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Petrogenesis of dolomitic marbles from Rongcheng in the Su‐Lu ultrahigh‐pressure metamorphic terrane, eastern China
Author(s) -
Ogasawara Y.,
Zhang R. Y.,
Liou J. G.
Publication year - 1998
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1046/j.1440-1738.1998.00177.x
Subject(s) - metamorphic rock , gneiss , geology , geochemistry , dolomite , hornblende , eclogite , terrane , biotite , mineral , mineralogy , materials science , quartz , metallurgy , subduction , paleontology , tectonics
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 TiO 2 , but those in (vi) are < 3 wt% and are regarded as later‐stage replacement products from olivine. Assemblages indicate uneven distribution of X CO 2 within an individual sample. Schreinemakers' analysis of assemblage Arg (Cal) + Dol + Chu + Di + Fo in the system CaO–MgO–SiO 2 –CO 2 –H 2 O with thermodynamic calculations indicates that Fo‐ and Chu‐bearing assemblages are stable at very low X CO 2 conditions, whereas Arg (Cal) + Dol + Di is stable at relatively higher X CO 2 conditions. When X CO 2 conditions are extremely low, Fo‐bearing and Chu‐bearing assemblages are stable at T > 800 °C and P > 2.2 GPa (X CO 2 = 0.01), and T > 730 °C and P > 2 GPa (X CO 2 = 0.005). If such extremely low X CO 2 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 X M g CO 3 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 CaCO 3 –CaMg(CO 3 ) 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 < X M g CO 3 at the eutectoid point during the decompressional stage: (i) Arg + Dol; (ii) Arg + Dol → Mg–Cal (eutectoid composition); (iii) Arg + Mg–Cal (X M g CO 3 < eutectoid composition); (iv) Mg–Cal; (v) Mg–Cal (low‐X M g CO 3 ) + exsolved Dol.