Hi Michele,

I would say that oxides do not enter the definition of gabbros… if there are some, then the name is mofified to oxide gabbros (when ox>5%).

Also saying that chemically gabbros are similar to basalts is sometimes true for some major elements (definitly not for compatible or incompatible trace elements), but usually not as those are most of the time cumulates. I would rather say that it corresponds to the plutonic equivalent to basalts, that crystallized slowly at depth without any reference to chemical composition.

Cheers

Lydéric

A simple question: what are the main criteria to bear in mind for the new/upgraded classification of igneous rocks? I do not understand the context of the newly proposed definitions of gabbro and diorite in Michele's statement.

Thank you Lyderic France . The reference to "iron oxides" is from Le Maitre (2002). I guess that none has found anomalous this information, so I would leave it.

Of course there are gabbros and gabbros, in the same way there are basalts and basalts. A basalt remains a basalt if it is rich or low in Nb or if its 87Sr/86Sr is higher or lower than BSE. It is not a matter of trace element content nor of isotopic ratios. In the new definition we would simply say that chemically gabbros resemble basalts, but are not exactly the same as basalts (i.e., are not confined in the strict field of the TAS diagram).

michele

Thanks Mattia. as members of the IUGS TGIR, we would like to work in a much more dynamic view compared to what has been done up to now. When the second edition of the IUGS book on the classification of igneous rocks went to the press, internet was still in its infant stage. Now we have powerful tools to exchange ideas and comments at the light speed, and I believe that ResearchGate is the best place where to do that. I hope you can contribute in this and all the other topics we are working on.

Ciao,

michele

Your definition for gabbro covers the modal classification using Ol-Opx-Cpx and quartz. According to the suggested definition, you restrict the presence of hornblende to diorite because hydrous minerals are rare in gabbro. These definitions will not cover the term "Hornblende gabbro", which is covered by the Plagioclase-Pyroxene-Hornblende modal classification of the classical "Le Matire" book. Do you want to consider the term "Hornblende gabbro" obsolete?!

What will be the plutonic rock name if the amphibole is associated with labradorite-bytownite plagioclase or if it composed of plagioclase and hornblende, but it plots in the basalt rather than andesite field on the TAS diagram?

I think it is better not add detailed names, such as gabbronorite, nortie, quartz gabbro and olivine gabbro to the definition of gabbro and make specific definitions for them as in the "Le Matire" book. Otherwise, you will need to add "hornblende gabbro" to you definition.

Thanks for the comment Yasser Abd El-Rahman . Nobody thought to delete the term "hornblende gabbro, of course". In the new scheme nobody thought to "restrict the presence of hornblende to diorite". Nothing of similar is written in the proposed scheme. Note, however, that the official IUGS definition, hornblende is not reported as typical mineral in gabbros. Please read again the present definition of gabbro I have reported in my first message. This is what we are dealing up to now.

A hornblende gabbro is and remains a gabbro. You can simply add the mafic mineral adjective you like (if that mineral is present), but the rock remains a gabbro. A hornblende gabbro can have quartz or foid and it makes no significant distinction. If in a gabbro is present olivine or quartz the petrogenetic significance is sensibly higher, and deserves a specific comment. Remember that the necessity to indicate olivine- or quartz-gabbro is reported in the actual IUGS scheme.

The word reported in the glossary of terms was simply to distinguish from other plagioclase-rich rocks. The aim is not to report a list of all the possible mafic minerals that can be found in gabbros.

I believe that glossary has to report also some commment that can be useful to allow distinction between that given name with similar rocks.

Cheers,

michele

Hi Michele,

It is nice to participate in this fruitful discussion. Actually, nomenclature of mafic and intermediate plutonites is a big concern when it comes to classifications, e.g. Le Maitre et al (2002) and others. I enjoyed the replies to your discussions and they no doubts include valuable comments and considerations that I agree with most of them. I have only few comments as follows:

1) It is better to use "Fe-Ti oxides" instead of "iron oxides" in the proposed definition. Of course Fe-Ti oxides and their primary and secondary intergrowths in gabbroic rocks comprise important minerals, which can be used as petrogenetic indicators, either in fresh rocks or in their metamorphosed counterparts.

2) I am inclined to the opinion of some colleagues who prefer to keep the name "hornblende gabbro", but in this case we need to consider what kind of hornblende, or generally amphibole, we talk about. Is it primary (magmatic) amphibole or secondary due to uralitization and deuteric alteration, which is still magmatic in late stage, and metamorphism is excluded?. I prefer to use the term "amphibole gabbro" that is sometimes very fresh and exclusively magmatic, and contains ferro-hornblende or oxy-hornblende essentially?. this should be considered, and therefore the usage of such nomenclature must be with ultimate care.

