North China Craton Research Articles

Triassic convergence and tectonic evolution of the West Qinling orogen: Constraints from reflection-seismology imaging

Although it is well known that the West Qinling orogen was created by the convergence and collision between the North China block (NCB) and the South China block (SCB), the process of the microplate convergence in the West Qinling orogen and its adjacent blocks remains poorly understood. Here we address this question by analyzing ∼ 400 km seismic-reflection-profile data to constrain the subsurface structure of the orogenic system. Our data reveal two north-dipping reflection sets sandwiched between a sub-horizontal Moho (16 s to 18 s, t.w.t.) and a decollement zone (∼6 s, t.w.t.) that separates the upper and mid-lower crusts. In detail, two sets of north-dipping reflectors occur beneath the Zoige basin (CDP 3000–4500, NR-1) and the southern West Qinling orogen (CDP 7000–10500, NR-2), which correlate to south-directed thrusts and are best interpreted as the result of the closure of the Paleo-Tethys Mianlue ocean in the early Triassic. This interpretation is consistent with the presence of an ocean separating the southern West Qinling orogen (S-WQO) from the northern West Qinling orogen (N-WQO). The origin of the required ocean is likely to be a branch of the Paleo-Tethys Mianlue ocean.

Rheological behavior of amphibolite facies migmatites during orogenesis: A case study from the North Qinling Belt, China

The rheological behavior of amphibolite-facies migmatites are key to understanding the evolution of middle–lower crust during orogenesis. The amphibolite-facies metamorphosed migmatites from the North Qinling Terrane are investigated to elucidate high temperature melt-present deformation and shear localization. The metatexite (plagioclase amphibolite and biotite plagiogneiss), diatexites, and deformed residuum (mylonitic amphibolite and layered amphibolite) are well-preserved in outcrops. The SW dipping and E-W striking foliation, and ptygmatic, asymmetrical folds indicate compressional deformation with top-to-the-WNW shearing. Zircon UPb geochronology indicates that crustal anatexis mainly occurred at ca. 400–386 Ma, associated with the collision between the North and South China Blocks. The occurrence of peritectic amphibole, the estimated anatectic conditions of 2.32–4.29 kbar and 595–691 °C and variation of mineral major compositions jointly demonstrate incongruent water-present melting reactions, which involved biotite- or amphibole-breakdown, while the deformation temperature is constrained at 605–644 °C. In the plagioclase amphibolite, the amphibole is dominated by (100)[001] dislocation slip, while plagioclase doesn't display a clear fabric. In the biotite plagiogneiss, plagioclase is dominated by strain-induced (010)[100] slip and quartz shows prism <a > slip. Rheological processes (brittle fracturing, melt(fluid)-assisted partial mylonitization and layering process) indicate that melt migration has an effect on the residuum. The formation of amphibole type-II fabric is controlled by cataclastic flow and rigid rotation during progressive deformation process and dissolution-precipitation processes lead to further weakening and strain localization.

Chronolgy and Geochemistry of the Sijiaying Iron Deposit in Eastern Hebei Province, North China Craton: Implications for the Genesis of High-Grade Iron Ores

