A possible explanation for the low biotite content is shown in the P-T diagram

(please see scan attached):

"Figure 7.8." on page 209 from the book 'Petrogenesis of Metamorphic Rocks' by Kurt Bucher and Martin Frey (1994) showing the "Low-pressure, high-temperature reactions and phase relationships in metapelites and metamorphism in the KFASH system ...".

At 580o Celsius and 4 Kbar you are almost exactly on the reaction curve "muscovite + biotite out / almandine + K-feldspar in".

Please check the ratio of garnet and K-feldspar compared to the amount of biotite in your sample.

Best regards,

Guenter Grundmann

Dear Prof. Grundman

Thank you for your answer. You are right about that reaction but the amount of garnet in low and there is no Kfs in the samples. It should be noted that, most of the biotites formed around the magnetite crystals. I could not find any suitable reaction for it. Is it possible that redox state limited formation of the biotites?

Another possible explanation for the relatively low biotite content in your meta-pelite samples is the significantly lower weathering resistance of biotite compared to muscovite during diagenesis. The smaller biotite fragments are replaced by chlorite. The muscovite, however, remains stable in a wide pressure / temperature range. The biotite formed around magnetite crystals (you mentioned) may be a new growth generation during amphibolite facies conditions.

Best regards,

Guenter Grundmann

Dear Mr. Miri,

you can no longer denominate this metamoprhic rock as being full part of the amphibolite facies regime. There are already signs of a retrograde process paving the way into the low-grade stage of regional metamorphism as indicated by the presence of chloritoid, muscovite and chlorite. As I have no full-blown picture of the entire setting I am reluctant to go any further with my explanation.

Kind regards

H.G.Dill

I agree with @Herald G. Dill.

Regards.

Dear Mohammed,

Please check if the biotites are replaced by muscovite.

Trisrota

Dear Prof. Grundmann, Prof. Dill and Trisrota

Thank you so much for your answers. I have found that Cld + Bt zone is generally absent from typical Barrovian sequence but can form in high-Al protholites.

Best regards