Metasomatism

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Related to metasomatically: contact metasomatism, metastatically

metasomatism

[‚med·ə′sō·mə‚ tiz·əm]
(petrology)
A variety of metamorphism in which one mineral or a mineral assemblage is replaced by another of different composition without melting.

Metasomatism

 

the replacement of certain minerals by others with substantial change in the chemical composition of the rock, occurring under the influence of solutions with high reactivity; the volume and solid state of the rock is usually preserved. A distinction is made between metasomatism of the magmatic stage that accompanies the intrusion of igneous rocks (for example, in connection with granitization) and postmagmatic metasomatism of the period when the rocks are cooling. Ore formation is associated with postmagmatic metasomatism. The chemical affinity of the solutions that cause metasomatism changes as the solutions cool. The following stages are distinguished in this process: the high-temperature alkaline stage (formation of skarn rocks, alkaline metasomatism), the acid stage (development of greisen and silicification), and the low-temperature alkaline stage (the development of calcareous, listvenite, beresite, and gumbeite rocks, alkaline metasomatism).

Infiltration metasomatism results from the transport of chemical components by a stream of solutions that filter through the rocks; diffusion metasomatism is associated with the diffusion of components in a relatively immobile solution that saturates the rock. At the boundary between two media that differ sharply in chemical affinity (limestones and quartzites, granites and ultrabasic rocks, and the like) there is counterdiffusion of different components (bimetasomatism).

Characteristic of the processes of metasomatism is the development of metasomatic zonation (with distinct borders between zones) resulting from the differential mobility of the components transported by the solutions. As the intensity of metasomatism grows, an increasing number of components become mobile and the number of minerals in the products of metasomatism decreases to the point where monomineralic rocks form.

REFERENCE

Korzhinskii, D. S. Teoriia metasomaticheskoi zonal’nosti Moscow, 1969.
References in periodicals archive ?
Emeralds occur at the Kagem mine mostly in metasomatically altered phlogopite-rich zones up to 3 meters wide that rim the numerous pegmatite dikes and quartz-tourmaline veins (genetically related to a granite pluton at depth) cutting through the talc-magnetite schist country rock.
When all of the primary pegmatites had crystallized, the alkaline solutions leached quartz from the pegmatite bodies and in rare cases even from the underlying granite and the metasomatically altered feldspars, micas etc.
Monazite-(Ce) is a characteristic mineral in the neighborhood of clefts in metasomatically altered rocks.