3) I am not in favour of using the nomenclature "quartz gabbro". I believe that primary quartz in gabbro, not diorite, is very rare or absent. In case of quartz presence in gabbro, possibility of being secondary silica should not be discarded. Of course, quart as a primary phase can be present in diorite, and hence we are all agree about the nomenclature of "quartz-diorite" I guess. Finally, the overlap with quartz-monzodiorite should be also considered.

Regards for all

Adel

Dear Adel A. Surour ,

Thanks for your comment. The term "iron oxides" is reported in the original definition. I agree with you that "Fe-Ti oxides" is much better. Probably we could change it with "oxy-spinel" considering the possible presence of other cation in the ulvospinel-magnetite solid solution.

Ok also for the hornblende. I have changed it, editing the description. In the Le Maitre (2002) version it is reported specifically "hornblende" rather than a generic "amphibole". Probably we could use the omnicomprensive term "amphibole".

As concerns the term "quartz gabbro" I have to remind that this is a term that appears for 35 years in the IUGS QAPF double triangle, so I would not change it.

Last, there is no possibility to overlap between diorite and monzodiorite, because these terms are associated to two distinc fields in the QAPF diagram.

Cheers,

michele

Dear Michele Lustrino

It is nice to know that you agree with some of my comments.

Let me stress again about the nomenclature of "quartz gabbro". I understand your point of view very well, and that this gabbroic variety exists in the IUGS QAPF triangle. This does not mean it will survive forever and as far as we have logic arguments it can be improved. Finally, I know that there is no possibility to overlap between diorite and monzodiorite in the sense of QAPF ternary diagram, and I mentioned it should be considered especially in case of altered gabbros and diorites. Minor secondary processes (alterations) such as silicification and others can affect and deviate the mineralogical composition, and the whole-rock chemical composition as well.

Cheers,

Adel

It is valuable to highlight the importance of an upcoming workshop that will likely address this matter and other issues at the 37th IGC in Busan. The workshop, titled "IUGS Updates on the Classification and Nomenclature of Igneous Rocks," will take place on August 30th from 10:00 to 12:00 at BEXCO, room C104.

Dear Michele Lustrino

A few remarks about the new definitions:

1) The requirement for a quartz content of more than 5% in quartz gabbro is not consistent with the QAPF diagram. For example, a sample contains quartz = 3–4%, plagioclase = 50%, alkali feldspar = 1%. When recalculated to 100%, these contents are transformed: Q (quartz) =5.5–7%, P (plagioclase) = 91–92.5%, A (alkali feldspar) =2%. The sample composition is located in the field of quartz gabbro with the modal quartz = 3–4%.

2) In my opinion, the oxides in the definition of gabbro and diorite is unnecessary. The oxide content may be minor. On the other hand, sulfides sometimes predominate among opaque minerals.

3) The chemical composition of diorites is equivalent to basaltic andesite, not andesite. Instead the chemical composition of quartz diorites is equivalent to andesite.

4) The problem with dark-colored minerals in diorite is more complicated. Why do we limit them to amphibole, biotite and augite? Diopside or hedenbergite may be present in diorites. What to do with orthopyroxene diorites that are not related to the deformed and recrystallized charnockitic suite? In our region, diorites and quartz diorites with an orthopyroxene content = 5–30% are called norites, or geologists suggest introducing their own name for these rocks. I would like a clearer definition of the term diorite to eliminate such discussions.

Thanks for the contribution Roman Terentiev .

A reply to each of your notes:

1) You write:

The requirement for a quartz content of more than 5% in quartz gabbro is not consistent with the QAPF diagram. For example, a sample contains quartz = 3–4%, plagioclase = 50%, alkali feldspar = 1%. When recalculated to 100%, these contents are transformed: Q (quartz) =5.5–7%, P (plagioclase) = 91–92.5%, A (alkali feldspar) =2%. The sample composition is located in the field of quartz gabbro with the modal quartz = 3–4%.

Of course, when dealing with QAPF, all the modal abundance have to be normalized to the fundamental minerals.

2) You write:

In my opinion, the oxides in the definition of gabbro and diorite is unnecessary. The oxide content may be minor. On the other hand, sulfides sometimes predominate among opaque minerals.

The reference to oxides is taken from the original (and official) definition reported by Le Maitre (1989 and 2002). I would not change it. I have never found sulfides prevaliling over oxides (oxy-spinels).

3) You write:

The chemical composition of diorites is equivalent to basaltic andesite, not andesite. Instead the chemical composition of quartz diorites is equivalent to andesite.

