Reversing the metamorphism process to determine the origin of the protolith, specially if it was a sedimentary rock, depending on the isotopic content and geochemistry of elements.
Protolith determination is an important task of metamorphic petrology and has been done in numerous studies. No single method will give you the answer, I recommend using and integrating multiple tools: field relations, stratigraphic study, hand sample descriptions of minerals and fabrics, thin section examination of minerals and fabrics, possible EMPA for determination of minerals and specific mineral chemistry, whole rock major-trace-and-REE geochemistry, isotope geochemistry, fluid inclusions, etc. Data should be plotted to compare with known analogs. I suggest reading of several articles in a journal such as Journal of Metamorphic Geology or Precambrian Research to get an idea of how different researchers use information to interpret protolith.
I don't think it is the right approach to use geochemistry in isolation to rebuild the protolith of the rock at interest, especially if it is the sedimentary rock. Because it may be possible to establish the protolith of a metamorphosed igneous rock with some certainty, but doing with the same certainty for the sedimentary rock is not possible, for they often conforms to igneous trends or atleast they will not conform with the trends of the supposed sedimentary correspondent. So what I suggest is to have a multiple approach: field occurrence, associated litho units, textures, structures and thin section studies along with the chemistry can help to rebuild the protolith and its environment.
Yes, there are many protoliths that, when metamorphosed, overlap in their characteristics. Metasediments aren't difficult to work with if you are careful and use the tools suggested.
One example: An arkosic sediment with quartz, ksp, and some mica -vs- a granitic rock with qtz, ksp, and some mica -vs- a rhyolitic/rhyodacitc tuff with high silica, K2O, and a little mica. With a medium to high grade metamorphism and recrystallization of the main components, there will be much similarity between the products. You need more than one determinative method to correctly identify the protolith. Field relations are important,so are textures of hand and thin section samples; combined with geochem and isotopes, you'll be able to support your interpretation.
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