The first and decisive point of any lithological/petrological classification scheme is a precise description of the mineral assemblage regarding the quantity of each of its components and of the texture concerning the size and orientation of each mineral group in space. Genetic aspects should be second in the row. Unfortunately, too often nomenclature is fraught with premature genetic connotations. Keep it simple and logical and leave the interpretation of its use with the user. You should not be dogmatic like the pope.

HGD

Yes indeed Harald. This is our main aim. The classification has nothing to do with petrogenesis. Petrography comes first (texture), than the types and amount of minerals and the possible presence of glass, then the chemical composition of whole-rock and of the single minerals.

I agree that from a petrographic point of view eclogites are metamorphic, but by definition igneous rocks are generated after solidification of a magma. The product of solidification of a basaltic magma at high depths cannot generate a basalt nor a gabbro, but a bimineralic assemblage identical to that of eclogites.

Michele

Dear Michele,

I find this point of great interest. Above all, if we think to Eclogite A type that is mostly found as nodules/xenoliths in alkaline rocks (basic and K-rich magmas, if I'm not wrong).

To explain their presence it is always invoked the presence of a subduction (recent, ancient, very old, proterozoic remnants, archean ghosts) and it is mirrored in the production of convoluted and, in my opinion, unrealistic models.

Opposite, declaring that they are a basaltic melt crystallized at a depth higher than plagioclase stability (i.e., P higher than the upper boundary of the Ab = Jd+qz solid-solid continue reaction, just to simplify as much as possible), it would be a clear, simple and working explanation.

I agree that from a common petrographic point of view, the bimineralic Eclogite A is considered a metamorphic product. However, I would like to stress the point it shows a typical "holocrystalline granular hypidiomorphic texture with a general isotropic fabric"... characteristics more close to a magmatic environment than to a metamorphic one.

To conlcude, I do not have the answer, but I'm very happy about the opening of this discussion. I will follow it!!!

FL

Caro Federico Lucci,

The problem here is not to find a solution for the genesis of the various types of eclogites. The problem is how to classify the product of crystallization of a basaltic melt at depths higher than 35-40 km. We cannot call this rock as basalt because it lacks plagioclase (not stable at those depths). Being this rock made up essentially of garnet and omphacitic clinopyroxene, I should be forced to classify it as eclogite,  but actually this name is now confined to the metamorphic rock category only.

What I would like to do is to emphasise that with the name of eclogite we can also indicate an igneous rock.

Cheers,

Michele

Hi Michele,

I agree with you Michele but what about Migmatite rock?

Cheers

Sarmad

Hi Sarmad Asi Ali ,

Migmatites are classified as very high grade metamorphic rocks. Migmatites are certainly metamophic rocks, independently on the mineral or whole-rock composition. Yes, by definition, metamorphism is a sub-solidus process, but the subcommission for the classification of metamorphic rocks accepted the production of melts as an extreme consequence of high-grade metamorphism. There are other cases of pyrometamorphism (i.e., production of melts as consequences of metamorphic processes) or cases where melts are produced by friction (e.g., pseudotachylites) or after meteorite impact (e.g., tektites).

The problem with eclogites is very different. They are correctly defined as metamorphic rocks, whose main constituents are garnet and Na-Al-Mg-Ca-rich clinopyroxene. However a basaltic magma crystallizing at depth (without any relevant evidence of metamorphic sub-solidus modification) has the same mineralogy of eclogites and cannot be called as basalt (because of the absence of plagioclase). I should be forced to call this rock as an igneous eclogite and I was asking what is the feeling of the scientific community in Research Gate.

Cheers,

michele

Thanks Michele,

Migmatites are composite rocks with both igneous and metamorphic features. They typically consist of a mix of metamorphic minerals like quartz and feldspar, with an igneous component such as granite or granitic gneiss. This results from partial melting of the rock, leading to the formation of a mixed texture.

Migmatites are a blend of both igneous and metamorphic rocks. They start as metamorphic rocks that partially melt and then solidify into a rock with characteristics of both types.

Cheers

Sarmad

Michele Lustrino I don't see your point. Why should we classify a magmatic eclogite as basalt? Just for the sake of applying a diagram which was devised for other purposes? According to the same reasoning, all granites (or almost) would be rhyolites, and they obviously are not. I'm concerned that this kind of approach would make a total chaos of the present IUGS classification instead of updating and enlarging it.

Yes, Yousif Mohammad, it seems we are tempted to look for processes instead of looking at rocks. This would be a mistake because the rocks are palpable reality, while the processes are interpreted (alas, sometimes imagined!).

Thanks to Sarmad Asi Ali for the explanation of what a migmatite is.

