ALBERT

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.

Notes: Abstract Petrogenetic grids for ultrahigh-pressure (UHP) metamorphism were calculated at different Xco2 conditions in the model system CaO-MgO-SiO2-CO2-H2O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P-T grids at Xco2= 0.01 and 0.5 are described. In the calculated P-T grid at Xco2= 0.01, four out of 10 stable invariant points, Co-En-Ms-Tc, Co-Di-En-Tc-Tr, Co-Di-Ms-Tc-Tr and Di-En-Ms-Tc-Tr lie within the stability field of coesite. If the fluid phase has Xco2= 0.5, no invariant point is stable under UHP conditions. Some magnesite-bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO2, Ms + Tr = Do + Fo + CO2+ H2O and Ms + Tc = Fo+ CO2+ H2O at Xco2= 0.01 and by reaction Ms + Tc = Fo + CO2+ H2O together with these three at Xco2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low Xco2 conditions include the following: Co-Do-Ms, Co-Di-Ms, Co-Di-Tc, Di-Ms-Tc, Di-En-Tc-, Di-En-Ms, Co-Di-En, Di-En-Fo, Di-Fo-Ms, Di-Do-Fo. Among them, talc-bearing assemblages are restricted to Xco2 〈 0.02 and their high-P limit is 31.7 kb (749°C) at Xco2= 0.01. Dolomite-magnesite-silica assemblages have large P-T stability fields even if Xco2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite-dolomite assemblage is restricted only to a calc-silicate rock interlayered with marble where Xco2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low Xco2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of Xco2 on the stability of coesite-dolomite-magnesite, diopside-enstatite-magnesite, diopside-talc assemblages is examined and the occurrence of coesite-dolomite, magnesite-bearing and talc-bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P-T grids.

Notes: Abstract Altered quartz-rich and nearly quartz-free eclogitic rocks and completely retrograde quartz-rich garnet amphibolites occur as blocks or lenses in gneisses at Weihai, northeastern tip of the Sulu ultrahigh-P belt. Eclogitic rocks with assemblage garnet ± clinopyroxene ± coesite + rutile have experienced three-stage metamorphic events including ultrahigh-pressure eclogite, granulite and amphibolite facies. Granulite metamorphic event is characterized by formation of the hypersthene + salite + plagioclase ± hornblende corona between garnet and quartz + clinopyroxene. P-T conditions for the three-stage recrystallization sequence are 840 ± 50°C, 〉28 kbar, about 760±50°C, 9 kbar, and ~650°C, 〈8 kbar respectively. Most country rock gneisses contain dominant amphibolite-facies assemblages; some garnet-bearing clinopyroxene gneisses recrystallized under granulite-facies conditions at about 740±50°C and 8.5 kbar; similar to granulite-facies retrograde metamorphism of the enclosed eclogitic blocks. Minor cale-silicate lenses within gneisses containing an assemblage grossular + salite + titanite + quartz with secondary zoisite and plagioclase may have formed within a large pressure range of 14-35 kbar. Eclogitic boudins and quartzo-feldspathic country rocks may have experienced coeval in situ UHP and subsequent retrograde metamorphism. The established nearly isothermal decompression P-T path suggests that this area may represent the interior portion of a relatively large subducted sialic block. The recognized UHP terrane may extend eastward across the Yellow Sea to the Korean Peninsula.

Notes: Abstract Minor epidote-zoisite, phengite, glaucophane, nyböite, talc, magnesite, and dolomite occur as matrix phases or as mineral inclusions in some ultrahigh-pressure (UHP) rocks from the Dabie-Sulu terrane. Some of these phases contain inclusions of coesite or coesite pseudomorphs and appear to have been in equilibrium with coesite at the time of formation. Their occurrences in the UHP rocks together with experimentally determined and calculated phase relations indicate that they are stable at mantle depths in relatively low-temperature environments. Because of the apparently dry nature of subducted continental protoliths of the Yangtze craton, small amounts of volatile components at depths exceeding 50 km along a cold subduction zone may have been stored mainly by these hydrous and carbonate phases. These minerals, in addition to some dense hydrous magnesian silicates, act as important carriers for H2O and CO2 recycled at mantle depths. Available petrological and geochemical data support limited or no fluid flow in this region. At very high pressures and low temperatures, the subducted sialic crust evidently served as a desiccating agent. Partial melting of the subducting slab, therefore, may not have occurred, and near absence of volatile expulsion from the subducting slab to the overlying mantle wedge + continental crust may have inhibited large-scale partial melting, accounting for the lack of a typical contemporaneous calc-alkaline magmatic arc.

