Protolith

A protolith is the original rock from which a metamorphic rock formed before metamorphism. It represents the pre-metamorphic parent material and can be igneous, sedimentary, or, less commonly, another metamorphic rock. The composition and texture of the protolith influence the mineral assemblages and textural changes that develop during metamorphism, though metamorphism often modifies or recrystallizes these features.

During metamorphism, existing minerals respond to changing pressure, temperature, and chemical conditions by recrystallizing and by forming new minerals. As a result, metamorphic rocks typically retain some chemical or textural clues of their protolith, while acquiring new features characteristic of their metamorphic conditions. The protolith concept helps petrologists interpret metamorphic histories, the inferred tectonic setting, and the evolution of Earth's crust.

Determining the protolith involves integrating mineralogical and chemical analyses, field relationships, and sometimes isotopic data. However, metamorphism and metasomatism can overprint or obscure original features, making protolith identification challenging. In some cases, the protolith is inferred from characteristic metamorphic facies or from remnants of original textures such as bedding in pelitic rocks or lithic components in metasedimentary rocks.

Common protolith–metamorphic rock relationships include pelitic protoliths yielding schists and gneisses, calcareous protoliths producing marbles, mafic protoliths forming amphibolites or eclogites, and quartz-rich sandstones becoming quartzites. The protolith concept remains central to understanding metamorphic processes, inheritance, and the sequential history of metamorphic rocks.