Yanshan Fold Research Articles

Petrogenesis and Tectonic Implications of the Early Triassic Nianzi Adakitic Granite Unit in the Yanshan Fold and Thrust Belt: New Constraints from U‐Pb Geochronology and Sr‐Nd‐Hf Isotopes

AbstractThe Nianzi granite unit, which includes the Nianzi, Xiaolianghou and Xiawopu granitic intrusions, is a significant component of the northern part of the North China Craton (NCC) and is situated in the Yanshan fold and thrust belt (YFTB). However, there is still debate regarding the tectonic evolutionary history of the YFTB during the late Permian to Triassic period, specifically regarding the timing of subduction and collision between the NCC and the Paleo‐Asian Ocean. The Nianzi granite unit exhibits unique petrological, geochronological and geochemical signatures that shed light on the tectonic evolutionary history of the YFTB. This study presents detailed petrology, whole‐rock geochemistry, together with Sr‐Nd isotopic, zircon U‐Pb dating and Lu‐Hf isotopic data of the granites within the Nianzi granite unit. Our findings demonstrate that the granites primarily consist of subhedral K‐feldspar, plagioclase, quartz, minor biotite and hornblende, with accessory titanite, apatite, magnetite and zircon. Zircon U‐Pb dating indicates that the Xiaolianghou granite was emplaced at 247.5 ± 0.62 Ma. Additionally, the adakitic characteristics of the Nianzi, Xiawopu and Xiaolianghou granitic intrusions, such as high Sr and Ba contents and high ratios of Sr/Y and (La/Yb)N, combined with negative Sr‐Nd and Lu‐Hf isotopes (87Sr/86Sr)i = 0.705681–0.7057433, εNd(t) = –21.98 to –20.97, zircon εHf(t) = –20.26 to –9.92), as well as the I‐type granite features of high SiO2, Na2O and K2O/Na2O ratios, enriched Rb, K, Sr and Ba, along with depleted Th, U, Nb, Ta, P and Ti, suggest that the Nianzi granitic unit was mainly derived from the partial melting of a thickened lower crust containing hydrous, calc‐alkaline to high‐K calc‐alkaline, mafic to intermediate metamorphic rocks. In light of these parameters, we further integrate our data with previous studies and conclude that the Nianzi granitic unit was generated in a post‐collisional extensional environment during the Early Triassic.

Apatite geochemical and Sr-Nd isotopic constraints on the source and petrogenesis of alkaline volcanic-plutonic ring complex

Alkaline rocks commonly occur as volcanic-plutonic ring complexes, but the petrogenetic mechanism for the coexistence between volcanic and plutonic rocks remains controversial. The Early Cretaceous Houshihushan alkaline ring complex in the Yanshan Fold and Thrust Belt, China, contains arfvedsonite granites surrounded by syenites, trachytes and alkali rhyolites. In situ trace elements and Sr-Nd isotopic compositions of apatites from these rocks are presented to constrain the petrogenetic link between coeval alkaline volcanic and plutonic rocks. Petrographic and geochemical features suggest that apatites in the trachyte crystallized from the melt at an early stage. Their initial 87Sr/86Sr ratios (0.7048–0.7052) and εNd(t) values (−21.9 to −17.6) well constrain the Sr-Nd isotopic compositions of the lithospheric mantle source for the trachyte, better than those of whole-rock (0.7114 and −18.6, respectively). In contrast, apatites in the syenites crystallized from host melt throughout the whole history of magma evolution. Their geochemical features and variable initial 87Sr/86Sr ratios (0.7084–0.7175) and εNd (t) values (−16.0 to −12.7) suggest that the syenites were formed by the cumulation of their parental magma, coupled with gradually mixed by felsic magma with a high initial 87Sr/86Sr ratio. However, a few and small-size apatite grains crystallized from parent magma of the arfvedsonite granites and alkali rhyolites suggest that in situ geochemistry of apatites is not feasible to constrain the petrogenetic process of high-silica alkaline rocks. Combined with whole-rock data, the geochemical features and Sr-Nd isotopic compositions of apatites indicate a crystal mush model to interpret the genetic link between syenites and arfvedsonite granites and alkali rhyolites. The early accumulations of mixing mafic and felsic magmas formed syenites at the margin, while the residual silicic melts solidified as arfvedsonite granites at the core. After the exposure of the intrusive rocks by rapid exhumation, the injection of high-temperature mantle-derived magma into the crystal mush promoted the extraction of the residual melt from the magma chamber. The mantle-derived magmas and residual melts directly erupted to the surface to form trachyte and rhyolite in the middle, eventually forming the alkaline ring complex.

