Yanshan Belt Research Articles

A new model for the segmentation, propagation and linkage of the Tan-Lu fault zone, East Asia

• The Tan-Lu Fault formed by coalescence of two kinematically independent segments. • The northern and southern Tan-Lu faults exhibit contrasting displacement profiles. • The northern and southern Tan-Lu faults propagated in opposite directions. • Southward indentation of the Siberian craton initiated the northern Tan-Lu Fault. • The northern and southern Tan-Lu faults linked in the early Late Jurassic. The Tan-Lu fault zone (TLF) is a major strike-slip fault with a long and complex history in East Asia, whose evolution provides a new perspective on the formation of large-scale faults (>1000 km long). Fault displacement analysis, geological mapping and U-Pb LA-ICP-MS dating have been performed to understand the evolution of the TLF. Along-strike displacement variation reveals that the TLF consists of two kinematically independent segments, the northern and southern TLF, with opposite long-term propagation directions. Structural and geochronological studies in the eastern Yanshan belt, located around the linkage area of the southern and northern TLF, indicates that NNE-trending sinistral strike-slip faults initiated at 167-164 Ma and were reactivated at 124-121 Ma. Structural analysis suggests that these early NNE-trending strike-slip faults transferred sinistral motion along the northern TLF into southward thrusting along the Yanshan belt, representing the Middle Jurassic southern termination of the northern TLF. Our studies suggest that the through-going TLF formed when the younger southward-propagating northern TLF merged with the older northward-propagating southern TLF in the Late Jurassic. A new model is thus proposed for the Mesozoic evolution of the TLF. The initiation and southward propagation of the northern TLF is interpreted to have resulted from the southward indentation of the Siberian craton into the amalgamated Central Asian Orogenic Belt and North China block. The divergent mega-splays of the northern TLF likely resulted from westward-younging formation during the clockwise rotation of northeast Asia. Coalescence of two genetically unrelated faults could be an alternative mode for large-scale fault formation.

Petrogenesis and tectonic implications of the Late Triassic intrusions in the North China Craton: Case study on the Huata complex in the western Yanshan

This paper presents geochemical and age data for Late Triassic lamprophyre, syenite, and granite in the Huata area of the western Yanshan belt of the North China Craton, to provide constraints on the origin and tectonic settings of these Late Triassic igneous rocks. The zircons from the Huata lamprophyres and syenites yielded a mean U Pb age of 203 and 206 Ma, respectively. The lamprophyres are enriched in incompatible elements, with (La/Yb) N = 30.57–33.89, (Gd/Yb) N = 3.54–3.64, and Sr-Nd-Hf isotopic signatures ( 87 Sr/ 86 Sr (initial) = 0.7053–0.7061, εNd(t) = −11.11 to −10.50, εHf(t) = −9.33 to −7.70), consistent with them originating from the low-degree melting of previously metasomatized lithospheric mantle in the garnet-stable area. The syenites display similar incompatible elements to the lamprophyres, but have relatively low (La/Yb) N = 14.26–17.23 and (Gd/Yb) N = 1.81–2.15 ratios. Their Sr-Nd-Hf isotopic compositions ( 87 Sr/ 86 Sr (initial) = 0.7045–0.7050, εNd(t) = −12.10 to −11.35 and εHf(t) = −8.64 to −6.81) were also similar to the lamprophyres, and they are considered to have been generated from the low-degree melting of lithospheric mantle in the garnet-spinel transition zone. The granites have major and trace element patterns and Sr Nd isotopes quite different from the syenites, but are similar to the crust-derived felsic rocks, being enriched in large ion lithosphere elements and depleted in high field strength elements. They are enriched in Nd isotope (εNd(t) = −16.63 to −15.18), and are inferred to have been generated by lower crust anataxis. The presence of igneous rocks in the Huata district indicates that the northern margin of the NCC had already entered a non-orogenic regime by the late Late-Triassic. • Lamprophyres and syenites derive from a metasomatized mantle. • Granites originate from the modified lower continental crust. • The tectonic regime transformed from post-collisional to intraplate at 206 Ma.

