Fluid generation and evolution during exhumation of deeply subducted UHP continental crust: Petrogenesis of composite granite–quartz veins in the Sulu belt,China

Composite granite–quartz veins occur in retrogressed ultrahigh pressure (UHP) eclogite enclosed in gneiss at General's Hill in the central Sulu belt, eastern China. The granite in the veins has a high‐pressure (HP) mineral assemblage of dominantly quartz+phengite+allanite/epidote+garnet that yields pressures of 2.5–2.1 GPa (Si‐in‐phengite barometry) and temperatures of 850–780°C (Ti‐in‐zircon thermometry) at 2.5 GPa (~20°C lower at 2.1 GPa). Zircon overgrowths on inherited cores and new grains of zircon from both components of the composite veins crystallized at c. 221 Ma. This age overlaps the timing of HP retrograde recrystallization dated at 225–215 Ma from multiple localities in the Sulu belt, consistent with the HP conditions retrieved from the granite. The ε_Hf(t) values of new zircon from both components of the composite veins and the Sr–Nd isotope compositions of the granite consistently lie between values for gneiss and eclogite, whereas δ¹⁸O values of new zircon are similar in the veins and the crustal rocks. These data are consistent with zircon growth from a blended fluid generated internally within the gneiss and the eclogite, without any ingress of fluid from an external source. However, at the peak metamorphic pressure, which could have reached 7 GPa, the rocks were likely fluid absent. During initial exhumation under UHP conditions, exsolution of H₂O from nominally anhydrous minerals generated a grain boundary supercritical fluid in both gneiss and eclogite. As exhumation progressed, the volume of fluid increased allowing it to migrate by diffusing porous flow from grain boundaries into channels and drain from the dominant gneiss through the subordinate eclogite. This produced a blended fluid intermediate in its isotope composition between the two end‐members, as recorded by the composite veins. During exhumation from UHP (coesite) eclogite to HP (quartz) eclogite facies conditions, the supercritical fluid evolved by dissolution of the silicate mineral matrix, becoming increasingly solute‐rich, more ‘granitic’ and more viscous until it became trapped. As crystallization began by diffusive loss of H₂O to the host eclogite concomitant with ongoing exhumation of the crust, the trapped supercritical fluid intersected the solvus for the granite–H₂O system, allowing phase separation and formation of the composite granite–quartz veins. Subsequently, during the transition from HP eclogite to amphibolite facies conditions, minor phengite breakdown melting is recorded in both the granite and the gneiss by K‐feldspar+plagioclase+biotite aggregates located around phengite and by K‐feldspar veinlets along grain boundaries. Phase equilibria modelling of the granite indicates that this late‐stage melting records P–T conditions towards the end of the exhumation, with the subsolidus assemblage yielding 0.7–1.1 GPa at <670°C. Thus, the composite granite–quartz veins represent a rare example of a natural system recording how the fluid phase evolved during exhumation of continental crust. The successive availability of different fluid phases attending retrograde metamorphism from UHP eclogite to amphibolite facies conditions will affect the transport of trace elements through the continental crust and the role of these fluids as metasomatic agents interacting with the mantle wedge in the subduction channel.     

Petrological evidence for shock‐induced high‐P metamorphism in a gabbro

Microlites (minute spherulitic, dendritic, skeletal, acicular and poikilitic crystals) diagnostic of crystallization in quenched melt or glass in fault rocks have been used to infer fossil earthquakes. High‐P microlites and crystallites are described here in a variably eclogitized gabbro, the wallrock to the coesite‐bearing eclogite breccia at Yangkou in the Chinese Su‐Lu high‐P metamorphic belt. The studied hand specimens are free of discernible shear deformation, although microfractures are not uncommon under the microscope. In the least eclogitized gabbro, the metagabbro, stellate growths of high‐P minerals on the relict igneous minerals are common. Dendritic garnet crystals (<1?5 μm) grew around rutile and/or phengite replacing ilmenite and biotite, respectively. Skeletal garnet also rims broken flakes of igneous biotite and mechanically twinned augite. Radial intergrowths of omphacite and quartz developed around relict igneous orthopyroxene and are rimmed by skeletal or poikilitic garnet where a Ti‐bearing mineral relict is present. Acicular epidote, kyanite and phengite crystallites are randomly distributed in a matrix of Na‐rich plagioclase, forming the pseudomorphs after igneous plagioclase. In the more eclogitized gabbro, the coronitic eclogite located closer to the eclogite breccia, all the igneous minerals broke down into high‐P assemblages. Thick coronas of poikilitic garnet grew between the pseudomorphs after igneous plagioclase and ferromagnesian minerals. The igneous plagioclase is replaced by omphacite crystallites, with minor amounts of phengite and kyanite. Thermodynamic modelling of the plagioclase pseudomorphs shows an increase in P–T in the wallrock from the metagabbro to the coronitic eclogite, and the P–T variation is unrelated to H₂O content. The fluid‐poor pressure overstepping scenario is unsupported both by phase diagram modelling and by whole‐rock chemical data, which show that the various types of eclogitized gabbro are all fairly dry. A large pressure difference of >2 GPa between the metagabbro and the coesite‐bearing eclogites ~20 m apart cannot be explained by the subduction hypothesis because this would require a depth difference of >60 km. The microlites and crystallites are evidence for dynamic crystallization due to rapid cooling because constitutional supercooling was unlikely for the plagioclase pseudomorphs. The lack of annealing of the broken biotite and augite overgrown by strain free skeletal garnet is consistent with a transient high‐P–T event at a low ambient temperature (<300 °C), probably in the crust. Therefore, the eclogitization of the wallrock to the eclogite breccia was also coseismic, as proposed earlier for the eclogite facies fault rocks. The outcrop‐scale P–T variation and the transient nature of the high‐P–T event are inconsistent with the other existing tectonic models for high‐P metamorphism. The fact that the less refractory but denser biotite is largely preserved while the more refractory but less dense plagioclase broke down completely into high‐P microlite assemblages in the metagabbro indicates a significant rise in pressure rather than temperature. Given that the metamorphic temperatures are far below the melting temperatures of most of the gabbroic minerals under fluid‐absent conditions, stress‐induced amorphization appears to be the more likely mechanism of the coseismic high‐P metamorphism.     