Late Mesozoic impact of paleo-Pacific subduction on the North China craton revealed by apatite U-Pb and fission-track double dating and trace element analysis in the eastern Yanshan fold belt, northeastern Asia

The late Mesozoic tectonic evolution of the eastern North China craton was intimately related with the subduction of the paleo-Pacific plate. This study sheds light on Late Jurassic−Early Cretaceous basin-mountain coupling during paleo-Pacific subduction based on low-temperature thermochronology and geochemical analyses from the easternmost Yanshan fold belt, located at the northern edge of the craton. We performed apatite U-Pb and fission-track double dating and trace element analyses of basement and sedimentary rock samples, integrated with zircon and apatite (U-Th)/He analyses on bedrock samples. Our results revealed that the easternmost Yanshan fold belt experienced two stages of rapid cooling and exhumation in the Middle Jurassic−earliest Cretaceous (ca. 170−140 Ma) and in the Early Cretaceous (ca. 140−90 Ma). The Middle Jurassic−earliest Cretaceous exhumation (ca. 170−140 Ma) is mainly recorded by bedrock along the margins of intermontane basins of the eastern Yanshan fold belt. This exhumation event is consistent with compressional Yanshan movement during the Middle Jurassic−earliest Cretaceous coeval with paleo-Pacific flat-slab subduction. Subsequent Early Cretaceous rapid cooling is ascribed to the progressive evolution of the metamorphic core complexes and associated tectonic exhumation of major plutons of northeastern Asia, which were likely controlled by rollback of the paleo-Pacific slab. A middle Cretaceous (ca. 110−90 Ma) tectonic exhumation event is revealed by very low lag-time values in middle Cretaceous strata. Our findings illustrate the potential of a thermochronology approach that combines single-grain double dating and trace element analyses in synmagmatic orogenic systems, which may find application to other orogenic settings worldwide.

Improved 3D Shallow‐Deep Vs Structure in Tongzhou, Beijing (China), Revealed by Dense Array Ambient Noise Tomography

AbstractShear‐wave velocity (Vs) structures can reveal the shallow sediment thickness and deep tectonic features of buried faults and geological units. They are important for reducing seismic and geological disasters in urban areas. Based on ambient noise data from the Tongzhou dense array (919 seismographic stations), we obtain a fine shallow‐deep (0–5 km) 3D Vs model, by jointly inverting the phase‐velocity dispersions of Rayleigh waves, including short‐period (0.3–2 s) multimode dispersions using the frequency‐Bessel transform method and the long‐period (2–6 s) fundamental‐mode dispersions using the fast marching method. Our results show that the Vs inhomogeneities agree well with the distribution of geological units. We use the 1 km/s isodepth of Vs as the reference thickness of quaternary sediments. Fengbo sag (FBS) and Dachang sag (DCS), which mainly show low velocity and density, have thick sediment thicknesses (approximately 550–600 and 320–420 m, respectively). The NE high‐velocity belt in Daxing high (DXH) has a thinner sediment thickness (∼230 m). Thus, FBS and DCS have a greater risk of earthquake hazards owing to the strong amplification effects of ground motion. Additionally, Vs distribution in the FBS, DCS, DXH, and Yanshan Fold Belt are spatially related to the medium density and buried faults (Nanyuan‐Tongxian, Daxing, and Xiadian faults). We infer that the Vs structures are associated with the controlling effects of these large normal faults and inhomogeneous strata density. The discontinuity of the NE high‐velocity belt in DXH probably results from the intense tectonic activity of Nankou‐Sunhe fault.

Influence of Sublacustrine Fan Depositional Model on Distribution and Morphology of Reservoirs: A Case Study in Eastern Slope of Liaoxi Uplift, Bohai Bay Basin, East China

