Mineral and fluid inclusions in zircon of UHP metamorphic rocks from the CCSD-main drill hole: A record of metamorphism and fluid activity

The Chinese Continental Scientific Drilling (CCSD) main drill hole (0–3000 m) in Donghai, southern Sulu orogen, consists of eclogite, paragneiss, orthogneiss, schist and garnet peridotite. Detailed investigations of Raman, cathodoluminescence, and microprobe analyses show that zircons from most eclogites, gneisses and schists have oscillatory zoned magmatic cores with low-pressure mineral inclusions of Qtz, Pl, Kf and Ap, and a metamorphic rim with relatively uniform luminescence and eclogite-facies mineral inclusions of Grt, Omp, Phn, Coe and Rt. The chemical compositions of the UHP metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks. Similar UHP metamorphic P–T conditions of about 770 °C and 32 kbar were estimated from coexisting minerals in zircon and in the matrix. These observations suggest that all investigated lithologies experienced a joint in situ UHP metamorphism during continental deep subduction. In rare cases, magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence, implying that the cores have been largely altered possibly by fluid–mineral interaction during UHP metamorphism.

Abundant H₂O–CO₂, H₂O- or CO₂-dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons, coexisting with low-P mineral inclusions. These fluid inclusions should have been trapped during magmatic crystallization and thus as primary. Only few H₂O- and/or CO₂-dominated fluid inclusions were found to occur together with UHP mineral inclusions in zircons of metamorphic origin, indicating that UHP metamorphism occurred under relatively dry conditions. The diversity in fluid inclusion populations in UHP rocks from different depths suggests a closed fluid system, without large-scale fluid migration during subduction and exhumation.     

The effect of composition and cation ordering on the compressibility of columbites up to 7 GPa

The unit-cell parameters of two columbite samples along the (Fe,Mn)Nb₂O₆ solid solution were measured by means of high-pressure single-crystal X-ray diffraction up to pressures of 7 GPa. The compressional behaviour of these minerals was studied as a function of composition and degree of order. The P–V data of all the samples were fitted with a third-order Birch–Murnaghan equation of state. For the two samples with different compositions but identical degree of order the substitution of Mn for Fe causes a decrease of the bulk modulus K _T0, from 153(1) to 146(1) GPa, without any effect on the pressure first derivative K′. For the two samples with the same composition, cation ordering causes an increase of the bulk modulus from 149(1) to 153(1) GPa and of the pressure first derivative from 4.1(2) to 4.8(3). The compressional behaviour is anisotropic with a linear axial compressibility scheme β _b > β _c ≥ β _a for all samples, regardless of composition and degree of order. Such anisotropy increases sligthly with increasing Mn content.     

Interpretation of subhorizontal crustal reflections by metamorphic and rheologic effects in the eastern part of the Pannonian Basin

The geologic origin of subhorizontal reflections, often observed in crustal seismic sections, was investigated by establishing metamorphic facies and strength of rocks in depth, and correlating these properties to seismic reflection sections from eastern Hungary. Estimation of the depths of metamorphic mineral stability zones utilized the principles developed by Fyfe et al. and known geothermal data of the area. The strength versus depth profile was derived by relating local seismic P -wave interval velocities to Meissner et al. 's activation energy. The results show that the series of subhorizontal reflections, observed in the Pannonian Basin, are a consequence of combined metamorphic and rheologic changes in depths. The synthesis of the integrated data set suggests that the retrograde alteration of the pre-Tertiary basement above the percolation threshold was made possible by the softening effect of shear zones and their water-conducting capacity. The subhorizontal reflections of highest energy, of the consolidated crust below the percolation threshold, originate in the depths of greenschist, amphibolite and granulite metamorphic mineral facies, which were formed in geothermal and pressure conditions similar to those existing today. These results imply the overprint of earlier (Variscan) metamorphic sequences of the crust by more recent retrograde metamorphic processes.     

