very simplistic (but usually effective): if its precursor was a mafic igneous rock, the amphibolite should NOT show a negative Eu anomaly in its REE pattern, whereas if the precursor was a post-Archean greywacke or shale a negatve Eu anomaly should show up. (Please remember: this is only a rule of thumb!)

Dear Chandan Kumar, i think you have to post some images of this amphibolite (rock and thin section, i mean).

Amphibolite as rock is a metamorphic rock from metamorfism applied to a mafic-intermediate magmatic rocks (or derivated from).

Amphibolitic facies, at contrary, defines a P-T windows in which metamorphic peak is achieved; so it means that both metapelitic and metabasic can suffer metamorphism in amphibolitic facies conditions).

Back to your question: if your rock is a real "Amphibolite" it could not be sedimentary in sensu strictu...

Texture and fabric of the rock at sure can help you, but remember that they could reflect exumation processes, or other younger events (deformation, alteration, hydrothermal transformation, and so on...).

I hope we can continue discussion with more details on the table.

Ciao

F.

Here's an old reference: The chemical distinction between ortho- and para-amphibolites

by B.E. Leake

J. Petrology (1964) 5 (2): 238-254.

doi: 10.1093/petrology/5.2.238

in any case i've found a citation i use with students:

the term “para-amphibolite” is outdated and generally is avoided since it is very hard to think that a carbonatic rocks (or a pelitic rock) shows such impurities, to produce amphibole and plagioclase as a mafic protolite.

De Vore, George (2003). Amphibolite. In: Encyclopedia of Sediments & Sedimentary Rocks (Ed. Middleton, V.). Springer. 593-594.

Dear Lucci,

Could you please send me the said article. . . .

I will be uploading the photos shortly. . .

I have studied such amphibolites in the Antarctic (Northern Victoria Land) and interpreted them as para-amphibolites, see attached copy.

Hello

I work with that kind of rocks. They can have a direct igneous origin (ortho...), a vulcanogenic origin or originate from a quartzite with carbonate ciment protolith.

All the above answers give important contributions to distinguish:

You can use the negative Europium anomaly and REE fractionation to show that the sedimentary contribution is important.

Use the Leacke diagrams (with Niggli numbers).

The ACF diagram with fields defined for the basic rocks (an exemple is attached).

Currently the term para-amphibolite tends to be replaced by the term "calc-silicate rock". Amphibolite refers to a metamorphic rock of igneous origin containing mainly amphibole and plagioclase (USGS).

Best regards,

Maria

Probably this paper might be interesting:

Linda LERCHBAUMER1)*), Urs KLOETZLI2) & Gerhard PESTAL3)

Schists and Amphibolites of the Kleinelendtal (Ankogel-Hochalm-Gruppe/Hohe Tauern, Austria) / New Insights on the Variscan Basement in the eastern Tauern Window____ Austrian Journal of Earth Sciences 2010_Volume 103/2б 138-152, Vienna

1) Bavarian Research Institute, University of Bayreuth, Universitätsstraße 30, D-95447 Bayreuth, Germany;

2) Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Wien, Austria;

3) Geological Survey of Austria, Neulinggasse 38, A-1030 Wien, Austria;

*) Corresponding author, [email protected]

All the above answers give important contributions to distinguish:They can have a direct igneous origin (ortho...), a vulcanogenic origin or originate from a quartzite with carbonate ciment protolith. this paper might be interesting:

Linda LERCHBAUMER1)*), Urs KLOETZLI2) & Gerhard PESTAL3)

Schists and Amphibolites of the Kleinelendtal (Ankogel-Hochalm-Gruppe/Hohe Tauern, Austria) / New Insights on the Variscan Basement in the eastern Tauern Window____ Austrian Journal of Earth Sciences 2010_Volume 103/2б 138-152, Vienna

∂18O-values in carbonates are significantly higher than in basalts.