The study on morphology and distribution of sublacustrine fan are necessary for the exploration of oil and gas, which could help to effectively predict the reservoirs of sublacustrine fans. In this paper, the distribution and geomorphology of sublacustrine fans of Dongying Formation in Liaoxi uplift (Bohai Bay Basin, East China) and their controlling factors (including shape and formations) have been analyzed in detail based on seismic, core, and logging data. The main conclusions achieved in this study are: (1) During the sequence of the third member of Dongying Formation (SQd3), two types of sublacustrine fan, including channelized fan (in channel shape) and non-channelized fan (in tongue shape and lobe shape), developed on the eastern slope of Liaoxi uplift, which inherited the characteristics of sediments structural maturity in braided river delta front (good sandstone sorting and high structural maturity); (2) Steep slope was favorable for forming tongue shape sublacustrine fans with large ratio of length and width, while gentle slope tended to deposit lobe shape fans; high mud content tended to form stable channels with strong erosion on the slope end, while high sand content tended to form continuous lobes with lobe shape; (3) In the basin with uplift, the beneficial combination among provenance, relative lake level change and paleomorphology, determines the development and distribution of sublacustrine fan, and the sublacustrine fan deposits are mainly concentrated in the TST. The Yanshan fold belt in the west provided sufficient sediments to the Liaodong Bay during LST for the development of a sublacustrine fan in the east slope of Liaoxi uplift since the Liaoxi uplift sunk into the water, with the result that the deposition of braided river delta front can overlap the uplift. The incised canyons in the Liaoxi uplift provided the channels for sediments entering into the eastern slope, and the main sedimentary location of lacustrine fans was between two stages of faults. This study could provide a theoretical basis for researching the characteristics and distribution of other sublacustrine fans in similar basin backgrounds.

Revisiting the Late Jurassic adakitic rocks in the Yanshan fold and thrust belts, North China Craton: Partial melts from thickened continental crust?

An integrated study of zircon U-Pb ages and Hf-O isotopes, whole rock and apatite geochemistry, and whole rock Sr-Nd-Hf isotopes were conducted for the Jurassic adakitic volcanic rocks from the Tiaojishan Formation in the Yanshan Fold and Thrust Belt (YFTB) of the North China Craton, to investigate their genesis, with attention to the role of crust thickness in achieving the adakitic traits. Zircon U-Pb dating gives eruption ages of 162–158 Ma for these rocks. These rocks have variable SiO 2 (55.7–68.1 wt%) and MgO (0.35–4.0 wt%), and adakitic trace element signatures including high Sr/Y (27.9–100) and (La/Yb) N ratios (9.6–27.6). Chemical variations in whole rock and apatite suggest a feldspar-dominated fractional crystallization, superimposed by variable mineral accumulation on whole rock compositions. Together with isotopic compositions that include evolved whole rock ( 87 Sr/ 86 Sr) i = 0.70447–0.70591, ε Nd (t) = −17.5 to −12.1, and ε Hf (t) = −22.2 to −11.2, and zircon ε Hf (t) = −26.8 to −11.2 and δ 18 O = 5.7–7.0 ‰, we attribute these rocks are sourced from mafic lower continental crust. By comparing the trace element signatures between the examined samples and modern continental arc magmatic rocks formed at variable crustal thickness, it is suggested that these rocks are most likely originated from a thickened crust. A further estimate based on whole rock Sr/Y and La/Yb gives a crust thickness of 56 ± 7 km. The crust thickening and associated uplift of the YFTB could be attributed to the westward subduction of the Paleo-Pacific plate. This led to a high altitude (∼3.8 ± 0.9 km) and cool climate habitat (a mean annual paleotemperature of 7 ± 7 °C) for the Yanliao Biota lived therein during this period, which may have played an important role in the earliest appearance of the feathered characteristic of dinosaurs. • A thickened lower continental crust source is responsible for the volcanic rocks. • Crust thickening and uplift are related to the subduction of the Paleo-Pacific plate. • Uplift-induced cooling may be important for the appearance of feathered dinosaurs.

Late Mesozoic Intracontinental Deformation in the Northern Margin of the North China Craton: A Case Study From the Shangyi Basin, Northwestern Hebei Province, China

During the late Mesozoic, the East Asian continent underwent a complex tectonic history due to multiple episodes of plate convergence. How the crust responds to the multiple plate convergence in the North China Craton (NCC) remains unclear. Here we undertook field geological investigations and fault-slip vectors analysis of the Shangyi Basin in the western Yanshan fold and thrust belt, northern margin of the NCC. Combined with new geochronological data, we delineate three phases of intracontinental deformation in the area: 1) NW-SE compression during the Late Jurassic to earliest Cretaceous (ca. 151–141 Ma); 2) NW-SE extension during the middle–late Early Cretaceous (ca. 135–110 Ma); and 3) NE-SW compressional deformation later than 110 Ma. The early NW-SE compression controlled the present bulk architecture of the basin, and the subsequent two tectonic events only caused limited reworking of the previous structures. Through balanced cross-section restoration, we estimate the horizontal shortening ratio of the crust in the study area is over 27% due to the NW-SE compression. Moreover, the contribution of tectonic shortening from the north side of the basin is greater than that from the south side. NW-SE compressional deformation is consistent in time with the episode B of the Yanshanian movement (Yanshanian B), which may be influenced by the subduction of the Paleo-Pacific plate beneath East Asia and the closure of the Mongol-Okhosk Ocean. Subsequent NW–SE extension is likely to be associated with the destruction of the NCC during the Early Cretaceous. Extension may result from the roll-back of the Paleo-Pacific plate and post-orogenic collapse of the Mongol-Okhotsk belt. The last NE-SW compressional event may be linked to the remote effect of the final collision between the Qiangtang and Lhasa terranes.

