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.
Is plagioclase a fundamental mineral in picrobasalts or could it lack? According to the term "basalt" I would imagine that plagioclase should be present (if not entirely glassy, of course), but this is not explained in Le Maitre (2002). Here the actual definition:
Picrobasalt: A chemical term for volcanic rocks, which will include certain picritic and accumulative rocks, which was introduced for TAS field Pc (Fig. 2.14, p.35). (Le Maitre, 1984, p.245)
Cheers,
michele
DEAR MICHELE
GOOD MORNING. AS YOU MENTIONED PICROBASALT IS REGARDED AS A PICRITIC ROCK (LE MAITRE ET AL. 2002). PICRITIC ROCKS COULD SOMETIMES COMPLETELY LACK PLAGIOCLASE. AN EXAMPLE IS THE EDIPSOS OCCURRENCE IN GREECE (SEE TSIKOURAS ET AL. 2008).
BEST REGARDS
IOANNIS
Yes Ioannis, but the term "basalt" is always associated to a volcanic rock with plagioclase (if it its not entirely glassy). To adopt a root name such as "basalt" with a prefix ("picro") would cause confusion, because the name should allow thinking a rock with plagiolase plus other things.
There are three options:
1) We change the name "picrobasalt" in something else (difficult to accept choice);
2) We change the definition of basalt as a volcanic rock with plagioclase (which I strongly disagree);
3) We leave this problem as it is.
Cheers,
michele
I THINK OPTION NUMBER 1 SOUNDS BETTER. IT IS DIFFICULT BUT NOT IMPOSSIBLE. LIKE MANY PETROLOGICAL TERMS, THE TERM PICROBASALT IS USED DIFFERENTLY BY EVERY GEOLOGIST. HOWEVER, YOU CAN ALWAYS FIND SOME OVERLAP IN THE DEFINITION.
BEST REGARDS
IOANNIS
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