Elements typical for an igneous origin are high Ni, V, Co, Cr, Ti, Fe, but several exceptions may exist due to fractional crystallization and/or later metasomatic overprint. It therefore may be a strategy to search for characteristic magmatic or sedimentary differentiation trends, which, however, is not necessarily straightforward and requires a large dataset.

An alternative idea: mafic tuffs are geochemically similar with their parental basalts and outline sedimentary textures and occur within a sedimentary sequence.

Dating of detrital or igneous minerals may be another approach – if available.

The "para"-amphibolites I studied were very rich in calc-silicate minerals (Ca-rich pyx, Ca-rich amph, epidote etc) and included calcite.

Dear Dr. Stella: I have used the diagrams proposed by Leake, B. E., but as I said in those diagrams the samples are indicating their igneous parentage!!!!!!!!

Dear Dr. Solveig, Dr. Maria, Dr. Tatyana and Dr. Issam: Thank you for the attachments. They really were of great help.

Dear Dr. Grimmer: I totally agree with you, but if the sedimentary rock has the precursor mafic rocks then aren't they going to be enriched in the said elements?!!

Dear Dr. Cemal: The presumed "Para"-amphibolites from our study area belongs to Archaeans and has no pyroxene. Plagioclase is present in negligible amounts. As for as the definition of amhibolites (whether para- or ortho-) is concerned, it should contain amphiboles and plagioclase along with some accessories. Suppose, there are no plagioclase (may be because of chemical weathering) can't we term it as amphibolites...................????????

Dear Chandan,

Based on your sample description, It seems that you have para-amphibolites. Anyway, Metamorphosed basalts create ortho-amphibolites and other chemically appropriate lithologies create para-amphibolites. The best way to distinguish between them is to make a full chenmical analysis for the major elements and compare it with the basalt chemical analysis if it is fit to basalt analyses that mean you have ortho-amphibolited otherwise you have para-amphibolites. If you need more help kindly send to your sample chemical analysis.

Best regards and wishes from Amman

Generally speaking, ortho-amphibolites are metamorphosed mafic igneous rocks, either volcanic (basalts) or plutonic (gabbros). Para-amphibolites are often derived from marls (calcareous shales). these two types of amphibolites can be distinguish by their geochemistry.

Orth-amphibolites have significantly higher MgO, FeO (or Fe2O3), TiO2 as well as higher transition elements (Ni, Co, Cr, Sc,...) contents than para-amphibolites.

Dear Dr. Ghrair and Dr. Demaiffe,

The samples have been analyzed for their whole rock geochemistry. The chemical values almost resembles that of basalts, but yet, their field occurrences and associated litho units, petrography, primary sedimentary structures as well as detrital quartz grains does not warrant for the igneous parentage.

Our theory is, in fact, any rock unit made up chiefly of hornblende, with a subordinate amount of plagioclase, whatever its origin may be, will indicate a basaltic composition. Metasediments having basaltic source rocks will obviously give the same end composition of basaltic composition and therefore, cannot be very clearly differentiated on the basis of their chemical composition.

How can you be sure that these are metasediments? Why can they not be sills that intruded parallel to bedding? I am sure you are in a metamorphic terrain where the rocks are deformed, ant it is possible that ductile deformation caused the apparent parallelism of rockk contacts. I canniot brlirver that you can still recognize detrital quatz grains in at least upper greenschist- to amphibolite-facies rocks.

Best wishes, Alfred

Well, if you happen to have a thin section, you can examine the zircons under SEM: if it is of sedimentary origin, there will be a lot of zircons with complex zoning, and if the rock is really old, zircons would have dark core. On the other hand, igneous and volcanic zircons would be much smaller, oval shaped, or prismatic. I can't say that this kind of discrimination is reliable, but in many cases, it helps to at least put some constrains.

