Channelized CO2-Rich Fluid Activity along a Subduction Interface in the Paleoproterozoic Wutai Complex, North China Craton

Greenschist facies metabasite (chlorite schist) and metasediments (banded iron formation (BIF)) in the Wutai Complex, North China Craton recorded extensive fluid activities during subduction-related metamorphism. The pervasive dolomitization in the chlorite schist and significant dolomite enrichment at the BIF–chlorite schist interface support the existence of highly channelized updip transportation of CO2-rich hydrothermal fluids. Xenotime from the chlorite schist has U concentrations of 39–254 ppm and Th concentrations of 121–2367 ppm, with U/Th ratios of 0.11–0.62, which is typical of xenotime precipitated from circulating hydrothermal fluids. SHRIMP U–Th–Pb dating of xenotime determines a fluid activity age of 1.85 ± 0.07 Ga. The metasomatic dolomite has δ13CV-PDB from −4.17‰ to −3.10‰, which is significantly lower than that of carbonates from greenschists, but similar to the fluid originated from Rayleigh fractionating decarbonation at amphibolite facies metamorphism along the regional geotherm (~15 °C/km) of the Wutai Complex. The δ18OV-SMOW values of the dolomite (12.08–13.85‰) can also correspond to this process, considering the contribution of dehydration. Based on phase equilibrium modelling, we ascertained that the hydrothermal fluid was rich in CO2, alkalis, and silica, with X(CO2) in the range of 0.24–0.28. All of these constraints suggest a channelized CO2-rich fluid activity along the sediment–basite interface in a warm Paleoproterozoic subduction zone, which allowed extensive migration and sequestration of volatiles (especially carbon species) beneath the forearc.

Strength Control Factors of Chlorite Schist under Schistose Structure

In tunnel engineering, it is important to understand the influence of schistose structure on the failure strength of chlorite schist. To explore the strength control factors of chlorite schist, this paper firstly analyzes the mineral composition and meso structure of chlorite schist of different weathering states. The results show that the mineral composition of chlorite schist is changed during the weathering process, and that chlorite is an anisotropic rock mass. Next, a series of uniaxial compressive tests were conducted on chlorite schist samples with different bedding angles (the angle between bedding plane and loading direction; θ=0°, 15°, 30°, 45°, 60°, 75°, and 90°), moisture conditions (dry and saturated), and weathering states (strongly weathered and weakly weathered). Based on the test data, the authors discussed the change laws of the rock strength with bedding angle, weathering state, and moisture condition. The main results are as follows: Chlorite schist is a low-anisotropy rock mass, whose compressive strength exhibited a V-shaped trend with the growing bedding angle; the schistose structure is the internal cause of the deformation and the anisotropic or transversely isotropic strength of the schist; the schistose structure is reshaped and further damaged by external factors (e.g. water softening and weathering effects) in engineering. The research findings help to improve the rock stability and support design in tunnel engineering.

Dehydroxylation of Minerals of Gadisunkapur Area with Special Reference to FTIR and Thermal Study

This paper attempts to explain the presence of OH group for micas along with some sulfate, chlorite, quartz and feldspar minerals. These minerals are found to be associated with barite vein, metasediments (quartz-chlorite schist), metarhyolite and varieties of granitic rocks of Gadisunkapur area of Hungund-Kushtagi Schist belt. The samples were collected in and around Gadisunkapur village of Bagalkote district, Karnataka and further processed to determine OH group anion in different minerals present based on FTIR and Thermal studies such as Thermogravimetric analysis (TGA) and Differential Thermal analysis (DTA). The high temperature dehydroxylation is investigated using this analysis. Furthermore, present study indicates the presence of different extra species (CO2, CO32- and OH-). The results show the incorporation of CO2 into the structures due to heating or enhanced by the dehydroxylation process.

