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Phase relations of siliceous marbles at ultrahigh pressure based on thermodynamic calculations: examples from the Kokchetav Massif, Kazakhstan and the Sulu terrane, China



Pseudosections were calculated for an ultrahigh pressure (UHP) siliceous calcite-dolomite marble from the Kokchetav Massif, Kazakhstan and a dolomite marble from the UHP unit of the Sulu terrane, China, in the system SiO2-Al2O3-MgO-FeO-CaO-Na2O-K2O-H2O-CO2 for the pressure (P)–temperature (T) range 1–7 GPa and 500–1300°C. These PT pseudosections show that lawsonite-clinopyroxene (T < 750°C) and garnet-clinopyroxene (T > 700°C) are typical assemblages of UHP siliceous marbles. Talc and zoisite can join these assemblages at P ≤ 3.5 GPa (T < 900°C). If phengite is additionally present its Si content can contribute to determine PT conditions for such marbles. The same is true concerning Ca contents in garnet. XCO2 of the mixed H2O-CO2 fluid phase, coexisting with UHP siliceous marbles, increases with rising T and is very low at UHP and T < 800°C. Densities of the studied marbles at UHP are around 3.1 g/cm3 and, thus, significantly lower than those of ordinary upper crustal rocks at identical PT conditions. The previously estimated exhumation paths of the two studied rocks (Sulu terrane: 3.5–1.0 GPa around 600°C; Kokchetav Massif: 7 GPa—1100°C to 0.65 GPa—700°C) are compatible with the newly calculated results. Copyright © 2010 John Wiley & Sons, Ltd.