Gold mineralization in Jiaodong Peninsula and destruction of North China Craton:Insights from Mesozoic granite

Large-scale tectonic magmatism and extensive gold mineralization have occurred in the eastern North China Craton(NCC)(Jiaodong and Liaodong peninsulas)since Mesozoic,which indicated that the region experienced decratonization process.The genesis type of granites evolved from S-type to I-type and A-type during Late Jurassic to Early Cretaceous,and thus reflects the evolution of geodynamics in the Late Mesozoic,indicating the varied subduction process of the Paleo-Pacific Plate(PPP)and the craton destruction.The evolution of geochemical features shows that the mantle beneath the Jiaodong Peninsula evolved from EM-II in Jurassic to EM-I in Early Cretaceous,demonstrating that the mantle switched from enriched to depleted.The gold of decratonic gold deposits in Jiaodong Peninsula mainly comes from the lithospheric mantle domains metasomatized by fluids derived from the metamorphism and dehydration of the subducted slab in the mantle transition zone.The rapid decomposition of minerals leads to the concentrated release of ore-forming fluids,and this process leads to the explosive gold mineralization during the craton destruction.Extensive magmatic uplift and extensional structures,triggered by the craton destruction in Early Cretaceous formed the extensional tectonic system,providing space for the decratonic gold deposits in Jiaodong Peninsula.

Characteristics and main controlling factors of helium resources in the main petroliferous basins of the North China Craton

At present, the main controlling factors of helium accumulation is one of the key scientific problems restricting the exploration and development of helium reservoir. In this paper, based on the calculation results of He generation rate and the geochemical characteristics of the produced gas, both the similarities and differences between natural gas and He resources in the Bohai Bay, Ordos and the surrounding Songliao Basin are compared and analyzed, discussing the main controlling factors of helium resources in the three main petroliferous basins of the North China Craton. It is found that the three basins of Bohai Bay, Ordos and Songliao have similar characteristics of source rocks, reservoirs and cap rocks, that's why their methane resource characteristics are essentially the same. The calculated ~4He generation per cubic metamorphic crystalline basement in the three basins is roughly equivalent, which is consistent with the measured He resources, and it is believed that the ~4He of radiogenic from the crust is the main factor controlling the overall He accumulation in the three basins;there is almost no contribution of the mantle-derived CH_4, which suggests that the transport and uplift of mantle-derived ~3He carried by the present-day magmatic activities along the deep-large faults is not the main reason for the mantle-derived ~3He mixing in the basins. Combined with the results of regional volcanic and geophysical studies,it is concluded that under the background of the destruction of North China Craton, magma intrusion carried a large amount of mantle-derived material and formed basic volcanic rocks in the Bohai Bay Basin and Songliao Basin, which replenished mantle-derived ~3He for the interior of the basins, and that strong seismic activities in and around the basins also promoted the upward migration of mantle source ~3He. This study suggests that the tectonic zone with dense volcanic rocks in the Cenozoic era and a high incidence of historical strong earthquakes history may be a potential area for helium resource exploration.

Destruction of the Northern Margin of the North China Craton in Mid-Late Triassic: Evidence from Asthenosphere-derived Mafic Enclaves in the Jiefangyingzi Granitic Pluton from the Chifeng Area, Southern Inner Mongolia

The petrology, geochronology and geochemistry of the mafic enclaves in the Mid-Late Triassic Jiefangyingzi pluton from Chifeng area, southern Inner Mongolia, in China are studied to reveal their petrogenetic relationship with the host pluton. Furthermore, the coeval magmatic assemblage and its petrogenesis on the northern margin of the North China craton(NCC) are studied synthetically to elucidate their tectonic setting and the implications for the destruction of the NCC. Zircon U-Pb dating reveals that the mafic enclaves formed at 230.4 ± 2.2 Ma, which is similar to the age of the host pluton. The most basic mafic enclaves belong to weak alkaline rocks, and they display rare earth element(REE) and trace element normalized patterns and trace element compositions similar to those of ocean island basalt(OIB). In addition, they have positive εNd(t) values(+3.84 to +4.94) similar to those of the Cenozoic basalts on the northern margin of the NCC. All of these geochemical characteristics suggest that the basic mafic rocks originated from the asthenosphere. Petrological and geochemical studies suggest that the Jiefangyingzi pluton and the intermediate mafic enclaves were formed by the mixing of the asthenosphere-derived and crust-derived magmas in different degrees. The Mid-Late Triassic magmatic rocks on the northern margin of the NCC could be classified into three assemblages according to their geochemical compositions: alkaline series, weak alkaline–sub-alkaline series and sub-alkaline series rocks. Petrogenetic analyses suggest that the upwelling of the asthenosphere played an important role in the formation of these Mid-Late Triassic magmatic rocks. Basing on an analysis of regional geological data, we suggest that the northern margin of the NCC underwent destruction due to the upwelling of the asthenosphere during the Mid-Late Triassic, which was induced by the delamination of the root of the collisional orogeny between Sino-Korean and Siberian paleoplates in Late Permian.

