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

✍ Scribed by Hans-Joachim Massonne

• Publisher: John Wiley and Sons
• Year: 2010
• Tongue: English
• Weight: 510 KB
• Volume: 46
• Category: Article
• ISSN: 0072-1050
• DOI: 10.1002/gj.1230

✦ Synopsis

Abstract

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.