Metabasalt

Metabasalt is basalt that has undergone metamorphism, recrystallizing its original minerals into new metamorphic ones while often retaining traces of its volcanic origin. Because the new minerals are commonly green (chlorite, actinolite, epidote), low-grade metabasalt is frequently called greenstone.

The specific minerals depend on the metamorphic grade: low grade yields chlorite and albite, higher grade yields amphibole and plagioclase (forming amphibolite), and high-pressure conditions can produce blueschist or eclogite.

Metabasalt is extremely common in ancient greenstone belts and in metamorphosed oceanic crust, recording the burial, heating, and deformation of former lava flows.

Metabasalt forms when basaltic rocks, such as lava flows, dikes, and oceanic crust, are subjected to elevated temperature and pressure during burial, regional metamorphism, or subduction.

The original basalt minerals (pyroxene, calcic plagioclase, olivine) recrystallize into stable assemblages: chlorite, epidote, actinolite, and albite at low grade; hornblende and plagioclase at medium grade; and high-pressure minerals like glaucophane (blueschist) or garnet and omphacite (eclogite) in subduction zones.

Metabasalt is widespread in Precambrian greenstone belts, ophiolite complexes, and ancient subduction zones worldwide, including the Canadian Shield, Scandinavia, and the Alps.

Identify metabasalt by its dark, fine-grained, often greenish appearance and its origin from basalt. Greenish hues from chlorite, epidote, and actinolite are a strong clue at low grade.

It is generally hard and dense. Relict volcanic features such as pillow structures or amygdales (filled gas bubbles) may survive, confirming a basaltic protolith. Foliation or schistosity may develop with deformation.

Look-alikes include unaltered basalt (which lacks green metamorphic minerals and shows fresh pyroxene), amphibolite (coarser, amphibole-rich, higher grade), and serpentinite (derived from ultramafic rock, softer and slicker).

Metabasalt, especially as greenstone, is used locally as crushed aggregate, dimension stone, and durable building material. Some ancient cultures prized fine-grained greenstone for axe heads and tools.

Greenstone belts made largely of metabasalt are major hosts for gold and base-metal deposits, making metabasalt important in mineral exploration.

Scientifically, metabasalt is key to understanding plate tectonics, the metamorphism of oceanic crust, and the geological history of Earth's oldest terranes. It is not a common gem material, though some compact greenstone is carved ornamentally.