Early Mesozoic polyphase contractional deformation in the Yanshan belt: Implications for the destruction of the north China Craton

• The northern North China Craton was in a contractional setting in early Mesozoic. • Post-collisional contractional deformation was due to the lithosphere delamination . • A decoupling process dominated the early stage of the destruction of the NCC. After the collision of the Mongolian arc terranes with the amalgamated Bainaimiao arc and North China Craton in the latest Permian to earliest Triassic, the northern North China Craton ended the Andean-type accretionary orogenesis. The nature of the subsequent post-collision tectonic setting in early Mesozoic in the northern North China Craton remains controversial. Some workers proposed that the northern North China Craton evolved into a post-orogenic extensional stage. However, this view was mainly built on geochemistry studies of Triassic plutonic rocks in northern North China Craton while it had never received support from studies of tectonic deformation. In contrast, our recent field mapping combining with geochronological studies in the Yanshan belt suggest that the northern North China Craton was in an overall polyphase multi-directional contractional settings in early Mesozoic. We have identified four stages of contractional deformation (D 1 -D 4 ) in this region. D 1 (250–231 Ma) is the intracratonic response to the collision of the Mongolia arc terranes with the NCC. D 2 (214 ∼ 207 Ma) and D 3 (204 ∼ 195 Ma) are post-collisional intracratonic contractional deformation which occurred synchronously with the lithosphere thinning. D 2 and D 3 and the intervened plutonism might be a crustal response to the delamination and sinking of the over-thickened lithosphere of the northern North China Craton. D 4 is the result of the west-ward subduction of the Izanagi plate beneath the eastern Asian continent plate in the Early Jurassic. These findings together with previous studies indicate that the northern North China Craton experienced a long period of polyphase contractional deformation before its final destruction in the Early Cretaceous. The early stage of the destruction of the North China Craton is characterized by coeval lithosphere thinning and crustal thickening.

Migration of Middle-Late Jurassic volcanism across the northern North China Craton in response to subduction of Paleo-Pacific Plate

The primary driver of magmatism, crustal deformation and metallogeny of the North China Craton in the Jurassic remains actively debated, either attributed to the subduction of the Paleo-Pacific (i.e., Izanagi) plate in the east or to the closure of the Mongol-Okhotsk Ocean in the north. This issue is addressed here by examining temporal and spatial variation of the contemporary volcanic rocks (i.e., Tiaojishan Formation) of the Yanshan belt in the northern part of the craton. These rocks are mainly calc-alkaline and intermediate-felsic in composition and show a highly coherent geochemical composition and unradiogenic zircon Hf isotope, indicating that they have similar origins and formed in a subduction-related setting. The eruption age of the Jurassic volcanic rocks decreases westward (i.e., inland-ward) from Western Liaoning (166–153 Ma), through Pingquan-Chengde-Luanping (162–153 Ma) and Jingxi-Xuanhua (158–149 Ma), to Yuxian (157–142 Ma). This temporal and spatial variation is consistent with the motion of the subducted Paleo-Pacific plate over that time. In addition to the westward migration, the earliest volcanic rocks in the Yanshan belt formed earlier than those in the adjacent southern Great Xing'an Range in the north. These observations suggest that the southward subduction and closure of the Mongol-Okhotsk plate played a limited role in generation of these volcanic rocks. Two episodes of Middle-Late Jurassic volcanism (166–153 Ma and 150–139 Ma) are observed in the northern Great Xing'an Range. They are interpreted as results of closure of the Mongol-Okhotsk Ocean and subduction of the Paleo-Pacific plate, respectively. Temporal-spatial variation of volcanic activity in the northern North China Craton and adjacent region demonstrates that the Paleo-Pacific subduction was the principal cause of the evolution of Eastern China during the late Mesozoic, whereas the Mongol-Okhotsk tectonic regime is largely confined to the northern Great Xing'an Range and the area north to it.