The Late Jurassic extensional event in the Yanshan fold and thrust belt (North China): New insights from an integrated study of structural geology, geophysics, and geochemistry of the Siganding granitic pluton

The Yanshan fold and thrust belt (YFTB) was built up by multiple contractional events, namely the “Yanshan Movement”, which was traditionally subdivided into the “Yanshan-A” and “Yanshan-B”. In contrast with the better constraint on “Yanshan-B” (ca.140 Ma), the timing of “Yanshan-A” is still in dispute. The Middle Jurassic contraction (172–160 Ma) was widely accepted, however, whether this contraction can last into 150 Ma is still unclear. In this article, to clarify the Late Jurassic tectonic regime in the YFTB, a multidisciplinary study, including structural geology, Anisotropy of Magnetic Susceptibility (AMS), gravity modeling, geochronology, and geochemistry, has been carried out to investigate the emplacement mechanism of the Siganding pluton (ca. 161–157 Ma) in the northern Beijing area. The magma was predominantly derived from the Precambrian continental lower crust. The magnetic foliations are pluton margin-parallel and outward dipping, resembling the fabric pattern of magmatic dome. The NE-SW striking magnetic lineations and NW-SE linear feeder zones revealed by gravity modeling indicate that the emplacement of the Siganding pluton was controlled by a weak NE-SW crustal stretching interpreted as a regional tectonic extension. Considering that the Late Jurassic (164–153 Ma) NE-SW oriented extension structures are not only documented in the YFTB, but also the entire eastern NCC, hereby we propose that the “Yanshan-A” is constrained at 172–164 Ma, instead of a wide range from the Middle Jurassic to Late Jurassic (172–150 Ma) as argued by previous workers. The extension could be ascribed to the oblique subduction of the Izanagi plate.

Geomorphological evolution and sediment dispersal processes in strike-slip and extensional composite basins: A case study in the Liaodong Bay Depression, Bohai Bay Basin, China

Abstract The geomorphological evolution and sediment dispersal processes of the Liaodong Bay Depression (LBD), a strike-slip and extensional composite basin in the Dongying Formation, were studied using 3D seismic, drilling, and logging data in an attempt to understand the diversity of sand-controlling factors and the complexity of the sand dispersal processes. The LBD was mainly supplied by sediments from the eastern side, which originated in the Yanshan Fold system (comprising the Dashi River, Liugu River, and Lin River systems) and sediments from the western side, which originated from the Jiaoliao Uplift system (comprising the Daqing River and Fuzhou River systems). Different sediment provenances had various sediment supply capacities depending on the composition of the source rocks. Sediments were injected into the basin through the channels identified from two interpreted southwest-to northwest-orientated seismic sections. Strata of the Dongying Formation could be divided into three sequences, and comprised shallow-deep lacustrine deposits, braided river delta deposits, fan delta deposits, and sublacustrine fan deposits. Based on the established sequence stratigraphic framework, the strata thickness distribution and sequence of geomorphological evolution could be restored from 3D seismic data of the LBD. By analyzing the distribution and migration of depocenters and the distribution of channels, sediment pathways in the basin could be determined under high flux sediment supply. In terms of five interpreted seismic sections from north to south, we demonstrated that in addition to immense changes in tectonic depositional style in the eastern areas of the basin, the sediment dispersal processes in different areas exhibited prominent discrepancies in sediment accumulations from diverse depositional systems. Tectonic activities, efficient sediment provenance, distribution of channels, and sequence geomorphology jointly controlled the sediment dispersal processes in the deposition areas and determined the potential reservoir distribution.