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: 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: Representative diamond-bearing gneisses and dolomitic marble, eclogite and Ti-clinohumite-bearing garnet peridotite from Unit I at Kumdy Kol and whiteschist from Unit II at Kulet, eastern Kokchetav Massif, northern Kazakhstan, were studied. Diamond-bearing gneisses contain variable assemblages, including Grt+Bt+Qtz±Pl±Kfs±Zo±Chl±Tur±Cal and minor Ap, Rt and Zrn; abundant inclusions of diamond, graphite+chlorite (or calcite), phengite, clinopyroxene, K-feldspar, biotite, rutile, titanite, calcite and zircon occur in garnet. Diamond-bearing dolomitic marbles consist of Dol+Di±Grt+Phl; inclusions of diamond, dolomite±graphite, biotite, and clinopyroxene were identified in garnet. Whiteschists carry the assemblage Ky+Tlc+Grt+Rt; garnet shows compositional zoning, and contains abundant inclusions of talc, kyanite and rutile with minor phlogopite, chlorite, margarite and zoisite. Inclusions and zoning patterns of garnet delineate the prograde P–T  path. Inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss. Other ultrahigh-pressure (UHP) indicators include Na-bearing garnet (up to 0.14 wt% Na2O) with omphacitic Cpx in eclogite, occurrence of high-K diopside (up to 1.56 wt% K2O) and phlogopite in diamond-bearing dolomitic marble, and Cr-bearing kyanite in whiteschist. These UHP rocks exhibit at least three stages of metamorphic recrystallization. The Fe-Mg partitioning between clinopyroxene and garnet yields a peak temperature of 800–1000 °C at P 〉40 kbar for diamond-bearing rocks, and about 740–780 °C at 〉28–35 kbar for eclogite, whiteschist and Ti-bearing garnet peridotite. The formation of symplectitic plagioclase+amphibole after clinopyroxene, and replacement of garnet by biotite, amphibole, or plagioclase mark retrograde amphibolite facies recrystallization at 650–680 °C and pressure less than about 10 kbar. The exsolution of calcite from dolomite, and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c. 420 °C and 2–3 kbar. A clockwise P–T  path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to UHP subduction-zone metamorphism within the diamond stability field. Tectonic mixing may have occurred prior to UHP metamorphism at mantle depths. During subsequent exhumation and juxtaposition of many other tectonic units, intense deformation chaotically mixed and mylonitized these lithotectonic assemblages.

Notes: The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic-ultramafic blocks, and abundant Jurassic-Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high-pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene-bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near-isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase-amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite-amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km-1 at intermediate to low pressures, and may represent the Sino-Korean hangingwall during Triassic subduction for formation of the ultrahigh- and high-P units to the south. Post-collisional intrusion of a mafic-ultramafic cumulate complex occurred due to breakoff of the subducting slab.

Notes: The Xugou garnet peridotite body of the southern Sulu ultrahigh-pressure (UHP) terrane is enclosed in felsic gneiss, bounded by faults, and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite. The peridotite is composed of variable amounts of olivine (Fo91), enstatite (En92−93), garnet (Alm20−23Prp53−58Knr6−9Grs12−18), diopside and rare chromite. The ultramafic protolith has a depleted residual mantle composition, indicated by a high-Mg number, very low CaO, Al2O3 and total REE contents compared to primary mantle and other Sulu peridotites. Most garnet (Prp44−58) clinopyroxenites are foliated. Except for rare kyanite-bearing eclogitic bands, most eclogites contain a simple assemblage of garnet (Alm29−34Prp32−50Grs15−39) + omphacite (Jd24−36) + minor rutile. Clinopyroxenite and eclogite exhibit LREE-depleted and LREE-enriched patterns, respectively, but both have flat HREE patterns. Normalized La, Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high-pressure crystal accumulation (+ variable trapped melt) from melts resulting from two-stage partial melting of a mantle source.Recrystallized textures and P–T estimates of 780–870 °C, 5–7 GPa and a metamorphic age of 231 ± 11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP metamorphism in the P–T forbidden zone with an extremely low thermal gradient (〈 5 °C km−1), and multistage retrograde recrystallization during exhumation. Develop of prehnite veins in clinopyroxenite, eclogite, felsic blocks and country rock gneiss, and replacements of eclogitic minerals by prehnite, albite, white mica, and K-feldspar indicate low-temperature metasomatism.