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1.

Recently, garnet pyroxenite enclaves within peridotites occurring near Raobazhai, Huoshan County, have been discovered. The garnet pyroxenite is small pods, decimeters in size, enclosed within intensively serpentinized peridotites. Major mineral components comprise: garnet (Prp25–35), sodium augite (Jd10–25) with a small amount of ilmenite. There are two stages of retrometamorphism: the retrogressive granulite facies mineral assemblage is superimposed by that of amphibolite facies. The host rocks of the garnet pyroxenite are spinel peridotites, including spinel harzburgite and lherzolite. Due to intensive serpentinitization, only 5%–40% of the relic olivine (Fo92–93) are preserved. The orthopyroxenes are Mg-rich (En87–93) with bending of cleavages and granulation at their margins showing intracrystalline plasticity. On the basis of garnet-clinopyroxene Fe−Mg exchange equilibrium geothermometry proposed by Ellis & Green (1979) and Krogh (1988)K D=4.06–5.28;T=793–919°C,P=1.5 GPa are estimated for the garnet pyroxenite. It is inferred that the peridotites are mantle rocks about 60 km in depth. During the exhumation of the orogenic belt, it was tectonically emplaced into the lower crust in the solid state and then uplifted to the shallow depth. Obviously, this kind of garnet pyroxenite must be petrogenetically related to its host rock. The REE distribution pattern and the Ni−Co−Sc diagram reveal that they are chemically equivalent to the basaltic melt and ultramafic residua respectively derived from partial melting of mantle rocks.

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2.
A new model is proposed for the structure of the Kaapvaal craton lithosphere. Based on chemical thermodynamics methods, profiles of the chemical composition, temperature, density, and S wave velocities are constructed for depths of 100–300 km. A solid-state zone of lower velocities is discovered on the S velocity profile in the depth interval 150–260 km. The temperature profiles are obtained from absolute values of P and S velocities, taking into account phase transformations, anharmonicity, and anelastic effects. The examination of the sensitivity of seismic models to the chemical composition showed that relatively small variations in the composition of South African xenoliths result in lateral temperature variations of ～200°C. Inversion of some seismic profiles (including IASP91) with a fixed bulk composition of garnet peridotites (the primitive mantle material) leads to a temperature inversion at depths of 200–250 km, which is physically meaningless. It is supposed that the temperature inversion can be removed by gradual fertilization of the mantle with depth. In this case, the craton lithosphere should be stratified in chemical composition. The depleted lithosphere composed by garnet peridotites exists to depths of 175–200 km. The lithospheric material at depths of 200–250 km is enriched in basaltoid components (FeO, Al2O3, and CaO) as compared with the material of garnet peridotites but is depleted in the same components as compared with the fertile substance of the underlying primitive mantle. The material composing the craton root at a depth of ～275 km does not differ in its physical and chemical characteristics from the composition of the normal mantle, and this allows one to estimate the thickness of the lithosphere at 275 km. The results of this work are compared with data of seismology, thermal investigations, and thermobarometry.  相似文献   

3.
In the Tarim Basin, black shale series at the bottom of Cambrian is one of the important marine facies hydrocarbon source rocks. This research focuses on the analysis of the isotope of noble gas of 11 cherts. The R/R a ratio of chert in the Keping area is 0.032–0.319, and 40Ar/36Ar is 338–430. In Quruqtagh the R/R a ratio is 0.44–10.21, and 40Ar/36Ar is 360–765. The R/R a ratio of chert increases with 40Ar/36Ar from the west to the east accordingly. They have evolved from the crust source area to the mantle source area in a direct proportion. Surplus argon 40ArE in chert is in direct proportion to the R/R a ratio, indicating that it has the same origin of excess argon as in fluid and mantle source helium. Comparison of the R/R a ratios between the west and the east shows that the chert in the eastern part formed from the activity system of the bottom hydrothermal venting driven by the mantle source, where the material and energy of crust and mantle had a strong interaction in exchange; whereas in the western part, chert deposited from the floating of hydrothermal plume undersea bottom, which is far away from the centre of activities of the hydrothermal fluid of ocean bottom. In addition, from noble gas isotope composition of chert, it is suggested that the ocean anoxia incident happened at the black shale of the Cambrian bottom probably because of the large-scaled ocean volcanoes and the following hydrothermal activities.  相似文献   

