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The ‘triple point’ paradigm of aluminosilicates revisited

The Al2SiO5 polymorphs (andalusite, kyanite and sillimanite), which are dominant constituents in aluminous metamorphic rocks, are gaining increasing importance as industrial sources for aluminium oxide, silumin and aluminium. The formation of the most promising deposits and occurrences of the Al‐rich rocks in the Transangarian segment of the Yenisei Ridge of the Siberian Craton are genetically related to the metamorphism of the And–Sil and Ky–Sil types. Reaction microtextures, chemical zoning in minerals, radiometric dating as well as the Р‐Т 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 Yenisey 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, notwithstanding the possibility that parts of the looping P‐T paths may have passed near the triple point. 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 aluminium silicate triple‐point assemblages, in which all three Al2SiO5 minerals are in stable equilibrium, are not possible in metapelitic rocks of a 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.