Anatexis


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As Anatexis ( Greek ανατηξις "melting"), also migmatisation , is defined as the partial melting of rocks of the earth's crust as a result of increase in temperature, pressure relief and / or fluid supply (z. B. of H 2 O , CO 2 ). Anatexis takes place in the deeper crust of the earth , mostly in the course of mountain building processes , e.g. B. in the Himalayas and the Alps , but also in the Thuringian Forest . The resulting rocks are called migmatites or migmatized gneiss , slate, etc.

The initial stage of melting is known as metatexis . The mobilization takes place only at the grain boundaries and affects only part of the mineral inventory ( partial melt ). In the higher stage, the diatexis , the dark ( mafic ) mineral components are increasingly melted, until magma and igneous rocks are finally formed . The more or less molten rocks ( metatexites and diatexites ) are summarized as migmatites (or anatexites ).

In granitic rocks, anatexis takes place under fluid-saturated conditions at temperatures above 650 ° C. Typical pressures are 0.5 to 1 GPa (corresponds to the pressure at a depth of 15 to 35 km). Mainly the quartz and the feldspars are melted. With increasing temperatures, granitic , granodioritic and quartz dioritic melts form successively . Basic rocks only melt at significantly higher temperatures. Since quartzites , amphibolites and calcareous silicate rocks are practically never found as anatexites in nature and are preserved unchanged as so-called resisters even in extensive migmatite areas, the maximum temperature that is reached in regional melting is estimated at around 800 ° C.

See also

literature

  • Siegfried Matthes, Martin Okrusch: An introduction to special mineralogy, petrology and deposit science . Springer, Berlin 2005, ISBN 3-540-23812-3 .