4.
Peridotite inclusions, crystal fragments, and kimberlite breccia at Green Knobs, New Mexico, have been studied to evaluate compositions and processes in the upper mantle below the Colorado Plateau. Most peridotite inclusions are spinel lherzolites and harzburgites, or their partly hydrated equivalents, in the Cr-diopside group. Orthopyroxene-rich websterites and olivine websterites comprise 3% of the peridotites and formed as cumulates. Typical anhydrous or slightly hydrated peridotites contain aluminous, calcic diopside (5–7% Al2O3), aluminous orthopyroxene (3–6% Al2O3), spinel, and olivine (near Fa9). Geothermometers based on different mineral pairs yield temperatures from above 1100°C to below 700°C in single rocks. High values, derived from pyroxenes with included exsolution lamellae, may approximate temperatures of primary crystallization. Low values, based on olivine-spinel and olivine-clinopyroxene pairs, approach upper mantle temperatures before eruption. In rare samples, some spinel grains are rimmed by garnet while others are not rimmed; garnet formation was controlled by nucleation kinetics. About one-third of the peridotites were deformed shortly before eruption, with effects ranging from mild cataclasis to the production of ultramylonites.Discrete crystals of garnet, olivine (near Fa8), and Cr-diopside represent garnet peridotite. Eclogites were not found. The garnet peridotite is more depleted than overlying spinel peridotite, and it is not a likely source for the minettes associated with the kimberlites.The mantle below Green Knobs consists of spinel peridotite from 45 to perhaps 60 km depth immediately underlain by more-depleted garnet peridotite. The position of the spinel-garnet transition may be fixed by kinetics. The kimberlite may have been produced when heat from ascending minette magma released volatiles from otherwise depleted garnet peridotite. Resulting gas-solid mixtures erupted along zones of deformation associated with Colorado Plateau monoclines. Sheared lherzolites formed during renewed movement along these zones.  相似文献   

5.
The homologues temperature of a crystalline material is defined as T/T_m, where T is temperature and T_m is the melting(solidus) temperature in Kelvin. It has been widely used to compare the creep strength of crystalline materials. The melting temperature of olivine system,(Mg,Fe)_2SiO_4, decreases with increasing iron content and water content, and increases with confining pressure. At high pressure, phase transition will lead to a sharp change in the melting curve of olivine. After calibrating previous melting experiments on fayalite(Fe_2SiO_4), the triple point of fayalite-Fe_2SiO_4 spinel-liquid is determined to be at 6.4 GPa and 1793 K. Using the generalized means, the solidus and liquidus of dry olivine are described as a function of iron content and pressure up to 6.4 GPa. The change of T/T_m of olivine with depth allows us to compare the strength of the upper mantle with different thermal states and olivine composition. The transition from semi-brittle to ductile deformation in the upper mantle occurs at a depth where T/T_m of olivine equals 0.5. The lithospheric mantle beneath cratons shows much smaller T/T_m of olivine than orogens and extensional basins until the lithosphere-asthenosphere boundary where T/T_m 0.66, suggesting a stronger lithosphere beneath cratons. In addition, T/T_m is used to analyze deformation experiments on olivine. The results indicate that the effect of water on fabric transitions in olivine is closely related with pressure. The hydrogen-weakening effect and its relationship with T/T_m of olivine need further investigation. Below 6.4 GPa(200 km), T/T_m of olivine controls the transition of dislocation glide from [100] slip to [001] slip. Under the strain rate of 10~(-12)–10~(-15) s~(-1) and low stress in the upper mantle, the [100](010) slip system(A-type fabric) becomes dominant when T/T_m 0.55–0.60. When T/T_m 0.55–0.60, [001] slip is easier and low T/T_m favors the operation of [001](100) slip system(C-type fabric). This is consistent with the widely observed A-type olivine fabric in naturally deformed peridotites, and the C-type olivine fabric in peridotites that experienced deep subduction in ultrahigh-pressure metamorphic terranes. However, the B-type fabric will develop under high stress and relatively low T/T_m. Therefore, the homologues temperature of olivine established a bridge to extrapolate deformation experiments to rheology of the upper mantle. Seismic anisotropy of the upper mantle beneath cratons should be simulated using a four-layer model with the relic A-type fabric in the upper lithospheric mantle, the B-type fabric in the middle layer, the newly formed A- or B-type fabric near the lithosphere-asthenosphere boundary, and the asthenosphere dominated by diffusion creep below the Lehmann discontinuity. Knowledge about transition mechanisms of olivine fabrics is critical for tracing the water distribution and mantle flow from seismic anisotropy.  相似文献   

