Metasomatism

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Related to metasomatised: Metasomatic Rocks, Altered igneous rock

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 ?
presents a complete study of primary fluid inclusions hosted in quartz and topaz from the Beauvoir rare-metal granite and the metasomatised stockwork (Massif Central, France).
Kaersutite and pargasite are amphiboles typical of alkaline basic rocks and metasomatised mantle xenoliths and their formation occurs early during the crystallization process (e.g.
Accordingly, we think that a lithospheric mantle metasomatised by crustallike (collisional?) components would be the source of these post-collisional basic magmas.
Our new data on the stable isotopic composition of the albitites and scapolite metagabbro support the interpretation that the original magmatic mafic and granitoid rocks were metasomatised by fluids reflecting a seawater origin or with a possible magmatic component.
In the Kragero area, single, metasomatised, large (>1 [km.sup.2]) albitites and scapolite metagabbro bodies have been mapped.
The marked differences in mantle source indicate that the Permian mantle beneath the western Tethys was heterogeneous and variably metasomatised, as suggested in previous studies (Gaggero et al, 2007; Gale, 2005; Lapierre et al, 1999).