compositions (i.e., SiO2 oversaturated). Rocks plotting in the basaltic andesite, andesite and dacite fields in the TAS diagram are subalkaline. Basalts can belong both to subalkaline and alkaline series, with distinction made based on modal or CIPW normative minerals, but statistically alkali basalts plot above the straight line with Na2O+K2O = 3 wt%; SiO2 = 45 wt% and Na2O+K2O = 5 wt%; SiO2 = 52 wt%. See alkaline series and alkaline basalt.
ALKALINE SERIES: A genetically linked series of rocks plotting above the straight line in the TAS diagram with coordinates: Na2O+K2O = 5 wt%; SiO2 = 52 wt% and Na2O+K2O = 10 wt%; SiO2 = 82 wt%. Rocks plotting in the trachybasalt, basaltic trachyandesite, trachyandesite, trachyte, basanite/tephrite, phonotephrite, tephriphonolite, phonolite and foidite/melilitite fields belong to the alkaline rock series. Commonly without CIPW quartz-normative compositions (i.e., SiO2-undersaturated to critically saturated). Basalts can belong both to alkaline and subalkaline series, with distinction made based on modal or CIPW normative mineralss, but statistically alkali basalts plot above the straight line with Na2O+K2O = 3 wt%; SiO2 = 45 wt% and Na2O+K2O = 5 wt%; SiO2 = 52 wt%. Alkaline series rocks have a minimum alkali content of 3 wt%. See subalkaline series and subalkaline basalt.
ALKALI: A prefix originally given to a rock which contains either: (1) modal foids and/or alkali amphiboles or pyroxenes or (2) CIPW normative foids or acmite. (Iddings, 1895b, p.183; Tomkeieff p.14). Now the prefix alkali is associated also to igneous rocks with modal or CIPW normative quartz. A concept based on the absolute alkali content (essentially Na2O+K2O) compared to all the other major oxides.
Comments welcome,
Michele
For the alkaline-subalkaline boundary in TAS, I use Miyashiro (1978) proposal. The reference is:
MIYASHIRO, A. 1978. Nature of alkalic volcanic rock series. Contributions to Mineralogy and Petrology, 66, 91–104, https://doi.org/10.1007/BF00376089.
See Bonin et al. (2020, GSLSP491, attached PDF) figure 2a. The boundary line (green dashed line) is nearly the same as the boundary between O1-O3 and S1-T groups of rocks in TAS, like Michele wrote. But it has the advantage to discriminate alkaline and subalkaline basalts, alkaline and subalkaline rhyolites. In addition, we can recall Rittmann's (1957) serial indices separating subalkaline, mid-alkaline and alkaline rocks (in blue in the figure), though they may be considered as nearly obsolete.
Bernard
Thanks Bernard.
I have updated the TAS scheme reporting the three literature alkaline-subalkaline division lines. As you can see, they are very close each other and, above all, they are close to the straight line of TAS diagram.
The scheme proposed by you on granitoid rocks deserves much respect, and I invite you to focus your activity in the TGIR on this argument in particular.
Cheers,
michele
Another group of igneous rocks that is not a 100% in consensus are apatite-magnetite-rich igneous rocks, which are often misread. For example: nelsonites and phoscorites, which are common in anorthosite and carbonatitic complexes, respectively. I have seen papers where both rocks are grouped for the same igneous province. Also, it is important to establish boundaries between magmatic and hydrothermal, for instance, Ap-Mt igneous rocks associated with IOA or IOGG type deposits which can be both from igneous or hydrothermal origin. How should these rocks be correctly named?
Thanks for the comment Felipe.
Many are the aspects in the classification of igneous rocks that need to be revised. What you report is certaingly worth of note, but out of place in this thread. We need to open a new discussion topic.
Note, however, that IUGS cannot go too much in detail. No comments on the petrogenesis is required, because in this case it will be impossible to reach a consensus. We only need simple tools to give "to each rock its proper name" (as Albert Streckeisen wrote nearly fifty years ago).
Cheers,
Michele
Hi Michele,
I hope you are ok. Sorry for interrupting and just continue the interesting discussion.
I thought this comment could contribute to the question, since these exotic rocks involve both subalkaline and alkaline derived rock series. I hope in this IUGS classification new version that petrogenesis will not be required for exotic or rare rocks.
Cheers!
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