Depending on the type of geotectonic environment, the mineralogical and geochemical characteristics of the basalts will reflect not only the composition of the original magma but also the intensity of the crustal contamination. If you examine the mafic volcaniclastic deposits that are seen on many LIPs as Ferrar, CAMP, Parana-Etendeka, Deccan, Siberia and Eimesh you will see that they have appearance of breccias composed of basaltic juvenile clasts showing strong vesicularity, supported by a matrix composed of fragments of quartz , fedspar, zircon, apatite, biotite, muscovite, etc., i.e,. very close to a sedimentary deposit. Even the matrix displays vesicularity. This matrix reflects largely the mineralogical composition of the sedimentary sequence with which the basic magma had contact during his ascension. The contact of the basic magma with the aquifer systems contained in the sedimentary sequences that fill intracratonic basin, can give rise to intensely explosive events, hidrovulcanic or phreatomagmatic. Thus, the lava flows interbedded with layers resembling silicilastic sedimentary rocks may be related to these processes. As final products the lavas became contaminated by the sedimentary sequence as well as the layers of 'sediments' will show evidence of that interaction and will be similar to breccias with blocks and bombs, or layers of 'sediments' containing too finely grained basaltic juvenile clasts and sideromelane shards (fine lapilli to fine-medium ash). The interflow beds (also known as intertrapp) produced in such processes will have a geochemical and mineralogical fingerprint very close to the deep sedimentary sequences with wich the basaltic magma interacted. So, be very careful with geochemical results in highly complex sequences, mostly in these ancient that were submitted to a lot of processes, including metamorphic.

Dear Licht,

But to understand the tectonics, we have to go again for the chemistry. And there are still many controversies regarding the role of chemistry in deciphering the Archaean plate tectonics......!!!!!!!!!!

Dear Chandan Kumar

To clarify what I want to explain, let's go back to the Parana Igneous Province. Few years ago, a colleague collected zircons from the basic flows. The U/Pb SHRIMP ages ranged from Archean to Cambrian, displaying the high degree of crustal contamination basic magma suffered during his trip from "chamber" to surface. If such "confusion" occurr in a 134.7 My volcano-sedimentary province, what we can expect in an Archean and highly metamorphic environment ?

Dear Licht,

I totally agree with your opinion. But shouldn't there be any hidden facts/clues, which can be used to overcome these confusions?

Possibly Cerium content may help to mark para-amphibolites

A detail chemical analysis will do.

Dear Gosso,

Could you please explain it in detail.........

Dear Chandan,

Could you please show us a micrograph of those rocks? it seems like you have retrogressed rocks. Do you have a good characterization of minerals by microprobe? I would check first whether the metamorphism makes sense across that "sedimentary sequence" and the origin of the "bedding".

Dear Chandan,

Just to share my recent experience in a similar confusion: we were recently drilling a Ni-Cu-PGE sulfide-bearing mafic-ultramafic intrusion hosted in a metasedimentary sequence, all of these metamorphosed under upper amphibolite- to granulite-facies metamorphism. We had amphibolite derived from (i) sedimentary calc-silicate rocks from country rocks, (ii) basalts from country rocks and (iii) mafic rocks from the intrusion edge (chilled margin). After a long review of drill holes, geological sections, mineralogy, and generation of PCAs we came out with a discriminant diagram using lithochemical analyses:

X: [(Ni_%/0.5)+(MgO_%/30)+(Cr_ppm/4000)+(Co_ppm/350)]

Y: [(TiO2_%/6)+(V_ppm/1000)+(Zn_ppm/1000)+(P2O5_%/2)]

The basaltic field plotted close to the Y axe (with Y values > 0.5), the mafic-ultramafic intrusion field close to the X axe (with X values > 0.5) and the sedimentary calc-silicate protoliths plotted close to 0,0 (X

Dear Jonas,

Thank you for the suggestion.😊

Could you please share any publications related to your said work.

I just need more details (may be some diagrams, pictures, etc.,) of your work.

Cheers,

Chandan

Hi Chandan,

There is nothing published... it was just a informal exercise to solve internal problems in the company. But of course I can share the Power Point presentation and the Excel file that directly plot the samples in the diagram.