6.
The Proterozoic anorogenic magmatic rocks are well developed in the Bayan Obo deposit region. They are composed of trachyte, magnesioarfvedesonite-feldspatite, potash-rhyolite, dacite, rhyolite, quartz porphyry and trachy basalt. A lot of high-K diabase veins (dykes) are also found. These anorogenic magmatic rocks are derived from the mantle. They have lower?Nd(t) (4.52-5.88) with T Nd DM = 1.54-1.92 Ga. Their Nd isotopic compositions and T Nd DM are consistent with those of ores, implying that the ore-forming materials were derived from these anorogenic magmatic rocks. The zircon U-Pb ages of the rocks are 1.8 Ga. Research results indicate that the Bayan Obo Group was replaced by the hydrothermal solution related to the anorogenic magmatic rocks, resulting in the formation of the deposit.  相似文献   

7.
A single garnet clinopyroxenite xenolith found at the Dish Hill basanite cone near Ludlow, California, has well developed unmixing and reaction textures like those found in garnet pyroxenite inclusions in Hawaiian, African and Australian basalts and like those of pyroxenites in some European alpine peridotites. Reconstructed pyroxene compositions suggest that before unmixing the rock consisted of clinopyroxene and about 10% garnet plus spinel, but all of the garnet may have been dissolved in clinopyroxene. Most or all of the garnet formed by exsolution from clinopyroxene and by reaction between clinopyroxene and spinel in an open system. Following exsolution, the rock was deformed and partly recrystallized in the solid state. Similarity of compositions of exsolved and recrystallized minerals suggests recrystallization at P-T conditions similar to those of exsolution.The rock is not the chemical equivalent of the host basanite and cannot represent magma of basanitic composition crystallized in the mantle. Its history of deformation and recrystallization, like that of accompanying spinel lherzolite inclusions, supports the idea that the garnet clinopyroxenite is an accidental inclusion derived from the upper mantle.  相似文献   

8.
Distribution of trace elements in spinel and garnet peridotites   总被引：6，自引：1，他引：5  
The distribution of trace elements in the upper mantle has been discussed on the basis of the trace element abundances in bulk rocks and constituent minerals of two spinel and garnet facies peridotite xenoliths in alkali basalts from eastern China. The data presented are consistent with the suggestion that highly incompatible elements (Rb, Ba, Th, U, Sr, Nb, Ta) mainly reside in intergranular components, and to a lesser extent in fluid inclusions in minerals. The LILE composition in olivine and orthopyroxene can be seriously affected by the presence of fluid inclusions. Consequently the subsolidus partitioning of the LILE cannot be used to infer the olivine-melt and orthopyroxene-melt partition coefficients for these elements. There is a significant difference in (Opx/Cpx)HREE ratios for spinel and garnet peridotites, suggesting a P-T control on equilibrium partition coefficients.  相似文献   

