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Common major error in many online professional geological references... claims that metamorphic rock "protoliths" are only sedimentary and igneous rocks.
However, Wikipedia "Metamorphic Rock" article correctly states that metamorphic rock "protoliths" include "... and existing metamorphic rocks." I proposed the major professional geological reference above, to support that. More major professional geological references available.
Wikipedia "Protolith" article and its references incorrectly claim metamorphic rock protoliths include only sedimentary and igneous rocks. That article needs corrections. Regardless of the many other online references that repeat this major error.
Proven cases over billions of years of plate tectonics show many major mountain belts have undergone multiple orogenies (mountain [re]building events) that clearly include re-metamorphism. Sometimes more intense "prograde" [re]metamorphism; sometime less intense "retrograde" [re]metamorphism. Cassiodorus8 (talk) 14:10, 15 March 2023 (UTC)[reply]
Including metamorphic rock as a type of protolith depends on one's definition of protolith. I think most geologists regard protolith as the unmetamorphosed parent rock, which therefore excludes metamorphic rocks (because they are not unmetamophosed). For example, in the prograde metamorphism sequence: shale --> schist --> gneiss, the shale is the protolith for both schist and gneiss; schist is not the protolith of gneiss because schist is already a metamorphic rock. It is important to remember that that protolith means first rock not previous rock. Gneiss can be classified as either orthogneiss (which is derived from an igneous protolith) or paragneiss (which is derived from a sedimentary protolith). There is no need for a third class (which would be derived from metamorphic rock). Archaean schists and gneisses that have been metamorphosed many times in their very long history are routinely described as having igneous or sedimentary protolith, not metamorphic protolith.
I think describing the opengeology textbook as a "major professional geological reference" is overstating its importance. It is merely another textbook for college students (and it is very short and simplified). Although the book does explicitly (and correctly) state that "... both igneous and sedimentary rocks can become metamorphic rocks. And metamorphic rocks themselves can be re-metamorphosed", another statement "Metamorphic source rocks, the rocks that experience the metamorphism, are called the parent rock or protolith, from proto– meaning first, and lithos- meaning rock" is too ambiguous. My interpretation of "Metamorphic source rocks" in this sentence is "igneous and sedimentary rocks (that are the sources of metamorphic rocks) are protoliths". Your interpretation seems to be "source rocks, which are metamorphic, can be protoliths" which I think does not reflect scientific consensus. The poor writing style of the authors is to blame for this ambiguity and misunderstanding. Another sentence "Metamorphism is the process that changes existing rocks (called protoliths) into new rocks with new minerals and new textures." and their definition in the glossary "Protolith: The rocks that existed before the changes that lead to a metamorphic rock, i.e. what rock would exist if the metamorphism was reversed." are also poorly worded, ambiguous and unnuanced. GeoWriter (talk) 17:59, 15 March 2023 (UTC)[reply]
p.s. right, seemed good reference, but not consensus. not pre-lith. deleted similar request for "metamorphic rock" article. time for me to go to pasture. best regards & wishes. Cassiodorus8 (talk) 18:42, 15 March 2023 (UTC)[reply]
p.p.s. regret most rock cycle diagrams erroneously lack feedback loop for metamorphic rocks' remetamorphism. also that common metamorphic rock definitions do not mention remetamorphism. Cassiodorus8 (talk) 18:54, 15 March 2023 (UTC)[reply]