Should I send by email? What's your email?

Jonas.

Send Your chemical analysis (15-20) without indications of mineral composition. I will try to divide method RHAT, or his unpublished options. I will send You the results. 'll see. Notify me of coincidence,or does not match with the results obtained by other methods. Let's try?

Hi Jonas,

It would be so kind of you. My email id is

[email protected].

cheers.........

Chandan

Dear Chandan,

Considering the information you sent I would considered to explore the continuity of amphibolite layer and their relationship with other structures that appear in the photographs. I have to say they looks highly deformed (you can see those veins that in some cases are parallel to the foliation and banding, but in some other are folded. Did you analyze the tectonometamorphic evolution of the sequence?

Considering the deformation degree I would be cautious about the sedimentary nature of the layering (could be in the past but...). A careful study of the transition amphibolite/sediments could be very helpful too. Even if metamorphism is non isochemical, gradual or steep transitions may reflect also the original relaionships.

About the composition try every graph like the classic ones of Laid (Cr,Ni...). Did you find also Fe-Mg amphiboles?... Sm-Nd analysis could also help you to track the origin, but again your rocks could be plutonic, subvolcanic, volcanic... so similar whole rock chemistry, very different geometry and relationship with other rock units...

I'd keep simple and double check regional context to complete the information about this sequence. Sometimes an outcrop with clear crosscuting relationship would be much more efficient than laboratory approaches (and cheaper! : -)

Last but not least , is it mandatory to distinguish between ortho and para-amphibolites here? I mean, to be 100% sure about that, or a loose clasification could be fine? In my experience there's no silver bullet to solve it but a reasonable group of independent evidences.

Juan

If derived from a sedimentary protolith, what could be a likely candidate that could account for the present day chemical composition?....a shale, a graywacke? what was its likely initial mineralogy?..is there microscopic textural evidence that support these observations?

Dear Nelson,

Of course there are microscopic textural evidence. And it has a bimodal source and hence, I suppose, you can't clearly differentiate based on their chemistry.

Actually in the field you can clearly make out the primary sedimentary structure like bedding planes. Microtexturally, the para-amphibolites are commonly fractured, broken and shattered and have halos. They also show interpenetration textures, especially platy forms, which could be due to compaction. At places, the fractures in hornblende grains are replaced by calcite. Some hornblende grains are altered to chlorite. Quartz grains are commonly sub-rounded to well-rounded. They occur within and around large crystals of hornblende. At places, quartz grains are segregated within which oriented hornblende crystals in the form of plates occur. Well-rounded clasts of quartz also commonly occur within calcite. Occurrence of sub-rounded to well-rounded monocrystalline quartz grains in the form of segregation and as individual grains within hornblende is a common feature. Sub-rounded to well-rounded shapes of quartz grains suggest their transportation and detrital origin. The quartz clasts within amphibolites and calcite have prominent relief and grain boundaries, and are corroded due to heat and fluid action during metamorphism, suggesting prior existence of quartz compared to amphiboles and calcite.

Dear Chandan

Following Stefan arguments...I did not see any rounded quartz grain in the micrographs you sent. You have recrystallized textures (in some parts they looks like foam ones) with quartz, plagioclase (some with clear zonning and so). It seems like amphiboles include a previous foliation but are somewhat interkinematic. There are some late fractures through which calcite appear. Again, photographs from outcrops show a high strained sequence with veins, folds and gneissic layering. If you claim for a sedimentary origin based on microtextural evidence, graywackes should have tons of proofs, could you show us some micrographs?. I'm not saying these are or are not paraamphibolites but whatever the idea you want to support it must be backed up by solid arguments. Once the sequence has been strained and metamorphosed it is much harder to find a final solution to your initial question, but this is very common

cheers

Juan

Dear Stefan and Juan,

Thank you very much for your answers. Those are really very interesting points you raised.

I will get back to you on to that very shortly.