9.
In this study, we have modeled the density(ρ) and bulk sound velocity(VΦ) profiles of the bottom lower mantle using the experimental thermal equation of state(EoS) parameters of lower-mantle minerals, including bridgmanite, ferropericlase,CaSiO3-perovskite, and post-perovskite. We re-evaluated the literature pressure-volume-temperature relationships of these minerals using a self-consistent pressure scale in order to avoid the long-standing pressure scale problem and to provide more reliable constraints on the thermal EoS parameters. With the obtained thermal EoS parameters, we have constructed the ρ and VΦ profiles of the bottom lower mantle in different composition, mineralogy, and temperature models. Our modelling results show that the variations of chemistry, mineralogy, and temperature have different seismic signatures from each other. The Fe and Al enrichment at the bottom lower mantle can cause an increase in ρ but greatly lower VΦ. A change in mineralogy needs to be considered with the lateral variation in temperature. The cold slabs will be shown as denser regions compared to the normal mantle because of the combined effect of a lower temperature and the presence of a denser post-perovskite at a shallower depth,whereas the hot regions will have a 1–2% lower ρ than the normal mantle. VΦ of both cold slabs and hot regions will be lower than the normal mantle when bridgmanite is the dominant phase in the normal mantle, yet they will be greater once bridgmanite transforms into post-perovskite in the normal mantle. Our modeling also shows that the presence of a(Fe, Al)-enriched bridgmanite thermal pile above the core-mantle boundary will exhibit a seismic signature of enhanced ρ and VΦ, but a reduced VS,which is consistent with the observed seismic anomalies in the large-low-shear-velocity-provinces(LLSVPs). The existence of such a(Fe, Al)-enriched bridgmanite thermal pile thus can help to understand the origin of the LLSVPs. These results provide new insights for the chemical and structure of the deepest lower mantle.  相似文献   

10.
The Piranshahr metaperidotites in the northwestern end of the Zagros orogen were emplaced following the closure of the Neotethys ocean. The ophiolitic rocks were emplaced onto the passive margin of the northern edge of the Arabian plate as a result of northeastward subduction and subsequent accretion of the continental fragments. The metaperidotites have compositions ranging from low-clinopyroxene lherzolite to harzburgite and dunite. They are mantle residues with distinct geochemical signatures of both mid-ocean ridge and supra subduction zone (SSZ) affinities. The abyssal peridotites are characterized by high Al2O3 and Cr2O3 contents and low Mg-number in pyroxenes. The Cr-number in the coexisting spinel is also low. The SSZ mantle peridotites are characterized by low Al2O3 contents in pyroxenes as well as low Al2O3 and high Cr-number in spinel. Mineral chemical data indicate that the MOR- and SSZ-type peridotites are the residues from ∼15–20% and ∼30–35% of mantle melting, respectively. Considering petrography, mineralogy and textural evidence, the petrological history of the Piranshahr metaperidotites can be interpreted in three stages: mantle stable stage, serpentinization and metamorphism. The temperature conditions in the mantle are estimated using the Ca-in-orthopyroxene thermometer as 1210 ± 26 °C. The rocks have experienced serpentinization. Based on the textural observations, olivine and pyroxene transformed into lizardite and/or chrysotile with pseudomorphic textures at temperatures below 300 °C during the initial stage of serpentinization. Subsequent orogenic metamorphism affected the rocks at temperatures lower than 600 °C under lower-amphibolite facies metamorphism.  相似文献   