Deep Structural Framework and Genetic Analysis of Gold Concentration Areas in the Northwestern Jiaodong Peninsula, China： A New Understanding based on High-Resolution Reflective Seismic Survey

The gold concentration areas in the northwestern Jiaodong Peninsula constituted an important gold metallogenetic region in Eastern China during the Mesozoic. The deep geological bodies＇ texture characteristic is important for exploring the resources thoroughly and understanding the metallogenic process. The detailed textures were revealed using high-resolution seismic profiles through the three major ore-controlling structures-Sanshandao fault zone, Jiaojia fault zone and Zhaoping fault zone. This study aims to establish a deep structural framework of this area. Based on their formation mechanism, the fault structures developed in the area can be divided into regional and local fault structures. The structural styles are characterised by superimposing their compressional, strike-slip and extensional multi-stage activities. The crust is cut by vertical structures corresponding to a left-lateral strike-slip fault system on the surface. Nearby these structures are the arc-shape structures formed by multi-stage magma intrusions into the upper crust. Bounded by the Tancheng–Lujiang and Muping–Jimo fault zones, the current Jiaodong block, developed a series of NE-trending strike-slip fault systems, was probably formed by the assemblage of several obliquely aligned blocks. The intensive magmatism and hydrothermal activity between the blocks induced large-scale mineralisation. It provides a new angle of view for understanding the cratonic destruction and large ore-concentration formed during the Mesozoic.

Geochronology and Geochemistry of Late Triassic Intrusive Rocks in the Xiuyan Area,Liaodong Peninsula,Eastern North China Craton:Petrogenesis and Implications for Lithospheric Thinning

The timing and mechanisms of lithospheric thinning and destruction of the North China Craton(NCC)remain controversial,and the overall geodynamics of the process are poorly understood.This paper documents Late Triassic igneous rocks including monzogranite,gabbro,and diorite from the Xiuyan District on the Liaodong Peninsula in the eastern NCC,which have LA-ICP-MS zircon U-Pb ages of 229.0±0.4 Ma,216.2±0.9 Ma,and 210.6±2.0 Ma,respectively.Monzogranite shows high-SiO_(2) adakite affinity,negative ε_(Hf)(t)values(-20.6 to-17.9),and old T_(DM2) ages(3.53-3.29 Ga),suggesting that their parental magma was derived from thickened Paleoarchean mafic lower crust and minor mantle materials that were also involved their generation.Gabbro is ultrapotassic,strongly enriched in LREEs and LILEs,depleted in HFSEs,and has evolved zircon Hf isotopes with negative ε_(Hf) of -10.04 to-5.85 and old T_(DM2) ages(2.59-2.22 Ga).These are diagnostic signatures of a crustal component,but their high contents of Mg O,Cr,Co,Ni indicate that the primary magma originated from enriched mantle.Diorite is enriched in LILEs and LREEs,depleted in HFSEs(with negative Nb,Ta,and Ti anomalies),and contains negative ε_(Hf)(t)values(-13.64 to-11.01).Compared with the gabbro,the diorite is relatively enriched in Nb,Ta and HREEs,and also contains younger T_(DM2) ages(2.11-1.94 Ga),suggesting that the diorite was formed by mixing between ancient lower crust-derived felsic magmas and asthenospheric mantle-derived magmas.Field observations,geochronology,geochemistry,and zircon Lu-Hf isotopes indicate that Late Triassic magmatism and tectonic activity resulted from deep subduction of the Yangtze Craton beneath the NCC in the Xiuyan area.This phase of tectonic activity was completed in the eastern NCC by the Late Triassic(216 Ma),and was subsequently followed by lithospheric thinning that began in the Late Triassic.

The big mantle wedge and decratonic gold deposits

The Circum-Pacific subduction zone is a famous gold metallogenic domain in the world,with two important gold metallogenic provinces,the North China Craton and Nevada,which are related to the destruction of the North China Craton and the Wyoming Craton,respectively.Their ore-forming fluids were possibly derived from the stagnant slab in the mantle transition zone.The oceanic lithospheric mantle usually contains serpentine layers up to thousands of meters thick.During plate subduction,serpentine is dehydrated at depths of<200 km and transformed into high-pressure hydrous minerals,known as Phases A to E,which carries water to the depth of>300 km.The overlying big mantle wedge is hydrated during the breakdown of these hydrous facies in the mantle transition zone.The dehydration of the subducted slab in the big mantle wedge releases sulfur-rich fluid,which extracts gold and other chalcophile elements in the surrounding rocks,forming gold-rich fluid.Because the cratonic geotherm is lower than the water-saturated solidus line of lherzolite,the fluid cannot trigger partial melting.Instead,it induces metasomatism and forms pargasite and other water-bearing minerals when it migrates upward to depths of less than 100 km in the cratonic lithospheric mantle,resulting in a water-and gold-rich weak layer.During the destruction of craton,the weak layer is destabilized,releasing gold-bearing fluids that accelerate the destruction.The ore-forming fluids migrate along the shallow weak zone and are accumulated at shallow depths,and subsequently escape along deep faults during major tectonic events,leading to explosive gold mineralization.The ore-forming fluids are rich in ferrous iron,which releases hydrogen at low pressure through iron hydrolysis.Therefore,decratonic gold deposits are often reduced deposits.

