| Реферат: | eng: Metapelites of highly aluminous bulk compositions, containing all three Al2SiO5 minerals, from Teya and Garevka complexes in the Transangarian region of the Yenisei Ridge (the western margin of the Siberian craton) were studied to determine their metamorphic evolution and elucidate distinctive features of regional geodynamic processes. Here we present geological, structural,, mineralogical, petrological, and isotopic-geochronological evidence of their polymetamorphic history and employ P-T-t constraints on the development of these complexes to argue that the Al2SiO5 triple-point assemblages, in which all three Al2SiO5 minerals are in stable equilibrium, are not possible. In recent years, andalusite, kyanite, and sillimanite, which are constituents of high-alumina schists, have become increasingly important in practical terms for the production of aluminum oxide, silumin, and aluminum for the expansion of the raw material base in industry. The studied metamorphic complexes are heterogeneous in pressure, which is expressed in the manifestation of prograde regional metamorphism of two facies series and retrograde synexhumation metamorphism. The first stage occurred as a result of the Grenville-age orogeny during late Meso-early Neoproterozoic (1 050-850 Ma) and was marked by low-pressure zoned metamorphism of the And-Sil type with with a metamorphic field gradient of dT/dH = 25-35 degrees C/ km typical of orogenic belts. At the second stage, the rocks experienced middle Neoproterozoic (801-793 Ma) collision-related medium-pressure metamorphism with a local pressure increase near the thrust faults and only minor heating, suggesting a low gradient of dT/dH = 7-14 degrees C/KM as a result of which the progressive replacement of And -> Ky +/- Sil and the formation of new mineral associations and deformation structures. The final stage evolved as a synexhumation retrograde metamorphism (785-776 Ma) with dT/dH <= 15 degrees C/km and recorded uplift of the rocks to upper crustal levels in shear zones. Principle differences in the direction of retrograde segments determining the summary trajectory of P-.-t paths are mainly controlled by mechanisms of exhumation in different geodynamic settings. Most rocks of the Teya Complex are characterized by clockwise P-T-t paths, while decompression cooling of the rocks of the Garevka Complex with counterclockwise movement is related to the rapid tectonic exhumation during extension and thinning of crust, which provide a sharp pressure drop in a still uncooled medium. Reaction microtextures, chemical zoning in minerals, radiometric dating as well as the P-T trajectories of these rocks provide evidence for the polymetamorphic history of the aluminosilicate triple-point assemblages (containing all three Al2SiO5 minerals). The metapelites with highly aluminous bulk compositions in the Yenisei Ridge record overprinting of early mineral assemblages during subsequent thermal events. The triple-point positions reported in earlier studies resulted from polymetamorphism (asynchronous growth of the polymorphs), when all aluminium-silicate minerals grew at different times in the metamorphic history of the rock and hence cannot be regarded as true triple-point parageneses. The occurrence of all three Al2SiO5 minerals in individual rocks was interpreted to be due to reaction relations with microtextural evidence of one polymorph replacing another. We illustrate that the aluminum silicate triple-point assemblages, in which all three Al2SiO5 minerals are in stable equilibrium, are not possible in metapelitic rocks of wide range of chemical composition. Hence such purported triple-point assemblages cannot represent a stable association and cannot be used to calibrate or test geothermobarometers.
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