ALBERT

Notes: The Maowu garnet-bearing ultramafic body (∼ 250 × 〉 50 m2) in the Dabie ultrahigh-pressure (UHP) terrane has several distinct petrologic characteristics: (i) most rocks are layers of garnet-bearing ultramafics including orthopyroxenite and clinopyroxenite with minor harzburgite and omphacite-rich layers; these compositional layers range in thickness from 5 cm to 1.6 m; (ii) rutile is ubiquitous and is most abundant in clinopyroxenite (up to 1–2 vol%); (iii) monazite is common as inclusions in silicates and as a matrix phase; (iv) exsolved plates of magnetite occur in olivine (Fo93), and monazite in apatite; (v) chromite occurs as fine-grained inclusions in enstatite and clinohumite; (vi) hydrous phases including talc, clinochlore and amphibole are common as inclusions in coarse-grained garnet; and (vii) major silicates are high in Mg/(Mg + Fe) values. Most of the ultramafic rocks are high in rare earth elements (REE), P, Cr and TiO2, and are significantly different from Mg–Cr or Fe–Ti garnet peridotites which are common in the western Alps, Western Gneiss Region, and the Bohemian Massif. Well-foliated orthopyroxenite is composed of Grt (Prp60–74) + En (En92–96; 0.05–0.14 wt% Al2O3) + Chl (2–3 wt% Cr2O3) ± clinohumite ± magnesite ± Di (Di94–97) + chromite. Omphacitic rocks contain mainly elongated omphacite (〉 90 vol%) with rare garnet (Prp45Alm41Gr13), rutile, apatite, and monazite. Omphacites (Jd63–67Ac6–12Aug20–31) display pronounced compositional zoning and contain inclusions of coesite relics and quartz pseudomorphs after coesite. Minor retrograde phases include talc/tremolite after Px, Chl after Grt, Cpx (Jd49Ac13Aug38) and Ab after omphacite. Garnet-bearing ultramafic and omphacitic rocks have been subjected to UHP metamorphism at pressures of ∼ 35–50 kbar and 750 ± 50 °C under extremely low XCO2 conditions (〈 0.001); a minor amphibolite facies overprint took place at P 〈 15 kbar and 650 °C. The protoliths may be Proterozoic ultramafic crustal cumulates that were subjected to metasomatism prior to Triassic subduction to mantle depths of more than 100 km during the collision of the Sino-Korean and Yangtze Cratons.

Notes: 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.

Notes: 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.