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A protolith is the original, unmetamorphosed rock from which a given metamorphic rock is formed (proto-: first; lithos: rock; both Greek).[1]

For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.

Sedimentary rocks have no protolith because they are made of sediment. However, the source of the sediment is termed its provenance.

Igneous rocks have no protolith because they form from magma.