| Реферат: | eng: The Al2SiO5 polymorphs (andalusite, kyanite, and sillimanite), which are widespread in aluminous metamorphic rocks, gain increasing importance as industrial sources for producing aluminum oxide, silumin, and aluminum. The polymetamorphic history of a series of metamorphic complexes of Al-rich metapelites in the Yenisei Range, with rocks containing the Al silicate triple-point assemblage (all three Al2SiO5 polymorphs) is illustrated by geological-structural, mineralogical-petrological, and isotope geochronological evidence. In the studied aureoles, the overprinting of earlier mineral assemblages with the later ones during various geodynamic events obviously follows from the reaction microtextures and chemical zoning in minerals, the P-T trajectories of these rocks, as well as radiometric dating. These indicator features show that the Al2SiO5 polymorphs sequentially grew as a result of a complex polymetamorphic history due to the changes in the tectonic settings. A comparative analysis of the results and literature data on other metamorphic regions worldwide shows that the Al2SiO5 minerals always (at least in the studied rocks) occur in reaction relationships with one another, grew at different time periods during the metamorphic history of the rocks, and hence, cannot be regarded as true triple-point assemblages, although some segments of the looping P-T paths may pass near the triple point. We conclude that the aluminum silicate triple-point assemblages, in which all three Al2SiO5 minerals are in stable equilibrium, are not possible in metapelitic rocks of any chemical composition. Hence, such purported triple-point assemblages cannot represent a stable association and cannot be used to calibrate or test geothermobarometers.
|