Decarbonation reaction of magnesite in subducting slabs at the lower mantle

High-pressure and temperature experiments (28–62 GPa, and 1,490–2,000 K, corresponding to approximately 770–1,500 km depth in the mantle) have been conducted on a MgCO₃ + SiO₂ mixture using a laser-heated diamond anvil cell combined with analytical transmission electron microscope observation of the product phases to constrain the fate of carbonates carried on the subducting basalt into the lower mantle. At these conditions, the decarbonation reaction MgCO₃ (magnesite) + SiO₂ (stishovite) → MgSiO₃ (perovskite) + CO₂ (solid) has been recognized. This indicates that above reaction takes place as a candidate for decarbonation of the carbonated subducting mid ocean ridge basalts in the Earth’s lower mantle.     

Calculated Phase Relations in the System Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O with Applications to UHP Eclogites and Whiteschists

Pressure–temperature grids in the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O and its subsystems have been calculatedin the range 15–45 kbar and 550–900°C, usingan internally consistent thermodynamic dataset and new thermodynamicmodels for amphibole, white mica, and clinopyroxene, with thesoftware THERMOCALC. Minerals considered for the grids includegarnet, omphacite, diopside, jadeite, hornblende, actinolite,glaucophane, zoisite, lawsonite, kyanite, coesite, quartz, talc,muscovite, paragonite, biotite, chlorite, and plagioclase. Compatibilitydiagrams are used to illustrate the phase relationships in thegrids. Coesite-bearing eclogites and a whiteschist from Chinaare used to demonstrate the ability of pseudosections to modelphase relationships in natural ultrahigh-pressure metamorphicrocks. Under water-saturated conditions, chlorite-bearing assemblagesin Mg- and Al-rich eclogites are stable at lower temperaturesthan in Fe-rich eclogites. The relative temperature stabilityof the three amphiboles is hornblende > actinolite > glaucophane(amphibole names used sensu lato). Talc-bearing assemblagesare stable only at low temperature and high pressure in Mg-and Al-rich eclogites. For most eclogite compositions, talccoexists with lawsonite, but not zoisite, in the stability fieldof coesite. Water content contouring of pressure–temperaturepseudosections, along with appropriate geotherms, provides newconstraints concerning dehydration of such rocks in subductingslabs. Chlorite and lawsonite are two important H₂O-carriersin subducting slabs. Depending on bulk composition and pressure–temperaturepath, amphibole may or may not be a major H₂O-carrier to depth.In most cases, dehydration to make ultrahigh-pressure eclogitestakes place gradually, with H₂O content controlled by divariantor higher variance assemblages. Therefore, fluid fluxes in subductionzones are likely to be continuous, with the rate of dehydrationchanging with changing pressure and temperature. Further, eclogitesof different bulk compositions dehydrate differently. Dehydrationof Fe-rich eclogite is nearly complete at relatively shallowdepth, whereas Mg- and Al-rich eclogites dehydrate continuouslydown to greater depth. KEY WORDS: dehydration; eclogites; phase relations; THERMOCALC; UHP metamorphism; whiteschists     

The Physico-Chemical Properties of a Subducted Slab from Garnet Zonation Patterns (Sesia Zone, Western Alps)

