Petrogenesis of dolomitic marbles from Rongcheng in the Su‐Lu ultrahigh‐pressure metamorphic terrane, eastern China

Abstract

Marbles are interbedded with biotite–hornblende gneiss in the Rongcheng area, Su‐Lu ultrahigh‐pressure (UHP) metamorphic terrane, eastern China. Both marble and gneiss include UHP eclogite layers and boudins. Seven dolomitic marbles were selected for petrologic investigation. Dolomitic marbles have assemblages of major constituent minerals: (i) Mg–Cal + Dol + Ol; (ii) Mg–Cal + Dol + Di + Ol; (iii) Mg–Cal + Dol + Di; (iv) Mg–Cal + Dol + Ti–Chu; (v) Mg–Cal + Ti–Chu + Di; and (vi) Mg–Cal + Dol + Ti–Chu + Di + Ol. Titanium–clinohumites of (iv) and (v) contain 3∼4 wt% in TiO2, but those in (vi) are < 3 wt% and are regarded as later‐stage replacement products from olivine. Assemblages indicate uneven distribution of XCO2 within an individual sample. Schreinemakers' analysis of assemblage Arg (Cal) + Dol + Chu + Di + Fo in the system CaO–MgO–SiO2–CO2–H2O with thermodynamic calculations indicates that Fo‐ and Chu‐bearing assemblages are stable at very low XCO2 conditions, whereas Arg (Cal) + Dol + Di is stable at relatively higher XCO2 conditions. When XCO2 conditions are extremely low, Fo‐bearing and Chu‐bearing assemblages are stable at T > 800 °C and P > 2.2 GPa (XCO2 = 0.01), and T > 730 °C and P > 2 GPa (XCO2 = 0.005). If such extremely low XCO2 is possible, the assemblages (i–v) could be regarded as products at UHP conditions excepting Mg–calcite transformed from aragonite + dolomite. Calcite–dolomite intergrowths are common in these assemblages. Mg–calcite inclusions were found in Ti–clinohumite of assemblage (iv) with well developed radial fractures. The maximum XMgCO3 value of 0.111 yields a solvus temperature of 698 °C. If this Mg–calcite formed from aragonite + dolomite, solvus thermometry gives a minimum P–T point along the equilibrium curve of the reaction aragonite + dolomite = Mg–calcite during the decompressional stage; T = 680 °C and P = 1.9 GPa. Along both the prograde and retrograde P–T paths for UHP dolomitic marbles, changes in model proportion among carbonate phases including aragonite, dolomite and Mg–calcite were proposed on the basis of the phase relations in CaCO3–CaMg(CO3)2. Aragonite + dolomite is a stable assemblage during the prograde stage. Appearance of the eutectoid point between aragonite and dolomite in the phase diagram plays an important role for phase changes in this system. After peak‐P, the following phase changes are predicted for the bulk compositions < XMgCO3 at the eutectoid point during the decompressional stage: (i) Arg + Dol; (ii) Arg + Dol → Mg–Cal (eutectoid composition); (iii) Arg + Mg–Cal (XMgCO3 < eutectoid composition); (iv) Mg–Cal; (v) Mg–Cal (low‐XMgCO3) + exsolved Dol.

The calcite → aragonite transformation in low‐Mg marble: Equilibrium relations, transformation mechanisms, and rates

“…Eutectoid decomposition textures, similar to those produced in our experiments, have been reported from low‐pressure/high‐temperature marbles metamorphosed at ∼480°C in the Hida Belt of Japan [ Imai et al , 1980] and from ultrahigh‐pressure marbles metamorphosed at ∼680°C in the Sulu Belt of China [ Ogasawara et al , 1998]. At both localities, during cooling, high‐Mg calcite (3–4 wt % MgO) decomposed into dolomite + low‐Mg calcite.…”

Diamond‐bearing and diamond‐free metacarbonate rocks from Kumdy–Kol in the Kokchetav Massif, northern Kazakhstan

“…Moreover dolomite‐rich marbles do not necessarily contain high MgCO 3 in calcite. These observations suggest the difficulty of Mg diffusion into Mg–calcite from well‐recrystallized dolomite through their grain boundaries ( Ogasawara et al 1998 ). Compositional variation of Mg–calcite from grain to grain and compositional heterogeneity in a grain scale may be explained by the possible source of Mg for Mg–calcite from fine‐grained dolomite inclusions in aragonite (or calcite).…”

“…For UHP dolomite and/or dolomitic marble, the following three processes should be considered during prograde and retrograde metamorphism: (i) aragonite recrystallization; (ii) dolomite recrystallization; and (iii) retrograde Mg–calcite formation from aragonite + dolomite ( Ogasawara et al 1998 ). When Mg–calcite began to form, both aragonite and/or dolomite in matrix were present as large crystals.…”

Metamorphic zoning and geodynamic evolution of an inverted crustal section (Karakorum margin, N Pakistan), evidence for two metamorphic events

“…However, in sample L6, Mg-calcite grains are found to have heterogeneous compositions, which could be explained by incomplete diffusion of Mg in a rapidly growing calcite at the expense of small dolomite inclusions (Ogasawara et al 1998). The observed compositional pattern is that of lamellae with Mg-rich calcite (Mg=1.8-5.8 wt%) within a Mg-poor calcite crystal (Mg=0.4-0.6 wt%) (Fig.…”