The Sijiaying iron deposit is located in the Eastern Hebei area of the southern section of the northern margin of the North China Craton (NCC) and is the largest single iron deposit in China. The deposit contains many banded iron formations (BIFs) and was proven to have more than 3 million tons of high-grade iron ore resources. This study carried out geochemistry and zircon U–Pb analysis of normal-grade iron ore, high-grade iron ore, and wall rock (biotite–leptynite, chlorite–sericite schist) in the Sijiaying deposit and discussed the genesis and metallogenic age of high-grade iron ore. BIFs have low concentrations of Al2O3 and TiO2 and high field strength element (HFSE) depletion, indicating almost no contamination via terrestrial debris. The standardized post-Archean Australian shale (PAAS) rare earth element (REE) distribution pattern indicates that the iron formation exhibits positive Eu, Y, and heavy rare earth element (HREE) anomalies and lacks negative Ce anomalies, indicating that the Sijiaying BIF was enriched with iron sources via high-temperature hydrothermal fluids from the seabed and deposited in an anoxic ancient marine environment. In addition, geological field work identified two types of high-grade iron ore in the mining area: primitive sedimentary and hydrothermally altered high-grade iron ore. Further ore geochemical research showed that the primitive sedimentary-type iron ore is similar in geochemistry to the BIF. In addition to low Eu/Eu* values, the hydrothermally altered high-grade iron ore shows geochemical characteristics similar to those of the BIF, suggesting that they share the same iron source but did not form at the same time. The total large ion lithophile element (LILE) (Sr, Ba, Pb) contents in primitive sedimentary-type high-grade iron ore are higher than those in hydrothermally altered high-grade iron ore, indicating that LILEs are carried away via fluids during the hydrothermal alteration process in normal-grade iron ore. The geochemical characteristics of biotite–leptynite and chlorite–sericite schist include high contents of SiO2 and Al2O3, light rare earth elements (LREEs), LILE enrichment (Rb, Ba, Sr, Zr), and HFSE depletion (Nb, Ta, P, Ti), characteristics that are similar to island arc volcanic rocks. The reconstruction of the original rock indicates that the wall rock is a product of volcanic sedimentary cycles in an island arc setting. Zircon cathodoluminescence images and LA–ICP–MS zircon U–Pb dating can be divided into four age groups (3283 Ma, 2547 Ma, 2500 Ma, and 2407 Ma), which correspond to the earliest volcanic activity in eastern Hebei, the main mineralization age of the Sijiaying BIF (the mineralization age of primitive sedimentary high-grade iron ore), a regional tectonic–metamorphic event, and the occurrence of migmatization (the mineralization age of hydrothermally altered high-grade iron ore), respectively. Therefore, the Sijiaying BIF and primitive sedimentary high-grade iron ores were deposited and mineralized at 2547 Ma, and the iron orebody was later altered via the hydrothermal solution at 2407 Ma, forming large-scale high-grade iron ores.

New record of the green algal fossil Proterocladus and coexisting microfossils from the Meso-Neoproterozoic Diaoyutai Formation in southern Liaoning, North China

The Mesoproterozoic-Neoproterozoic transition represents a crucial interval of evolutionary innovation and phylogenetic diversification. A spotty fossil record indicates that several clades of multicellular eukaryotes, including red and green algae, as well as fungi, may have diverged and diversified at this transition. However, the Meso-Neoproterozoic fossil record remains poorly documented, particularly at the assemblage level, greatly limiting our understanding of the marine ecosystem in this critical time interval. Here we report a well-preserved assemblage of organic-walled microfossils from the Meso-Neoproterozoic Diaoyutai Formation (ca. 1050–950 Ma) in Southern Liaoning Province of the North China Block. Raman geothermometric analysis of these organic-walled microfossils suggests that the Diaoyutai Formation experienced relatively low-grade metamorphism, with apparent peak metamorphic temperatures of ∼ 215 ℃. The Diaoyutai assemblage consists of 14 species belonging to 9 genera, as well as several unnamed or unidentified forms. In addition to simple forms such as aggregated coccoidal and unbranched filamentous fossils that are often interpreted as cyanobacteria, a moderate diversity of eukaryotes is also observed. Probable eukaryotic fossils in this assemblage include the branched multicellular alga Proterocladus antiquus that has been previously interpreted as a green alga, several morphologically complex spheroidal taxa (e.g., Germinosphaera bispinosa, Pterospermopsimorpha insolita, P. pileiformis and Squamosphaera colonialica), and relatively large leiospheres (50–700 μm in diameter). We note that the occurrence of Proterocladus antiquus in the Diaoyutai assemblage predates its previously reported occurrence from the overlying Nanfen Formation. Overall, the Diaoyutai assemblage presents a glimpse of the coastal marine ecosystem at the Meso-Neoproterozoic transition, which consisted of benthic microbial mats dominated by filamentous cyanobacteria, millimeter-sized benthic green algae with thallus elevating above the sediment–water interface, and micrometer-sized planktonic prokaryotic and eukaryotic organisms. From the perspective of primary producers, the rise of photosynthetic eukaryotes in the Meso-Neoproterozoic represents a transformative innovation to modernize the coastal marine ecosystem in terms of ecological functions.

