Abstract:Granulite terrains exposed on the North China craton are the exhumed Precambrian lower crust, whereas the granulite-facies xenoliths entrained in volcanic rocks are representatives of the Phanerozoic lower crust. The comparative studies of granulite terrains and granulite xenoliths provide important constraints on the evolution of the lower crust beneath the North China Craton. Based on data available on the granulite terrains and granulite xenoliths, this paper discusses the modification of the lower crust beneath the North China Craton during the Phanerozoic mainly from the perspectives of zircon U-Pb ages and their Hf isotopic compositions. The lower crust was also significantly modified in company with the destruction of the mantle lithosphere since the Mesozoic, with the time and degree of modification varying from region to region. Magmatic underplating played the most important role in modifying the pre-existent ancient crust, while the remelting of the ancient crust was probably another way to modify the lower crust.

Abstract:The North China Craton is the most obvious and most typical area for the study of the destruction of the craton. The timing, scale and geodynamics of decratonization are hot issues among the geoscientists both in China and abroad. Large-scale magmatism is one of the geological responses to decratonization. The ages, sources of magmas and petrogenetic processes of magmatic rocks can be used to constrain the timing, scale and possible geodynamics of decratonization. Mesozoic intrusive rocks are widely distributed in the eastern North China Craton. Based on research works conducted in the past ten years or so, the authors summarize zircon U-Pb ages of the Mesozoic intrusive rocks in eastern North China Craton, which show mainly three stages of magmatism in eastern North China Craton, i.e., Triassic, Jurassic and Early Cretaceous. Geochemical features were used to trace their magma sources and petrogenesis, and the transition of the nature and thickness of the lithosphere and the spatial scale of decratonization were further constrained. On such a basis, the possible geodynamic mechanism of decratonization of the North China Craton is discussed in combination with regional geology.

Abstract:Oxygen isotopic records from olivine crystals in Mesozoic mafic rocks and xenoliths in Shandong Province indicate an important contribution of both subducted continental crust and oceanic crust to the modification of the lithospheric mantle beneath the North China Craton (NCC). The early Cretaceous lithospheric mantle that has higher δ¹⁸O (6.0‰～7.2‰) than normal mantle was mainly metasomatized by melts/fluids derived from the subducted Yangtze crustal rocks, which included the lower mafic and upper felsic components with subordinate marine carbonates. In contrast, some low-MgO peridotite xenoliths in the late Cretaceous lithospheric mantle that has lower δ¹⁸O (4.1‰～5.3‰) than normal mantle were ascribed to the modification of subducted oceanic crust that had been hydrothermally altered prior to the subduction. The incorporation of recycling crustal rocks into the lithospheric mantle through plate subduction was an important mechanism for the modification and destruction of the lithospheric mantle beneath the NCC, e.g., the introduction of the fertile continental crustal components led to the refertilization of the lithospheric mantle that made it more fusible in response to thermal perturbation. The large-scale early Cretaceous mafic magmatism in NCC resulted probably from the breakoff or detachment of the entire subducted continental slab. The subduction of the paleo-Pacific Ocean since late Mesozoic has exerted a significant impact on the mantle replacement and accretion, resulting in the coexistence of "old" and "newly accreted" mantle domain beneath the NCC.

Abstract:An electronic backscattered diffraction (EBSD) study of the sieve-textured reaction rim between orthopyroxene and basalt in the Hannuoba mantle xenoliths was conducted in Damaping area, Hebei Province. Some conclusions have been rearched: ① The reaction rim has a complex multi-layer structure composed of a basalt layer, an olivine-rich layer and an intergrowth layer of olivine and clinopyroxene; ② The Mg^# of the olivine grains in the reaction rim increases toward the orthopyroxene grain while the Mg^# of the clinopyroxene grains in the reaction rim remains stable; ③ There is a well-defined crystallographic topotactic relationship between orthopyroxene and clinopyroxene, i.e., (100)_opx//(100)_cpx, (010)_opx//(010)_cpx, (001)_opx//(001)_cpx; ④ The crystallographic distribution of olivine grains is nearly at random and shows no relationship with orthopyroxene and clinopyroxene. This sieve-textured reaction rim can be interpreted as the result of a multistage reaction process. In the first step, the orthopyroxene reacts with the melt to form clinopyroxene which made the reaction melt more silicic. As the melt gets saturated with silica, olivine precipitates from the reaction melt. Comparing to the reaction between a silica-saturated melt and a peridotite, the reaction between a silica-understaturated melt and a peridotite is probably easier to proceed due to the formation of sieve-textured reaction rims. The sieve-textured reaction rims provide a feasible channel for the migration of the melt along otherwise closed grain boundaries at high pressure, weaken the surrounding peridotite through the reduction of mineral grain size, and maintain a continuous erosion and destruction of the peridotite during the upward migration of the melt. This process may lead to a quick rejuvenation of lithospheric mantle.