Who says this? Nobody has never certified this interpretation. Probably it is your idea, but if you have any official information, please share it with me. Note that basaltic andesites are much more similar to basalts than andesites (for example, many of the CFB - Continental Flood Basalts - are actually basaltic andesites, but are defined as "basalts s.l.").

4) You write:

The problem with dark-colored minerals in diorite is more complicated. Why do we limit them to amphibole, biotite and augite? Diopside or hedenbergite may be present in diorites. What to do with orthopyroxene diorites that are not related to the deformed and recrystallized charnockitic suite? In our region, diorites and quartz diorites with an orthopyroxene content = 5–30% are called norites, or geologists suggest introducing their own name for these rocks. I would like a clearer definition of the term diorite to eliminate such discussions.

Again, the reference to hornblende is what is officially reported in Le Maitre (2002). I have difficulties to believe to diorites with primary diopside. Again, if you have evidence of rocks with this mineralogy, I kindly invite you to share with me the source of these data. Hedembergite is usually a quite rare mineral, in particular in diorites. I would be happy to read articles dealing with these minerals in diorites, thanks.

Amphibole (very often green hornblende) is, by far, the most abundant mafic mineral in diorites.

Note that at page 20 of Le Maitre (2002) orthopyroxene diorites are classified as norites, but within the family of charnockitic rocks.

To conclude: if I posted this question it was indeed for the same reason you reported at the end of your message. We all hope to reach a consensus among the vast majority of igneous petrologists worldwide, but we all know that it will be impossible to have 100% consensus even on one single definition.

Cheers,

michele

Dear Michele and TGIR,

Thank you for your work on improving these definitions. This will be very useful to the community and for teaching purposes. I have a suggestion: explicitly include in the definition that the difference in the range of chemical compositions between gabbros and basalts largely reflects cumulate processes.

This distinction is particularly important for students, given the common simplification that basalts are the intrusive/ compositional equivalents of gabbros. The difference in compositional range, as indicated in the revised definition, already shows that this is not exactly true.

Based on Niu et al. (2002)'s definition of gabbros as "cumulate equivalents of basalts,” you might consider something like this: “Petrogenetically, gabbros are cumulate equivalents to basalts, hence their compositions are not strictly confined to the basalt field in the TAS diagram.”

That said, I'm not sure whether genetic implications are against the norms for IUGS definitions. In this case, I would state that "Chemically, gabbros are similar to basalts," rather than equivalent.

Excerpt from Niu et al. (2002): “Bowen (1928) first suggested that gabbros were formed by crystal sorting and accumulation from cooling basaltic melts. Four decades later, Wager and Brown (1968) illustrated in great detail various types of cumulate textures developed during the formation of gabbros in response to cooling of basaltic magmas. Perhaps most, if not all, modern igneous petrologists would accept the concept by Bowen and the conclusions by Wager and Brown, but ironically, some modern igneous petrology textbooks maintain “gabbroic rocks are intrusive/compositional equivalents of basalts but have phaneritic grain size” (e.g., Best, 1995), implying that the slow cooling, and thus greater grain size, is the sole difference between gabbros and basalts. A simple conceptual argument is that if gabbros are indeed the product of fractional (vs. equilibrium) crystallization, then we cannot avoid the conclusion that gabbros are cumulate, not melt, equivalents, although gabbros could trap a significant amount of interstitial melt (e.g., Coogan et al., 2001).”

As a second suggestion, perhaps you could also consider a "Fe-Ti oxide gabbro" or "Oxide gabbro" classification when there are more than 5% oxides. These terms are already used in the literature, particularly for layered complexes, but as far as I know, there is no official IUGS definition.

Cheers, David

Dear All,

I shouldn't have written because my knowledge of lithology is minimal (recognition of stones is minimal). I have not been able to practice my profession for thirty years, presumably for political reasons. In the last thirty years, during which I have been able to imagine an original geological motel, I have come to the following conclusion:

The circumstances (mechanism) of the formation of the rocks should be decisive in this case. The petrological division taught at Romanian universities between 1960-1990, which changed and became more complicated after the 90s (I think this happened globally), was a great fit for these.

The definition of the rock has become more confusing compared to earlier times based on the new schemes. For example, the adaptation to the plate tectonics scheme combined with the adherence to the mineralogical content eliminated the possibility of easily associating the so-called rocks with the natural real conditions.

It's true, it doesn't belong here, but here, for example, by relegating the use of the relative time scale to the background, they dealt a cruel blow to the ability to create a correct geological vision.