Dehydroxylation of Minerals of Gadisunkapur Area with Special Reference to FTIR and Thermal Study

This paper attempts to explain the presence of OH group for micas along with some sulfate, chlorite, quartz and feldspar minerals. These minerals are found to be associated with barite vein, metasediments (quartz-chlorite schist), metarhyolite and varieties of granitic rocks of Gadisunkapur area of Hungund-Kushtagi Schist belt. The samples were collected in and around Gadisunkapur village of Bagalkote district, Karnataka and further processed to determine OH group anion in different minerals present based on FTIR and Thermal studies such as Thermogravimetric analysis (TGA) and Differential Thermal analysis (DTA). The high temperature dehydroxylation is investigated using this analysis. Furthermore, present study indicates the presence of different extra species (CO2, CO32- and OH-). The results show the incorporation of CO2 into the structures due to heating or enhanced by the dehydroxylation process.

Tracing Highly Siderophile Elements through Subduction: Insights from High-pressure Serpentinites and ‘Hybrid’ Rocks from Alpine Corsica

The highly siderophile elements (HSE) include the economically critical platinum group elements (PGE; Os, Ir, Ru, Rh, Pt, Pd, Au and Re), gold and rhenium. The HSE are redox sensitive in mantle and seafloor environments and have a strong affinity to iron and sulphur, therefore their distribution within the subducted mantle lithosphere record changes to oxidation state and sulphidation. The mobility of the HSE during subduction has important implications for Re–Os isotopic signatures in the mantle, and the formation of Cu–Au arc-related ore deposits. In this study, subducted rock samples from Alpine Corsica are used to track the HSE in serpentinites and hybrid ultramafic–mafic rocks through the subduction cycle. A comparison of bulk-rock HSE concentrations with those in pre-subduction analogues provides insights into the transfer of the HSE throughout the subduction cycle. Serpentinites subducted to blueschist–eclogite-facies conditions have similar HSE concentrations to primitive upper mantle (PUM) concentrations, therefore it is concluded that the HSE are not mobilized from serpentinites on the scale of the whole-rock or greater. Therefore, as suggested in previous studies, crustal lithologies may be more important contributors of the HSE to the sub-arc mantle, particularly Pt, Pd and Re. In contrast, HSE concentrations in hybrid rocks (talc schist and chlorite schist) deviate from protolith concentrations. Rhenium is higher in the talc schist, and Ir and Ru are lower in the chlorite schist than in the PUM, or possible mafic protoliths. Mineral parageneses place temporal constraints on the growth of hosts to the HSE (sulphides, oxides and metal alloys), and record changes to the activities of oxygen and sulphur (aO2–aS2), and hence redox conditions, from pre-subduction to exhumation. Laser ablation inductively coupled plasma mass spectrometry was used to determine the HSE concentrations in sulphides and oxides, and the detection of small (∼2–25 µm2) platinum group minerals utilized high-resolution SEM mapping techniques. The prograde and retrograde sulphides have lower HSE concentrations compared with sulphides from pre-subduction settings. Therefore, the redistribution of the HSE on a mineral scale from sulphides to alloys and/or other sulphides has occurred within the serpentinites, which may reflect more reducing conditions during serpentinization or subduction, consistent with the results of thermodynamic modelling. In contrast, the mineral assemblages in the hybrid rocks imply an increase in the extent of sulphidation and oxidation, and higher fluid:rock ratios during exhumation, coincident with Re enrichment in the talc schist, and a decrease in the concentrations of Ir and Ru in the chlorite schist, at length scales greater than those of the rock samples. Therefore, hybridization of lithologies at the slab–mantle interface may enhance the transfer of the HSE to the sub-arc mantle. If Re transfer from the slab to the sub-arc mantle is possible, this questions the robustness of Re–Os isotope signatures as tracers of crustal recycling.

Geochemistry and origin of zircon in chlorite schists of the Ronda peridotites (Betic Cordilleras, southern Spain)