Mesozoic-Cenozoic extension of the Bohai Sea： contribution to the destruction of North China Craton

The Bohai Sea is a Late Mesozoic-Cenozoic feature of the basin-mountain system located in eastern North China Craton （NCC）. The Late Mesozoic thinning of the lithosphere signals the early destruction of the NCC. The onset of the destruction was due to the delamination of thick lithosphere of the craton, represented by the NW- trending grabens in an en-echelon arrangement west to Tanlu Fault, and by the NNE-trending grabens within the Tanlu Fault Zone. The Late Mesozoic NW-trending grabens are overprinted by structures related to the Cenozoic NE-trending pull-apart basin with very thick Mesozoic-Cenozoic sediments in the eastern NCC. C- frequency diagrams of growth faults and the extension factor （fl） of four sections across the basin suggest that the extension migrated from the margin to the center of Bohai Sea, and that the Mesozoic and Cenozoic extension factors for Bohai Sea are higher than that of the margin. These evidences suggest that the greatest extension occurred in the center of Bohai Sea, which is consistent with the thinnest crust being found in the center of the sea. The extension ratios and tectonic evolution of the Bohai Sea suggest that it is the key region for the destruction of the NCC, as evidenced by the topography. However, the NCC experienced two stages of destruction with the late stage related to the tectonic regime of Northeast Asia.

Crustal deformation and dynamics of Early Cretaceous in the North China Craton

The Early Cretaceous represents a peak period of the North China Craton(NCC)destruction.A comprehensive analysis of crustal deformation during this period can reveal processes and dynamics of the destruction.The peak destruction of the NCC was associated with intense extension whose representative deformation products are metamorphic core complexes(MCCs),extensional domes and rifted basins.These MCCs occurred along both northern and southern margins of the NCC,and resulted from synchronous extension and magmatism,showing difference from the typical orogen-type MCCs in many aspects.The MCCs of the Early Cretaceous were replaced by extensional domes under relatively weak extension and uplift.In contrast to a major depression-type basin of the Early Cretaceous in the western NCC,rifted basins of the same age in the eastern NCC appeared as medium-to small-scale ones extensively.In the eastern NCC,the rifted basins north of the Bohai Bay are characterized by a feature similar to an active rift whereas those south of the Bohai present similarity to a passive rift.Various sorts of extensional structures developed during the peak destruction indicate a stable stress state of NE-SE extension over the entire central to eastern NCC,consistent with the plate margin-driven stress field.Spatial distribution of the extensional structures presents an 1800 km wide back-arc extension region in the central to eastern NCC,consistent with the Paleo-Pacific slab rollback model following flat subduction.Temporal-spatial variation of initial extension and volcanic activity during the peak period also supports the rollback model right after the flat oceanic slab.The crustal deformation evolution demonstrates that the peak destruction of the NCC took place after the B-episode compression of the Yanshan Movement of the earliest Early Cretaceous and terminated with onset of the C-episode compression of the earliest Late Cretaceous.

U-Pb Geochronology, Elemental and Sr-Nd Isotopic Geochemistry of the Houyaoyu Granite Porphyries： Implication for the Genesis of Early Cretaceous Felsic Intrusions in East Qinling

The Early Cretaceous Houyaoyu granite porphyries are located in the south margin of the North China Craton. Field observations, petrography, geochronology, major and trace elemental and Sr-Nd isotopic compositions are reported to elucidate the genesis of the Houyaoyu granite porphyries. SIMS zircon U-Pb analyses for the Houyaoyu granite porphyries yield two concordant ages of 133.2±2.3 （2σ） and 131±1.1 （2σ） Ma, respectively. Major and trace elemental compositions indicate that these porphyries are high-K I-type granites with high contents of SiO2, K20, Rb, U, Pb, low Nb, Ta, Ti, and P. Initial S：Sr/S6Sr ratios range from 0.708 3 to 0.709 7, and εNd（t） values range from -9.13 to -12.3, with corresponding two-stage depleted-mantie Nd model ages （T2DM） varying from 1.57 to 1.91 Ga. This suggests that the Houyaoyu granite porphyries were predominantly derived from ancient lower conti- nental crust, with minor involvement of mantie-derived components. On the basis of the tectonic evolution of the Qinling Orogen and geochemical characteristics of the Houyaoyu granite porphyries, it is proposed that they were formed in an extensional tectonic setting related to lithospheric destruction of the North China Craton, and produced Mo and Pb-Zn mineralization in East Qinling Orogen. KEY WORDS： East Qinling, granite porphyries, ancient lower continental crust, destruction of North China Craton.