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.
Once having classified a rock to the alkaline series, the next step is usually to identify the alkali ratio to choose adjectives such as sodic-potassic-ultrapotassic.
Present IUGS definition:
The glossary section of the present IUGS classification of igneous rocks does not report any information on the adjectives "sodic", "potassic" or "ultrapotassic". The third edition has to fill this gap. IUGS only provides some info on the mildly alkaline rocks in the TAS diagram (i.e., those falling in the trachybasalt, basaltic trachyandesite and trachyandesite fields). The simple (but far from being satisfying) IUGS rule defines sodic a rock with Na2O wt% content (minus 2 wt%) higher than its K2O wt% and potassic if the Na2O wt% content (minus 2 wt%) is lower than its K2O wt%. For example, a rock with 5 wt% Na2O and 2 wt% K2O is considered as sodic, whereas a rock with 3 wt% Na2O and 2 wt% K2O is considered potassic. Remember that this distinction is considered valid only for the three mildly alkaline compositions (trachybasalts, basaltic trachyandesites and trachyandesites). Nothing is said about other compositions (e.g., ultrabasic, acid and strongly alkaline rocks).
In addition, IUGS considers a rock as "ultrapotassic" if molar K2O/Na2O is >3 (see section 2.7.2 of Le Maitre, 2002). No information is reported on the equivalent sodic term (i.e., “ultrasodic”). To conclude, IUGS reports only a short comment on the adjective “transitional” to be addressed to basalts only. According to IUGS the term “transitional basalt” should be avoided (it is not reported in bold in the Glossary of terms section). The IUGS definition for “transitional basalt” is “A variety of basalt transitional between typical tholeiitic basalt and alkali basalt. It consists of olivine, Ca-rich augite, plagioclase and titanomagnetite plus variable, but small, amounts of alkali feldspar. Ca-poor pyroxenes are absent.”.
To conclude, present IUGS rules are:
Sodic: (Na2O wt% - 2 wt%) > K2O wt% (valid for trachybasalts, basaltic trachyandesites and trachyandesites only).
Potassic: (Na2O wt% - 2 wt%) < K2O wt% (valid for trachybasalts, basaltic trachyandesites and trachyandesites only).
Transitional: No indication reported.
Ultrapotassic: molar K2O/Na2O >3
Ultrasodic: No indication reported.
Not definitive proposal of the IUGS TGIR:
1. Albeit not completely correct, we propose to deal with major oxides, not molar concentration, because it is much easier to manage oxides, without any special calculation.
2. The Na2O + K2O ratio has to be >3 wt%. We emphasize that this threshold value is not sufficient to avoid non-alkaline rocks (for example, non-alkaline acid rocks have Na2O + K2O up to 7), but it is the minimum ratio in case of basic-ultrabasic compositions.
3. MgO has to be >3 wt% to avoid major changes associated with fractional crystallization, following Foley et al., 1987 (Earth-Sci. Rev.). This means that rocks such as phonolites and trachytes could not be classified as ultrapotassic-potassic-transitional-sodic ultrasodic.
4. Ultrapotassic: K2O/Na2O >2 (Na2O/K2O 3 wt% (following Foley et al., 1987, Earth-Sci. Rev.).
5. Potassic: K2O/Na2O between 1 and 2.
6. Transitional: K2O/Na2O between 1 and 0.5.
7. Sodic: K2O/Na2O between 0.5 and 0.25 (Na2O/K2O between 2 and 4).
8. Ultrasodic: K2O/Na2O 4); Na2O
DEAR MICHELE
I AGREE WITH MOST IUGS TGIR PROPOSALS FOR ALKALINE ROCKS. NEW USEFUL TERMS LIKE ULTRASODIC SHOULD BE ADDED. REGARDING ULTRAPOTASSIC ROCKS, THE K2O/Na2O RATIO (FOR K2O>3) SHOULD BE >2 BASED ON THE RELEVANT FOLEY ET AL. 1987 ARTICLE. THIS IS OBVIOUS SINCE, FREQUENTLY, CLASSIC ULTRAPOTASSIC ROCKS LIKE OROGENIC LAMPROITES HAVE A K2O/Na2O>2 BUT NOT EXCEEDING 3. ALSO, I AM NOT SURE ABOUT NOT INCLUDING RIFT-GENERATED PHONOLITES AND LOCAL HIGHLY ULTRAPOTASSIC VARIETIES OF TRACHYTE SUCH AS TIMANITES (UDORATINA ET AL. 2010) AND DOIRANITES (ASVESTA 1992, PE-PIPER ET AL. 2004) IN THE ALKALINE ROCKS DUE TO LOW MgO. THE EXPERT IGNEOUS PETROLOGIST ALAN WOOLLEY FOR EXAMPLE HAS INCLUDED PHONOLITES AND ULTRAPOTASSIC TRACHYTES IN ALL OF HIS "ALKALINE ROCKS AND CARBONATITES OF THE WORLD" VOLUMES. FINALLY, IT SHOULD BE MENTIONED THAT ALKALINE ROCKS COMMONLY CONTAIN PYRIBOLES.