3-D electrical structure of the lithosphere beneath the Yanshan Belt and its adjacent areas, North China

The North China Craton is one of the oldest continental nuclei in the world. However, extreme lithospheric modification has occurred since the Mesozoic, of which the most typical area is the Yanshan orogenic belt. In this research, a 3-D lithospheric electrical structure beneath the Yanshan belt and adjacent areas was obtained using magnetotelluric array data, which reveals the regional heterogeneity. Notably, the crust and upper mantle (< 100 km) within the Yanshan belt, especially in its southern part, are characterized by high resistivity (> 1000 Ωm). These features are interpreted to be a relic cratonic lithosphere mainly composed of highly depleted olivine. A lithosphere-scale relatively high-conductivity zone (~ 100 Ωm, near EW-trending), which roughly lies beneath the Miyun-Xifengkou-Qinglong fault coincides with the boundary of Archean greenstone belt and also the Mesozoic granitic zone, probably indicating a reactivated Precambrian lithospheric weak/shear zone within Yanshan belt. In contrast, the lower crust and upper mantle of the Bohai Bay basin show large areas of high conductivity, which may result from the modification of the crust and mantle by the upwelled asthenospheric materials. Additionally, the epicenters of several historic destructive earthquakes, including the 1976 Tangshan earthquake (M7.8), are mainly located near the high- and low-resistivity boundary at the conjunction of the Yanshan belt and the Bohai Bay basin. The fragmentation of brittle crustal rocks mainly happened in where the tectonic stresses concentrated, which might result from the rheological difference between the high-resistivity ancient and the high-conductivity modified lithosphere. In these places, intense seismicity would be induced.

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.

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.

Linking Mesozoic lode gold deposits to metal-fertilized lower continental crust in the North China Craton: Evidence from Pb isotope systematics

The source(s) of lode gold deposits formed in Precambrian cratons related to accretion/collision and cratonic reactivation, formerly attributed to either supercrustal volcano-sedimentary sequences or deep crustal/sub-crustal origin, remain controversial largely because gold deposits are spatially related to metamorphosed rocks, but geochemical data somewhat indicate a poorly understood deep source. Reconciling such conflicts is important to better understand the main factor controlling the formation of ore deposits and their genetic link with specific tectonic settings. Giant Late Mesozoic lode gold provinces in the North China Craton (NCC) were formed ca. 1.7 Ga later than cratonization metamorphism, and contemporaneous with intensively felsic to mafic magmatism related to cratonic reactivation. In this study, we conduct a comprehensive Pb isotope study on major gold deposits from the eastern Yanshan belt, northern margin of the NCC. In order to constrain the source(s) of gold, we attempt to map regional Pb isotope variations of lower continental crust (LCC), and develop a two stage quantitative model (punctuated by three prominent geological events at 2.80 Ga, 1.85 Ga and 0.16 Ga) to reproduce time-integrated Pb isotopic signatures of deep-seated lithospheric reservoirs to make a comparison with Pb isotopic signatures of the gold mineralization. Gold-bearing pyrites within different types of host rocks have relatively uniform Pb isotopic ratios, which are significantly different to high-grade metamorphosed host rocks, but similar to those of spatially associated Late Mesozoic granitic rocks. Our data show that the Pb isotopic signatures of gold deposits vary consistently with presumed regional Pb isotopes of the LCC during the Late Mesozoic. Lead isotopic heterogeneity of the LCC was likely caused by underplating of mafic magmas derived from mantle sources. During underplating, highly chalcophile elements (e.g. Au, Ag and Cu) were concentrated at the base of the LCC due to sulfide saturation from mafic magmas. Integrating petrological, geochemical, geochronological, and considering chalcophile element solubility, we propose a new genetic model to describe the formation of Late Mesozoic gold deposits in the NCC: (1) early formation of sulfide-bearing cumulates with high Au/Cu ratios during magma differentiation at the base of the LCC; (2) subsequent fluid-fluxed remelting of these cumulates at the onset of lithospheric extension and release and ascent of ore-forming fluids to the site of precipitation. Considering the metallogenic characteristics of other Au ore-forming systems, we suggest that metal fertilization in deep-seated reservoirs and subsequent tectonic decompression are important factors controlling the development of Au-rich ore deposits worldwide. This study demonstrates how Pb isotope can be employed to trace source(s) of gold deposits.

