The ‘triple point’ paradigm of aluminosilicates revisited

Abstract

The Al 2 SiO 5 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 Al 2 SiO 5 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 Al 2 SiO 5 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 Al 2 SiO 5 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. K E Y W O R D S Al 2 SiO 5 minerals, Al-rich metapelites, triple point, polymetamorphism, Yenisei Ridge, Siberian Craton 1 | INTRODUCTION The polymorphic modifications of Al 2 SiO 5 , namely kyanite, andalusite and sillimanite, are among the most important indicators of metamorphism in pelitic lithologies. Having the same chemical composition, they are characterized by different crystal structures stable at different P-Tconditions. Based on various trends of the evolution of temperature with depth and the relationships between these trends on the P-T diagram with the stability fields of Al 2 SiO 5 polymorphs, different 'baric' types of metamorphism are distinguished (for example, Miyashiro, 1961;Reverdatto, Likhanov, Polyansky, & Sheplev, 2019). Thus, andalusite is stable at low pressures and temperatures; with pressure increase, it is replaced by kyanite, and with temperature increase, andalusite and kyanite are replaced by sillimanite, resulting in the formation of zoned metamorphic complexes of low and moderate pressures. These conditions are mostly located in part of P-T space traversed by many crustal metamorphic sequences, so that inferences about P-T conditions can be made based on the occurrence of one or more of the polymor...

Metamorphic Conditions and Raman Spectroscopic Carbonaceous Material Thermometry of Host Schists of Olympiada and Eldorado Gold Deposits (Yenisey Ridge, Russia)

“…The rocks do not have any evidence of early HT/LP metamorphism which was mentioned in the nearest area [20] or the grade of early metamorphism was not higher than greenschist facies. This shows that the early stage of metamorphism described over the whole YR [44] probably has a patchy distribution.…”

“…Detailed studying of metamorphic processes of Transangarian part of YK was performed along Tatarka thrust (Figure 1, review [44] and references therein). Fe-and Al-rich metapelitic schists of the Korda formation show petrological evidence of two superimposed metamorphic events.…”

Alumina Hosts in Fe- and Al-Rich Metapelites from Transangaria (Yenisey Ridge, East Siberia): Distribution, Composition, and Mining Potential

“…Fe-and Al-rich metapelites with sillimanite-group rock-forming minerals, staurolite and chloritoid, and with high economic contents of Al 2 O 3 (X average ≈ 20 wt%) occupy large areas in the Transangarian segment of the Yenisey Ridge due to a unique combination of protolith compositions and conditions of metamorphism [12][13][14][15][16][17][18][19]. The alumina-rich metamorphic rocks in the region may result from multistage And-Sil and Ky-Sil metamorphism of Fe-and Al-rich Early Proterozoic sediments.…”

New Evidence for Grenville and Valhalla Tectonic Events at the Western Margin of Siberian Craton: The Example of the Garevka Metacomplex, Yenisey Ridge

“…На первом этапе сформировались зональные комплексы низких давлений And-Sil типа гренвильского возраста при обычном для орогенеза метаморфическом градиенте dT/dH = 25-35 °С/км . На втором этапе эти породы подверглись неопротерозойскому (~800 млн лет) коллизионному метаморфизму умеренных давлений Ky-Sil типа с низким dT/dH  12 °С/км [Likhanov, Santosh, 2020]. Заключительные стадии развития коллизионного орогена в регионе маркируются дайковыми роями бимодального пояса, представленного анорогенными грани-тоидами и внутриплитными базитами рифтогенной природы с возрастами внедрения 797-792 млн лет, фиксирующими начало распада Родинии , 2019.…”