Yanshanian Orogeny Research Articles

Chau Thoi and Nui Gio volcanic rocks: A remnant of the Loei Phetchabun volcanic belt in Viet Nam

Introduction: Extrusive volcanic rocks, such as dacite, andesite, basalto-andesite, basalt… of Chau Thoi and Nui Gio hills in Bien Hoa and Binh Phuoc provinces, southern Viet Nam, characterize volcanic island arc rocks. These rock suites formed as the convergent tectonic between the Indochina and Sibumasu geological blocks.
 Methods: Geochemical data of rock samples collected on the field were examined and analyzed by the Academia Sinica I E S (Institute of Earth Science, Taiwan and processed with a GCD kit (Geochemical Data Toolkit) to ascertain their characteristics and geotectonic setting. Result: Geochemical data both in major elements and trace elements of the Chau Thoi – Nui Gioshow a specific characteristic of a volcanic island arc environment.
 Discussion: Chau Thoi and Nui Gio rocks are suitable to correlate to the Permian Thailand Loei Phetchabun volcanic belt. However, at the current time, Chau Thoi and Nui Gio rocks have been classified as Deo Bao Loc formation - late Jurassic to early Cretaceous in ages - belong Truong Son magmatic belt. This magmatic belt resulted from the Yanshanian orogeny by the subduction of the Paleo-Pacific oceanic plate beneath the Eurasia (Indochina) continental plate. More studies needed to be performed, specially geochronological data to support the study.
 Conclusion: Chau Thoi and Nui Gio rock suites characterize volcanic island arc rocks, products of a convergence tectonic between Indochina and Sibumasu blocks. They are remnants of the Thailand Loei Phetchabun volcanic belt, the first time reported in Vietnam.

A Study on the Heterogeneity Characteristics of Geological Controls on Coalbed Methane Accumulation in Gujiao Coalbed Methane Field, Xishan Coalfield, China

Commercial exploration and exploitation of coalbed methane (CBM) in Gujiao coalbed methane (CBM) field, Xishan coalfield, have rapidly increased in recent decades. The Gujiao CBM field has shown strong gas distribution heterogeneity, low gas content, and wide distribution of wells with low production. To better understand the geological controlling mechanism on gas distribution heterogeneity, the coal reservoir evolution history and CBM accumulation process have been studied on the base of numerical simulation work. The burial history of coal reservoir can be classified into six stages: shallowly buried stage; deeply buried stage; uplifting stage; short-term tectonic subsidence stage; large-scale uplifting stage; and sustaining uplifting and structural inversion stage. Mostly, coal seams have experienced two-time thermal metamorphisms with twice hydrocarbon-generation processes in this area, whereas in the southwest part, the coal seams in there suffered three-time thermal metamorphisms and hydrocarbon-generation processes. The critical tectonic events of the Indosinian, Yanshanian, and Himalayan orogenies affect different stages of the CBM reservoir accumulation evolution process. The Indosinian orogeny mainly controls the primary CBM generation. The Yanshanian orogeny dominates the second and third gas generation and migration processes. The Himalayan orogeny mainly affects the gas dissipation process and current CBM distribution heterogeneity.

Genesis of the Xinfang Gold Deposit, Liaodong Peninsula: Insights from Fluid Inclusions and S-Sr Isotopic Constraints

The North China Craton (NCC) has been continuously reactivated since the Mesozoic and this decratonization is responsible for its economically important gold mineralization in the Mesozoic. The Early Cretaceous (110–130 Ma) gold mineralization in the NCC has been well-studied due to its significance, but little attention has been given to other episodes of gold mineralization related to polyphased reactivation of the NCC. The Xinfang mesozonal gold deposit (143 Ma) in the Liaodong Peninsula is related to the one of the episodes of the Yanshanian orogeny. The orebodies of the Xinfang gold deposit were controlled by the low angle transpressive fault systems and hosted by the Neoarchean monzogranitic gneiss. Fluid inclusion microthermometry reveals that the mineralizing temperatures range from 220 to 280 °C, with salinities from 6 wt.% NaCl eqv. to 15 wt.% NaCl eqv., pressures from 199 to 321 Ma. The S isotopic characteristics of sulfides not only record a heterogeneous source including magmatic or gneissic sulfur but also record inter-mineral isotope fractionation. The initial 87Sr/86Sr values of pyrite (0.713 480–0.729 031) indicate a radiogenic crustal origin for the sources. The metamorphic dehydration of the underlying basement resulted in the genesis of the Xinfang gold deposit. We summarize three episodes of gold mineralization in the Liaodong Peninsula related to continuous reactivation of the NCC, which indicates a great exploration potential of this area.

