Abstract

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 Р–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.

中文翻译：

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Al 2 SiO 5“三点”组合的不稳定性作为富铝异质岩中多态性的结果

摘要

Al ₂ SiO ₅多晶型物（黄铁矿，蓝晶石和硅线石）广泛存在于铝质变质岩中，作为生产氧化铝，硅铝矿和铝的工业来源，其重要性日益提高。叶尼塞山脉中一系列富含铝的变质岩的变质史，包括含有硅酸铝三点组合（所有三个Al ₂ SiO ₅多晶型）的岩石，通过地质，构造，矿物学，岩石学和同位素年代学证据。在研究的金相中，在各种地球动力学事件中，较早的矿物组合与较晚的矿物组合的叠印显然来自于矿物的反应微观结构和化学区划，这些岩石的Р–T轨迹以及辐射测年。这些指示特征表明，由于构造背景的变化，由于复杂的多变质史，Al ₂ SiO ₅多晶型物依次生长。对全球其他变质区的结果和文献数据的比较分析表明，Al ₂ SiO ₅矿物总是（至少在所研究的岩石中）彼此发生反应关系，并且在该变质史的不同时期生长。岩石，因此，尽管循环的P – T的某些部分不能被视为真正的三点组合路径可能会通过三点附近。我们得出的结论是，在任何化学成分的变质岩中，铝硅酸盐三点组合（其中所有三种Al ₂ SiO ₅矿物均处于稳定平衡）是不可能的。因此，这种声称的三点组合不能代表稳定的关联，也不能用于校准或测试地热气压计。