Regardless of my background knowledge, I have deciphered the formation of gold nuggets and the formation of a type of mineral, certain earthquakes can be easily predicted with my ideas. (I want to share all of this on my Geo-intuition YouTube channel, if there are subscribers, or people who buy from my polo shirt, or if there are enough people who buy my republished book, Eszme)

As an outsider, I don't want to say that what you want to do is not good. But you should look into how the Taoists criticized similar scholastics of their time. The point is that whatever you create, you can create a clear picture of where you use it, it should be better than what existed before.

Best regards,

Laszlo

Hi Laszlo,

Sorry, but I did not fully understand your comment.

IUGS purpose is not to provide hints for building petrogenetic models. The main scope of the TGIR is to provide simple tools to classify correctly an igneous rock. Full stop. Petrogenetic models come out from the mind of each researcher, of course, not from IUGS Secretary or from any of the TGIR members.

A gabbro is a gabbro independently if it formed long an oceanic ridge or in a subduction setting. This is essential. If we do not accept this, we will never reach any consensus in the petrological community. We cannot name a rock based on plate tectonics, it is a great error and I hope nobody accept this link. Only after having correctly classified a rock, it is possible to start building a petrological model, based on petrographic, mineral chemical and geochemical constraints. Similarly, there is no link between the classification of a rock and its age.

At the end, thank you for not saying that what we are doing is not good. Now I feel much better. I prefer to remain a scientist, leaving Taoism to other people.

Michele

Dear David Debruyne,

Thanks for the comment.

Effectively we could add some comment on the cumulitic origin of gabbros, but note that this implicitly is a petrogenetic comment, and we want to leave petrogenesis outside the book. You know that there are different, and often contrasting, interpretation on the origin of each rock type. If we start proposing a specific petrogenetic model for a given rock type, we should do that for all the others. I agree that most of gabbros are cumulitic, but this is not always true (for example, there are gabbros without positive Eu anomalies). The example you give is from mid oceanic setting, which is very different from what may happen in mid continental plates, continental rifts, subduction settings or back-arc basins.

Students have first to learn how recognise a gabbro and how to distinguish a gabbro from a diorite (and from a basalt). Then they can go more in detail trying to understand what geological process led to the formation of that special kind of texture and paragenesis. But this come after, and is not contemporaneous with the identification of a rock as a gabbro (or as a diorite).

You report an article of Yaoling Niu (2002) where he says that “Petrogenetically, gabbros are cumulate equivalents of basalts”. Well, we are doing no petrology here. This is a much more basic approach (basic in terms of simplicity, not in terms of SiO2 content…).

I agree with you in changing “equivalent” with “similar”. I have already made this change in the definition at the beginning of the thread.

Another interesting aspect is related to the possibility to add a new official term of the gabbro family, i.e., “oxide gabbro”. This definition is relatively common in literature, but it is not explained in the IUGS book. Note, however, that often oxide gabbros are mineralized variants of “normal variants”, with oxide content up to 25%, which is clearly a post-magmatic modification of an original rock. Alternatively, it can be the result of an accumulation of oxides in the bottom of a magma chamber. In that case, we should provide some indication on when to classify a gabbro as an “oxide gabbro”, because a cumulitic rock is and remains an igneous rock. Instead of placing 5% the threshold of oxides to classify a gabbro as an “oxide gabbro”, I would propose to increase this amount to 10%, considering that many gabbros can have 5% oxides (in that case we should classify most of gabbros as oxide gabbros. Does it sound good for you? I have added a short sentence to the original message in this thread.

Cheers,

michele

Michele Lustrino ,

What you want to say can be understood, but somehow is contradictory:

''IUGS purpose is not to provide hints for building petrogenetic models. The main scope of the TGIR is to provide simple tools to classify correctly an igneous rock. Full stop. Petrogenetic models come out from the mind of each researcher, of course, not from IUGS Secretary or from any of the TGIR members.'

I hope you accept: that every rock has its own objective history of formation: The same material ingredients always produce the same result under the same conditions, unless one of the conditions changes. Somehow incomprehensible, it cannot be understood that this is not a primary goal. If this is not a primary goal: How the 'main scope of the TGIR is achieved: is to provide simple tools to classify correctly an igneous rock, if it does not consider this somewhat fundamental criterion?

I do not intend to analyze the third paragraph in more depth, although it would be worthwhile. Instead, let's look at the essence of its opening sentences in the light of the following article that easily refutes subduction:

Presentation The phenomenon of subduction is incompatible with Earth's su...

The sentences:

'A gabbro is a gabbro independently if it formed long an oceanic ridge or in a subduction setting. This is essential. If we do not accept this, we will never reach any consensus in the petrological community.'

Finally, it is worthwhile for any natural scientist to consider the following simple principle: It applies to any branch of natural science that it can only be accepted as a science if it is based on a sufficiently well-observed natural phenomenon and which is described with an appropriately objective philosophy of nature.

