Metabasites

Metabasites are metamorphic rocks derived from basaltic rocks, primarily through regional or contact metamorphism. They represent the metamorphic equivalents of basaltic crust and, in practice, include rocks that preserve basalts' mafic composition after metamorphism.

Mineral assemblages in metabasites vary with metamorphic grade. At greenschist facies, they typically contain chlorite, actinolite, epidote, and albite. In amphibolite facies, the protolith evolves toward hornblende (or other amphiboles), plagioclase, and pyroxene. Granulite facies introduces orthopyroxene, clinopyroxene, garnet, and plagioclase, while high-pressure eclogite facies yields garnet and omphacite with little plagioclase. Textures range from fine-grained to layered or banded, and metamorphism often preserves some original igneous layering.

Occurrence and significance include presence in metamorphosed oceanic crust and ophiolites, as well as in subduction complexes where basaltic rocks experience high pressures. Metabasites provide key records of pressure–temperature history and tectonic processes such as subduction, crustal thickening, and mélange formation.

Classification and terminology note that metabasite is a broad, sometimes loosely defined term. Some geologists distinguish metabasites from more specific rock names (such as amphibolite, granulite, or eclogite) by referring to the basaltic protolith. In many contexts, metabasite serves as a general label for basalt-derived metamorphic rocks across a range of grades.