Timing of deposition in the Dengzhangzi and Guojiadian Basins of the Yanshan fold-thrust belt, North China

ABSTRACT The Triassic-Jurassic stratigraphy in the Yanshan fold-thrust belt is dominated by nonmarine deposits, and the division of this stratigraphy remains controversial among geologists. Age results from biostratigraphic and isotope chronological studies are inconsistent, especially for the Dengzhangzi and Guojiadian Basins in the eastern part of the Yanshan belt. We present four new detrital zircon U–Pb isotope datasets for the Triassic-Jurassic stratigraphic units and one for the Permian unit in the basins. The geochronological results show that the andesitic breccia in the Permian strata was deposited at ca. 273.78 ± 1.68 Ma, the Laohugou Formation was deposited in the Anisian-Ladinian Stage, the andesitic lava in the Shuiquangou Formation erupted at ca. 238.56 ± 0.91 Ma, and the conglomerate in the Guojiadian Formation was deposited from ca. 170 ± 5 to 163.0 ± 2.2 Ma. Field observations and facies analyses demonstrate that the Laohugou, Shuiquangou and Dengzhangzi Formations predominantly consist of subaqueous debris flow deposits, subaerial fluvial deposits, and fan delta deposits while the Guojiadian Formation is generally composed of fluvial and subaerial mass-flow deposits that formed in braided channels. Here, we propose a more precise and comprehensive Triassic-Jurassic stratigraphic division scheme based on age constraints and facies analysis. With this new stratigraphic sequence, we compare the Mesozoic stratigraphic units in the study area with those in other basins in the Yanshan fold-thrust belt in terms of the lithological associations, regional unconformities and zircon U-Pb dating results. This comparison indicates that the Indosinian and Yanshanian movements exerted a far-reaching influence on the development and evolution of the Dengzhangzi and Guojiadian Basins in the study area, forming several important regional unconformities and syntectonic conglomeratic units. These new age data on the stratigraphy are of fundamental significance for understanding the Mesozoic tectonic evolution of Yanshan.

Late Jurassic‐Early Cretaceous Deformation in the Western Yanshan Fold‐Thrust Belt: Insights From Syntectonic Sedimentation in the Chicheng Basin, North China

AbstractThe Yanshan fold‐thrust belt is a significant intraplate deformation belt along the northern margin of North China Craton and records multiple phases of deformation during the Mesozoic. However, the specific tectonic evolution of the Yanshan belt during the Mesozoic has long been debated. In particular, many arguments exit regarding whether a contractional or extensional context dominated the Yanshan belt in the Late Jurassic‐Early Cretaceous. One reason for this argument is the poor understanding of the tectono‐sedimentary relationship preserved in the sedimentary records. A lack of evidence of syntectonic sediments makes it more difficult to reconstruct this relationship. The Chicheng basin in the western Yanshan belt contains complete Upper Jurassic‐Lower Cretaceous succession and is bounded by marginal faults. New regional geological mapping, detailed sedimentary facies analysis, and recognition of two growth strata packages help reconstruct the tectono‐sedimentary relationship in the Chicheng basin. Additionally, zircon U‐Pb dating provides constraints on the timing of the deformation. Our results suggest that the dynamic sediment dispersal system in the Chicheng basin during the Late Jurassic‐Early Cretaceous were controlled by two phases of thrusting, including the activities of the Shangyi‐Pingquan fault during 162–147 Ma, and the Fengning‐Longhua fault during 140–135 Ma. Therefore, we propose that the western Yanshan belt was in a contractional context during the Late Jurassic‐Early Cretaceous. Our findings indicate that the tectonic setting of the Yanshan belt experienced a prominent transition from E‐W trending structures to NE‐SW trending structures and that this transition was probably related to subduction of the Paleo‐Pacific plate beneath the East Asian continent.