Regards,

Laszlo

Dear Michele, thank you for these considerations. A threshold of 10% indeed sounds more useful for classifying oxide gabbros.

Cheers,

David

Dear Michele and IUGS Task Group on Igneous Rocks,

About 10 years ago, as part of en effort to create a standardized nomenclature for rock classification in the Chilean Geological Survey, we reviewed the IUGS plutonic rock systematics as published in Le Maitre et al. (2002). In the area of gabbroic rocks, we found there are issues, so I support your effort to reexamine the system. Having said that, the work of Le Maitre et al. is the culmination of more than 4 decades of work by hundreds of petrologists. It is a monumental piece of work that has no parallel for the classification of metamorphic and sedimentary rocks and deposits. I believe making changes should be done thoughtfully, with great care and deliberation.

First, I wish to offer some friendly criticism of your original post. I think you have inaccurately represented the current IUGS definitions for gabbro and diorite. The definitions you cite are taken from the glossary section of Le Maitre. The glossary is a compilation of 1637 terms that include both obsolete terms and terms that are accepted for use in the IUGS system. The petrological descriptions of the accepted terms are inconcise and frequently inconsistent with the definition of the same term as expressed in the main text of Le Maitre. For example, for diorite there is no mention of the common mafic minerals in the definition of diorite where defined on page 24 of the text, and for gabbro there is no mention of accessory minerals in its definition.

Although not specifically stated in the introduction to the Le Maitre glossary, I believe that the language used there indicates that the part that precedes “now” is describing the generally accepted definition prior to the IUGS work, and that the part that follows “now” is referring to how it is NOW defined in the new IUGS system, which is usually followed by citing a diagram or section of the main text where one can find a more complete definition. This writing structure is repeated for most of the IUGS accepted terms in the glossary where the paragraph ends with a final sentence that begins “Now defined . . .”

On page 24 of the text, the separation of gabbro and diorite is straightforward and unambiguous. Both have a QAP modal composition that places them in field 10 of the QAPF diagram and a color index of >10%, but diorites have plagioclase with an An0-50 content and gabbros (sensu lato) An50-100. The use of visual aids, such as the QAPF diagram is helpful, but it is also desirable to have a verbalized version of the definition that does not require referencing a diagram, particularly for use in a glossary, for example. So, combining the information in the QAPF diagram (including information in the figure caption) and the text description of Field 10, I think a current complete IUGS definition would be:

Gabbro (sensu lato): a relatively coarse-grained plutonic rock with a QAP modal composition of 90% plagioclase in the feldspar fraction, color index >10% and 10% and 10% and

Dear Andrew Tomlinson,

Many thanks for your detailed reply. Below my comments (in italics) to your single notes.

Dear Michele and IUGS Task Group on Igneous Rocks,

[...]

First, I wish to offer some friendly criticism of your original post. I think you have inaccurately represented the current IUGS definitions for gabbro and diorite. The definitions you cite are taken from the glossary section of Le Maitre. The glossary is a compilation of 1637 terms that include both obsolete terms and terms that are accepted for use in the IUGS system. The petrological descriptions of the accepted terms are inconcise and frequently inconsistent with the definition of the same term as expressed in the main text of Le Maitre. For example, for diorite there is no mention of the common mafic minerals in the definition of diorite where defined on page 24 of the text, and for gabbro there is no mention of accessory minerals in its definition.

As a young researcher, what I would do is to go directly to the glossary of terms. If, as you write, the glossary is "frequently inconsistent with the definition of the same term as expresses in the main text" it would be a serious problem. Luckily I do not believe this is the situation. The definitions reported in the glossary of terms are almost 100% compatible with what is reported in the main text. Any difference between what is written in the glossary and what is reported in the main text needs to be deleted.

[...]

On page 24 of the text, the separation of gabbro and diorite is straightforward and unambiguous. Both have a QAP modal composition that places them in field 10 of the QAPF diagram and a color index of >10%, but diorites have plagioclase with an An0-50 content and gabbros (sensu lato) An50-100. The use of visual aids, such as the QAPF diagram is helpful, but it is also desirable to have a verbalized version of the definition that does not require referencing a diagram, particularly for use in a glossary, for example.

Ok, but I can assure you that only a few people go through the main text, while the great majority limit their attention to what is reported in the glossary section. I feel we can simply increase the details in the glossary, fully matching what is written in the main text. In the new version we propose, there is no disagreement with what is reported in the main text. We were aware of what was reported in the main text, but we simply wanted to underline that some important concepts (e.g., the type of plagioclase in gabbros and diorites) were missing in the glossary.