11.
Garnet compositions are used to understand mantle petrogenesis and to reconstruct the lithostratigraphy of the shallow mantle (<200 km). However, garnets in polymict peridotites from the Kaapvaal craton (>2500 Ma) have a centimeter-scale elemental and stable isotopic variability suggestive of a mixed mantle provenance. The chemical heterogeneity of the garnets is similar to that reported from rocks sampled over a considerable depth and temperature range within the lower lithosphere. For example garnets found in polymict peridotites are similar to garnets found in sheared and granular peridotites, ‘cold’ and ‘hot’ lherzolites, peridotitic (P-type) diamond inclusions, and garnets from polybaric (50-200 km) peridotites (i.e. spinel, garnet and diamond facies). These data indicate that the Kaapvaal cratonic root has been disturbed by complex processes possibly associated with crack propagation and entrainment that juxtaposed garnet-bearing lithologies of diverse petrogenesis, provenance and depth. This has preserved chemical disequilibrium in the high pressure minerals in what is, in effect, a mantle breccia possibly associated with kimberlite precursors.  相似文献   

12.
The VPREMOON seismic reference Moon model (Garcia et al., 2011) has been tested with respect to the thermal regime and chemical composition of the mantle. Based on a self-consistent thermodynamic approach and petrological models of the lunar mantle covering a wide range of concentrations of CaO, Al2O3, and FeO, we convert the P- and S-wave velocity profiles to the temperature–depth profiles. The solution procedure relies on the method of the Gibbs free energy minimization and the equations of state for the mantle material which take into account the effects of phase transformations, anharmonicity, and anelasticity. We find that regardless of the chemical composition, the positive P- and S-wave velocity gradient in the lunar mantle leads to a negative temperature gradient, which has no physical basis. For adequate mantle temperatures, the P- and S-wave velocities should remain almost constant or slightly decrease with depth (especially VS) as a result of the effects of the temperature, which grows faster than pressure. These findings underscore the importance of the relationship of the thermodynamics and physics of minerals with seismology.  相似文献   

13.
High-precision Nb, Ta, Zr, Hf, Sm, Nd and Lu concentration data of depleted mantle rocks from the Balmuccia peridotite complex (Ivrea Zone, Italian Alps) were determined by isotope dilution using multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) and thermal ionisation mass spectrometry (TIMS). The Zr/Hf ratios of all investigated samples from the Balmuccia peridotite complex are significantly lower than the chondritic value of 34.2, and the most depleted samples have Zr/Hf ratios as low as 10. Correlated Zr/Hf ratios and Zr abundances of the lherzolites preserve the trend of a mantle residue that has been depleted by fractional melting. This trend confirms experimental studies that predict Hf to behave more compatibly than Zr during mantle melting. Experimentally determined partition coefficients imply that the major Zr and Hf depletion most likely occurred in the spinel stability field, with (DZr/DHf)cpx≈0.5, and not in the garnet stability field, where (DZr/DHf)grt is probably close to one. However, minor amounts of melting must have also occurred in a garnet facies mantle, as indicated by low Sm/Lu ratios in the Balmuccia peridotites. The Nb/Ta ratios of most lherzolites are subchondritic and vary only from 7 to 10, with the exception of three samples that have higher Nb/Ta ratios (18–24). The overall low Nb/Ta ratios of most depleted mantle rocks confirm a higher compatibility of Ta in the mantle. The uniform Nb/Ta ratios in most samples imply that even in ‘depleted’ mantle domains the budget of the highly incompatible Nb and Ta is controlled by enrichment processes. Such a model is supported by the positive correlation of Zr/Nb with the Zr concentration. However, the overall enrichment was weak and did barely affect the moderately incompatible elements Zr and Hf. The new constraints from the partitioning behaviour of Zr–Hf and Nb–Ta provide important insights into processes that formed the Earth’s major silicate reservoirs. The correlation of Zr/Hf and Sm/Nd in depleted MORB can be assigned to previous melting events in the MORB source. However, such trends were unlikely produced during continental crust formation processes, where Sm/Nd and Zr/Hf are decoupled. The different fractionation behaviour of Zr/Hf and Sm/Nd in the depleted mantle (correlated) and the crust (decoupled) indicates that crustal growth by a simple partial melting process in the mantle has little effect on the mass budget of LREE and HFSE between crust and mantle. A more complex source composition, similar to that of modern subduction rocks, is needed to fractionate the LREE, but not Zr/Hf and the HREE.  相似文献   