New Late Jurassic to Early Cretaceous Paleomagnetic Results From North China and Southern Mongolia and Their Implications for the Evolution of the Mongol‐Okhotsk Suture

AbstractTo better constrain the evolution of the Mongol‐Okhotsk suture, we carried out new paleomagnetic studies on Sharilyn Formation (~155 Ma) and Tsagantsav Formation (~130 Ma) in southern Mongolia, Amuria Block (AMU), and Tuchengzi Formation (~140 Ma) and Dadianzi/Yixian Formation (~130 Ma) in the Yanshan belt, North China Block (NCB). A total of 719 collected samples (from 100 sites) were subjected to stepwise thermal demagnetization. After a low‐temperature component of viscous magnetic remanence acquired in the recent field was removed, the stable high‐temperature components were isolated from most samples. The high‐temperature components from each rock unit passed a fold test and a reversal test, indicating their primary origins. The corresponding paleomagnetic poles were thus calculated. For AMU, the ~155 Ma pole is at 74.7°N/232.5°E (A95 = 3.7°), the ~130 Ma pole at 74.6°N/194.7°E (A95 = 2.9°); for the NCB, the ~140 Ma pole is at 82.7°N/208.6°E (A95 = 4.3°), the ~130 Ma pole at 80.5°N/197.4°E (A95 = 2.3°). By combining our new results with the published data, we refined the 155–100 Ma segment of the apparent polar wander paths for AMU and NCB, which can demonstrate that these two blocks have been tectonically coherent (AMU‐NCB) during 155–100 Ma. Comparison of the apparent polar wander paths, however, revealed a latitudinal plate convergence of 14.3° ± 6.9° and ~19.0° relative rotation between Siberia and the AMU‐NCB after ~155 Ma. Large‐scale latitudinal convergence likely ceased by ~130 Ma, although some relative rotation between them continued along the Mongol‐Okhotsk suture until ~100 Ma.

Syn-tectonic sedimentation and its linkage to fold-thrusting in the region of Zhangjiakou, North Hebei, China

The timing of the “Yanshanian Movement” and the tectonic setting that controlled the Yanshan fold-and-thrust belt during Jurassic time in China are still matters of controversy. Sediments that filled the intramontane basins in the Yanshan belt perfectly record the history of “Yanshanian Movement” and the tectonic background of these basins. Recognizing syn-tectonic sedimentation, clarifying its relationship with structures, and accurately defining strata ages to build up a correct chronostratigraphic framework are the key points to further reveal the timing and kinematics of tectonic deformation in the Yanshan belt from the Jurassic to the Early Cretaceous. This paper applies both tectonic and sedimentary methods on the fold-and-thrust belt and intramontane basins in the Zhangjiakou area, which is located at the intersection between the western Yanshan and northern Taihangshan. Our work suggests that the pre-defined “Jurassic strata” should be re-dated and sub-divided into three strata units: a Late Triassic to Early Jurassic unit, a Middle Jurassic unit, and a Late Jurassic to early Early Cretaceous unit. Under the control of growth fold-and-thrust structures, five types of growth strata developed in different growth structures: fold-belt foredeep type, thrust-belt foredeep type, fault-propagation fold-thrust structure type, fault-bend fold-thrust structure type, and fault-bend fold-thrust plus fault-propagation fold composite type. The reconstructed “source-to-sink” systems of Late Triassic to Early Jurassic, Middle Jurassic and Late Jurassic to early Early Cretaceous times, which are composed of a fold-and-thrust belt and flexure basins, imply that the “Yanshanian Movement” in our study area started in theMiddle Jurassic. During Middle Jurassic to early Early Cretaceous times, there have been at least three stages of fold-thrust events that developed “Laramide-type” basement-involved fold-thrust structures and small-scale intramontane broken “axial basins”. The westward migration of a “pair” of basement-involved fold-thrust belt and flexure basins might have been controlled by flat subduction of the western Paleo-Pacific slab from the Jurassic to the Early Cretaceous.