So, combining the information in the QAPF diagram (including information in the figure caption) and the text description of Field 10, I think a current complete IUGS definition would be:

Gabbro (sensu lato): a relatively coarse-grained plutonic rock with a QAP modal composition of 90% plagioclase in the feldspar fraction, color index >10% and 10% and 10% and

What about these definitions in the main text?

Field 10 (diorite, gabbro, anorthosite) — the three root names in this field are separated according to the colour index and the average composition of their plagioclase – anorthosite (M 10%, plagioclase An0-50), gabbro (M >10%, plagioclase An50-100). Plagioclase-rich phaneritic rocks may be further subdivided, according to the relative abundances of their orthopyroxene, clinopyroxene, olivine and hornblende as shown in Fig. 2.6. Either of the two synonymous terms dolerite or diabase may be used for medium-grained gabbros rather than the term microgabbro, if required.

Gabbro (sensu stricto) = a plagioclase-rich phaneritic rock with a QAP modal composition made up of 90% plagioclase in the feldspar fraction, >10% clinopyroxene,

Dear Michele Lustrino

my remark:

The chemical composition of diorites is equivalent to basaltic andesite, not andesite. Instead the chemical composition of quartz diorites is equivalent to andesite.

Your Answer:

Who says this? Nobody has never certified this interpretation. Probably it is your idea, but if you have any official information, please share it with me. Note that basaltic andesites are much more similar to basalts than andesites (for example, many of the CFB - Continental Flood Basalts - are actually basaltic andesites, but are defined as "basalts s.l.").

my remark-2:

Yes, you are right, there are no such comparisons in English literature. However, one of the authors of the 2002 IUGS, S. Efremova (Efremova, Stafeev, 1985), back in the days of the Soviet Union, published a monograph with such a comparison (diorite are similar to basaltic andesite, quartz diorite are similar to andesite - Fig. 82, page 282). This is the diagram of the Terminological Commission of the Petrographic Committee of the USSR Academy of Sciences.

There is a subject for discussion here. In your new proposed definition, diorite are similar to andesite. What volcanic rock would be similar to quartz diorite, dacite? I think no. In a frequently cited diagram, Middlemost (1994) compares a field of basaltic andesite with gabbroic diorite (gabbrodiorite). However, in the current version of the IUGS, gabbrodiorite is a variety that does not form a wide field ("Gabbrodiorite. Now used for rocks which are mineralogically on the boundary of gabbro and diorite, i.e. their plagioclase is An50" (IUGS, 2002)). That is, on the TAS diagram, gabbrodiorite should not form its own wide field, but a narrow strip. What will be the analogue of basaltic andesite (SiO2= 52-57 wt%) if diorite is actually immediately followed by gabbro (an analogue of basalt)? A simple mass balance calculation shows that quartz diorites (according to your definition: Q=5-20, An20-50, Amp+Bt=10-40) fall in the range of SiO2=56-64 wt%, and quartz-free diorites have SiO2

Thank you again Roman.

I did not know the Efremmova and Stafeev book. It is written in cyrillic, so it is impossible for me to get out any useful information. It is a pity, because I believe it could have been a very useful contribution in igneous petrology. Unfortunately this is not the only case of Soviet researchers forced to publish in cyrillic, well knowing that the results of their studies would have never reached a worldwide attention.

I am wondering why Efremova did not ask to add more information regarding this aspect (i.e., a connection between QAPF-identified plutonic rocks and TAS-identified volcanic rocks).

I believe that it is reasonable to propose a similarity between:

1) gabbro and basalts,

2) diorites and basaltic andesites,

3) quartz diorites and andesites.

Let me discuss about this with the other TGIR members.

Cheers,

michele

Another point to discuss:

According to IUGS there could be also foid-bearing andesites (Fig. 2.11) and foid-bearing diorites (field 10' in Fig. 2.5). Has anybody heard of them?

michele

Typos:

Orthopyroxene gabbro: >5% orthopyroxene.

Clinopyroxene norite: >5% clinopyroxene.

Diorite: plagioclase with An0-50 composition.

An=0-10(20) is more typical for granites.

Thank you Roman.

I also added clinopyroxene>orthopyroxene (in orthopyroxene gabbro) and orthopyroxene>clinopyroxene in clinopyroxene norite.

michele

Michele,

Thanks for your reply. Below are a few comments (without italics) to your comments. I have removed most of my orignal text to short it. Where I have retained the original text for clarity of the discussion I have placed it in brackets:

As a young researcher, what I would do is to go directly to the glossary of terms.

It’s unforunate, but I suspect you are probably right. This is certainly justification for improving the current descriptions in the glossary.