Sm–Nd Isochron Age Constraints of Au and Sb Mineralization in Southwestern Guizhou Province, China

Southwestern Guizhou, China, is famous for hosting clusters of Carlin-type Au, Sb, and Hg-Tl deposits. These deposits are thought to be the products of a low-temperature hydrothermal metallogenic event. Calcite and fluorite are common and widespread gangue minerals in Au and Sb deposits, respectively. Ore-related calcite commonly coexists with stibnite, realgar, and orpiment at the periphery of high-grade orebodies in Au deposits, while ore-related fluorite is generally intergrown with stibnite in Sb deposits. In this study, ore-related calcite and fluorite samples from representative Au (Zimudang) and Sb (Dachang) deposits, respectively, were separated, and the rare earth element (REE) concentrations, Sm/Nd isotope ratios, and Sm–Nd isochron ages were analyzed. This study aims to determine the formation ages of the calcite and fluorite and to constrain the age of low-temperature metallogenic event in Southwestern Guizhou. The calcite and fluorite samples contain relatively high total concentrations of REEs (8.21–22.5 μg/g for calcite, 21.7–36.6 μg/g for fluorite), exhibit variable Sm/Nd ratios (0.51–1.01 for calcite, 0.35–0.49 for fluorite), and yield Sm–Nd isochron ages of 148.4 ± 4.8 and 141 ± 20 Ma, respectively. These ages are consistent with the age range constrained by the low-temperature thermochronology of zircon (132–160 Ma), crosscutting relationships of stratigraphy or intrusions (96–160 Ma), and previous dating results (135–150 Ma) in Southwestern Guizhou. Collectively, the ages obtained in this study add new evidence to previous geochronology studies, such that the low-temperature hydrothermal mineralization in Southwestern Guizhou can be constrained to 135–150 Ma, corresponding to the Yanshanian orogeny, which was associated with a weak extensional tectonic environment.

Middle Jurassic orogeny in the northern North China block

Multiple shortening took place during the Mesozoic in the periphery of the North China block (NCB), a major tectonic grain of East Asia, but no consensus exists as to the timing, intensity, and causes of individual events. This work focuses on the first phase shortening in Jurassic, which represents the initiation of Yanshanian orogeny, by investigating the structural evolution of the Yanshan belt in the middle segment of the northern NCB. The first-phase shortening used to be regarded as a weak compressive event for it is recorded by parallel or low-angle unconformities between Middle and Upper Jurassic strata in some places. Our recent field investigations, however, identified some pronounced angular unconformities separating the Upper Jurassic strata from the underlying highly folded units in the northern Yanshan belt. The new observations demonstrate that the first-phase shortening was actually a strong contractional event. The co-existence of parallel and angular unconformities in the southern and northern Yanshan belt can be explained by different thrusting processes. It is argued that lateral variations of initial basement dips must have played a crucial role in controlling structural evolution of the overlying pre-Middle Jurassic sedimentary succession. It is shown that the first-phase thrusting and folding system was thin-skinned and predominantly verge northwestward. Deformational intensity also weakened toward the north and northwest. The first-phase shortening is constrained to occur in a short time span from 170 to 165 Ma based on new radiometric ages of strata above and below the unconformity, and presumably took place simultaneously in the northern NCB. The first-phase shortening was likely driven by horizontal tectonic push from the southeast. It is suggested that low-angle subduction of the paleo-Pacific plate might be responsible for the Middle Jurassic orogeny in the northern NCB and other parts of East Asia.

Yanshanian Orogeny During North China's Drifting Away From the Trench: Implications of Numerical Models

AbstractTectonics in North China and adjacent areas were dominated by the Pacific Plate subduction since the Jurassic. Although the extension‐dominated tectonics in North China since the Early Cretaceous can be well explained by the Pacific Plate subduction, it is still not well understood why North China and adjacent areas underwent intense compression (Yanshanian orogeny) during the Late Jurassic and why these regions rapidly switched to extension‐dominated tectonics during the Early Cretaceous. Plate reconstructions suggest that East Asia was moving rapidly in the direction opposite the trench during the Jurassic when North China was undergoing strong compressional deformation. This relationship between back‐arc deformation and overriding plate's motion seems to contradict previous compilations and geodynamic models. We suggest that large‐scale mantle convection and exotic terrane accretions along the East Asian margin have jointly contributed to the Yanshanian orogeny across North China during the Jurassic. We verify this hypothesis with numerical geodynamic models. Our numerical experiments also suggest that large‐scale downwelling mantle flow beneath Asia has promoted continued subduction beneath the Asian margin despite the subduction was often punctuated by collisional and accretionary events.