Crust–mantle interaction and craton destruction: evidence from Late Mesozoic plutons in the North China Craton

Late Mesozoic granitoids, widely distributed in the Yanshan Belt of the North China Craton (NCC), are important markers of cratonic destruction and lithospheric thinning. We report new petrological, geochemical, zircon U–Pb and Lu–Hf isotopic data on two late Mesozoic granitic plutons from the central NCC near the Trans-North China Orogen. We report zircon U–Pb emplacement ages of 162 – 156 Ma and 132 Ma from the Siganding and Fangshan plutons respectively, indicating Late Jurassic–Early Cretaceous magmatism. The presence of a dusty zone, and discontinuous zoning, in plagioclase phenocrysts and the occurrence of mafic microgranular enclaves in both granitoid plutons suggest mingling and mixing of felsic and basic magmas. The rocks investigated in this study show high Na 2 O + K 2 O and high Mg# (Mg# = Mg/(Fe + Mg)) and are metaluminous, with enrichment in light rare earth elements and large ion lithophile elements (Rb, Ba, U, and Sr) and depletion in high strength field elements (Th, U, Nb, Ta, P, and Ti). They also display negative εHf( t ) values and high Sr/Y ratios, comparable with adakitic rocks, suggesting that the felsic magmas of these intrusions were probably derived from the partial melting of the Palaeoproterozoic (two-stage zircon Hf-depleted model ages between 2.5 and 1.9 Ga) thickened lower crust of the NCC, whereas the mafic magma was probably derived from subcontinental lithospheric mantle. We correlate the magmatism with lithospheric thinning beneath the NCC and associated crust–mantle interaction. It is inferred that lithospheric thinning was of limited extent in Late Jurassic (162 – 156 Ma) times. The magma flare-up at around 130 Ma was produced by large-scale melting of the enriched subcontinental lithospheric mantle with development of extensive intracontinental adakitic magmatic rocks derived from thickened ancient lower continental crust with variable involvement of subcontinental lithospheric mantle. Supplementary material: Mineral assemblages and abundance, representative electron microprobe analyses, LA-ICP-MS zircon U–Pb age data and LA-MC-ICP-MS Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3815308

The Early Cretaceous Shangzhuang layered mafic intrusion and its bearing on decratonization of the North China Craton

AbstractThe North China Craton (NCC) is one of the classic examples of decratonization through extensive lithospheric destruction during Mesozoic time. Among the various pulses of magmatism associated with cratonic erosion are the rare mafic intrusions in the Yanshan Belt. Here we investigate the Shangzhuang layered intrusion belonging to this suite, which is characterized by compositional layering with troctolite, noritic gabbro and gabbro/gabbroic anorthosite/gabbrodiorite from the bottom to top. The different lithologies of this intrusion exhibit close field relationships, similar chemical patterns and overall identical Lu–Hf isotopes indicating a co-magmatic nature. The fine-grained gabbros occurring near the margin of the intrusion display U–Pb ages similar to those of the other rocks and are considered to represent the composition of the parent magma, characterized by Fe, Mg and Ti enrichment. The magma was sourced from low-degree partial melting of spinel lherzolite sub-continental lithospheric mantle, which had been enriched by crust–mantle interaction and metasomatic fluids derived from the Mongolian oceanic slab subduction beneath the NCC during Late Palaeozoic time. In addition, limited asthenospheric or deeper-mantle materials were also locally mixed with the enriched mantle as the final source component. Our zircon U–Pb data constrain the emplacement age of this intrusion asc. 128–123 Ma in Early Cretaceous time, and correlates with the regional extensional tectonics betweenc. 135 and 115 Ma in the eastern and central NCC. Mantle upwelling associated with this event resulted in the thermal and chemical erosion of the lithospheric mantle, and emplacement of the parent magma of this layered intrusion.

Mesozoic felsic volcanic rocks from the North China craton: Intraplate magmatism associated with craton destruction

