Metasomatism

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Related to metasomatized: 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 ?
These fluid inclusions were analysed in the quartz and topaz from the metasomatized stockwork.
Finally, it remains remarkable that the geochemical signature of the RMG is still preserved in the primary L1-type fluid inclusions from the metasomatized La Bosse stockwork, despite the distance from the granitic cupola (>100 m) and interaction with the host-rocks and external fluids.
The host rock of this type of beryl occurrence is similar to beryl (emerald) deposits that are formed via syn- to post-tectonic reactions under low-grade regional metamorphism and within biotite schist tectonically overlain by ophiolite, such as in southern Egypt (Abdalla and Mohamed, 1999) and Habachtal, Austria, (Grundmann and Morteani, 1989) where the source of chromium is believed to be metasomatized ophiolitic rocks (Groat et al., 2008).
The Eskisehir green color beryl crystals formed in schists via the upthrust in the Upper Cretaceous of chromium and magnesium-rich metasomatized ophiolites onto metamorphic units.
In the present context, the different westward angular velocity of the lithospheric fragments in the South American plate, as defined by the "second order plate boundaries" (e.g., the Pylcomaio lineament [18]) as well as the different rotational trends at S1924[degrees] Latitude, would favour decompression and melting at different times of variously metasomatized (wet spot) portion of the lithospheric mantle with variable isotopic signatures [24].
Tirone, and P Comin-Chiaramonti, "Clinopyroxenes from metasomatized spinel-peridotite mantle xenoliths from Nemby (Paraguay): crystal chemistry and petrological implications," Mineralogy and Petrology, vol.
When the metasomatized mantle wedge melts, LIL elements strongly partition into the melt, whereas HFS elements preferentially remain in stable accessory minerals.
Alternatively, these features can be inherited from mafic magma derived from metasomatized mantle, and the differentiated products from such magma reproduce the jagged pattern.
The Geological Significance of the Estremely Hydrothermal Metasomatized Auriferous Ore Rb-Sr Isochron (in Chinese with English abstract).
Therefore, it is not necessary to invoke an active mantle plume to generate the basalt volcanism in this area, but the melt generation could be also attributed to lithospheric extension associated with passive upwelling of variously metasomatized upper mantle.
In the second, the most important ore-forming phase, the brecciated limestone and dolomite were metasomatized by ascending iron-rich solutions, and transformed into large bodies of carbonates consisting mostly of siderite.