Abstract:Mantle peridotite xenoliths entrained in the Cenozoic alkaline basalts from Kuandian of eastern North China Craton provide important constraints on the nature and evolution of the lithospheric mantle beneath this area. Two types of xenoliths can be defined based on the Mg^# of their olivines: Type 1 low-Mg^# lherzolites are characterized by low Mg^# in olivines (89.8～90.3), high TiO₂ (0.38%～0.57%), Al₂O₃ (4.41%～6.87%), FeO^T (2.46%～3.73%), MnO (0.08%～0.11%) content and low Cr^# (7.42～14.2) in the coexisting clinopyroxenes, and have undergone low degrees of partial melting; these characteristics are similar to those of low-Mg^# peridotites in the Late Cretaceous and Cenozoic basalts, thus probably representing newly accreted lithospheric mantle. Type 2 high-Mg^# harzburgites are characterized by high Mg^# in olivines (91.0～92.3), low TiO₂ (0.03%～0.33%), Al₂O₃ (2.27%～5.49%), FeO^T (2.04%～2.40%), MnO (0.07%～0.08%) content and high Cr^# (15.3～25.8) in the coexisting clinopyroxenes and have undergone high degrees of partial melting; these refractory geochemical characteristics are completely different from those of low-Mg^# lherzolites, but similar to those of high-Mg^# peridotite xenoliths from Archean and Proterozoic lithospheric mantle in the craton, therefore are considered to be relicts of ancient lithospheric mantle. The nonexistence of obvious differences in equilibrium temperatures of low-Mg^# and high-Mg^# peridotites suggests that there is no significant stratification within the sub-continental lithospheric mantle beneath Kuandian, but there exists mixture between the newly accreted mantle and the ancient lithospheric mantle. Trace element characteristics of the Kuandian peridotite xenoliths indicate that they have undergone multiple complex mantle metasomatism, and the metasomatic agents include not only silicate melts/fluids but also carbonate melts; both melts/fluids released by Pacific slab subduction and the heat perturbation and upwelling of hot asthenospheric melts caused by subduction were involved in their provenance, and thus Pacific slab subduction might have played an important role in the destruction of the North China Craton.

Abstract:Mantle xenoliths entrained in volcanic rocks are direct samples that could provide important information on both compositions and thermal state of the sub-continental lithospheric mantle (SCLM). Garnet lherzolites trapped by lamproites that were emplaced during the Early Ordovician at Dahongshan of Hubei Province display abundant characteristics of both trace elements and isotopes. Their p-t estimates indicate that the SCLM has a thickness of～110 km. Mantle xenoliths trapped by the Early Jurassic basalts in the hinterland of South China (Ningyuan and Daoxian) have fertile compositions, representing the mantle residues that were subjected to low degrees of partial melting. The whole-rock Re-Os isotopes of Ninyuan mantle xenoliths support the existence of juvenile mantle in these areas during the Mesozoic, which resulted from the accretion of the asthenosphere. Therefore, the enriched mantle that had probably existed beneath Ningyuan during the Paleozoic was completely delaminated and replaced by juvenile mantle. The mantle delamination was geodynamically related to the large-scale lithospheric extension, which occurred since～225 Ma. The Cenozoic basalts widely distributed along the coastal areas of South China contain abundant mantle xenoliths. p-t estimates based on mineral composition reveal that the lithospheric mantle has a thickness of 80～90 km, with a hot geothermal gradient. Both whole-rock and sulfide Re-Os isotopes support the existence of ancient mantle residues of Paleoproterozoic ages beneath the coastal areas of South China during the Cenozoic. Therefore, the Cenozoic lithospheric extension in this area only triggered partial removal of the ancient lithospheric mantle.