Garnets in continentally derived high-pressure (HP) rocks ofthe Sesia Zone (Western Alps) exhibit three different chemicalzonation patterns, depending on sample locality. Comparisonof observed garnet zonation patterns with thermodynamicallymodelled patterns shows that the different patterns are causedby differences in the water content of the subducted protolithsduring prograde metamorphism. Zonation patterns of garnets inwater-saturated host rocks show typical prograde chemical zonationswith steadily increasing pyrope content and increasing XMg,together with bell-shaped spessartine patterns. In contrast,garnets in water-undersaturated rocks have more complex zonationpatterns with a characteristic decrease in pyrope and XMg betweencore and inner rim. In some cases, garnets show an abrupt compositionalchange in core-to-rim profiles, possibly due to water-undersaturationprior to HP metamorphism. Garnets from both water-saturatedand water-undersaturated rocks show signs of intervening growthinterruptions and core resorption. This growth interruptionresults from bulk-rock depletion caused by fractional garnetcrystallization. The water content during burial influences significantly thephysical properties of the subducted rocks. Due to enhancedgarnet crystallization, water-undersaturated rocks, i.e. thoselacking a free fluid phase, become denser than their water-saturatedequivalents, facilitating the subduction of continental material.Although water-bearing phases such as phengite and epidote arestable up to eclogite-facies conditions in these rocks, dehydrationreactions during subduction are lacking in water-undersaturatedrocks up to the transition to the eclogite facies, due to thethermodynamic stability of such hydrous phases at high P–Tconditions. Our calculations show that garnet zonation patternsstrongly depend on the mineral parageneses stable during garnetgrowth and that certain co-genetic mineral assemblages causedistinct garnet zonation patterns. This observation enablesinterpretation of complex garnet growth zonation patterns interms of garnet-forming reactions and water content during HPmetamorphism, as well determination of detailed P–T paths. KEY WORDS: dehydration; high-pressure metamorphism; Sesia Zone; subduction; thermodynamic modelling     

Ca-rich Garnet-Clinopyroxene Rocks at Hujialin in the Su-Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates?

Layers of Ca-rich garnet–clinopyroxene rocks enclosedin a serpentinite body at Hujialin, in the Su–Lu terraneof eastern China, preserve igneous textures, relict spinel ingarnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite,Fe-rich spinel, and also amphibole in clinopyroxene. In termsof their major and trace element compositions, the studied samplesform a trend from arc cumulates towards Fe–Ti gabbros.Reconstructed augite compositions plot on the trend for clinopyroxenein arc cumulates. These data suggest that the rocks crystallizedfrom mantle-derived magmas differentiated to various extentsbeneath an arc. The Ca-rich garnet + diopside assemblage isinferred to have formed by compressing Ca-rich augite, whereasthe relatively Mg-rich cores of garnet porphyroblasts may haveformed at the expense of spinel. The protolith cumulates weresubducted from near the crust–mantle boundary (c. 1 GPa)deep into the upper mantle (4·8 ± 0·6 GPaand 750 ± 50°C). Negatively sloped P–T pathsfor the garnet–clinopyroxene rocks and the corollary ofcorner flow induced subduction of mantle wedge peridotite arenot supported by the available data. Cooling with, or without,decompression of the cumulates after the igneous stage probablyoccurred prior to deep subduction. KEY WORDS: arc cumulates; Ca-rich garnet; garnet–clinopyroxene rocks; Su–Lu terrane; UHP metamorphism     

中国大陆科学钻探工程主孔榴辉岩的岩石学研究

中国大陆科学钻探工程5000m主孔位于苏鲁超高压变质带南部。该钻孔0～2000m主要由榴辉岩、片麻岩、石榴石橄榄岩和少量片岩和石英岩组成。累积厚度达1000多米的榴辉岩具有不同的矿物组成、不同的矿物含量和不同的全岩化学成分,可划分成富Si的石英榴辉岩、富Ti的金红石榴辉岩、富Al的多硅白云母和蓝晶石榴辉岩、富Mg的双矿物榴辉岩和具有正常玄武岩成分的普通榴辉岩。榴辉岩的原岩包括基性层状侵入岩和变质表壳岩。榴辉岩全岩成分对石榴石和绿辉石中某些化学组分的含量有明显控制,而且直接影响到变质条件估算的准确性。扩散成分环带的广泛发育表明超高压矿物在早期退变质过程中发生了成分再平衡。这一事实以及具有成分生长环带石榴石变斑晶的存在,为榴辉岩形成在更高温度(>940℃)和更高压力(>4.5GPa)条件下提供了有力的证据。     

赣北中元古界双桥山群低变质强变形行为与格林威尔期造山作用

广泛分布于赣北地区的中元古界双桥山群，以低级区域变质作用、强烈构造变形、具陆内造山带构造变形特征；其成果系在华夏古陆与扬子古陆的碰撞拼接过程中，其应力向陆内传递的结果；时间大致在1100－800Ma，与全球格林威尔期造山作用相当。