BEST REGARDS
IOANNIS
Dear Joannis,
this is actually what is proposed. To define a rock as ultrapotassic we propose to follow the suggestion of Foley et al. (1987), i.e., K2O >3 wt% and K2O/Na2O >2, as in the scheme proposed in this thread. In addition we should consider also MgO >3 wt%, to exclude that the relatively high K2O could be simply an effect of prolonged fractionaly crystallization of K2O-poor mineral assemblage. For example, K2O-rich trachytes of the Phlegrean Fields volcano cannot be considered as ultrapotassic, because strongly fractionated melts with very low MgO.
Note that the original definitions for ultrapotassic rocks considered K2O/Na2O ratios be >3. Foley et al. (1987) reduced this ratio to 2 to include several rocks in the "ultrapotassic" family.
I did not find any word referring to "timanite" or "doiranite" in the Le Maitre (2002) book.
At the end I underline that "pyribole" is not a mineral name reccommended by IMA, so I would not quote it in the new edition of the book.
Cheers,
michele
"Doiranite" is a rock from Greek Macedonia. The reference is:
Osswald K. 1938: Geologische geschichte von Griechisch—Nord- 832 makedonien. Geologische landesanstalt von Griechenland. 833 Nationale Druckerei, Athen 3, 1—141.
"Timanite" is a recent rock name. The oldest reference is not Udoratina et al. (2010), but:
Malkov B. A., Fillippov V. N., Shvet-sova I.V. 2006. Timanit — unikalnaya vysokotitani-staya ultrakalievaya raznovidnost trahita: Sred-nii Timan, pozdnii paleozoi (Timanite —unique high-titanium ultrapotassium variety of trahite: Middle Timan, Late Paleozoic). Vestnik of Institute of Geology 12, 13—21. I am afraid that this reference has not easy access.
In addition, note that it may be mistaken with "Timanites", a Devonian Ammonoid , whose genus name is more ancient (Schellwien, 1898).
"Trahite", referred to in Malkov et al., is unknown in Google.
Bernard
DEAR BERNARD
GOOD AFTERNOON. YOU DID A GREAT SEARCH, BOTH INITIAL REFERENCES ARE RIGHT. THE TWO HIGH-K ROCK TYPES HAVE SOME DISTINCT CHARACTERISTICS NOT FOUND IN COMMON TRACHYTES. THE DOIRANITES FROM MACEDONIA IN GREECE HAVE SANIDINE MEGACRYSTS AND QUARTZ OCELLI SURROUNDED BY MINUTE BIOTITE LATHS (POINTING TO LIQUID IMMISCIBILITY). THE 1ST WORLD WAR MONUMENT ON THE HILL BEHIND THE VILLAGE OF DOIRANI IS MADE OF DOIRANITE (SEE PHOTO ATTACHED). THE TIMANITES ON THE OTHER HAND HAVE HIGH TiO2 AND CONTAIN PRIMARY INTERMETALLIDES, NATIVE ELEMENTS, WUSTITE, SULPHIDES WHICH ALL EXISTED BEFORE K-METASOMATISM. I THINK THESE ROCK TYPES COULD BE INCLUDED IN THE NEW IUGS GLOSSARY FOR IGNEOUS ROCKS (I HAVE FOUND 5 REFERENCES FOR DOIRANITE). FOR MORE INFORMATION ABOUT DOIRANITE YOU COULD CONTACT DR. ASVESTA AND PROF. GEORGIA PE-PIPER. FOR TIMANITE DETAILS YOU COULD CONTACT DR. MALKOV ET AL. AND DR. UDORATINA ET AL. FINALLY, I THINK THAT THE RUSSIAN GEOLOGISTS MEANT BY TRAHITE THE ROCK TRACHYTE.
BEST REGARDS
IOANNIS
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