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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
Author(s) -
Massonne HansJoachim
Publication year - 2011
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.1230
Subject(s) - massif , geology , terrane , dolomite , lawsonite , phengite , geochemistry , mineralogy , eclogite , subduction , tectonics , seismology
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.