metasomatisme

Metasomatisme, or metasomatism, is a geological process in which rocks undergo chemical alteration due to the interaction with external fluids during metamorphism or diagenesis. The defining feature is mass transfer of chemical components between fluids and the rock, leading to the formation of new mineral assemblages and changes in bulk composition. This can occur while textures are preserved or progressively recrystallized, and it often creates localized zones of metasomatic rocks along fractures, faults, or contact aureoles.

Mechanisms and agents

Hydrothermal, magmatic, or basinal fluids transport ions such as Na, K, Ca, Al, Si, Fe, Mg, CO3, Cl, F and S. Through dissolution of some minerals and precipitation of others, original rocks are replaced or enriched, sometimes forming sharp chemical boundaries called metasomatic fronts. Temperature and pressure conditions, fluid salinity, and the nature of fluid pathways control the specific mineral shifts.

Common manifestations

Different metasomatic styles include albitization (Na-rich replacement of feldspars), potassic metasomatism (enrichment in K-bearing minerals like K-feldspar and micas), carbonate metasomatism (introduction or redistribution of carbonate minerals), and silicification ( Quartz impregnation). Metasomatism is often associated with Contact metamorphism near igneous intrusions, subduction-related fluids, or hydrothermal vein systems, and can produce economically important ore-related rocks such as skarns and greisen halos.

Relation to metamorphism

Metasomatism represents a chemical reorganization with substantial mass transfer, distinguishing it from pure metamorphic recrystallization, though it frequently occurs in tandem with metamorphic processes and can define late-stage alteration halos around mineral deposits.