:-)

Cheers

Chandan

Dear Kumar

During the metemorphic process, many elements are highly mobile . For this reason, you can find some clues showing origin of amphibolite. I mean that I agree with stefan krause and his consideration. Another way to solve this problem is confocal raman spectrometer. I think that you can find good clues for solving origin problems.

Several Discriminant diagrams, suggested by Leake (1964, 1969) and Gokhale & Gothe (1978), and discriminant unction equation of Shaw and Kudo (1965), have been used to distinguish ortho and para amphibolites. You may also see the article of C. Bhattacharyya (1987, Indian Journal of Earth Science) for this.

Please read ,,,,,Discriminant function equation......there is spelling error

You can use REE geochemistry :D

Ortho-amphibolites have higher Cr,Ni and Ti contents and low k ratios, but since most of basic igneous rock have low Cr, Ni and Ti content and also alkali metamorphism normally disturbs k ratios, therefore there should be number of analysis and compare data with other places to give reliable trend.

See paper by Phil Orville in AJS (1960's?), if you haven't seen it already.

Ed Ghent

Dear Dr. Kumar, here a couple of papers of para-amphibolite in greywacke (not all of them are in chem sed rxs!) that may relate to both your tectonic and lithological settings, and complement all the technical very useful information above.

Archean-Proteroz, interbanding of sed rxs and amphibolite:

1. "Unique amphibolite–iron formation assemblage in the

Kawaweyak Lake area, Manitoba"

http://www.manitoba.ca/iem/mrd/geo/field/roa07pdfs/GS-6.pdf

2. "Geological investigations in the Manasan Falls area, Thompson

Nickel Belt, Manitoba"

http://www.manitoba.ca/iem/mrd/geo/field/roa11pdfs/GS-9.pdf

3. Theye et al. 2012 'Significance of amphibole compositional zoning in para-amphibolite"

http://meetingorganizer.copernicus.org/EMC2012/EMC2012-388.pdf

Hope it helps,

Andres Urrutia

You can use the CaO-MgO-FeO triangular diagram or the mg-c-al-alk triangle of Niggli which differentiate between Para- and ortho-amphibolite. Also relict texture of igneous rocks such as relict ophitic texture is typical of ortho-amphibolite.

I have a question

What is the differences between amphibole schist and para-amphibolite in amphibolite metamorphic facies?

I fully agree with Prof. Stephen Kraus suggestions/observations.  I am sure you may find chilled margin(s).  Many people somehow over look this aspect.  Also, can you send /show a photomicrograph of the amphibolite?  I wonder whether it could be an ultramafic rock retrograded to amphibolite. KNRAO

Dear Chandan,

 You have plenty of useful comments and suggestions, particularly those of Stephen Kraus. In 1960s and 1970s, there was a hot controversy regarding the ortho versus para  origin of banded amphibolites of the type you are discussing. BE Leake and his colleagues (including BW Evans) discussed the problem in considerable detail. Geochemistry, including Cr, Ni, Ti, was used in the distinction, apart from other arguments. You may like to read a paper on the origin of amphibolites in the kohistan magmatic arc of the NW Himalaya (MQ Jan, 1988. Geochemistry of amphibolites....Mineralogical Magazine 52, 147-59). 

 In general, I would think that amphibolites should be derived from mafic precursors. They can be sills or tuffaceou beds alternating with sedimentary beds.You did not mention whether your rocks are thin-bedded or not. In thin-bedded marly rocks, amphibolite bands can develop during metamorphism.

Finally, the rounded shape of the quartz grains in amphibolite grade metamorphism cannot be alluded to transport. At this grade of metamorphism, quarts as well as other minerals would be the product of metamorphic recrystallization.

Best,

Qasim

The difference between the two types was explained in the following links

https://academic.oup.com/petrology/article-abstract/5/2/238/1576422?redirectedFrom=PDF

http://www.j-igu.in/igu%2021-5%20(web)/paper6.pdf

Best Regards Dr. Chandan Kumar B

Yes, it's possible. Some variation diagrams are also useful.