Migmatites are classified as metamorphic rocks and not as igneous rocks. Their protoliths can be igneous or sedimentary. As all high-grade metamorphic rocks, T-P conditions can lead to pass the solidus of the system inducing partial melting. Anyway they are and remain high-grade metamorphic rocks.

Note, however, that this thread was addressed to a completely different topic.

Cheers,

Michele

Thanks to Gavril Săbău for your reply.

Not clear why you do not "see" my point. You ask me why we should classify a magmatic eclogite as basalt? Follow my reasoning:

1. A melt with a basaltic composition is generated at mantle depths for several reasons;

2. A portion of this basaltic melt reaches the surface and crystallizes as a basalt;

2. Another portion of this basaltic melt solidifies at 30 km depth at lower continental crust conditions to form a gabbro pluton;

3. A third portion of this basaltic melt do not reach 30 km depth and crystallizes at depths of 50 km, which can be uppermost lithospheric mantle or lowermost continental crust in overthickned orogenic areas;

4. In the last case, the basaltic melt crystallization cannot lead to the formation of any basalt or gabbro, because plagioclase is not stable at those pressures;

5. The mineral paragenesis formed during the crystallization of such a basaltic melt is essentially made up of a pyrope-rich garnet and an omphacitic clinopyroxene;

6. After 100 Myr, erosion allows these deeply crystallized melts to the surface. What a future geologist will see is a rock he/she wuold classify as eclogite;

7. Somebody should explain that this eclogite is instead the result of a normal crystallization process (i.e., it is an igneous rock), and not the result of a metamorphic change of an original igneous rocks (gabbro or basalt).

8. In conclusion: eclogites can have an igneous origin. This is written in any place and in any book, because these rocks are classified only withing the IUGS book of metamorphic rocks and never in the IUGS book of igneous rock.

Is it now clear my point? Not clear why, according to you "all granites (or almost) would be rhyolites". This comment is out of topic here. Similarly it is not clear why, according to you "this kind of approach would make a total chaos of the present IUGS classification instead of updating and enlarging it". What do you mean with "chaos"? I am trying to explaining how to approach the systematics of igneous rocks, adding a new term (eclogite) in this family.

Regards,

michele

Yes Yousif Mohammad

Now: Eclogites are exclusively metamorphic rocks

In future (hopefully): Eclogites are essentially metamorphic rocks, but it is possible to find also igneous variants.

Cheers,

Michele

Hi Gavril Săbău , I again do not understand your approach. You write: "Yes, Yousif Mohammad, it seems we are tempted to look for processes instead of looking at rocks. This would be a mistake because the rocks are palpable reality, while the processes are interpreted (alas, sometimes imagined!).".

I am not looking for processes (at least in this discussion). I am looking a way to classify correctly rocks. Up to now, if somebody found a rock with garnet and omphacitic clinopyroxene the correct definition was "a metamorphic rock called eclogite". What I propose is "a metamorphic or igneous rock called eclogite". To infer if this rock belongs to the metamorphic or igneous category we have to look in detail its texture, the composition of the minerals and the relation with country rocks. My skill and profession is, for 30 years, to look at rocks. Not clear where is the mistake.

Michele

Hi Michele,

Thanks for considering my reply.

A magma is a magma and a rock is a rock. Classifying them in a mixed framework could lead to confusion or even to dead ends. Basalt is an effusive rock, so a holocrystalline garnet pyroxenite is definitely not a basalt, no matter how many arguments are invoked, though it conceivably might have originated from a basaltic magma. Or we can dramatically change the present classification, with no foreseeable benefit.

The strength and ultimately the success of any classification is imparted on one hand by its criteria, comprehensive yet as simple and easily recognizable as possible, and a clear hierarchy among them. Ideally, these two qualities have to reflect themselves reasonably in the definition of the individual species and their hierarchical relationships. Combining descriptive features with genetic interpretations at the same hierarchical level might serve a purpose in many instances, but could be misleading in even more instances.

One of the cornerstones of the present classification of the plutonic igneous rocks is the mineralogical content, exposed in the well-known, widely-applied and very successful QAP(F) diagram. It does not apply to volcanic rocks for obvious reasons, instead the relatively simple chemical TAS diagram is doing an excellent job. But the recourse to chemical diagrams for classification purposes where direct assessment of the mineralogy is possible in my opinion would be both unnatural and confusing.

Just to mention it, going one level up in the hierarchy, the structural criteria and the geological background prevail in the distinction of plutonic from effusive and subvolcanic rock.