Multiple episodes of tectono-thermal disturbances in the Huayangchuan U-Nb-Pb polymetallic deposit in the Xiaoqinling region, central China and their significances on metallogeny

The Huayangchuan U-Nb-Pb polymetallic deposit is hosted in a suite of Neoarchean to Paleoproterozoic gneisses in the Xiaoqinling region, central China. The deposit is composed mainly of discrete small-scale carbonatite veins emplaced largely along shallow-lying brittle fractures. Various regimes of deformation from ductile flow to brittle fracturing are documented in the deposit and its adjacent areas. These include an earliest phase of contracting deformation (D1) manifested as well-developed boudinages, abundant relict rootless folds and penetrative compositional bands, i.e., gneissosity generally trending E-W, a subsequent phase of disperse dextral ductile shear (D2) along the E-W-trending gneissosity, and a following phase of general NE-SW compression (D3) resulting in local contraction deformation bands and abundant sinistral ductile shear along the E-W-trending gneissosity, all of which are affected by an overwhelming NNE-SSW compression (D4) with hemi-ductile to brittle deformation behaviors. The deformation after D4 is generally brittle and may arise from an earlier regime of dominant compression (NW-SE) (D5) and a later regime of extensive extension (D6). Multiple thermal events are superimposed on gneisses as revealed by 40Ar/39Ar dating on biotite collected from various lithologies in the deposit and its adjacent areas, which yields ages ranging from Late Triassic (238–203 Ma), through earliest Cretaceous (139–134 Ma) and middle Cretaceous (117–92 Ma), to latest Cretaceous (72 Ma) and even early Cenozoic (53 Ma). This sequence of thermal disturbances unlikely predates the D4 deformation given that it is the last ductile deformation experienced by the Precambrian rocks. Ore-bearing carbonatite veins have their shallowly lying attitudes consilient with the D4 and D5 deformation and ages compatible with the first two thermal overprints. Accordingly, a two-stage mineralization model is proposed for the Huayangchuan polymetallic deposit with an early phase of ore veining in the Late Triassic to latest Jurassic during the nearly N-S collision between the South and North China blocks and a late phase of ore veining or more possibly reworking in the Early Cretaceous related to the Yanshanian orogeny due to a generally NW-directed subduction of the Paleo-Pacific oceanic plate beneath the eastern Asia continent.

Geological control of the eastern Great Wall: Mountain-basin relationships in the eastern North China Craton

The E–W trending Yanshan belt, an intraplate fold-thrust belt located in the northern North China Craton, has experienced several episodes of Mesozoic deformation, which resulted in the widely distributed magmatism and mountain-basin tectonics that completely re-shaped the topography of the eastern North China Craton. The eastern part of the famous Chinese Great Wall was built on the high range of the southeastern Yanshan mountain belt juxta-posed to the plain, which directly relates to the Bohai Bay basin. Our study focuses on which tectonic processes created such mountain-basin couple in Mid-Late Mesozoic times. The U-Pb LA-ICP-MS dating of zircons yield ages of 114 to 201 Ma for various granites and 115 to 116 Ma for volcanic rocks from Yixian and Jiufotang Fms. The detrital zircons from the Lower Cretaceous sandstones yield four age groups of 2587 to 2460 Ma, 2222 to 1828 Ma, 297 to 190 Ma and 187 to 100 Ma, which are all sourced from the Qinglong and surrounding areas and indicating that the Qinglong area started to uplift after the Middle Jurassic. The Qinglong area underwent multiple deformation by NE–SW compression in the Middle-Late Jurassic times, WNW–ESE compression in the Late Jurassic to Early Cretaceous, ENE–WSW extension in the Early Cretaceous and NNW–SSE compression in the Late Cretaceous during the final stage of the Yanshanian orogeny. Meanwhile, widely distributed granite intrusions and emplacement of the Upper Jurassic-Lower Cretaceous volcanic rocks indicate a large amount of magma input into the area. The Qinglong area with the Great Wall along its southern margin close to adjacent plain to the south was uplifted to form the Qinglong highland and surrounding related basins by combination of the following three processes: the multiple tectonic Late Jurassic and Late Cretaceous shortening processes related to Yanshanian orogeny, Early Cretaceous regional extension triggered by slab-retreat of the Paleo-Pacific ocean and inflation of large amount magma at depth during Jurassic and Early Cretaceous.