The North China craton preserves the history of crustal growth and craton formation during the early Precambrian, followed by extensive lithospheric thinning in the Mesozoic associated with large-scale magmatism and metallogeny. The timing and petrogenesis of the voluminous Mesozoic magmatic rocks are important in understanding the mechanism of craton destruction. Here, we investigate a suite of volcanic rocks including basalt and trachybasalt, basaltic andesite and andesite, dacite and trachydacite, and rhyolite from the Yanshan belt in the northern part of the North China craton and provide evidence for intraplate magmatism along a paleosuture. We present bulk chemistry, zircon U-Pb geochronology and rare earth element data, and Lu-Hf isotopes from the volcanic suite and attempt to constrain the timing of magmatism and source characteristics. The zircon U-Pb data show two age peaks at 175−165 Ma and 155−145 Ma. Geochemically, the rocks are calc-alkaline with arc-related features. The andesites show adakitic affinities with high Sr contents (up to 1140 ppm), high Sr/Y (up to 76.2) and La N /Yb N ratios (up to 21.7), lack of a negative Eu anomaly, extreme depletion in Y and Yb, and relatively low MgO contents (1.3−4.4 wt%), indicating that they were likely derived from the partial melting of thickened lower continental crust. The Zr/Ba ratios indicate interaction between lower crust and primitive magma, possibly through magma underplating at the crust-mantle boundary. However, there was no arc-related tectonic setting in the study area during the Jurassic, which indicates that the elemental and isotopic compositions are possibly inherited from the basement rocks generated during the Paleoproterozoic subduction-accretion-collision process in the North China craton, subsequent to the 2.5 Ga cratonization event. Our zircon Hf isotope data also confirm the incorporation of Paleoproterozoic reworked material. The magmatism and craton destruction along a paleosuture were induced by mantle upwelling through far-field tectonics of the Pacific plate subduction. Our study presents a case where source components have strongly influenced the geochemical imprint of the magmas, with intraplate volcanics preserving a continental arc magmatic signature.

Out-of-sequence thrusting in polycyclic thrust belts: An example from the Mesozoic Yanshan belt, North China Craton

The EW trending Yanshan belt, an intraplate fold-thrust belt located in the northern North China Craton that has experienced several episodes of deformation widely separated in time, is characterized by out-of-sequence thrusts. According to detailed mapping in the central Yanshan belt, five geometric and stratigraphic criteria used to aid in determining whether a thrust has an out-of-sequence geometry or not can be recognized. They are (1) unconformable relationships, (2) inclination of fault surfaces, (3) irregular changes in apparent offset along strike, (4) short fault length relative to apparent offset, and (5) in-sequence geometry. With the help of these criteria, two generations of out-of-sequence thrusts that postdate the original in-sequence thrusting in the central Yanshan belt are recognized. The ancestral southward verging fold-and-thrust belt that formed prior to 180 Ma was deformed and cut by two younger generations of faults that are probably more deeply rooted and are constrained to between 172–165 Ma and 152–135 Ma. A series of thrusts with opposite vergence formed during the last period, resulting in abundant abnormal field relationships such as younger-on-older thrust relations, fold truncation, and cutting down-section. The nature and occurrence of faults in the Yanshan belt implies that superimposed deformation, a common feature in polycyclic orogenic belts, is a mechanism for the generation of out-of-sequence thrusting. This adds to mechanisms already described in the literature, such as maintaining constant critical taper at an orogenic scale, inhibition of the deformation front, and lateral changes in the nature of the decollement horizons.

燕山赤城地区中元古界硅质岩地球化学特征

燕山赤城地区中元古界硅质岩地球化学特征

The Shangzhuang Fe-Ti oxide-bearing layered mafic intrusion, northeast of Beijing (North China): Implications for the mantle source of the giant Late Mesozoic magmatic event in the North China Craton

The Early Cretaceous Shangzhuang Fe-Ti oxide-bearing layered mafic intrusion in the Yanshan Belt northeast of Beijing is coeval with the giant Late Mesozoic igneous province in the eastern part of the North China Craton (NCC). This magmatic event was associated with lithospheric thinning and thus the igneous rocks have been used to characterize the nature of the Mesozoic mantle beneath the NCC. The Shangzhuang mafic pluton intruded a large granodioritic complex and crystallized at ~850–872°C at a depth of 13–14 km. It is composed, from the base upward, of troctolite, Fe-Ti oxide-bearing gabbronorite and gabbro. Rocks from this intrusion have low initial 87Sr/86Sr ratios (0.7053–0.7058), negative initial εNd values (−9.4 to −10.7), highly differentiated LREE and nearly flat HREE patterns indicative of an EMI-like mantle source unaffected by upper crustal contamination. The occurrence of Fe-Ti oxide ore layers, magnetite-ilmenite exsolution lamellae in hornblende and high TiO2 contents of the silicate rocks indicate that they formed from Fe- and Ti-rich ferrobasaltic magmas, which may have been generated by addition of magmas from a deeper mantle source. The presence of orthopyroxene, high-Mg ilmenite (up to 8.5 wt %), hornblende, biotite and high oxygen fugacities calculated from coexisting titanomagnetite-ilmenite pairs can be explained by derivation from an enriched EMI-type mantle modified by fluids from a subducted slab and mixed with asthenospheric or deeper-mantle materials in an extensional setting. Exposure of the complex occurred during large-scale uplift (at least 13 km) and exhumation of the Yanshan orogenic belt in the Early Cretaceous. Chemical metasomatism triggered by water and enhanced by heat from a deep magma source may have played an important role in removing the ancient cratonic root, generating partial melting of the lithospheric mantle and producing coeval magmatic activity in the Mesozoic eastern NCC.