14.
High-resolution P wave tomography shows that the subducting Pacific slab is stagnant in the mantle transition zone and forms a big mantle wedge beneath eastern China. The Mg isotopic investigation of large numbers of mantle-derived volcanic rocks from eastern China has revealed that carbonates carried by the subducted slab have been recycled into the upper mantle and formed carbonated peridotite overlying the mantle transition zone, which becomes the sources of various basalts. These basalts display light Mg isotopic compositions(δ26 Mg = –0.60‰ to –0.30‰) and relatively low87 Sr/86 Sr ratios(0.70314–0.70564) with ages ranging from 106 Ma to Quaternary, suggesting that their mantle source had been hybridized by recycled magnesite with minor dolomite and their initial melting occurred at 300-360 km in depth. Therefore, the carbonate metasomatism of their mantle source should have occurred at the depth larger than 360 km, which means that the subducted slab should be stagnant in the mantle transition zone forming the big mantle wedge before 106 Ma. This timing supports the rollback model of subducting slab to form the big mantle wedge. Based on high P-T experiment results, when carbonated silicate melts produced by partial melting of carbonated peridotite was raising and reached the bottom(180–120 km in depth) of cratonic lithosphere in North China, the carbonated silicate melts should have 25–18 wt% CO2 contents, with lower Si O2 and Al2 O3 contents, and higher Ca O/Al2 O3 values, similar to those of nephelinites and basanites, and have higher εNdvalues(2 to 6). The carbonatited silicate melts migrated upward and metasomatized the overlying lithospheric mantle, resulting in carbonated peridotite in the bottom of continental lithosphere beneath eastern China. As the craton lithospheric geotherm intersects the solidus of carbonated peridotite at 130 km in depth, the carbonated peridotite in the bottom of cratonic lithosphere should be partially melted, thus its physical characters are similar to the asthenosphere and it could be easily replaced by convective mantle. The newly formed carbonated silicate melts will migrate upward and metasomatize the overlying lithospheric mantle. Similarly, such metasomatism and partial melting processes repeat, and as a result the cratonic lithosphere in North China would be thinning and the carbonated silicate partial melts will be transformed to high-Si O2 alkali basalts with lower εNdvalues(to-2). As the lithospheric thinning goes on,initial melting depth of carbonated peridotite must decrease from 130 km to close 70 km, because the craton geotherm changed to approach oceanic lithosphere geotherm along with lithospheric thinning of the North China craton. Consequently, the interaction between carbonated silicate melt and cratonic lithosphere is a possible mechanism for lithosphere thinning of the North China craton during the late Cretaceous and Cenozoic. Based on the age statistics of low δ26 Mg basalts in eastern China, the lithospheric thinning processes caused by carbonated metasomatism and partial melting in eastern China are limited in a timespan from 106 to25 Ma, but increased quickly after 25 Ma. Therefore, there are two peak times for the lithospheric thinning of the North China craton: the first peak in 135-115 Ma simultaneously with the cratonic destruction, and the second peak caused by interaction between carbonated silicate melt and lithosphere mainly after 25 Ma. The later decreased the lithospheric thickness to about70 km in the eastern part of North China craton.  相似文献   

