amphibolites

Amphibolites are metamorphic rocks in which hornblende (an amphibole) and plagioclase feldspar dominate the mineral assemblage. They are typically dark-colored and contain little or no quartz. The texture is usually granular, but may be porphyroblastic with hornblende and plagioclase crystals of differing sizes. Accessory minerals such as biotite, magnetite, ilmenite, or garnet occur depending on grade and composition.

Amphibolites form under medium- to high-grade metamorphic conditions, within the amphibolite facies, from mafic protoliths such as basalts, gabbros, or basaltic andesites. In metamorphism, pyroxene is partly or wholly replaced by hornblende, while plagioclase remains stable; the precise mineral assemblage reflects pressure, temperature, and chemical environment. The relative absence of quartz distinguishes amphibolites from quartz-rich metamorphic rocks; when quartz is present in significant amounts, the rock may be described as quartz-bearing amphibolite in informal contexts.

Amphibolites occur globally in regional metamorphic belts and in ophiolitic terrains where mafic crust has undergone metamorphism. They serve as indicators of amphibolite-facies conditions and help interpret tectonic settings such as subduction zones and collisional orogens. Nomenclature-wise, rocks whose dominant mineralogy is hornblende and plagioclase are classified as amphibolites; those with substantial quartz or other mineral assemblages may be reclassified as related metamorphic rock types.