If, as you write, the glossary is "frequently inconsistent with the definition of the same term as expresses in the main text" it would be a serious problem. Luckily I do not believe this is the situation. The definitions reported in the glossary of terms are almost 100% compatible with what is reported in the main text.

The fact that two seasoned researchers (you and I) have different interpretations as the intentions in the glosssary illustrates the ambiguity there. Either way I think we can agree that the current glossary definitions are incomplete and don’t communicate well the modal definitions that are contained in the ternary diagrams.

Any difference between what is written in the glossary and what is reported in the main text needs to be deleted.

Agreed, so mention of accessory minerals in the glossary, such as Fe-Ti oxides, should either be deleted or it needs to be made clear they don’t form part of the definition of the rock, even though they may be universally present in these rock types.

Ok, but I can assure you that only a few people go through the main text, while the great majority limit their attention to what is reported in the glossary section.

Again, unfortunate.

I feel we can simply increase the details in the glossary, fully matching what is written in the main text. In the new version we propose, there is no disagreement with what is reported in the main text.

I can’t agree on this point. For example, there is no mention of hornblende, biotite or augite in the definition of diorite in the main text. If you intend on keeping this part in the glossary as auxillary information, there needs to be a clear explanation of what criteria define the rock type and what is a description of common attributes that, although common, are not necessary for its definition. Otherwise, a young researcher too bothered to look up the definition in the text might think an orthopyroxene diorite is not a diorite.

We were aware of what was reported in the main text, but we simply wanted to underline that some important concepts (e.g., the type of plagioclase in gabbros and diorites) were missing in the glossary.

As I said before, I think improvements can also be made in the main text. For example, Gill (2010, Igneous Rocks and Processes, Wiley-Blackwell, pg 94) uses the term clinopyroxene troctolite and claims that it conforms to the logic of the IUGS nomenclature eventhough it is not presented in Le Maitre. To me it isn’t clear that clinopyroxene troctolite is a valid term. In the Plag-Px-Ol ternary of Le Maitre, a troctolite with >5% pyroxene would place it in the olivine gabbro-olivine gabbronorite-olivine norite field. Clearly there is ambiguity in the Le Maitre work which is leading to confusion in the community. I think more needs to be done than just improve the glossary.

[Gabbro (sensu lato): a relatively coarse-grained plutonic rock with a QAP modal composition of 90% plagioclase in the feldspar fraction, color index >10% and

Michele,

Here are my comments (in italics) concerning the definitions you posted yesterday.

Field 10 (diorite, gabbro, anorthosite) — the three root names in this field are separated according to the colour index and the average composition of their plagioclase – anorthosite (M 10%, plagioclase An0-50), gabbro (M >10%, plagioclase An50-100). Plagioclase-rich phaneritic rocks may be further subdivided, according to the relative abundances of their orthopyroxene, clinopyroxene, olivine and hornblende as shown in Fig. 2.6. Either of the two synonymous terms dolerite or diabase may be used for medium-grained gabbros rather than the term microgabbro, if required.

I see you’ve substituted the name gabbro in the latter part with “plagioclase-rich phaneritic rocks”. By doing so you lose the chain of thought in the paragraph. The first sentence mentions anorthosite, diorite and gabbro. Of these three, anorthosite is the most plagioclase-rich rock, so it sounds like you are referring to anorthosites. To keep the referencial link between the sentences, you need to use the same term in both.

As explained in Le Maitre, they are referring to gabbro sensu lato (gabbroic rocks) in their description of field 10. In your other post you indicated that you don’t like using the term gabbroic rocks for rocks that may not have clinopyroxene. How about using “gabbro and related rocks”?.

Field 10 (anorthosite, diorite, gabbro and related rocks) — the three root names in this field are separated according to the colour index and the average composition of their plagioclase – anorthosite (M 10%, plagioclase An0-50), gabbro and related rocks (M >10%, plagioclase An50-100). Gabbro and related phaneritic rocks may be further subdivided, according to the relative abundances of their orthopyroxene, clinopyroxene, olivine and hornblende as shown in Fig. 2.6. Either of the two synonymous terms dolerite or diabase may be used for medium-grained gabbros rather than the term microgabbro, if required.

Gabbro (sensu stricto) = a plagioclase-rich phaneritic rock with a QAP modal composition made up of 90% plagioclase in the feldspar fraction, >10% clinopyroxene, 90% plagioclase in the feldspar fraction,

Would be easier to have the colour index featuring in the definition of diorite versus gabbro, since it is easier to distinguish in the field than the An% of plagioclase.