15.
Adakitic rocks in continental settings are commonly considered to be formed by partial melting of thickened or delaminated lower crust. Investigations on this kind of rocks can provide important information about crustal evolution complementary to information from other rocks. This paper reports adakitic granodiorite of the Lingxi pluton in the interior of the Cathayisa Block. LA-ICP-MS zircon U-Pb dating shows that it was formed in the late Early Cretaceous(100±1 Ma). The granodiorite has geochemical features of adakitic rocks derived from partial melting of the thickened lower crust, e.g., high SiO2(mainly ranging from 64.4 to 68.9 wt.%) and Sr(624–894 ppm) contents, Sr/Y(49.9–60.8) and La/Yb(23.4–42.8) values, low Y(10.3–17.1 ppm), Ni(5.62–11.8 ppm) and MgO(mostly from 0.86 wt.% to 1.57 wt.%) contents and weak Eu anomaly. It has initial 87Sr/86 Sr ratios of 0.7086–0.7091, εNd(t) values of.6.2 to.5.9 and zircon εHf(t) values mostly of.10.1 to.7.6. Based on the geochemical characteristics and simple modelling, it is suggested that the most likely generation mechanism of the Lingxi granodiorite is partial melting of a thickened Proterozoic lower continental crust at a pressure ≥12 kbar(or crust thickness ≥40km), leaving a garnet-bearing amphibolite residue. Combining our results and previous studies of the tectonic evolution of the Cathaysia Block, we propose that the crust was thickened to over 40 km by a compressive event occurring during the late Early Cretaceous, which is supported by the observation that there is an angular unconformity between the Upper Cretaceous Series and the early Lower Cretaceous or the Jurassic rocks. After this event, the Cathaysia Block experienced a lithospheric extension and thinning probably driven by the high-angle paleo-Pacific subduction. With the attenuation of lithosphere, the lower crust was heated to partial melting by upwelling asthenospheric materials, resulting in generation of the Lingxi granodiorite and other coeval granitoids in the Cathaysia Block. This study provides new information on the crustal evolution of the Cathaysia Block during the Early Cretaceous.  相似文献   

16.
The Xushen gas field, located in the north of Songliao Basin, is a potential giant gas area for China in the future. Its proved reserves have exceeded 1000×108 m3 by the end of 2005. But, the origin of natural gases from the deep strata is still in debating. Epimetamorphic rocks as a potential gas source are widely spreading in the northern basement of Songliao Basin. According to pyrolysis experiments for these rocks in the semi-confined system, gas production and geochemistry of alkane gases are discussed in this paper. The Carboniferous-Permian epimetamorphic rocks were heated from 300°C to 550°C, with temperature interval of 50°C. The gas production was quantified and measured for chemical and carbon isotopic compositions. Results show that δ 13C1 is less than ?20‰, carbon isotope trend of alkane gas is δ 13C1<δ 13C2<δ 13C3 or δ 13C1<δ 13C2>δ 13C3, these features suggest that the gas would be coal-type gas at high-over maturity, not be inorganic gas with reversal trend of gaseous alkanes (δ 13C1>δ 13C2>δ 13C3). These characteristics of carbon isotopes are similar with the natural gas from the basin basement, but disagree with gas from the Xingcheng reservoir. Thus, the mixing gases from the pyrolysis gas with coal-typed gases at high-over maturity or oil-typed gases do not cause the reversal trend of carbon isotopes. The gas generation intensity for epimetamorphic rocks is 3.0×108–23.8×108 m3/km2, corresponding to R o from 2.0% to 3.5% for organic matter.  相似文献   

17.
High-pressure (HP) and ultrahigh-pressure (UHP) eclogites exposed in collisional orogens are widely regarded to record the history of crustal rocks that were subducted to mantle depths and exhumed back to the surface. Insight into subduction and exhumation processes plays an important role in understanding the nature and evolution of subduction zones, geodynamics and plate tectonics. In contrast to continental-type HP to UHP metamorphic rocks that are dominated by felsic lithology, oceanic-type HP to UHP metamorphic rocks are dominated by mafic eclogites and thus have greater density, and their exhumation needs to overcome large barriers and may involve complicated tectonic processes. The exhumation of HP to UHP rocks is mainly influenced by the internal buoyancy, however, the external tectonic forces (such as channel flow) also act as effective exhumation drivers; in addition, effects of tectonic settings (such as slab rollback and breakoff) should take into account. The HP-UHP metamorphic terrane in Southwestern Tianshan, which mainly comprises of metasediments with interlayered metamafic lenses and blocks, represents a typical accretionary mélange associated with deep subduction of oceanic crust. However, the exhumation mechanisms of these once deeply buried HP-UHP rocks are still under discussion. Based on the field occurrences, petrographic features, peak metamorphic P-T conditions and ages of the eclogites/blueschists and their metasedimentary country rocks, a “sediment-type subduction channel” model is advocated in this study to appraise/decipher the formation and evolution of Southwestern Tianshan HP-UHP metamorphic belt. Poly-cyclic metamorphic eclogites record the multistage burial-exhumation cycling manipulated by convective flow in a channel-like interface between the plates, giving robust evidence for the presence of a subduction channel. In addition, this study summarizes some remaining geotectonic problems and research perspectives concerning the Southwestern Tianshan HP-UHP metamorphic belt.  相似文献   