Geological process of Late Paleozoic shale gas generation in the eastern Ordos Basin, China: Revelations from geochemistry and basin modeling

During the Meso-Cenozoic, the eastern Ordos Basin (EOB) underwent a complex tectonothermal process influenced by the destruction of North China Craton, significantly affecting the thermal maturation of the Late Paleozoic source rocks. Tectonothermal modeling and maturation of coal-bearing shales were performed in the EOB to reveal the geological process of shale gas generation. Organic geochemical data for shales show that TOC values vary from 0.96 wt% to 21.36 wt% and Ro values range from 1.56% to 2.47%, as well as that organic matter mainly consists of type III kerogen—indicating excellent potential, and sufficient maturity, for gas generation. Thermal modeling suggests that heat flow in the EOB significantly increased as a result of intense magmatic activities during the Yanshanian orogeny, with peak values of 108 and 128 mW/m2 in the northern and southern regions, respectively. Consequently, great variations in maturation history are seen in different regions of the EOB, with the Taiyuan and Shanxi shales reaching the wet gas and dry gas stages, respectively, in the northern region and all shales reaching the dry gas stage in the southern region. Hydrocarbons were generated at 237 Ma and peaked around 162 Ma, whereas hydrocarbon expulsion began at 230 Ma and peaked at 131 Ma. Rapid subsidence in the Triassic brought the shales to the oil generation window and Early Cretaceous magmatic activities led to rapid maturation of shales, again reaching the gas generation stage. Accordingly, extremely high levels of thermal maturity are well documented in hydrocarbons' alkane composition and stable carbon isotope ratio. Gas geochemical data (δ13C1 vs. C1/C2+3, δ13C1 vs. C2+) imply that that hydrocarbons are from highly mature thermogenic gas generated mainly from humus, and this offers great support for thermal maturation modeling. Regionally, coal-bearing shales have been steadily uplifted and eroded since the Late Cretaceous, causing partial destruction of gas reservoirs. The tectonothermal evolution and hydrocarbon generation of EOB were controlled primarily by magmatic thermal effects resulting from the westward subduction of the Pacific plate.

Late Jurassic Leucogranites of Macau (SE China): A Record of Crustal Recycling During the Early Yanshanian Orogeny

The Yanshanian Orogeny consists of Jurassic to Cretaceous compressional-extensional cycles related to the subduction of the Paleo-Pacific Ocean beneath East Asia. Numerous granitic plutons were emplaced across southeast China at distinct intervals over a period of ~100 million years that migrate from the interior to the coast. A key region to help constrain the secular evolution of granitic magmatism related to the Yanshanian Orogeny is the Peal River estuary as three distinct Mesozoic magmatic belts congregate. The islands of Taipa and Coloane, Macau Special Administrative Region, are located along the western bank of the Pearl River estuary and are composed of spessartine-bearing biotite leucogranite. In situ zircon geochronology yielded Late Jurassic weighted-mean 206Pb/238U ages of 160.0 ± 2.0 Ma and 161.5 ± 2.0 Ma that correlate to the late Early Yanshanian Orogeny. Inherited zircons of Middle Triassic to Middle Jurassic age were also identified. The rocks are peraluminous, ferroan, calc-alkalic to calcic and classify as post-collisional granite. Geochemical modeling indicates that the chemical variability of the rocks is related to hydrous fractional crystallization under reducing conditions (dFMQ -1) at ~7 km depth. The Sr-Nd isotopes (ISr = 0.71156 to 0.72477; eNd(t) = -7.9 to –8.6), zircon Hf isotopes (eHf(t) = -3.8 to -8.8), incompatible trace element ratios, and whole rock composition indicate that the parental magma was derived primarily from a sedimentary source. It is likely that the islands of Taipa and Coloane form a coherent plutonic complex that may be a member of larger batholith that extends across the Pearl River estuary to Hong Kong. The emplacement of the leucogranites is attributed to decompressional melting associated with a period of crustal relaxation or tensional plate stress that occurred during the transition from low angle subduction to high angle subduction of the Paleo-Pacific plate.

Caledonian (Early Paleozoic) veins overprinted by Yanshanian (Late Mesozoic) gold mineralization in the Jiangnan Orogen: A case study on gold deposits in northeastern Hunan, South China