Marlina Elburg,

Unfortunately, the color index range of diorite and gabbro is the same, 10-90. In Streckeisen’s seminal paper of 1976, “To each plutonic rock its proper name” (Earth Science Reviews), he recounts that in constructing the current system they considered plagioclase composition, mafic mineralogy, paragenetic relationships, and even chemical composition. He emphasized that color index should not be used and goes on to state that the distinction of diorite and gabbro by plagioclase composition at An50 “is quite firmly entrenched in the literature”.

I think historical usage and precedence are important considerations when evaluating these definitions. In fact, under Section 2.1 of Le Maitre et al. (2002) on guiding principles of classification, point 7 reads:

“The classification should follow as closely as possible the historical tradition so that well-established terms, e.g. granite, basalt, andesite, are not redefined in a drastically new sense.”

Streckeisen (1976) also mentions the importance of this principle and lists the authors of textbooks and papers that they consulted in determining the traditional use of terms.

Nevertheless, I understand your point very well, as a regional geologist whose principal task is making maps, I give a preliminary assignment based on the color index and a final assignment when I’ve had an opportunity to check the plagioclase composition in thin section.

Cheers,

Andrew

The gabbro vs. diorite distinction is actually one of my favourite questions to ask in student seminars and faculty position interviews.

Very interesting discussion above, and there are many good observations and valuable opinions by everyone above, but some of the points made and contrasts drawn seem flawed to me.

Gabbros form from slow cooling of basaltic magma under plutonic conditions and are thus coarse-grained - this is the classical concept of gabbro vs. dolerite vs. basalt. Yet many gabbros are cumulates from basaltic liquids, not just slowly cooled magma. Both mechanisms to form gabbros are valid.

Yes, diorites commonly have amphibole (hornblende, typically), and we are talking about primary magmatic amphibole/hornblende. There are gabbros with primary magmatic hornblende, though commonly gabbros lack amphibole. These hornblende gabbros, like oxide gabbros and quartz gabbros, are named after accessory (not essential) minerals, hornblende or Fe-Ti oxides or quartz as the case may be. Nothing wrong there. (And we assume that we are not talking about secondary hornblende or xenocrystic quartz, etc.)

Gabbros with ol-px etc. much more abundant than plag are the leucogabbros (not diorites), and the reverse are melagabbros. These modifier terms exist and are good.

There are diorites with SiO2 contents in the basalt/gabbro range, owing to high contents of amphibole etc. See for example Cucciniello et al. 2022 on Girnar, J. Petrol. It would be wrong to label these rocks gabbro because of they plot in the gabbro field on TAS (and TAS was intrinsically not meant for such rocks).

Feldspathoidal diorites exist. Please see Pande et al. 2017 Int. J. Earth Sci. for an example.

Colour index overlaps for gabbro and diorite and cannot distinguish them. I have seen anorthosites ranging from pure white to pure black. In fact colour index appears the weakest criterion, worse than nothing.

I stand with the classical distinction involving plagioclase composition, and the common and abundant hornblende in diorites unlike in gabbros (e.g., Hatch, Wells, Wells and many others including QAPF). Plagioclase composition (irrespective of how much or no amphibole etc.) should be key, and there seems no better way.

Sorry for my goof-up above. Plagioclase-rich gabbros are leucogabbros, which grade into anorthosites, and the reverse are melagabbros.

The rest seems correctly stated on a re-read.

DEAR COLLEAGUES

GOOD MORNING TO YOU ALL. I THINK WE SHOULD THANK MICHELE LUSTRINO ET AL. FOR THE CHANCE TO DISCUSS MUCH DEBATED SUBJECTS IN IGNEOUS PETROLOGY. GABBROS AND DIORITES ARE ONE SUCH CASE LEADING TO MORE CONFUSING TERMS LIKE GABBRODIORITE (A GABBROID WITH An50 PLAGIOCLASE AND SECONDARY URALITIC HORNBLENDE).

DEAR MICHELE, REGARDING THE POSTING OF OTHER ARGUMENTS IN RESEARCHGATE BY TGIR MEMBERS, I PROPOSE THAT YOU INITIATE A LAMPROPHYRE CLAN VS LAMPROPHYRE FACIES DISCUSSION.

BEST REGARDS

IOANNIS

Thank you Johannis.

We are organizing a new discussion on lamproite/lamprophyre definition soon. It will certainly be a very hot topic... ;-)

michele

DEAR MICHELE

GOOD MORNING. I THINK THAT THE LAMPROPHYRE CLAN VS LAMPROPHYRE FACIES DISCUSSION SHOULD START THIS WEEK SO THAT THERE IS ENOUGH TIME FOR SOME CONCLUSIONS UNTIL THE 37IGC IUGS TGIR PRESENTATION OF UPDATES ON THE 30/8/2024 (AS MENTIONED BY MATHEUS DENEZINE).

BEST REGARDS

IOANNIS