Gold source and ore-forming process of the Linglong gold deposit, Jiaodong gold province, China: Evidence from textures, mineral chemical compositions and sulfur isotopes of pyrite

The Linglong gold orefield is located northwest of the Jiaodong gold province, in the eastern North China Craton (NCC) area. It consists of over 10 gold mines with a total gold resource of approximately 1000 t, and gold mineralization occurs in gold-quartz veins. The Linglong gold deposit underwent four stages of ore-forming processes namely: Pyrite-quartz (stage I) – milky-white quartz vein with euhedral pyrites, Quartz-pyrite (stage II) – smoky quartz veins with euhedral-subhedral pyrites, Polymetallic sulfides (stage III) – quartz veins with large amounts of polymetallic sulfides (pyrite, pyrrhotite, sphalerite, galena), hematite, and native gold, and the Quartz–carbonate (stage IV) – calcite-quartz veins. This study focuses on the investigation of pyrites from three main mineralization stages (I-III), exploring their textures, in-situ major and trace element concentrations, and in-situ sulfur isotopic compositions.The study examined pyrite through various methods including BSE and EMPA major element, LA-ICP-MS trace element, and LA-MC-ICP-MS sulfur isotope analyses. Pyrite from three distinct stages of ore-forming processes were analyzed. Pyrites from stage I (Py-I) exhibited euhedral crystals with homogeneous brightness in the BSE, lacked mineral inclusions, porous microtextures, and oscillatory zones. Py-I possessed low contents of Au, Ag, As, Bi, Cu, Sb, Zn, Co, and Ni. Pyrites from stage II (Py-II) showed fine-coarse grains and euhedral-subhedral features with slight fractures, similar to Py-I, however, had the higher concentrations of As and Sb. Pyrites from stage III (Py-III) were found in the quartz-polymetallic sulfide veins and exhibited subhedral to anhedral features and more significant concentrations of metallic elements, particularly Au and As. The δ34S values of Py-I, Py-II and Py-III were in the ranges of 6.76‰–7.47‰, 6.11‰–7.27‰ and 6.07‰–8.33 ‰, respectively. Analysis of the texture and geochemical composition of pyrite suggested that invisible gold was primarily present in pyrite in the form of solid solution (Au+), accompanied by a small amount of nano-form gold. The ore-forming fluids that led to this mineralization were mostly derived from mantle-derived mafic magma mixed with some minor metamorphic sedimentary material. A decrease in fluids temperature, water–rock interactions and fluid boiling led to the depositions of sulfides and gold. In the Late Mesozoic, lithospheric delamination of the eastern block of the NCC triggered decompression and melting of the upwelling asthenospheric mantle resulting in partial melting of the lithospheric mantle. This lithospheric mantle-derived mafic magma provided the ore-forming fluids for large-scale gold mineralization in the Jiaodong gold province.

Paleozoic Tectonic Switch in the North Qinling Orogenic Belt: Constraints from the Paleozoic Granites from the Northern Qinling Migmatite Terrane

AbstractThe variations in source rocks and melting conditions of granites can provide essential clues for the crustal magmatic response in orogenic process. Based on geochronology, whole‐rock and mineral chemistry, this paper reveals two different granites in the Northern Qinling migmatite complex, which reveal obvious differences in source region and melting condition. The older granodiorite (402 ± 0.8 Ma) displays typical Na‐rich adakite affinity, i.e., high Na2O/K2O (2.04 to 2.64) and Sr/Y (96 to 117) ratios, they have relative evolved isotopic compositions (εNd(t) = –0.52 to –0.04; zircon εHf(t) = –0.06 to +7.78). The younger leucogranite (371 ± 2 Ma) displays higher SiO2 (72.32 to 73.45 wt%), lower (TFeO + MgO + CaO + TiO2) contents (&lt;2 wt%) and depleted Sr‐Nd‐Hf isotopic compositions (i.e., εNd(t) = +2.6 to +3.0; zircon εHf(t) = +5.94 to +14.12), as well as high 10000 × Ga/Al and TFeO/MgO ratios, indicating that they represent highly fractionated I‐type granites that derived from melting of juvenile crust. The variations in source rocks and melting condition of the two granites indicating a tectonic switch from compression to extension in 400 to 370 Ma, this switch is later than that in the eastern section of the North Qinling, indicating a scissor collision process between the South Qinling and North China Craton (NCC) in Devonian era.