The Jiangnan Orogen, situated between the Yangtze and Cathaysia blocks in South China, is an important gold-polymetallic metallogenic belt in China. Like many gold deposits hosted in Precambrian rocks in eastern China, the timing of gold mineralization in the Jiangnan Orogen, mostly hosted by Neoproterozoic rocks, has been controversial. A number of previous geochronological studies have suggested that mineralization ages range from Neoproterozoic to Cretaceous, with Caledonian (Early Paleozoic) and Yanshanian (Jurassic – Cretaceous) being considered most important. As Caledonian is the most important period of regional metamorphism, and Yanshanian is the most important period of granitic magmatism, the timing of mineralization is closely related to the genetic nature (orogenic versus post-orogenic) of the gold mineralization. This paper addresses these problems by studying the ages and characteristics of ore-forming fluids of two major gold deposits (Wangu and Huangjindong) in the central part of the Jiangnan Orogen, in northeastern Hunan. Muscovite Ar-Ar dating suggests that the bulk of the quartz veins in the Wangu and Huangjindong deposits were formed at ca. > 399 – 397 Ma, i.e., during the Caledonian orogeny, whereas gold mineralization (and associated veining) took place at ca. 142 – 130 Ma, i.e., during the Yanshanian orogeny. Fluid inclusions in the barren “Caledonian veins” are characterized by the H2O-NaCl-CO2 system, homogenization temperatures of 266 – 276 °C and salinities of 5 – 5.4 wt% NaCl equivalent, similar to those found in many orogenic-type gold deposits, whereas the fluid inclusions in the gold-bearing “Yanshanian veins” (with abundant sulfides) overprinting the barren veins have a H2O-NaCl composition system, homogenization temperatures of 169 to 256 °C, and salinities from 8.7 to 15.2 wt% NaCl equivalent. It is proposed that both the Caledonian and Yanshanian veining events are controlled by the same fracture systems, with the former being formed in a relatively deep, orogenic deformation-metamorphic environment, and the latter in a relatively shallow, extensional environment accompanied by extensive granitic magmatism. Reactivation of the Caledonian structures during the Yanshanian orogeny, together with availability of sulfur and gold, possibly associated with granitic intrusions, played a key role in the formation of the gold deposits in northeastern Hunan and possibly elsewhere in the Jiangnan Orogen.

Secular Variation of Early Cretaceous Granitoids in Kyushu, SW Japan: The Role of Mélange Rocks as a Possible Magma Source

The Early Cretaceous volcanic-arc granitic rocks from Kyushu, SW Japan are contemporaneous with the granitic rocks of the Yanshan Orogeny (SE China) along the eastern Eurasian continental margin. The secular geochemical variations of the whole-rock major elemental and Sr-Nd isotope data of the Early Cretaceous granitic rocks from Kyushu, SW Japan, as well as the zircon and apatite saturation temperatures, shows distinct changes during the Albian (~115 Ma to ~100 Ma) as: (1) the mASI value of the rocks (i.e. Shiraishino granodiorites) decreases below 1, (2) the Sr-Nd isotopic data are relatively constant (87Sr/86Sri = 0.70471 to 0.70573; Nd(t) = +0.2 to +1.9) within different rock types including granites, granodiorites tonalites and adakitic rocks (i.e. the Shiraishino granodiorites), following the increase of 87Sr/86Sri and decrease of Nd(t) from Berriasian, and (3) higher maximum temperatures at ~105 Ma. The secular changes indicate that important geodynamic changes occurred in the arc system of SW Japan as it changed from subduction-accretion during the Jurassic to continental arc during the Early Cretaceous. Thermodynamic partial melting modelling demonstrates that the Albian granitic rocks can be derived from melange rocks, such as chlorite-actinolite schists, at moderate depth and variable redox conditions. It is concluded that the genesis of the Early Cretaceous granitic rocks from Kyushu, SW Japan, may be related to upwelling of the asthenosphere and hot corner flow into the mantle wedge caused by slab rollback, which followed a shallowing of the subduction angle and subsequent flat-slab subduction during the Late Jurassic. The resultant heat induced the partial melting of the melange rocks that formed on and were transported from the subducted plate interface.

Crustal evolution events in the Chinese continent: evidence from a zircon U-Pb database

ABSTRACT The zircon U-Pb chronology database provides a good opportunity to obtain important zircon growth peak periods in the Earth's history so as to study the origin and evolution of the crust. It should be noted that research preference affects the objectivity of zircon sampling, leading to hot data in the database and age statistics. To evaluate the influence of hot data on statistical results, the W and Y indexes are introduced. Using a Gaussian model of multipeak fitting of zircon U-Pb age frequencies, we identify seven major growth peaks in zircons from the Chinese continental crust, which are 2498.95, 1855.82, 828.88, 444.29, 249.46, 131.96, and 58.21 Ma. Due to differences in the time scales of zircon growth peaks, these peaks can be divided into two categories: first-order zircon growth peaks (I) and second-order zircon growth peaks (II), which represent longer and shorter time scales, perhaps due to different kinds of geological dynamics, respectively. In addition, there are clear correspondences between these ages and various geological events recognized by most scholars, namely, the Wutai orogeny, Lvliang orogeny, Jinning orogeny, Caledonian orogeny, Indosinian orogeny, Yanshanian orogeny, and Himalayan orogeny, respectively.

