Muscovite can contain 1-2% of Na, but not enough to make the 4% analyzed. If you are sure about your analysis, I would recommend to double check if you do not have paragonite in your sample, and it can be very small / sparse over your sample, often in epitaxial growth with muscovite. This white mica is VERY similar (optically) to muscovite and you won't be able to distinguish it from muscovite on the microscope. You will certainly need microprobe analysis. Maybe a WDS (or EDS) mapping for Na will highlight the presence of a Na-rich phase (if, of course, you have access to an electron microprobe or an SEM equipped with EDS)?

Plagioclase (albite) is also possible, and if your sample is right at the feldspar-in mineral isograd, it is likely that feldspar will form very tiny grains, very similar to quartz with possibly no twinning (so again, can be hard to distinguish from quartz).

Other Na-bearing mineral could be an amphibole, which is relatively rare in "pure" micaschist, but can occur if you had a metamarl or something like this.

Regarding the plagioclase-paragonite-muscovite relation, you can check my paper. Beside focusing on Ar-Ar ages, it contains several microphotographs and WDS mapping of several samples with coexisting muscovite and paragonite.

http://petrology.oxfordjournals.org/content/52/4/691.full

(I can send you a PDF of it if you don't have access to JPet, just let me know).

Best,

Julien

Thank you Julien:)

I have a possibility to analyse by EDS and WDS microprobe. There is really hard to distinguish paragonite and muscovite, thats true (kinds of white mica at all).

This rock suppose to be a metapelite, rather than metapsamite or metamarl and there are no amphiboles at all. However, huge amount of muscovite (much more than biotite) might suggest Na-bearing even given 4%, I think.

The PDF file looks interesting, you have been used Theriak-Domino, I saw. Do you think this software good to make P-T paths? What do you think about Thermocalc or Perple-X capabilities?

Metapelite are always very useful for thermobarometric calculation (lots of mineral to yield precise PT-conditions, most of them being well characterized, garnet for zoning and PT path calculation...). However, as Sobhi mention, you need to make sure your high-Na analyses is not the results of a contamination. If so, any thermobarometric calculation will be wrong. Effectively, pure paragonite contains ca. 8% Na2O, so you would need at least 50% of paragonite in your sample to match this whole rock analysis number (unless there is also some other Na-bearing phase, such as plagioclase, as pure albite can contains nearly 12% Na2O).

What kind of PT range are you thinking here? Paragonite is usually stable until max. 600-620C at medium pressure (6-8 kbar), and less at low pressure (4 and below)...

Concerning the choice of Theriak-Domino, Thermocalc or Perple_X, this is up to you. Theriak-Domino is very easy to use, and has two internally consistent databases, one based on Berman's researches (from TWQ), the other from Holland & Powell (from Thermocalc). Unfortunately this has been a while since this databases haven't been updated (Berman since a very long time, and Holland & Powell has newer database but it has not been re-written from Theriak Domino). I've never used Thermocalc, but I've been told it is pretty complex to obtain a useful P-T grid. The other choice, Perple_X, is good. It has the advantage of working "as easily" as Theriak-Domino, and contains more recent databases. It also includes more models for both solid-solution and standard state of minerals (enthalpy, entropy...). HOWEVER, you need to make the choice of which database and model to use for each minerals, which can yield to some problem if you select inconsistent model (e.g. choose the Berman model for biotite, and then Holland & Powell for garnet). Just need to be aware of this. The documentation and the program itself helps you making the right choice.

Good luck!

J.

Thank you Julien.

I'm expecting P-T range in amphibolite facies metamorphism, as well as the nearest amphibolite bodies in the field. Thus 500-600oC and 6-8 kbar is possible. Moreover, there is a lot of staurolite phase in paragenesis with biotite and muscovite or garnets. Al2O3 value vary about 15%. Therefore the range of temperature conditions is rather narrow. I'm researching actually mica schists and metabasites at all (mostly ortho- and para-amphibolites). I'm now using Petrograph, GCDkit, NewPet and I would use arguably Thermocalc or Theriak-Domino Software.

Thank you for useful advices !

Is it possible to send a thin-section image?

Yes it is but only after 4th May because I'm on a conference.

I've completely forgotten about this topic after conference...

I attach thin section images of the Na-bearing mica schist. The whole rock is foliated and large garnet porphyroblasts are centered the images.