amphibolite

Amphibolite is a metamorphic rock dominated by hornblende and plagioclase feldspar, typically dark in color and medium to coarse in grain size. It forms mainly from mafic rocks such as basalt, diabase, or gabbro through metamorphism, though variations can occur depending on the protolith and conditions.

The principal minerals in amphibolite are hornblende (or other amphiboles) and calcium-rich plagioclase. Accessory minerals such as quartz, biotite, garnet, titanite, magnetite, and sometimes ilmenite may be present. The rock often exhibits a foliated or banded texture, reflecting alignment of minerals during regional deformation.

Amphibolite corresponds to the amphibolite facies of regional metamorphism, indicating medium to high temperatures, roughly 500–750°C, with pressures that vary widely. Higher-grade, garnet-bearing varieties can form at higher pressures. The assemblage is characteristic of metamorphosed mafic rocks and contrasts with the lower-grade greenschist facies, which features chlorite and actinolite instead of hornblende.

Occurrence and significance: amphibolites are widespread in mountain belts and ancient greenstone terrains, and they provide essential clues about the thermal and tectonic history of a region. They are also used as indicators of metamorphic grade and crustal evolution in geological mapping and petrological studies.