Growth structures and growth strata of the Qianjiadian Basin in the western Yanshan fold and thrust belt, North China

The northwestward subduction of the Izanagi Plate beneath the eastern Eurasian Plate during the Late Mesozoic caused a series of compressional deformation events in the Yanshan fold and thrust belt (YFTB), but the tectonic deformation timing, tectonic properties and relationship between tectonic uplift and sediment accumulation in the intermontane basins continue to be debated. For this reason, the sedimentology, sediment provenance, and basin structure of the Qianjiadian Basin (QJB) in the northern Beijing region during the Late Jurassic and Early Cretaceous were studied in detail. The results suggest that a fault-propagation fold-type (FPFT) growth structure and growth strata developed on the western edge of the QJB and that the top part of Member 2 and Member 3 of the Tuchengzi Formation are growth strata controlled by the limb rotation mechanism. In two small thrust faults in the QJB, the Mesoproterozoic Xiamaling Formation is thrust over the Tuchengzi Formation, and these faults may have controlled the development of the fault-bend fold-type growth strata. An analysis of the “source-to-sink” process suggests that the hanging wall succession of the Shangyi-Pingquan fault (SPF) was the main source area of the Late Jurassic to early Early Cretaceous strata in the QJB. A zircon 206Pb/238U age of 140.8±2.4 Ma for the volcanic rocks at the bottom of the FPFT growth strata represents the timing of the initiation of FPFT growth structure development. The discovery of the FPFT growth structure and growth strata in the QJB indicate that the QJB was an intermontane flexural basin controlled by fold and thrust structures during the early Early Cretaceous. Near the E-W-trending SPF, the NE-SW-trending Qianjiadian thrust fault (QJTF) and two small intrabasinal thrust faults may constitute a unified right-lateral strike-slip system that formed in response to the northwestward flat subduction of the Izanagi Plate beneath the East Asian continent during the Jurassic-Early Cretaceous.

Geometry and kinematics of the Baodi fault and its implication on the structural relationship between Yanshan Fold Belt and Bohai Bay Basin

Geometry and kinematics of the Baodi fault and its implication on the structural relationship between Yanshan Fold Belt and Bohai Bay Basin

Revisiting of the Yanshanian basins in western and northern Beijing, North China

We present new laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb ages and provenance data for the Yanshanian basins in western and northern Beijing, North China, with the aims of demonstrating the relationship between basin evolution and “Yanshanian tectonism” and constraining the Late Jurassic–Early Cretaceous tectonic evolution of the region. The inherited Yanshanian basin in western Beijing has a Late Jurassic–Early Cretaceous tectonic history similar to that of typical Yanshanian basins in the Yanshan Fold–Thrust belt (YFTB), and it became inactive at ∼138 Ma. The Qianjiadian basin (QJB) in northern Beijing is only infilled by the sediments of the Tuchengzi Formation. Our new zircon U–Pb ages (160–156 Ma) for the pyroclastic and volcanic rocks of the Tuchengzi Formation suggest that formation of the QJB commenced at ∼160 Ma, consistent with the main episode of “Yanshanian tectonism”. The QJB developed in the footwall of the Pingquan-Gubeikou thrust fault (PGF), and records the syn-tectonic unroofing and erosion of the hanging wall of that fault. It was later reformed by the post-depositional Qianjiadian thrust fault (QJF) along the western margin. The formation and development of the QJB in northern Beijing and the inherited Yanshanian basin in western Beijing were related to the “Yanshanian tectonism”, especially at ca. 160 Ma, when the intense deformation and large-scale magmatism occurred. The tectonic transition from N–S to NW–SE shortening is attributed to the northwest-ward subduction of the Paleo-Pacific Plate, and is constrained to ∼156–135 Ma in the YFTB and eastern China.

Collision-Induced Late Permian–Early Triassic Transpressional Deformation in the Yanshan Tectonic Belt, North China

Crustal deformation occurred widely in the Yanshan tectonic belt, north of the North China Craton, and its time and mechanism remain as open problems. On the basis of new structural data and 40Ar-39Ar dating on hornblende and biotite from the rocks in the belt, a dynamic link between the deformation and plate interaction is established. Abundant structures in the Fengning-Longhua fault zone indicate that it is a transpressional fault zone including both strike-slip (dextral) and compressional (south-directed) components and is kinematically conformable to the Huade-Chifeng fault zone in the Neimeng Uplift and to the Pingquan-Gubeikou and Miyun-Xifengkou-Jinxi transpressional fault zones in the Yanshan fold and thrust belt, suggesting coupled deformation in the overall Yanshan tectonic belt during the Late Permian–Early Triassic. This phase of deformation coincides well in age and in kinematics with the Late Permian collision between the North China Block and Mongolia arc terranes, implying that the most intense Late Permian–Early Triassic transpressional deformation in the Yanshan belt was developed as a result of the convergence of the Central Asian Orogenic Belt.