Present-Day Three-Dimensional Deformation across the Ordos Block, China, Derived from InSAR, GPS, and Leveling Observations

The Ordos Block in China experiences tectonic activity and frequent earthquakes due to compression from the Tibetan Plateau and extension from the North China Block. This has prompted the construction of a high-resolution three-dimensional (3D) deformation field to better understand the region’s crustal movement. Considering the limitations of the existing geodetic observations, we used InSAR, GPS, and leveling observations to create a high-precision 3D deformation field for the Ordos Block. Spherical wavelet decomposition was used to separate tectonic and non-tectonic deformation signals. Short-wavelength non-tectonic deformation fields revealed complex surface deformation patterns caused by groundwater, oil, gas extraction, and coal mining. Long-wavelength tectonic deformation fields showed subsidence in the southern margin of the block, while the interior and northeastern margins were uplifted. By combining imaging results from the seismic velocity structure and magnetotellurics, we infer that the upwelling of deep materials beneath the northeastern margin leads to surface uplift with tensile strain rates. The crustal uplift in the area south of 38°N matches the thickening of the lower crust. The weak subsidence and eastward horizontal movement disappearing near 108°E at the southern margin support the existence of asthenosphere flow beneath the Qinling orogenic belt.

Magma system and equilibrium depth of the Cenozoic basalts in the central North China craton

Magma system and equilibrium depth of the Cenozoic basalts in the central North China craton

A metamorphic devolatilization model for the granitoid-hosted Wulong lode gold deposit, Liaodong Peninsula, Eastern North China Craton: Evidence from geology, fluid inclusions, and O-Sr-Li isotopes

The genesis of the gold deposits in the eastern North China Craton (NCC) has long been disputed. Many would relate the gold mineralization to magmatism. The Wulong in the Liaodong Peninsula is a classic example in the eastern NCC and was considered to be of magmatic hydrothermal origin previously. Nevertheless, we propose a metamorphic devolatilization model to explain its genesis based on geological, fluid inclusion and O-Sr-Li isotopic study. The geological profiles, temperatures, salinities, and O isotopes of the ore-forming fluids at Wulong show no spatial variations over a 600 m vertical depth interval. These evidence convincingly prove its vertically extensive feature, which is similar to orogenic gold deposits, but different from magmatic hydrothermal systems. The initial Sr isotopes of the ore-forming fluids (0.71435 to 0.71578) fall into the range of metabasalts from the Liaohe Group (the Paleoproterozoic basement), indicating that metabasalts are the probable sources. Lithium and its isotopic modelling further suggest that metamorphic devolatilization of metabasalts (with chain silicates Ch, phyllosilicates Ph, and quartz Qtz as endmembers; Ch, Li ≈ 5 ppm, δ7Li ≈ 8‰; Ph, Li ≈ 63 ppm, δ7Li ≈ −1‰; Qtz, Li ≈ 10 ppm, δ7Li ≈ 30‰) could account for the Li concentrations (5–41 ppm) and δ7Li value of the fluid (21.4‰) of the Wulong gold deposit. Rayleigh fractionation in a closed system led to large Li isotopic variation of the ore-forming fluids (21.4 to 11.0‰). The metamorphic devolatilization model based on the vertically extensive feature, Sr isotopic constraint and Li isotopic modelling is in striking contrast with previous qualitative isotopic studies that support a magmatic hydrothermal origin for the Wulong and other granitoid-hosted gold deposits in the eastern NCC. Integrated with geological characteristics and radiogenic isotopes, this study here shows that Li isotopes can provide new constraints on the genesis of orogenic gold deposits.

