Dear Mrs. Chaudhuri,

I take in this case a tripartite approach.

1. Have a look at the metamorphic lithologies associated with your rock under study; are there meta-carbonates, metabiolites...etc. Sometimes you can get an impression of the hidden paleoenvironment. Even by the presence of strange minerals like scapolite (ref. palaeo-sabkha..) you can get a hint.

2. Use ACF and AFK diagrams where the protoliths are shown by reference fields

3. Try and find ultrastable minerals like rutile, tourmaline and zircon. A very subtle investigation of them microchemically and morphological may in some cases shed some light on the parent material.

Take a multiple approach encompassing a geological (1.), chemical (2) and mineralogical (3) method.

I wish you much success

H.G.Dill

Dear Dr. Dill,

Thank you for your suggestion.

Best

Trisrota

Take a look at Seymour et al. 2018, "Laramide subduction and metamorphism of the Orocopia Schist, northern Plomosa Mountains, west-central Arizona: Insights from zircon U-Pb geochronology". https://doi.org/10.1130/G45059.1

You could use the U-Pb zircon geochronology depth-profiling technique highlighted in the paper. Getting accurate dates on 100 or so zircon cores should elucidate whether the schist is detrital in origin or more or less a single magmatic event. You'll also get the bonus of identifying later metamorphic events!

Good luck.

Thanks Evan.

Dear Trisrota Chaudhuri

Hello

Your studied are normal medium to high grade metapelites and their mineral assemblages in common and normal, dont worry, There similar case in Precambrian metamorphic rocks of central Iran (Jandagh or Jandaq complex). Formation and present of K-Feldspar and also plagioclase in these rocks is normal phenomena.

It is likely that the protolith of the quartzofeldspathic schist is greywacke. K-feldspar rarely occur in metapelites except at high grade of metamorphism, with the likelihood of partial melting being initiated. However, K-feldpar is a common mineral in metagreywackes.

Thanks Mahmoud and bhimbola