The Calcite-Dolomite Solvus Temperature and T-X(CO2) Evolution in High-Grade Impure Marble from Thongmön Area, Central Himalaya: Implications for Carbon Cycling in Orogenic Belts

Abstract

Impure dolomitic marble from the Great Himalayan Sequences (GHS) in Thongmön area, central Himalaya, is first systematically reported here concerning its petrographic features, textural relations, and fluid evolution. The Thongmön impure marble is characterized by the assemblage of calcite + dolomite + forsterite + spinel + phlogopite + clinohumite ± diopside ± retrograde serpentine. Three groups of calcite and dolomite occurring both as inclusions and in the matrix were identified: group I is represented by relatively magnesium-rich calcite (Cal) (CalI:XMg = 0.10–0.15) and almost pure dolomite (Dol) (DolI:XMg = 0.47–0.48), corresponding to the Cal-Dol solvus temperatures of 707–781 °C; group II is characterized by vermicular dolomite exsolutions (DolII:XMg = 0.45–0.46) in Mg-rich calcite and Mg-poor calcite (CalII:XMg = 0.05–0.08) adjacent to DolII, and the recorded solvus temperatures are 548–625 °C; group III is represented by nearly pure calcite (CalIII:XMg = 0.003–0.02) and Ca-rich dolomite in the matrix (DolIII:XMg = 0.33–0.44). Isobaric T-X(CO2) pseudosection at a peak pressure of 15 kbar in the system K2O-CaO-MgO-Al2O3-FeO-SiO2-H2O-CO2 suggests that the peak fluid composition of the Thongmön forsterite marble is restricted to X(CO2) < 0.04 at T > 780 °C due to a potential infiltration event of H2O-rich fluid. Alternatively, the forsterite marble is a retrograde product subordinated to the GHS exhumation process, and its fluid composition is relatively CO2-rich (0.6 < X(CO2) < 0.8 at 5 kbar, 750 °C) at a nearly isothermal decompression stage. In either case, we suggest that the carbon flux contributed by metacarbonate rocks in an orogen setting to the global carbon cycling must be considered.