A pioneering riparian community dominated by Nystroemiaceae gymnosperms from the Guadalupian (middle Permian) of China

A new fossil plant community is documented from the Upper Shihhotse Formation on the North China Block with an extremely unusual composition and diversity. Zircon U–Pb dating yields an age of 268.3 ± 2.3 Ma during the mid-Roadian to early Wordian stages of the Guadalupian. The community occurs in two beds and is dominated by fossils of the gymnosperm family Nystroemiaceae, which account for up to 99% of specimens recovered and comprise stems, branches, roots, ovuliferous complexes of Nystroemia pectiniformis and N. sp., and leaves of Chiropteris reniformis. The two different species of Nystroemia are distinguished on size and apical features of ovules, with N. pectiniformis having two apical bicornuate horns, whereas N. sp. has apical auricular extensions. Leaves of C. reniformis are megaphyllous and possess a long petiole and net venation. In the Cathaysian flora, different species of Nystroemiaceae are distinguished by features of their ovules and habit, but all bear leaves of C. reniformis which suggests evolutionary modularity and different rates of evolution between the ovules and leaves. Both N. pectiniformis and N. sp. are shown to have been short, uptight trees. Sedimentary evidence shows the community grew in a meandering fluvial setting with repeated river channel changes. Compared with contemporaneously diverse Cathaysian floral communities in the Upper Shihhotse Formation, the vegetation has a very restricted diversity and is a rare occurrence of a gymnosperm-dominated community. The Nystroemiaceae demonstrates an r-selective ecological strategy as pioneering plants adapted to living in unstable habitats and newly opened gaps in the riparian zone.

Geophysical‐Geochemical Modeling of Deep Crustal Compositions: Examples of Continental Crust in Typical Tectonic Settings and North China Craton

AbstractThe chemical composition of the deep continental crust is key to understanding the formation and evolution of the continental crust. Constraining the chemical composition of present‐day deep continental crust is, however, limited by indirect accessibility. This paper presents a modeling method for constraining deep crustal chemical structures from observed crustal seismic structures. We compiled a set of published composition models for the continental crust to construct functional relationships between seismic wave speed and major oxide content in the crust. Phase equilibria and compressional wave speeds (VP) for each composition model were calculated over a range of depths and temperatures of the deep crust. For conditions within the alpha(α)‐quartz stability field, robust functional relationships were obtained between VP and major oxide contents of the crust. Based on these relationships, observed VP of the deep crust can be inverted to chemical compositions for regions with given geotherms. We provide a MATLAB code for this process (CalcCrustComp). We apply this method to constrain compositions from deep crustal VP of global typical tectonic settings and the North China Craton (NCC). Our modeling results suggest that the lower crust in subduction‐related and rifting‐related tectonic settings may be more mafic than platforms/shields and orogens. The low VP signature in the deep crust of the NCC can be explained by intermediate crustal compositions, higher water contents, and/or higher temperatures. The chemical structure obtained by this method can serve as a reference model to further identify deep crustal features.

Cu–S Isotopes of the Main Sulfides and Indicative Significance in the Qibaoshan Cu–Au Polymetallic Ore District, Wulian County, Shandong Province, North China Craton

