Abstract
The effect of the composition of the sources and the thermal and baric evolution of metasomatic solutions on the character of metasomatic products was studied by numerically simulating solution-rock interaction. The rock sources of the solutions assumed in these simulations were shale, granite, and granodiorite. The relative acidity of the rocks and solutions interacting with them was suggested to estimate using the parameter \(A_R = \log \left( {a_{R^ + } /a_{H^ + } } \right) + \frac{1} {2}\log a_{H_2^O } \) (where R = K, Na). The values of this parameter in rocks differently vary with temperature and pressure and are controlled by buffer equilibria of minerals composing these rocks. The acidity of the solution relative to the source increases at cooling because of the more rapid dissociation of acids than salts and bases. However, the solution can be either more acidic or more alkaline relative to protoliths of other composition. Solutions derived from various sources but cooling in similar regime produce metasomatites of different composition. As the solution cools, feldspar metasomatites can grade into quartz-muscovite ones, which corresponds to the vertical temperature zoning of metasomatic aureoles. The alkalinity of solutions relative to their sources increases at their decompression, but the solutions may be less alkaline relative to protoliths of other composition. In contrast to cooling regime, solutions derived from all of the examined types of sources at decompression produce one of the feldspars and Ca, Mg, and Fe minerals, such as biotite, chlorite, amphibole, and magnetite. Quartz-free varieties of feldspathic metasomatites can be produced at either cooling or decompression but only by alkaline solutions. The potassic or sodic character of feldspathization at decompression depends on the composition of the source and the P-t conditions of solution generation.
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Original Russian Text © A.B. Kol’tsov, 2015, published in Geokhimiya, 2015, No. 2, pp. 144–161.
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Kol’tsov, A.B. Effect of the sources and evolution of solutions on the composition of metasomatites. Geochem. Int. 53, 133–149 (2015). https://doi.org/10.1134/S0016702914120040
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DOI: https://doi.org/10.1134/S0016702914120040