Petrogenesis of impure dolomitic marble from the Dabie Mountains, central China

Petrogeneses of impure dolomitic marble and enclosed eclogite from the Xinyan area, Dabie ultrahigh-pressure (UHP) metamorphic terrane, central China were investigated with a special focus on fluid characteristics. Identified carbonate-bearing UHP assemblages are Dol + Coe ± Arg (or Mgs) ± Ap, Dol + Omp ± Coe ± Ap ± Arg (or Mgs), Phen + Omp + Coe + Dol ± Coe + Dol ± Arg and Dol + Coe + Phen + Rt ± Omp 9p\ Arg 9p\ Ap. Retrograde asssemblages are characterized by symplectitic replacement of Tr-Ab and Di-Ab after omphacite, and Phl-Pl symplectite afeter phengite. The P-T conditions of UHP metamorphism were estimated to be P >2.7 GPa and T >670°C bu the occurrence of coesite inclusions in garnet in enclosed eclogite and garnet-clinopyroxene geothermometer. The P-T conditions of initial amphibolitization were estimated to be 620 < T < 670°C and 1.1 < P < 1.3 GPa by calcite-dolomite solvus thermometer and mineral parageneses. Phase relations in P-T-XCO₂ space in the systems NaAl-CMSCH and KCMASCH were calculated in order to constrain fluid compositions. Compositions and parageneses of UHP-stage minerals suggest the presence of fluid in UHP and exhumation stages. Occurrence of retrograde low-variance assemblages indicates that fluid composition during amphibolitization was buffered. A metastable persistence of magnesite and very restricted occurrence of calcite, magnesite and dolomite suggest a low fluid content in the post-amphibolitization stage.