With a focus on the Cu isotope geochemistry of chalcopyrite, this paper analyzed the Cu isotope geochemistry of the Qibaoshan crypto-explosive breccia-type Cu–Au polymetallic ore district in Wulian, Shandong Province, North China Craton (NCC). Combined with the results of the in situ sulfur isotope analysis of sulfides, a certain reference and evidence for the study of the genetic mechanism of the epithermal-porphyry Cu polymetallic metallogenic system were provided. The results of the in situ isotope analysis show that the δ34S values of the main sulfides in the Qibaoshan Cu–Au polymetallic ore district range from −6.81‰ to +3.82‰ and are likely to be attributed to the mixing of the derived mantle with the surrounding sedimentary rock assimilation. The ore-forming mechanism may be related to the progressive cooling and transition of the earliest hydrothermal fluids that were dominated by H2S under relatively reducing conditions, followed by a gradual transition from oxidation to reduction. The Cu isotopic composition of the sulfides in ores (δ65Cu = +0.169‰–+0.357‰) decreases with depth, which is likely caused by the upward transport of heavier Cu isotopes. The upper part of the crypto-explosive breccia pipe in the Qibaoshan area may be relatively more gaseous, resulting in the enrichment of δ65Cu. As the gas phase decreases and the liquid phase increases with depth, the δ65Cu value gradually decreases. This indicates the transition from a low-temperature phyllic alteration to a high-temperature K-feldspar alteration. Large, concealed pluton intrusions or orebodies may be present at a depth of the Qibaoshan area. The heavy δ65Cu characteristic is a potential indicator for tracing the fluid activity of the porphyry system and searching for Cu mines. The results provide a reference for the study of the genetic mechanisms of the epithermal-porphyry Cu polymetallic metallogenic system.

Volcanic phosphorus supply boosted Mesozoic terrestrial biotas in northern China

The Mesozoic terrestrial Jehol Biota of northern China exceeds the biomass and biodiversity of contemporaneous Lagerstätten. From 135 to 120 Ma, biotic radiation may have responded to the peak destruction of the North China Craton. However, the direct mechanistic link between geological and biological evolution is unclear. Phosphorus (P), a bio-essential nutrient, can be supplied by weathering of volcanics in terrestrial ecosystems. The middle–late Mesozoic volcanic-sedimentary sequences of northern China are amazingly rich in terrestrial organisms. Here we demonstrate episodic increases in P delivery, biological productivity, and species abundance in these strata to reveal the coevolution of volcanism and terrestrial biotas. A massive P supply from the weathering of voluminous volcanic products of craton destruction thus supported a terrestrial environment conducive to the high prosperity of the Jehol Biota. During the nascent stage of craton destruction, such volcanic-biotic coupling can also account for the preceding Yanliao Biota with relatively fewer fossils.

Circum-cratonic Triassic syenite province of North China craton reveals heterogeneous lithospheric mantle related to dual subduction

Circum-cratonic Triassic syenite province of North China craton reveals heterogeneous lithospheric mantle related to dual subduction

Circum-cratonic Triassic syenite province of North China craton reveals heterogeneous lithospheric mantle related to dual subduction

Circum-cratonic Triassic syenite province of North China craton reveals heterogeneous lithospheric mantle related to dual subduction

First occurrence of caryocaridids (Crustacea, Phyllocarida) in the Ordovician of North China

Although caryocaridids experienced a significant biodiversity boost during the Ordovician and spread over broad trans-equatorial areas, they have been rarely reported from Siberia, Tarim and North China. Herein, we describe a new species of caryocaridids, Soomicaris ordosensis sp. nov., from the Upper Ordovician of western Inner Mongolia, China, which represents the first report of caryocaridids from the North China craton and extends the palaeogeographical distribution of caryocaridid phyllocarids to the northwestern margin of the Palaeo-Tethys. Detailed morphological and morphometric studies show that these specimens can be ascribed to different ontogenetic stages. After close inspection, we infer that Soomicaris ordosensis sp. nov. should have experienced a long pre-adult time, with several moulting stages, during which time the posterior margin of carapace gradually extends backward to form the posterodorsal spine, and the posterior margin of carapace gradually transitions from convex, linear, and finally to a rightward-sloping sigmoid curve. The taphonomic features of caryocaridids from the Upper Ordovician in the western Inner Mongolia suggest in-situ and parautochthonous preservation. The carapaces have experienced weak phosphatic mineralization during diagenesis. The Ordovician caryocaridids were most likely stenothermic which preferred to inhabit the cool- to cold-water environments from the upper mesopelagic to the lower epipelagic zones, and showed the polar emergence phenomenon.