Abstract:Garnet pyroxenite xenoliths from the Hannuoba Cenozoic basalt were formed by magmatic underplating in the uppermost mantle (40～45 km), and represent the crust-mantle transition zone. The electrical conductivity of sintered garnet pyroxenite WD958 was measured at 1.2 GPa and 380～900℃, using a Solartron 1260 Phase-Gain Analyzer. The temperature dependence of electrical conductivity (σ) can be fitted by an Arrhenius equation: σ=σ₀ exp(-ΔH/kT), where T is in Kelvin and k is the Boltzmann constant. Values of the pre-exponential factor (σ₀) and activation enthalpy of electric conductivity (ΔH) of sample WD958 are 97.5 S/m and 1.27 eV, respectively. The water contents of minerals were analyzed using the Fourier transform infrared spectrometry. The average water content in clinopyroxene is 117×^-6 H₂O, whereas olivine is very dry (<1×10^-6H₂O) and the water content in garnet cannot be determined due to alteration. The laboratory-derived electrical conductivity of mantle minerals shows that the calculated conductivity using the Hashin-Shtrikman average can match the measured values by assuming a mixture of hydrogen-bearing clinopyroxene, dry garnet and dry olivine. This demonstrates the contribution of both small polaron conduction and proton conduction mechanisms to the bulk conductivity of garnet pyroxenite, and the sample can be regarded as a resistive matrix with non-interconnected conductive inclusions. If the water partition equilibrium between minerals is preserved at the in situ depth (40～45 km), the electrical conductivity of garnet pyroxenite will be enhanced by 1 order magnitude and the proton conduction mechanism becomes predominant. For the lithosphere with a high geothermal gradient, the temperature at the Moho depth could reach 1 000 ℃ and garnet pyroxenite is characterized by high conductivity. In contrast, under normal geothermal gradients, garnet pyroxenite shows conductivity as low as spinel lherzolite. During magmatic underplating, therefore, the electrical crust-mantle boundary will vary with temperature and water concentration.

Abstract:Anomalous zones of high electrical conductivity and pronounced electrical anisotropy have been detected by geophysical electromagnetic deep surveys in some regions of the uppermost mantle; nevertheless, their origin remains a long-standing controversial issue. Several conduction mechanisms have been proposed, such as the presence of partial melts, H-bearing olivine, graphite and sulfides in grain boundaries, Fe-rich garnet and hydrous minerals like micas. The differences between these models have significantly different impacts on the composition, structure and physical-chemical properties of the uppermost mantle, and even on the triggering of earthquakes and the styles of mantle convection and other dynamical processes. Based on recent studies of petro- logy, geochemistry, geophysics, numerical modeling and laboratory high-pressure and high-temperature experiments, this paper deals with various problems involved with the available models. The authors hold that electrical anomalies in some regions of the uppermost mantle may be related to its macro-scale heterogeneities, including the local enrichment of Fe³⁺- and H₂O-rich augites (and other pyroxenite analogues) in the form of dykes/veins and/or the local presence of remnants from recycled crustal materials (e.g., conductive subduction-associated oceanic and continental crust and/or delaminated continental deep crust stretched into high aspect ratio bands/lenses due to mantle flow). Electrical anomalies in different regions of the uppermost mantle may have different origins, and thus eeking for a single unique explanation seems unnecessary.

Abstract:Different types of magmatic deposits, such as Cu-Ni-PGE sulfide deposits and V-Ti-Fe oxide deposits, have been formed in the late-Permian Emeishan mantle plume magmatic system. They show the trends of increase of basic rock proportion and the decrease of PGE content from PGE-rich, Cu-Ni-PGE, through Cu-Ni-rich sulfide deposits, to V-Ti-Fe oxide deposits. Cu-Ni-PGE sulfide deposits are characterized by negative anomalies of Nb and Ta, relatively high contents of H₂ and magmatic volatiles, whereas V-Ti-Fe oxide deposits are characterized by positive anomalies of Nb, Ta and Ti, and negative anomalies of Zr and Hf, as well as higher contents of H₂O, CO₂ and H₂, and lower contents of magmatic volatiles. They both are enriched in strongly incompatible elements and light REE. Sr-Nd isotopic compositions of whole rocks from these deposits are similar to those of Emeishan basalts. Sr, Nd, Os, C and He isotopic compositions indicate different degrees of crustal contamination. High Ti basalts and V-Ti-Fe oxide deposits show low degrees of crustal contamination, whereas some low Ti basalts and Cu-Ni-PGE sulfide deposits display high degrees of crustal contamination during magmatic evolution. These data suggest that the formation of these magmatic deposits might have been related to the Emeishan basaltic system. The formation of oxide deposits was probably related to fractional crystallization, high contents of H₂O and elevated oxygen fugacity, whereas sulfide mineralization might have been formed in reduced magmatic media by fractionation and crustal contamination during the magmatic evolution.