Jurassic intracontinental deformation of the central North China Plate: Insights from syn-tectonic sedimentation, structural geology, and U[sbnd]Pb geochronology of the Yungang Basin, North China

Jurassic tectonism resulted in late Mesozoic intracontinental deformation of the North China Plate. A fold-and-thrust belt, related growth strata, and volcanic materials have been identified in the Jurassic Yungang Basin, which can be used to constrain the timing and scale of faulting and sedimentation. The age of volcanic materials in the growth strata and the sedimentary cycle of Jurassic strata indicate that the late Middle Jurassic Yungang Formation was the earliest sedimentary response to the onset of Jurassic deformation (i.e., the Yanshanian orogeny) from 170 to 160 Ma. Deformation involved detachment and fault propagation folding, which produced a marginal thrust belt and limb rotation of anticline. Restored cross-sections show that the northern basin experienced more intense shortening than the southern basin, consistent with field observations, possibly controlled by the Qifengshan strike-slip fault in the center of the fold-and-thrust belt. Fault striations, conjugate joints, and asymmetric folds show that the Yungang Basin underwent NW–SE-trending compression during the Middle–Late Jurassic. Therefore, we propose that the eastern Ordos Block was in a compressional setting during the Middle–Late Jurassic, due to flat subduction of the Paleo-Pacific Plate beneath the East Asian continent.

Decoding stratigraphic and structural evolution of the Songliao Basin: Implications for late Mesozoic tectonics in NE China

An analysis of the structural styles and sedimentary filling of the Songliao Basin (SB) in the late Mesozoic was carried out based on a detailed interpretation of a large number of 2D and 3D seismic reflection profiles. Using precise stratigraphic chronology, it was established that the SB mainly experienced five stages of evolution in the late Mesozoic, including thermal uplifting in the Late Jurassic, rifting in the Early Cretaceous, symmetrical subsidence early in the early Late Cretaceous, asymmetrical subsidence in the middle Late Cretaceous, and inversion at the end of Cretaceous. The structures contained listric faults and planar domino-style normal faults in the rifting stage; dense normal faults with small throw, as well as negative flower structures in the symmetrical subsidence stage; positive flower structures in the asymmetrical stage; and compression and inversion structures (e.g., angled unconformities and folds) in the inversion stage.Two angular unconformities identified in this area, T5 and T02, may reflect orogenic events caused by Paleo-Asian Ocean closure and subduction of the Paleo-Pacific Ocean Plate beneath northeastern Asia from SE to NW. The compressive stress of the Yanshanian orogeny in the Yanshan tectonic belt originated from the subduction of the Paleo-Pacific Ocean Plate rather than the closure of the Mongol-Okhotsk Ocean. Although the SB had already been influenced by Paleo-Pacific Ocean Plate subduction in the middle Late Cretaceous, the stress did not cause the inversion of the SB. The SB was affected by Paleo-Pacific Ocean Plate subduction during the deposition of the second member of the Nenjiang Formation in the Upper Cretaceous; however, significant orogenic events and obvious inversion of the SB did not take place until the end of the Late Cretaceous.

Enrichment origin of critical elements (Li and rare earth elements) and a Mo-U-Se-Re assemblage in Pennsylvanian anthracite from the Jincheng Coalfield, southeastern Qinshui Basin, northern China

The Qinshui Basin is the largest reservoir of commercial coalbed methane in China. The No. 15 seam, one of the major coal seams for coalbed methane exploration in the Jincheng Coalfield, SE Qinshui Basin, has recently been found to be enriched in the critical element Li. This study further investigates the inorganic geochemistry of the No. 15 coal and associated rocks and reveals the enrichment origin of critical elements in these horizons. The Jincheng No. 15 coal is rich in Li, Mo, U, Se, Re, and rare earth elements and Y (REY). Concentrations of Mo, U, Se, and Re are higher in the two topmost coal benches, probably due to marine transgression in the early diagenetic stage after the peat had been established. The enrichment of Li and REY in a few lowermost coal benches is suggested to be derived from hydrothermal fluids that were related to magmatic activity (110–140 Ma) during the Yanshanian Orogeny (Late Jurassic-Early Cretaceous), or a Late Triassic-Early Jurassic abnormal thermal event occurring at 190–215 Ma. Light REY (La, Ce, Pr, and Nd) in the Jincheng No. 15 coal are predominantly associated with phosphate minerals. Heavier REY (Sm to Lu) show a mixed phosphate and organic affinity in the studied coal samples. Non-coal samples are characterized by light REY- and HREY-enrichment, which is probably inherited from the source material in the Trans North China Orogen.Trace elements in the coal can be highly enriched in the ash if the coal is combusted due to the low to medium ash yield of the coal. Critical elements, including Li, Ga, and REY, in the Jincheng No. 15 coal are considered as promising material for industrial recovery and have been previously under-estimated. On the other hand, environmentally sensitive elements including As, Hg, and Pb, while having concentrations within the normal range compared with average worldwide coals, can be highly enriched in the coal combustion residues if the coal is combusted, and thus careful consideration needs to be given to the utilisation of such coals.