Experimental study on the hydrological performance of green roofs in the application of novel biochar

Biochar has the potential to be a soil amendment in green roofs owing to its water retention, nutrient supply, and carbon sequestration application. The combined effects of biochar and vegetated soil on hydraulic performance (e.g., saturated hydraulic conductivity, retention and detention, and runoff delay) are the crucial factor for the application of the novel biochar in green roofs. Recent studies investigated soil water potential (i.e., suction) either on vegetated soil or on biochar-amended soil but rarely focused on their integrated application. With the purpose of investigating the hydraulic performance of green roofs in the application of biochar, the combined effect of biochar and vegetated soil on hydrological processes was explored. Artificial rainfall experiments were conducted on the four types of experimental soil columns, including natural soil, biochar-amended soil, vegetated natural soil, and vegetated biochar-amended soil. The surface ponding, bottom drainage and the volumetric water content were measured during the rainfall test. Simulation method by using HYDRUS-1D was adopted for estimating hydraulic parameters and developing modelling analysis. The results indicated that the saturated hydraulic conductivity of vegetated soil columns were higher than bare soil columns. The addition of biochar decreased the saturated hydraulic conductivity, and the magnitude of decrease was much significant in the case of vegetated soil. The influence of vegetation on permeability is more prominent than biochar. The vegetated biochar-amended soil has the highest retention and detention capacity, and shows a preferable runoff delay effect under heavy rain among the four soil columns. The results from the present study help to understand the hydrological processes in the green roof in the application of biochar, and imply that biochar can be an alternative soil amendment to improve the hydraulic performance.     

Dissolved organic matter in surface runoff in the Loess Plateau of China: The role of rainfall events and land-use

Exploring the chemical characterization of dissolved organic matter (DOM) is important for understanding the fate of laterally transported organic matter in watersheds. We hypothesized that differences in water-extractable organic matter (WEOM) in soils of varying land uses and rainfall events may significantly affect the quality and the quantity of stream DOM. To test our hypotheses, characteristics of rainfall-runoff DOM and WEOM of source materials (topsoil from different land uses and gullies, as well as typical vegetation) were investigated at two adjacent catchments in the Loess Plateau of China, using ultraviolet–visible absorbance and excitation emission matrix fluorescence with parallel factor analysis (PARAFAC). Results indicated that land-use types may significantly affect the chemical composition of soil WEOM, including its aromaticity, molecular weight, and degree of humification. The PARAFAC analysis demonstrated that the soils and stream water were dominated by terrestrial/allochthonous humic-like substances and microbial transformable humic-like fluorophores. Shifts in the fluorescence properties of stream DOM suggested a pronounced change in the relative proportion of allochthonous versus autochthonous material under different rainfall patterns and land uses. For example, high proportions of forestland could provide more allochthonous DOM input. This study highlights the relevance of soils and hydrological dynamics on the composition and fluxes of DOM issuing from watersheds. The composition of DOM in soils was influenced by land-use type. Precipitation patterns influenced the proportion of terrestrial versus microbial origins of DOM in surface runoff. Contributions of allochthonous, terrestrially derived DOM inputs were highest from forested landscapes.     

Displacement analytical prediction of shallow tunnel based on unified displacement function under slope boundary

For slope condition of ground surface, the asymmetrical deformation about the vertical center line and the horizontal center line of the tunnel cross section can be formed. A unified displacement function expressed by the Fourier series is presented to express the asymmetrical deformation of the tunnel cross section. Five basic deformation modes corresponding to the expansion order 2 are a complete deformation mode to reflect deformation behaviors of the tunnel cross section under slope boundary. Such this complete displacement mode is implemented into the complex variable solution for analytically predicting tunneling-induced ground deformation under slope boundary. All of these analytical solutions are verified by good agreements of the comparison between the analytical solutions and finite element method results. A parameter study is carried out to investigate the influence of deformation modes of the tunnel cross section, geometrical conditions of the tunnel and the slope angle, and “Buoyancy effect” on the displacement field. Finally, the proposed method is consistent with measured data of the Hejie tunnel in China qualitatively. The presented solution can provide a simplified indication for evaluating the ground deformation under slope condition of ground surface.