metabasalts

Metabasalts are metamorphosed basalts, the metamorphic equivalents of basaltic volcanic rocks. They form when basalt or basaltic rocks are subjected to heat and pressure during metamorphism, typically at low to intermediate grades associated with regional tectonism or contact with intrusions. Metabasites are common in ancient geological terranes, especially greenstone belts, and also occur in ophiolite complexes where oceanic crust has been emplaced and subsequently metamorphosed.

The mineralogy and texture of metabasalts vary with metamorphic grade. In greenschist facies, they commonly host chlorite, actinolite or tremolite, albite, epidote, and quartz, often preserving a relict basaltic texture but with recrystallization and foliation. In amphibolite facies, metabasalts recrystallize to a coarser assemblage dominated by hornblende (or other amphiboles) and plagioclase, with stronger deformation and foliation. Higher-grade metamorphism can produce granulite-facies assemblages, incorporating minerals such as garnet or orthopyroxene, though such stones are less typical for basalts. Water-rich metamorphism can also alter primary minerals to serpentine, chlorite, or other hydrous phases during early stages.

Field relations and significance: Metabasalts commonly occur with other metabasites, metavolcanic rocks, and metagabbros in greenstone belts and ophiolites, recording ancient volcanic activity and crustal processes. They provide insights into the metamorphic history and tectonic evolution of their terranes and can be associated with various ore mineralization in some greenstone belt environments.