Further paleomagnetic results from the ~ 155 Ma Tiaojishan Formation, Yanshan Belt, North China, and their implications for the tectonic evolution of the Mongol–Okhotsk suture

A new paleomagnetic study on well-dated (~155Ma) volcanic rocks of the Tiaojishan Formation (Fm) in the northern margin of the North China Block (NCB) has been carried out. A total of 194 samples were collected from 26 sites in the Yanshan Belt areas of Luanping, Beipiao, and Shouwangfen. All samples were subjected to stepwise thermal demagnetization. After removal of a recent geomagnetic field viscous component, a stable high temperature component (HTC) was isolated. The inclinations of our new data are significantly steeper than those previously published from the Tiaojishan Fm in the Chengde area (Pei et al., 2011, Tectonophysics, 510, 370–380). Our analyses demonstrate that the paleomagnetic directions obtained from each sampled area were strongly biased by paleosecular variation (PSV), but the PSV can be averaged out by combining all the virtual geomagnetic poles (VGPs) from the Tiaojishan Fm in the region. The mean pole at 69.6°N/203.0°E (A95=5.6°) passes a reversal test and regional tilting test at 95% confidence and is thus considered as a primary paleomagnetic record. This newly determined pole of the Tiaojishan Fm is consistent with available Late Jurassic poles from red-beds in the southern part of the NCB, but they are incompatible with coeval poles of Siberia and the reference pole of Eurasia, indicating that convergence between Siberia and the NCB had not yet ended by ~155Ma. Our calculation shows a ~1600-km latitudinal plate movement and crustal shortening between the Siberia and NCB after ~155Ma. In addition, no significant vertical axis rotation was found either between our sampled areas or between the Yanshan Belt and the major part of the NCB after ~155Ma.

Cretaceous anomalous subsidence and its response to dynamic topography in the Songliao Basin, Northeast China

To understand the formation mechanisms and evolution of the Songliao Basin, we applied backstripping and strain rate inversion methods to 20 wells and reinterpret two seismic sections in the Songliao Basin. The obtained data were used to reconstruct the tectonic subsidence history and further assess the potential subsidence mechanisms of this area. The predicted post-rift subsidence based on the uniform stretching model that followed earlier lithospheric thinning events was much lower than the subsidence provided by backstripping. The anomalous subsidence during the post-rift stage was between 200m and 800m in the Songliao Basin, and the depocenter of anomalous subsidence moved eastward between ca. 110 and 80Ma before returning to the west between ca. 80 and 65Ma. Regional variations in the subsidence rates suggest a possible deficit in the negative buoyancy (mantle loading) that is induced by the sinking slab beneath the Songliao Basin. In addition, the anomalous post-rift subsidence was primarily driven by downward drag pressure from the subducting western Izanagi slab and asthenospheric mantle flow beneath the Songliao Basin. The basement rifting in the Songliao Basin during the early Cretaceous and the normal faulting that occurred during the post-rift stage resulted in a secondary contribution to the anomalous post-rift subsidence. Unlike the Songliao Basin, several early Cretaceous rift basins south of the Yanshan Belt were only filled during the syn-rift subsidence, and the anomalous post-rift uplift of the Yanshan belt was driven by deep mantle flow.