This work deals with scarce chlorite schists scattered through the Ronda peridotites (Betic Cordilleras, Spain). These schists have unusually high zircon contents, which contrast with the usual lack of this mineral in ultramafic rocks. From field data and detailed petrographic, geochemical, and geothermometric studies, we focused on the origin of the zircon, a relevant issue for the interpretation of geochronological results. The chlorite schists appear as concordant sheets with granite dikes and as blackwall zones between dikes and serpentinized peridotites. As the intrusion age of the dikes and chlorite schist zircon crystallization (ca. 22 Ma) is slightly older than the age of serpentinization and related chlorite schist formation (ca. 19 Ma), we propose that the chlorite schists are tied to the intrusion of the granite dikes and the subsequent serpentinization of peridotites. Trace and rare earth elements alone are not indicative of the magmatic or hydrothermal origin of the zircon, but the combination of information about zircon morphology, melt inclusions, geothermometry, and the structural relationships between granite dikes and chlorite schists points to late magmatic melts for the zircon origin. We suggest that high-temperature melts saturated in F and Cl acted as Zr carriers under low-pH conditions. A change of the pH conditions, due to hydrothermal alkaline fluids incoming for the concomitant peridotite serpentinization, would have led to zircon crystallization and concentration at the apical zones of the dikes, and to rodingitization before the extensive observed chloritization.

PETROGRAFIE A MINERALOGIE KRUPNÍKOVÉHO TĚLESA NA LOKALITĚ ZADNÍ HUTISKO U VERNÍŘOVIC (HRUBÝ JESENÍK)

The paper deals with mineralogy and petrology of a soapstone body at the locality Zadní Hutisko near Vernířovice in the Hrubý Jeseník Mts. (NE part of the Bohemian Massif). The soapstone body is located in the northern spur of the Sobotín Massif composed dominately of amphibolites. The soapstone body shows a very distinct symmetrical internal zonation. Four main types of rocks can be distinguished in the direction from the centre to the edge of the body: i) soapstone (talc + dolomite); ii) talc schist; iii) tremolite schist; iv) chlorite schist. Common accessory minerals of the rocks are apatite and magnetite. Formation of the studied soapstone body may have been caused by metasomatic alteration of peridotite (serpentinite). Epidote-chlorite granofels with a variable content of amphibole (magnesiohornblende or tschermakite) and with a high content of opaque ore minerals (magnetite, hematite, and also ilmenite) was found at the contact of the soapstone body with amphibolite. Epidote-chlorite granofels has been formed by alteration of amphibolite.

Mineralogie krupníkového tělesa v Medvědím dole u Koutů nad Desnou v Jeseníkách

Soapstone bodies are relatively common in the Sobotín Massif in the Hrubý Jeseník Mts. (NE part of the Bohemian Massif). There was a small soapstone body found near Kouty nad Desnou in the northern spur of the Sobotín Massif composed dominately of amphibolites. The article focuses on mineralogy and petrology of this body and rocks in its surroundings (mainly hornblendite). Studied soapstone body is composed of these four types of rocks: i) talc schists with variable content of chlorite (clinochlore) and amphibole (actinolite, magnesiohornblende); ii) soapstone (mineral association talc + dolomite); iii) chlorite schist; iv) actinolite schist. Chromite is a typical accessory mineral of soapstone body rocks as well as hornblendite. This soapstone body does not show mineralogical and petrographical zonality typical for well-known soapstone bodies of the Sobotín Massif – former soapstone deposits Smrčina and Zadní Hutisko. However there is no mineralogical or petrographical difference between those deposits and studied soapstone body. Formation of the soapstone body at Kouty nad Desnou is probably connected to hydrothermal alteration of hornblendite running along shear zones.

POLARIMETRIC SIGNATURES IDENTIFICATION FOR DIFFERENT FEATURES IN RADARSAT-2 POLSAR IMAGE: A CASE STUDY OF HALAYIB AREA, EGYPT

In fully polarized SAR (PolSAR) data the returned signal from a target contains all polarizations. More information about this target may be inferred with respect to single-polarization. Distinct polarization separates targets due to its different backscattering responses. A Radarsat-2 PolSAR image acquired on December 2013 of part of Halayib area (Egypt) was used in this study. Polarimetric signatures for various features (Wadi deposits, Tonalite, Chlorite schist, and Radar penetrated areas) were derived and identified. Their Co-polarized and Cross-polarized signatures were generated, based on the calculation of the backscattered power at various ellipticity and orientation angles. Graphical 3D-representation of these features was provided and more details of their physical information are depicted according to their different polarization bases. The results illustrate that polarimetric signatures, obtained due to factors like surface roughness, dielectric constant and feature orientation, can be an effective representation for analyzing various features. The shape of the signature is significant and can also indicate the scattering mechanisms dominating the features response.