18.
The compressional behavior of the MgCr_2O_4 spinel has been investigated with the CASTEP code using density functional theory and planewave pseudopotential technique. We treated the exchange-correlation interaction by both the local density approximation(LDA) and generalized gradient approximation(GGA) with the Perdew-Burker-Ernzerhof functional. Our simulation was conducted for the pressure range of 0–19 GPa. We obtained the isothermal bulk modulus(K_T) of the MgCr_2O_4 spinel as 181.46(48) GPa(GGA; low boundary) or 216.1(11) GPa(LDA; high boundary), with its first derivative(K'_T) as 4.41(6) or 4.5(1), respectively. The oxygen parameter u is not constant but negatively correlated with P, and decreases by about 0.5–0.6% for the investigated P range. The component polyhedra have different compressibilities, increasing in the order of(O_4)_1CrO_6(O_4)_2O_6MgO_4. The Mg-O bond in the MgO_4 tetrahedron is much more compressible than the Cr-O bond in the CrO_6 octahedron.  相似文献   

19.
The transport of water in subduction zones   总被引：9，自引：0，他引：9  
The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low(?5?C/km), lawsonite may carry water into great depths of ?300 km. In the hot subduction zone where the geothermal gradient is high(25?C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of 80 km to form felsic melts, into which water is highly dissolved. In this case, only a minor amount of water can be transported into great depths. A number of intermediate modes are present between these two end-member dehydration modes, making subduction-zone dehydration various. Low-T/low-P hydrous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of ?60–80 km to release large amounts of water. In contrast, the low-T/low-P hydrous minerals are replaced by low-T/high-P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80–160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultrahigh-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately.  相似文献   

20.
The paper continues a cycle of petromagnetic investigations of epicontinental deposits at the Mesozoic-Cenozoic (K/T) boundary and is devoted to the study of the Gams section (Austria). Using thermomagnetic analysis, the following magnetic phases are identified: goethite (T C = 90–150°C), hemoilmenite (T C = 200?300°C), metallic nickel (T C = 350–360°C), magnetite and titanomagnetite (T C = 550–610°C), Fe-Ni alloy (T C = 640–660°C), and metallic iron (T C = 740–770°C). Their concentrations are determined from M(T). In all samples, ensembles of magnetic grains have similar coercivity spectra and are characterized by a high coercivity. An exception is the lower coercivity of the boundary clay layer due to grains of metallic nickel and iron. With rare exceptions, the studied sediments are anisotropic and generally possess a magnetic foliation, which indicates a terrigenous accumulation of magnetic minerals. Many samples of sandy-clayey rocks have an inverse magnetic fabric associated with the presence of acicular goethite. The values of paramagnetic and diamagnetic components in the deposits are calculated. According to the results obtained, the K/T boundary is marked by a sharp increase in the concentration of Fe hydroxides. The distribution of titanomagnetite reflects its dispersal during eruptive activity, which is better expressed in the Maastrichtian and at the base of the layer J. The along-section distribution of metallic iron, most likely of cosmic origin, is rather uniformly chaotic. The presence of nickel, most probably of impact origin, is a particularly local phenomenon as yet. The K/T boundary is not directly related to an impact event.  相似文献   

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