protoliths

A protolith is the original rock from which a metamorphic rock forms during metamorphism. It can be any rock type—igneous, sedimentary, or even another metamorphic rock—that experiences elevated temperature and pressure, leading to mineralogical and textural changes. The protolith’s chemistry and mineralogy strongly influence the metamorphic reactions and the final mineral assemblage of the metamorphic rock.

Common protoliths and their typical metamorphic products illustrate the concept. Shale and other fine-grained sedimentary rocks commonly yield slate, phyllite, and then schist or gneiss as metamorphic grade increases. Limestone and dolostone typically reform as marble. Sandstone often becomes quartzite. Igneous rocks, such as granite or diabase, can produce high-grade metamorphic rocks like granitic gneiss or schist under regional metamorphism, and migmatites when partial melting occurs.

During metamorphism, minerals recrystallize and new minerals form as rock textures are reoriented and healed under elevated temperature and pressure. Fluids may drive metasomatic exchanges that change bulk composition in some domains. The protolith also governs the presence of certain minerals and the types of reactions that occur, influencing the index minerals used to define metamorphic facies and pressure–temperature paths.

Interpreting metamorphic rocks often requires identifying the protolith or constraining its possible types, because the same metamorphic rock could have originated from several different protoliths. The protolith’s age and composition thus provide essential context for understanding tectonic setting and crustal evolution.