Deformation characteristics and formation mechanism of the Yunmengshan metamorphic core complex

The Yunmengshan metamorphic core complex in the middle part of the Yanshan Fold and Thrust Belt records crust extension processes of the eastern North China Craton during its peak destruction. Development of the metamorphic core complex was controlled by the generally NNE-striking Dashuiyu Shear Zone. The shear zone dips SE and becomes shallower NE-wards, leading to exposures of a ductile shear zone in the southern and middle parts and brittle faults in the northern part. Exposure structures, microstructures, and quartz C- axis fabrics indicate that the ductile shear zone belongs to an extensional shear zone with a top-to-the-SE shear sense. Deformation temperatures of 300-520 ℃ suggest a midcrustal origin for the ductile shear zone. A ductile deformation belt in the footwall of the shear zone is only as wide as 1-3 km, indicating no widespread mid-crustal ductile flow in the region during the deformation. Zircon U-Pb dating of dykes and plutons as well as hornblende and biotite 40 Ar/ 39 Ar dating demonstrate that the metamorphic core complex originated at 135 Ma and experienced intense shearing of the Dashuiyu Shear Zone, development of the supradetachment basins, and synkinematic intrusion during 135-125 Ma. The metamorphic core complex was subjected to rapid exhumation during 125-114 Ma when the Dashuiyu Shear Zone suffered continuous activity and passive doming. The shear zone and its hanging wall were cut or replaced by a series of brittle faults when they were uplifted to a brittle regime, showing that exhumation took place in continuous extensional activities. The metamorphic core complex turned into slow exhumation in an extensional regime in the following latest Early Cretaceous. The evolution history suggests that the Yunmengshan metamorphic core complex was developed by the rolling-hinge model, a common formation mechanism for intraplate metamorphic core complexes in the North China Craton, under the continuous NW-SE extension during the Early Cretaceous (135-100 Ma).

Late Mesozoic compressional to extensional tectonics in the Yiwulüshan massif, NE China and its bearing on the evolution of the Yinshan–Yanshan orogenic belt: Part I: Structural analyses and geochronological constraints

With a cratonic nucleus, the North China Craton (NCC) experienced a complex tectonic evolution with multiphase compressional and extensional events during Mesozoic times. Along the northern part of the NCC, the Yinshan–Yanshan fold and thrust belt was a typical intraplate orogen. Jurassic and Cretaceous continental sedimentation, magmatism, widespread intraplate characterize the Yinshan–Yanshan orogenic belt. The geodynamic significance of these tectonic events is still in dispute. In the western part of the Liaoning province, the Yiwulüshan massif crops out at the eastern end of the Yinshan–Yanshan orogenic belt. The Yiwulüshan massif presents an elliptical domal shape with a NE–SW striking long axis. The structural evolution of this massif brings new insights for the understanding of the Mesozoic plutonic–tectonic history of the NCC. A multidisciplinary study involving structural geology, geochronology, Anisotropy of Magnetic Susceptibility (AMS) and gravity modeling have been carried out. The presentation of the new results splits into two parts. Part I (this paper) deals with field and laboratory structural observations, and presents the main geochronological results. The AMS, gravity modeling data will be provided in a companion paper (Part II). The early compressional deformation (D1) corresponds to a Late Jurassic to Early Cretaceous southward thrusting. The subsequent deformation is related to the Early Cretaceous exhumation of the Yiwulüshan massif. A detailed structural analysis allows us to distinguish several deformation events (D2, D3, and D4). The Cretaceous extensional structures, such as syntectonic plutons bounded by ductile normal faults, metamorphic core complexes, and half-graben basins are recognized in many places in East Asia. These new data from the Yiwulüshan massif constitute a link between Transbaikalia, Mongolia, North China and South China, indicating that NW–SE extensional Mesozoic tectonics occurred throughout the entire region.