Origin of the Xiaohekou skarn copper deposit and related granitoids in the Zha-Shan ore cluster area, South Qinling, China

The Xiaohekou copper deposit is one of the few typical skarn-type, commercial deposits in the Zha-Shan ore cluster area in the eastern part of South Qinling. The ore-related granitoids in the Xiaohekou mining district include a number of small plutons of granodiorite porphyry and granite porphyry, as well as numerous dykes. They are characterized by relatively low SiO2 (65.9–73.3 wt%) and MgO (0.26–1.72 wt%) contents, variable Mg# (29.4–64.0) values, high K2O (3.11–5.75 wt%) content and negligible δEu (1.00–1.15) values. The samples also exhibit depletion in Cr, Ni, Nb, Ta, and Ti content, and high Sr/Y (46.6–64.7) and La/Yb (17.0–25.0) ratios. These features are similar to those of high-K calc-alkaline adakite-like magma formed by partial melting of thickened lower crust in continental collision zones or intracontinental settings. The majority of the samples have TDM2(Hf) of 1.5–1.2 Ga, corresponding to weakly negative εHf(t) values (−4.75 to −0.13) typical of crustal material, but more than one quarter have TDM1(Hf) of 0.80–0.71 Ga, corresponding to positive εHf(t) values of +0.07 to +2.45, indicating that the granitoids resulted from partial melting of Mesoproterozoic lower crustal rocks with significant input from enriched lithospheric mantle-derived magmas. Zircons from the Xiaohekou granitoids have ΔFMQ values concentrated around −0.62 to +5.54, most of which show the magma fO2 of FMQ to HM, in accordance with the widespread occurrence of magnetite, hematite and specularite in the Xiaohekou skarn system, which is an indicator of relatively high fO2 for a magmatic–hydrothermal system. The granitoids exhibit similar or even higher Ce4+/Ce3+ (average of 452) and Eu/Eu* (average of 0.72) values in comparison with those of fertile Cu deposits worldwide, indicating that the Xiaohekou granitoids may possess the potential to form a large magmatic–hydrothermal Cu deposit. Zircons of the granitoids show obvious positive correlations between oxidation indices (Ce and Eu anomalies, Ce4+/Ce3+ ratios, and ΔFMQ values) and εHf(t) values, implying the involvement of an oxidized mantle component which might have caused the fO2 elevation and played an important role in the formation of Cu deposits. The zircon U–Pb ages (141.3 ± 1.3 and 138.1 ± 2.0 Ma) of the Xiaohekou ore-related granitoids obtained in this study are similar to the zircon U–Pb and molybdenite Re–Os ages yielded from porphyry Mo mineralization at the Chigou and Lengshuigou deposits in the Zha-Shan ore district, indicating that they are possibly part of a single tectonic, magmatic, and metallogenic event in the Yanshan orogeny. The ore-related granitoids in the Xiaohekou area were formed in a post-collisional compression–extension transition regime during the Early Cretaceous Qinling intracontinental orogeny. The orogeny can be attributed to the far-field effect of a switch from pre-Mesozoic Tethyan tectonics to Late Mesozoic Pacific tectonics. This triggered a stress field transition from N–S to E–W trending and from intracontinental subduction thickening to large-scale lithospheric delamination and thinning.

Late Mesozoic tectonostratigraphic division and correlation of the Bohai Bay basin: Implications for the Yanshanian Orogeny

We present the results of Mesozoic sequences of the Bohai Bay basin in North China, based mainly on geochronology and interpretations of seismic profiles and logging data. Five tectono-stratigraphic sequences are defined: Lower-Middle Triassic, Lower-Middle Jurassic, Upper Jurassic, Lower Cretaceous and Upper Cretaceous. Based on an analysis of detrital zircons, the clastic rocks recorded two intervals of Jurassic magmatic activity, during 180–175 and 160–152 Ma, which can be correlated to the Nandaling and Tiaojishan Formations in the Yanshan area, respectively. However, since Jurassic volcanic rocks have not yet been found in the Bohai Bay basin, we speculate that these two stages of activity were mainly concentrated around the periphery of the North China Craton (NCC) rather than within. Based on an analysis of zircons from volcanic rocks and pyroclastics, the early Cretaceous magmatism in the Bohai Bay basin can be divided into two stages, 125–120 and 110–100 Ma, which can be correlated to magmatism in the eastern part of the NCC. The zircon ages indicate an absence of volcanic activity during the late Tuchengzi and Zhangjiakou periods which may correlate to the uplift of the Bohai Bay basin in the late Late Jurassic. Comparison of the development of Mesozoic basins and sedimentary strata in the central-eastern part of Yanshan tectonic belt and the the Bohai Bay basin indicates that the two areas are generally comparable, but with substantial differences. The central-eastern part of the Yanshan structural belt lacks Early-Middle Triassic strata, and the Bohai Bay basin lacks late Jurassic-early Cretaceous strata. Based on research results from late Mesozoic sedimentary structures in the central and eastern parts of the Yanshan tectonic belt, we infer that episode A of the Yanshanian Orogeny was weak in the Bohai Bay basin and its periphery, while episode B of the Yanshanian Orogeny had a strong influence on the Bohai Bay basin and its periphery. The available data reveal differences in the expression of these episodes: the Bohai Bay basin is characterized by vertical uplift, and the northern margin of NCC is characterized by horizontal compression uplift.

