Protolith

A protolith is the preexisting rock from which a metamorphic rock forms. Its original composition and texture influence the metamorphic rock’s mineral assemblage and structure.

Metamorphism: The solid-state mineralogical and structural transformation of rock under changed temperature, pressure, or fluid conditions. It is the process that changes a protolith into metamorphic rock.
Igneous rock: Rock formed when magma or lava cools and solidifies. Igneous rocks can become protoliths for metamorphic rocks.
Metaconglomerate: A conglomerate altered by metamorphism, commonly retaining deformed pebbles in a recrystallized matrix. Its pebble-rich precursor is often recognizable even after deformation.
Metamorphic rock classification: The grouping of metamorphic rocks by mineral composition, texture, and origin. Protolith information can separate rocks with similar metamorphic textures but different origins.
Orthogneiss and paragneiss: A distinction between gneiss derived from igneous rock and gneiss derived from sedimentary rock. The contrast makes protolith identity explicit even when both rocks share gneissic texture.
Metamorphic facies: A set of mineral assemblages characteristic of a particular range of pressure and temperature. The conditions imposed on a protolith help determine which facies develops.
Sedimentary rock: Rock formed from deposited sediment or from minerals precipitated from water or air. Sedimentary rocks commonly supply the material later metamorphosed.
Metabasalt: Basalt transformed by metamorphism while retaining chemical or textural evidence of its volcanic origin. It demonstrates how an igneous protolith can acquire a new mineral assemblage.
Geological mapping: The recording and interpretation of rock units, structures, and geological relationships across an area. Recognizing protoliths helps map the original rocks beneath metamorphic overprinting.
Metamorphic rock: Rock altered in the solid state by changes in temperature, pressure, or fluid conditions. A protolith is the precursor; metamorphic rock is the transformed product.

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