Abstract:Located in the northeastern margin of Tarim Plate,the Pobei mafic-ultramafic intrusion consists of several intrusive bodies, such as Poyi, Posan, Poqi, and Poshi, in which the Poyi and Poshi intrusive bodies show the better metallogenic potential. In the past, the Pobei intrusion was considered to have been geodynamically formed in the post-collisional setting or the mantle plume setting. In this paper, systematic mineralogical and geochemical data of the Poyi intrusive body were used to reveal the geodynamic setting. The results show that the Fo values of the olivine range from 83.17 to 90.2, the orthopyroxene minerals are mainly bronzite, whereas the clinopyroxene minerals are mainly augite and endiopside; the Cr^# and Mg^# of chrome spinel range from 0.45 to 0.69 and from 0.11 to 0.41, respectively, and show the evolution trend from Al and Mg richness to Cr and Fe richness. The trace elements display obvious Nb, Ta, Ti negative anomalies and Pb positive anomalies. The variations of major elements are mainly controlled by fractional crystallization/cumulate action of olivine, pyroxene and plagioclase The MgO content and FeO content of parent magma are estimated to be 16.09% and 10.38%, respectively. The initial crystallization temperature of parent magma is estimated from 1 331℃ to 1 411℃, indicating that the parental magma is picritic magma. The Pobei intrusion might have been derived from a high degree of partial melting of matasomatized lithospheric mantle during the activity of Tarim mantle plume.

Abstract:OH incorporated in nominally anhydrous minerals (NAMs) has received increasing attention due to its profound influence on the chemical and physical properties of minerals. Olivine is the most abundant mineral phase in the upper mantle, and a trace amount of water in its structure would lead to a large effect on the rheology and conductivity of the upper mantle. Thus, the knowledge of the mechanisms of H incorporation in olivine's structure and the mechanisms and rate of H diffusion in its lattice is helpful to understand the water circulation between the surface and the deep earth and to explain the rhelogically and magnetotellurically observed data. In this paper, the authors compiled studies of olivine during the last two past decades on how H associated with different point defects correlate with the apparent infrared absorption bands, how these different H diffuse through the lattice and how fast these diffusive fluxes occur. The problems existent in these researches are also pointed out in this paper.

Abstract:The Huojihe molybdenum deposit in the Xiao Hinggan metallogenic belt is a large porphyry deposit discovered in recent years. Based on the analysis of characteristics of mineralization, the authors conducted the LA-ICP-MS U-Pb dating of zircon from metallogenic granitic complex and Re-Os dating of molybdenite in Huojihe area. LA-ICP-MS U-Pb ages of zircons from medium-fine grained granodiorite and porphyritic granite are 181.0±1.9 Ma (n=15, MSWD=4.7) and 193.6±1.4 Ma respectively. The molybdenite Re-Os isochrones age is 176.3±5.1 Ma (n=6, MSWD=1.3), with weighted average model age of 181.2±1.8 Ma. The ages from the two methods are close to each other, suggesting that they might have been formed in the same petrogenetic ore-forming system. The petrogenesis started at 193.6±1.4 Ma and the mineralization in the magma occurred at 181.2±1.8 Ma. The ore-forming age of the Huojihe molybdenum deposit is close to the molybdenite Re-Os isochron age of the Wunugetu Cu-Mo deposit and the Lanjiagou and Yangjiazhangzi molybdenum deposits. There existed an early Jurassic magmatic-mineralization event in Northeast China. Trace element and isotope show that granodiorite and porphyritic granite in Huojihe belong to A-type granites. Petrogenesis and mineralization of Huojihe molybdenum deposit were related to changes of the tectonic environment from crust extruding to extending. The mineral source was derived from magma.

Abstract:It is usually difficult to ascertain the shape of folds in multiply-deformed high-grade metamorphic gneisses and schists. If such rocks contain porphyroblasts, a new approach is possible because of the way through which porphyroblast grew was affected by crenulation versus reactivation of compositional layering. Isoclinally folded rocks in Texas Creek, Arkansas River region of Central Colorado contain relics of fold hinges; nevertheless, it is very difficult to ascertain whether they are antiforms or synforms because of the effects of younger refolding and the locally truncated nature of coarse compositional layering. Using the asymmetry of overprinting foliations relative to the five FIA sets in this region, the authors have revealed that an isoclinal fold, which was previously interpreted as a synform, is actually an antiform, which was developed during the first stage of porphyroblast growth (around 1 500 Ma).