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

Abstract

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 ± Arg and Dol + Coe + Phen + Rt ± Omp ± Arg ± Ap. Retrograde assemblages are characterized by symplectitic replacement of Tr–Ab and Di–Ab after omphacite, and Phl–Pl symplectite after phengite. The P–T conditions of UHP metamorphism were estimated to be P > 2.7 GPa and T > 670 °C by 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.