Structural style and kinematics of the Taihang-Luliangshan fold belt, North China: Implications for the Yanshanian orogeny

AbstractThe Middle–Late Jurassic to earliest Cretaceous fold belts of the Yanshanian orogen in North China remain enigmatic with respect to their coeval deformation histories and possible relationship to the contemporaneous Cordilleran-style margin of eastern Asia. We present geological mapping, structural data, and a >400-km-long, strike-perpendicular balanced cross section for the Taihang-Luliangshan fold belt exposed in the late Cenozoic central Shanxi Rift. The northeast-southwest–trending Taihang-Luliangshan fold belt consists of long-wavelength folds (∼35–110 km) with ∼1–9 km of structural relief cored by Archean and Paleoproterozoic metamorphic and igneous basement rocks. The fold belt accommodated ≥11 km of northwest-southeast shortening between the Taihangshan fault, bounding the North China Plain, in the east and the Ordos Basin in the west. Geological mapping in the Xizhoushan, a northeast-southwest–oriented range within the larger Taihangshan mountain belt, reveals two major basement-cored folds: (1) the Xizhou syncline, with an axial trace that extends for ∼100 km and is characterized by a steep to overturned forelimb consistent with a southeast sense of vergence, and (2) the Hutuo River anticline, which exposes Archean–Paleoproterozoic rocks in its core that are unconformably overlain by shallowly dipping (<∼20°) Lower Paleozoic rocks. In the Luliangshan, Mesozoic structures include the Luliang anticline, the largest recognized anticline in the region, the Ningjing syncline, which preserves a complete section of Paleozoic to Upper Jurassic strata, and the Wuzhai anticline; together, these folds are characterized by a wavelength of ∼45–50 km. Shortening in the Taihang-Luliangshan fold belt is estimated to have occurred between ca. 160 Ma and 135 Ma, based on the age of the youngest deformed Upper Jurassic rocks in the Ningjing syncline, previously published low-temperature thermochronology, and regional correlations to better-studied Yanshanian fold belts. The timing of basement-involved deformation in the Taihang-Luliangshan fold belt, which formed >1000 km from the nearest plate margin, corresponds with the termination of arc magmatism along the eastern margin of Asia, implying a potential linkage to the kinematics of the westward-subducting Izanagi (paleo-Pacific) plate.

Coal-bed methane geology of the No. 2 coal seam in Fengfeng Coalfield, North China

The Fengfeng Coalfield is an important coal production base in the Hebei Province of North China. It has been reported that higher coal-bed methane (CBM) contents exist in this area showing good prospects for the exploration of CBM. However, there are few studies on CBM investigation in Fengfeng Coalfield. In order to better understand the geological controls on CBM in the Fengfeng Coalfield, geological surveys and laboratory experiments were conducted that included coal petrology analysis, proximate analysis, mercury intrusion, low-temperature liquid nitrogen adsorption, coal permeability analysis, and methane adsorption/desorption. The results show that the organic compositions are dominated by vitrinites, followed by inertinites, and minor liptinites. The CBM contents are range between 0.83 and 11.52 m3/t, with an average of 4.72 m3/t. The moisture contents are range from 0.76 to 2.54%, with an average of 1.21%, and the ash yields are range from 11.46 to 24.5%, with an average of 17.07%. Low-temperature nitrogen gas adsorption indicates the micropores provide most of the pore specific surface and the pore volume is mainly determined by small pores and mesopores. The geologic structures, coal-bearing strata, and the coal depositional environments were studied through field geological research and laboratory tests. The coals were deposited in a river-dominated shallow delta environment to form the mudstones or siltstones of the coal roof and floor that were used to seal the CBM. The CBM were mainly generated during stages of the Hercynian–Indosinian and the Early-Middle Yanshanian orogeny. Underground water flowed to the Dashucun Mine and its adjacent mines in the northeast of the Gushan Anticline to form a semi-closed hydrodynamic system that created the CBM emission and preservation. The coal rank increases from the southern to the northern areas. Then, it can be speculated that the CBM formation and preservation conditions in the north are better than in the south. The best prospective targets for CBM are in the northern areas. The results of this study may be useful for CBM exploration in North China.