One interesting fact about metamorphic rocks is that they were different rocks prior to being heated and squeezed. In many cases we can infer the lithology of the rock that was metamorphosed (the PROTOLITH) to produce the metamorphic one.
The primary non-foliated metamorphic rocks are easily interpreted. Marble is made of calcite (or some other carbonate) and this mineral is common in only one possible protolith: limestone (or some other carbonate rock).
Similarly, quartzite is made only of quartz, and only quartz sandstone has enough of that mineral and no others to be a reasonable protolith.
Among foliated rocks slate is the obvious candidate for such analysis. It is made of clay minerals and it has only been metamorphosed to a very low grade. It is reasonable to suppose that the protolith was also made of clays, and the only candidate is a mudstone or shale.
Actually, many other rocks probably began as mudrock protoliths. Throughout the phyllite and schist grades that amount of aluminum required to make micas and related metamorphic minerals is high enough that clays are the most likely source. The same could be true for at least some gneisses.
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Gneiss is such a high grade rock that there are other possible protoliths. because the minerals in gneiss also form in igneous rocks (that is, because gneiss forms at temperatures comparable to igneous temperatures) they can also serve as protoliths. Thus a granite or a diorite might have been the protolith for a gneiss.
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