The crust-mantle transition beneath Northeast China from P–wave receiver functions

Introduction: The continental lithosphere of the Northeast China Plain (NECP) is probed using P–wave receiver function analysis of passive seismic data. The NECP is much–discussed as it includes different geological provinces of varying tecto–magmatic origin and dates back from Archean to Holocene. Quantifying the tectonic and magmatic influences on the structure and composition of the lithosphere puts important constraints on evolution of NECP. For this, we explore 75 sites across NECP using receiver function analysis.Methods: A recently developed technique of inverting for 1–D S–wave velocity profiles beneath seismic stations that is based on the principles of Bayesian statistics (hierarchical transdimensional Bayesian Inversion; HTBI) is applied to receiver functions from the NECP. In addition, an improved crustal thickness–compressional to shear wave velocity ratio (H–κ) analysis was conducted to retrieve the crustal thickness and Vp/Vs ratio of the region. These estimated point measurements are integrated and systematically studied for a regional view of the current crustal architecture.Results and Discussion: We observe a laterally varying and highly complex lithosphere beneath the NECP. A shallower crust–mantle transition (≤32 km) characterises the Precambrian North China craton and Late Mesozoic–Cenozoic Songliao Basin from the adjacent Central Asian Orogenic Belt and the Changbaishan Volcanic field (35–40 km). Beneath the latter, low Vp/Vs ratios (~1.65) are obtained, whereas all other regions feature ratios in excess of 1.75. Multiple velocity gradients are observed at crustal depths within the craton, in contrast to the adjacent orogen, which indicates a higher degree of crustal complexity of the former. The width of the crust–mantle transition across the NECP is found to be mainly intermediate (2–7 km) and occasionally sharp (≤2 km). From our observations, we infer that there is a substantial difference between the eastern North China Craton’s lithospheric architecture and the rest of the NECP, with most of the NECP exhibiting more complexity than previously reported.

Recycling of continental crust materials: Geochemical constraints from post-collisional alkaline intrusive rocks in the Dabie orogen

Post-collisional alkaline intrusive rocks from the Dabie orogen were studied for their whole-rock major-trace elements and Sr-Nd-Hf-Pb isotopes, as well as zircon U-Pb ages and Hf-O isotopes. The results provide geochemical constraints on the nature of their mantle sources and thus insight into crust-mantle interaction in the continental collision zone. The alkaline intrusive rocks are composed of syenite and nepheline syenite. Syn-magmatic zircon U-Pb dating by LA-ICP-MS for them yielded Early Cretaceous ages of 131.3 ± 1.4 Ma to 122.6 ± 0.6 Ma, coeval with the post-collisional magmatism in the Dabie orogen. One relict zircon with U-Pb age of 211 Ma is consistent with the timing of metamorphism for the ultrahigh-pressure (UHP) metamorphic rocks in this orogen. They have arc-like trace element distribution patterns, such as enrichment in LILE (large ion lithophile element) and LREE (light rare earth element) but depletion in HFSE (high field strength element), and enriched whole-rock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7077–0.7131 but negative εNd(t) values of −16.0 to −9.4 and εHf(t) values of −17.5 to −12.7. Their syn-magmatic zircons have three groups of Hf-O isotope compositions comparable to those of UHP metamorphic rocks in Central-South Dabie and North Dabie, which represent the upper and middle continental crust of the subducted South China Block, respectively. In this regard, slab–mantle interaction is evident during the Triassic continental collision. We suggest that the melts derived from the subducted South China Block reacted with the lithospheric mantle wedge of the North China Block, resulting in phlogopite-bearing metasomatites, whose partial melting would generate the post-collisional alkaline intrusive rocks during the Early Cretaceous.

