Protolith refers to the precursor lithology of a metamorphic rock. For example, the protolith of a slate's a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith's a major aim of metamorphic geology. Sedimentary rocks have no protolith because they're made of sediment. However the source of the sediment's termed its provenance. Igneous rocks have no protolith because they form from magma.… (More on Protolith)