Rapid exhumation of basement rocks along the northern margin of the North China craton in the early Jurassic: evidence from the Xiabancheng Basin, Yanshan Tectonic Belt

AbstractEarly Mesozoic Basins in the Yanshan Fold–Thrust Belt (YFTB), located along the northern margin of the North China Craton (NCC), record significant intraplate deformation of unknown age. In this article, we present evidence for the rapid exhumation of high‐grade basement rocks along the northern margin of the NCC in the Early Mesozoic. U–Pb geochronology of detrital zircons constrains the maximum depositional ages of syntectonic sedimentary units that formed during the unroofing of basement rocks and plutons in the Xiabancheng Basin. In the Early Mesozoic, the Xiabancheng Basin recorded a dramatic transformation in depositional environments, related to a significant change in the regional tectonic setting. In this study, the tectonic evolution of the YFTB is established from paleocurrent data and U–Pb zircon ages of sandstone and granitic gravels of the Xingshikou Formation, Xiabancheng Basin. The paleocurrent direction of meandering fluvial facies in the Triassic Liujiagou and Ermaying Formations are from east to west. In contrast, the overlying Xingshikou Formation consists of alluvial fan facies with paleocurrent directions from north‐northwest to south‐southeast. The lower and middle segments of the Xingshikou Formation record rapid exhumation of basement rocks along the northern margin of the NCC. U‐Pb ages of detrital zircons within the Xingshikou Formation are characterized by three major U–Pb age groups: 2.2–2.5 Ga, 1.7–1.8 Ga and 193–356 Ma. From 193 Ma to 356 Ma, a subsidiary peak occurs at 198 ± 5 Ma, constraining the sedimentation age of the Xingshikou Formation to the Early Jurassic. Zircon from the Wangtufang pluton in the northern portion of the Xiabancheng Basin yields U–Pb ages of 191 ± 1 Ma and 207 ± 1 Ma. Within error, these crystallization ages are identical to detrital zircon ages of 206 ± 1 Ma and 206 ± 2 Ma obtained for granitic gravel clasts in the Xingshikou Formation. Thus, the Wangtufang pluton and surrounding basement rocks must have experienced rapid uplift and exhumation during the Early Jurassic. The onset of exhumation along the northern margin of the NCC occurred at ca. 198–180 Ma.

Petrogenesis of silica-saturated and silica-undersaturated syenites in the northern North China Craton related to post-collisional and intraplate extension

Whole-rock major and trace element and Sr-, Nd- and Hf-isotope data, together with zircon U–Pb and Hf-isotope data, are reported for three alkaline intrusions (Liangjia, Jianfang and Hekanzi) in the Yanshan Fold and Thrust Belt in the northeastern North China Craton, in order to investigate their sources, petrogenesis and tectonic implications. The Liangjia and Jianfang plutons are mainly composed of medium-grained quartz syenites, whereas, the Hekanzi complex consists of nepheline-bearing pyroxene-biotite syenites. Zircon U–Pb dating gives emplacement ages of 254–246Ma for the Liangjia and Jianfang silica-saturated syenites and 226–224Ma for the Hekanzi silica-undersaturated syenites. The Late Permian silica-saturated alkaline rocks have high silica contents (SiO2=69.2–75.7wt.%) and low MgO (0.04–0.47wt.%) concentrations, and are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) but with negative Eu, Ba and Sr anomalies and depletion in high field strength elements (HFSEs), with significant negative Nb, Ta, P and Ti anomalies. Geochemical data and Sr-, Nd and Hf-isotopic compositions indicate that they were mainly derived from partial melting of lower crustal materials with extensive pyroxene and plagioclase fractionation. The Triassic silica-undersaturated syenites have relatively high MgO abundances (up to 5.3wt.%) at basic to intermediate silica contents. They are enriched in LREE and LILE and depleted in HREE and HFSE, with negative Nb, Ta and Ti anomalies. They have relatively low initial 87Sr/86Sr ratios and relatively negative whole rock εNd(t) and εHf(t) values and zircon εHf(t) values. All these geochemical features indicate that they were the result of crustal assimilation and fractional crystallization of a mafic magma derived from an amphibole-clinopyroxene-rich lithospheric mantle via small degree partial melting. Based on the regional geology, the Late Permian syenites would form in a post-collisional extension setting, whereas, the Triassic syenites were most likely generated in an intraplate extensional setting, providing time constraints on the geodynamic transition from post-orogenic to intra-plate extension.

