protolithtypen

Protolithtypen, often called protolith types, are the original rocks from which metamorphic rocks form during metamorphism. The protolith may be igneous, sedimentary, or, in some cases, itself already metamorphosed. The composition, texture, and chemical signature of the protolith influence the mineral assemblages that develop under elevated temperature and pressure, as well as the fluids involved, and thus control the character of the resulting metamorphic rock.

Common protoliths are igneous rocks such as granites and basalts and sedimentary rocks such as shales, sandstones, and limestones. In practice, many metamorphic rocks record protoliths that derive from these original rocks. In some cases the protolith itself was previously altered by metamorphism, yielding metasedimentary or metavolcanic precursors that were later re-metamorphosed.

Determination of a protolith involves petrographic analysis, whole-rock chemistry, isotopic data, and consideration of the regional geology. The type of protolith helps explain the metamorphic mineral assemblages and textures observed, such as slate or phyllite from shale, schist or gneiss from granitic or volcanic precursors, and marble from limestone.

Understanding protolithtypen provides insights into past environments, tectonics, and metamorphic histories, aiding reconstructions of regional geology.