Going back to the magmatic eclogite and how we should name it if we are lucky enough to find it, I suggest you leave the decision for the moment it becomes necessary. It is obvious that a basaltic composition would display an eclogitic assemblage if formed under the right conditions; therefore it needs a motive, means and opportunities. Thus a fertile mantle source has to undergo a relatively high degree of partial melting at considerable depths and at least part of the resulting melt has to consolidate at these depths. Though theoretically possible, it is quite hard to imagine a permissive geotectonic framework for this chain of events. So I think it's premature to name and classify such an "igclogite", especially after the advent of "arclogites", which, however, are recognized as metamorphic rocks.

Another problem is what a user expects from a classification - too vast a subject to be discussed here, but primarily being able to apply it according to her/his/its/their/NA needs.

Dear Michele Lustrino

I finally read this entire discussion and have a few comments. I have no objection to the fact that part of the eclogites may represent basaltic magma, solidified under mantle conditions. If this is going to be part of a book focused on nomenclature, then it is necessary to state what distinguishes these rocks from metamorphic eclogites. Boundaries are a must. Then there are certainly also problems, what to do with near-eclogites, i.e. strongly garnetic rocks, sometimes with a small content of plagioclase or with other, less alkaline clinopyroxene. And then also what to do with strange eclogites that also contain orthopyroxenes (griquaite). I know eclogitic rocks basically only from the "Variegated Series of Moldanubicum", where they are segregations or bodies in connection with serpentinites and often also granulitic rocks. Apparently an old ophiolite zone. So I don't know if you're opening pandora's box.

Thanks for your comment Dalibor Matýsek, very useful for the discussion.

The new edition of the IUGS book will be, as the first two ones, simply devoted to systematics, trying to avoid as much as possible any connection with petrogenetic aspects (i.e., why that kind of rocks formed in that place at that time). We have only to provide simple tools to give rocks a name. Then the rest (petrology) is up to researchers sensibility. Everybody knows that there are so many petrogenetic processes for each rock type that an encyclopaedia would be not sufficient.

First of all we should underline that there are metamorphic (the great majority) and igneous eclogites. The easiest way to distinguish the two could be based on the texture. All the eclogites with igneous textures are igneous rocks, all the others with metamorphic textures are metamorphic rocks. Easy to write, much more difficult to evaluate, because of transitional types, of course.

Considering the presence of different types of metamorphic (i.e., real) eclogites, the idea to propose additional boundaries and fields could be very difficult. This is particularly true considering that there are different types of basaltic magma, and each of them could crystallize at depth producing different types of "igneous" eclogites. Examples are the different Na2O content of clinopyroxene, which I should expect much higer in alkaline basaltic magmas with respect to subalkaline variants. Similarly, also the TiO2 content can vary from ~0.1 to ~4-5 wt%, and the folowing rutile content of "igneous" eclogite can vary (as well as the composition of spinel solid solution minerals. Also the garnet of the "igneous" eclogite could have a high to low MgO (pyrope) content, considering the degree of evolution of the basaltic magma.

According to Le Maitre (2002) griguaites have no feldspars and are a sort of garnet clinopyroxenites, so this should be out of the topic here.

Effectively your comment emphasizes that "igneous" eclogites could be simply classified as ultramafic rocks with clinpyroxenite or garnetite compositions. Note that clinopyroxenites are actually defined as igneous rocks, but the term "garnetite" does not appear in Le Maitre (2002), while it appears in Fettes and Desmond (2007) as a metamorphic rock.

What a confusion!

michele

Yes, it's chaos and a big one.

In the Czech Republic, the term griquait is most often interpreted as garnetic pyroxenite, garnetic websterite or eclogite with both ortho and clinopyroxenes. Orthopyroxene should be present.

I see a significant complication in the fact that most eclogites, in Bohemia and Moravia, probably all underwent some retrograde metamorphism (more like several different stages), so some textural features are rather illusory. It will be the same with ultrabasics. They are all serpentinites, hundreds of massifs, only with rare remnants of orthopyroxene and olivine.

If the serpentinites are interpreted as metamorphosed (serpentinized) ultrabasics, I see no reason why the garnet inlays should not be interpreted in the same way. But why pyroxene survives serpentinization, I don't know for sure now. Isn't that so?

Dalibor M.

Dear Michele Lustrino

I see that I arrived pretty late at the discussion, but I would like comment. I would agree with the point made by Harald G. Dill about classification schemes, but, in this case, where, despite purely descriptive origin, the name is now very closely associated with the process, I believe that adopting your proposal would end up creating more confusion, instead of creating more clarity. I would follow what you said in your answer to Dalibor Matýsek, i.e., classify "igneous" eclogites as ultramafic rocks with clinpyroxenite or garnetite compositions.

