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 SiO 2 ‐Al 2 O 3 ‐MgO‐FeO‐CaO‐Na 2 O‐K 2 O‐H 2 O‐CO 2 for the pressure ( P )–temperature ( T ) range 1–7 GPa and 500–1300°C. These P – T 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 P – T conditions for such marbles. The same is true concerning Ca contents in garnet. X CO 2 of the mixed H 2 O‐CO 2 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/cm 3 and, thus, significantly lower than those of ordinary upper crustal rocks at identical P – T 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.