Petrogenesis of an Alkali Syenite-Granite-Rhyolite Suite in the Yanshan Fold and Thrust Belt, Eastern North China Craton: Geochronological, Geochemical and Nd-Sr-Hf Isotopic Evidence for Lithospheric Thinning

The Yanshan Fold and Thrust Belt in eastern China has been intruded by a series of alkalic igneous rocks, ranging in composition from granite and rhyolite to syenite and trachyte. Laser ablation inductively coupled plasma mass spectrometry U^Pb analyses of zircon from three alkaline suites yield Early Cretaceous ages of 130^117Ma. Three groups of rocks have been identified based on their mineralogical, geochemical and Sr^Nd^Hf isotope characteristics. The alkali granites and rhyolites are ferroan and have low Al2O3, MgO, CaO, Sr, Ba and Eu concentrations and high SiO2, total Fe2O3, K2O, Nb, Ga, Ta, Th and heavy rare earth element abundances and Ga/Al ratios. Geochemical data and Sr-, Ndand zircon Hf-isotopic compositions [(Sr/Sr)i1⁄4 0 7050^0 7164, eNd(t)1⁄4 8 4 to 13 6 and eHf(t)1⁄4 5 7 to 16 8] indicate that they were probably generated by shallow dehydration melting of biotiteor hornblende-bearing granitoid crustal source rocks and then mixed with contemporaneous magma from a mantle and/or lower crustal source. Ferroan syenites have distinct geochemical features from those of the alkaline granites and rhyolites, suggesting that they were produced by clinopyroxene and plagioclase fractionation of melt derived from an enriched mantle source, mixed with lower and upper crustal-derived magmas. The magnesian syenites and trachytes have Sr-, Ndand zircon Hf-isotopic compositions that are distinct from those of the ferroan syenites. They were mainly derived from partial melting of lower crustal materials, mixed with enriched mantle-derived alkali basaltic magma.The emplacement of an alkali syenite^granite^rhyolite suite, coeval with the formation of metamorphic core complexes and pull-apart basins in eastern China, indicates they formed in an extensional setting, possibly as a result of lithospheric thinning.

Geochemistry of Middle Oligocene-Pliocene sandstones from the Nanpu Sag, Bohai Bay Basin (Eastern China): Implications for provenance, weathering, and tectonic setting

Major, trace and rare earth elements of 37 sandstones of the Middle Oligocene-Pliocene from the Nanpu Sag, Bohai Bay Basin were analyzed to evaluate their provenance, weathering, tectonic setting. Low-intermediate values of the Chemical Index of Alteration (CIA) reveal that these sandstones experienced relatively weak chemical weathering. Trace element ratios and REE characteristics show that the source rocks mainly consisted of felsic magmatic rocks that formed in the Yanshan Fold Belt during Mesozoic Yanshanian period. Tectonic setting discrimination diagrams show that the sandstones were deposited in a reactivated inner-continental setting, supporting the origin of the Bohai Bay Basin as being related to subduction of the Pacific plate towards the Eurasian plate.

Constraints on the timing of uplift of the Yanshan Fold and Thrust Belt, North China

This paper reports in situ U–Pb ages and Hf isotopic data on detrital zircons from Paleozoic to Late Mesozoic strata in the Xishan area near Beijing that provide important constraints on the evolution and paleogeography of the northern part of the North China Craton (NCC). Zircons from Carboniferous to Late Mesozoic sandstones are characterized by three major groups of U–Pb ages (2.5–2.3 Ga, 1.9–1.7 Ga and Late Phanerozoic ages). The Neoarchean and Paleoproterozoic zircons, and some Phanerozoic zircons, have negative ε Hf( T) values, similar to igneous zircons from intrusive rocks known from the NCC, indicating that the source of the sandstones was locally derived. However, Late Triassic to Late Jurassic (205–158 Ma) sandstones contain a group of Phanerozoic zircons with positive ε Hf( T) values, distinct from known igneous zircons in the NCC, suggesting an exotic source. The nearest potential source is the Xing-Meng Orogenic Belt (XMOB) to the north of the NCC, where considerable amounts of juvenile crustal materials exist. It is concluded that there are two sudden shifts in source provenances of the Xishan sediments; from the NCC before the Late Triassic, to a mixed source from the NCC and XMOB during the Late Triassic to Late Jurassic, and then from the NCC again during the Late Jurassic (< 158 Ma). The first shift in source resulted from the exhumation and denudation of rocks due to uplift of the XMOB, which coincided with post-collisional extension in the Late Triassic and intraplate extension of the NCC in the Middle to Late Jurassic. The second source shift resulted from the uplift of the Yanshan Fold and Thrust Belt, thus constraining the timing of this to the Late Jurassic.