Magnesium isotope fractionation in differentiation of mafic-alkaline-carbonatitic magma and Fe-P-REE-rich melt at Bayan Obo, China

The Bayan Obo area in the North China Craton (NCC) contains the largest rare earth element (REE) resources in the world. The key to this gigantic and efficient REE accumulation has been the focal point of century-long research efforts. Intense differentiation of mantle-derived alkaline magmas is envisaged as a highly viable process for REE enrichment based on the first discovery of mafic-alkaline-carbonatitic suite in the Bayan Obo region. Geochronological and Sr-Nd-Hf-O isotopic analyses revealed that these rocks derived from a common mantle source and evolved with magmatic differentiation at 1.31 Ga to 1.32 Ga. Elemental geochemistry and Mg isotopes in this magma series (alkaline diabase sill, syenite dyke and carbonatite) unambiguously support the differentiation model. The earliest ferrocarbonatite, with nearly undifferentiated REE concentrations, has mantle-like δ26Mg (−0.27 ± 0.03‰), and therefore can be considered as the original product of the mantle derived alkaline magma at Bayan Obo. The subsequent magnesiocarbonatites have a LREE enrichment feature, which is consistent with the alkaline diabase sills. The magnesiocarbonatites possess light Mg isotopic composition (from − 0.42‰ to − 0.50‰), whereas the alkaline diabase sills have heavy Mg isotopic composition (from − 0.03‰ to − 0.05‰). This rock assemblage should present two end members, which differentiated likely by immiscibility in the chamber from a common alkaline magma. The subsequent syenite dyke has no obvious Mg isotopic variation (from − 0.44‰ to − 0.39‰) during differentiation from the above alkaline magma. Notably, as the most important REE hosts, fine-grained ferrocarbonatites show large Mg isotopic variations (from − 0.85‰ to + 0.13‰) and present positive correlations with total MgO and CaO contents, but negative with total Fe2O3, P2O5 and REE contents. In fine-grained ferrocarbonatite, closely associated apatite, monazite, hematite and magnetite minerals occur as a special intergrowth texture. This unique paragenetic assemblage is likely the mineralization record of a Fe-P-REE-rich melt, which shows particular affinity of hydrothermal fluid and light Mg isotope in carbonatitic magma. By contrast, magnesian siderite phenocrysts as the early cumulates in carbonatitic magma are enriched in heavy Mg isotope (from − 0.07‰ to + 0.21‰). Immiscibility of Fe-P-REE-rich melt and fractional crystallization of carbonate minerals are responsible for large Mg isotope fractionation and giant REE enrichment in the Bayan Obo deposit.

Early Precambrian mafic dykes in western Shandong province, North China Craton: constraints on the chronology, genetic model and tectonic significance

AbstractIn the Late Neoarchaean, the lithosphere of the North China Craton (NCC) experienced a strong extensional event, which is of great significance for understanding the evolution of the continental crust in the Precambrian. In this study, a suite of mafic dykes from Shandong province in the northeastern NCC were investigated to determine the nature, timing and source of rift-related magma activities using zircon U–Pb data, whole-rock geochemistry and Nd–Hf isotopes. Zircon U–Pb dating of four dolerites by laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) yielded weighted mean 207Pb/206Pb ages in the range 2509 ± 6.1 to 2537 ± 6.2 Ma (2σ, 95 % confidence interval). The mafic dykes are classified as alkaline rocks based on their K2O + Na2O contents (6.78–7.21 wt %) and belong to the shoshonitic series according to their K2O contents (3.23–3.36 wt %). The dolerites show low concentrations of light rare earth elements ((La/Yb)N between 7.17 and 8.55), positive Eu anomalies (Eu/Eu* between 1.12 and 1.27), positive Ba, K, Pb, Sr, Eu, Dy and Lu anomalies, and depleted U, Nb, Pr, Ta, P, Nd and Ti anomalies. The dykes are characterized by low initial (87Sr/86Sr)i (∼0.6969), positive εNd(t) values (0.2–0.8) and εHf(t) values (0.5–8.6) and relatively old mean Nd and Hf model age (2.73 Ga). Collectively, the data suggest that the mafic dykes were derived from the partial melting (10–20 %) of an isotope-depleted garnet–lherzolite mantle source that was hybridized through interaction with subducted lower crustal material. The parental magmas of these dykes underwent a certain number of crustal contaminations during magma ascent. The mafic magmatism represented in the form of the dyke swarms is considered to be a response to widespread lithospheric extension which affected the NCC at c. 2.5 Ga during the Neoarchaean.