Cheers

Caio

Dear Michele and all, I arrived late as well but I wish to add some enthropy to the discussion: if we consider the high-pressure magmatic crystallization of a basaltic magma (P> 1.5 GPa), the resulting rock is composed by garnet+clinopyroxene (+/- olivine). However, we cannot classify it as eclogite because the resulting HP-HT magmatic cpx is not omphacitic. It will be a very aluminous augite with high Ca-Tschermak and high jadeite content (but not as much as omphacite). So, the rock should be classified as garnet clinopyroxenite. Additionally the eclogite xenoliths in kimberlites are mostly interpreted as recycled pieces of subducted Archean to Paleoproterozoic oceanic crust, not as HP cumulates.

Dear Alessandra,

your comment is very welcome.

Not clear why you write that a basaltic melt crystallizing at depth cannot have an omphacitic cpx. If a basaltic rock brought at >1 GPa pressure can stabilize omphacitic cpx, I do not understand why a basaltic melt, with the same composition, cannot stabilize the same type of omphacitic cpx.

In addition you know much better than me that omphacitic cpx in eclogites have a very wide chemical composition with large variations in the jadeitic component.

I also note that the term "clinopyroxenite" does not refer to a specific cpx composition. According to Le Maitre (2002) a clinopyroxenite is "An ultramafic plutonic rock consisting almost entirely of clinopyroxene. Now defined modally in the ultramafic rock classification". It is plutonic (but it is difficult to chategorize such kind of rocks), without any indication on the jadeite component of the clinopyroxene.

The jadeite component of pyroxenes depends on the composition of the frozen basaltic melt, btw.

To conclude, I want to stress out that in the definition of eclogite, basalt and any other igneous rocks we have to avoid any petrogenetic information (i.e., cumulate, xenolith, mantle or crustal origin). We have only to provide simple tools to give a rock a name.

Ciao,

michele

Dear Michele,

thank you for your reply and the stimulating discussion! I agree with you about the need of providing simple tools for rock classification and nomenclature, avoiding as much as possible (petro)genetic informations. However, I must admit that I do not like the idea of classifying HP magmatic rocks as eclogites and I would limit this term only to metamorphic rocks corresponding to the IUGS definition of eclogite (Desmond, 2007). For HP rocks derived from basaltic melts (> 1.5-2GPa), we need to find another name. I am aware that omphacite may have a range of jadeite contents, but natural examples of HP rocks crystallized from basaltic melts do not generally contain omphacitic cpx (see garnet-pyroxene xenoliths from oceanic mantle, e.g. Keshav et al., 2007, doi:10.1093/petrology/egm035 or subcontinental, off-cratonic mantle, e.g. Scott et al, 2024 https://doi.org/10.1093/petrology/egae085), in agreement with HP experimental phase relationships. For such rocks we could simply use the classification of pyroxene-rich ultramafic rocks (clinopyroxenites, websterites, etc.) + prefix garnet. On the other hand, this is the nomenclature commonly used in the literature.

ciao

Alessandra

Dear Alessandra,

In order to classify a rock as of igneous type, a researcher should first check its petrography. If the texture is typical of metamorphic rocks (e.g., granoblastic) the rock should not be classified as igneous. This is basic igneous petrology.

What happens if a basaltic melt crystallizes at pressures higher than 1 GPa? Officially, as all the rocks formed from the solidification of a magma, it should be classified as igneous, but if the texture is granoblastic it should be classified as metamorphic. That is the problem and the shortcircuit.

At the same time such kind of rock should be classified as igneous (because associated to crystallization from a magma) and metamorphic (because not with textures typical of igneous rocks).

michele

P.S. Note that many pyroxene-rich ultramafic rocks are officially classified among the igneous rock family, but they have actually metamorphic textures (many, not all). Classical examples are peridotites, which are igneous rocks with metamorphic texture...

m

I am well aware that the texture of these HP ogneous rocks are metamorphic, with rare exceptions. Similarly, when we are dealing with mantle peridotites the textures are granoblastic, porphyroclastic,etc., but IUGS classification for igneous ultramafic rocks is applied. So, you will agree with me that we can classify HP magmatic products in the mantle just as we do with peridotites. I prefer to use an igneous name for (recrystallized) pyroxene-garnet rocks than a metamorphic name like eclogite which requires the occurrence of omphacitic cpx. A compromise is needed...

Alessandra

Yes, a compromise in needed, but the first rule (actually recommendation) of IUGS is to check at the rock texture to define a rock if it belongs to the igneous clan. Our idea is to add a comment in the new edition of the book, reporting that, despite the textures of many mantle rocks are typical of metamorphic assemblages, we continue to classify them in the igneous rock family for a kind of inertia (and to avoid riots...).

michele