BGHMS: A Geohistory Modeling System for Cratonic Basins

The BGHMS system described in this paper was designed to model the geohistory for cratonic basins and written in turbo BASIC language with a user-friendly interface for data inputs and outputs on IBM PC-compatible computers. The major mathematical models in this system include the models of compaction correction. unconformity restoration. sea-level change and paleo-water-depth correction. and tectonic subsidence modeling. Two geological events of sediment compaction and erosion are considered in this system. It consists of three modules for data input, geohistory modeling and graph conversion and output. containing more than twenty subroutine programs, is an easy-to-use. menu-driven program system with many obvious advantages, and is a useful tool for hydrocarbon geologists to make quantitative studies on geohistory and to make exploration decisions in regional evaluation.

Deformation patterns and hydrocarbon potential related to intracratonic strike-slip fault systems in the east of Central Uplift Belt in the Tarim Basin

Strike-slip faults,proven to be closely linked to hydrocarbon migration and accumulation,are wide-spread in the Tarim Basin Craton.Deformation patterns in the eastern part of the Central Uplift Belt of the Tarim Basin are analyzed in detail to understand the deformation model and genetic mechanism of strike-slip faults,how their patterns impact reservoir formation and influence oil and gas potential.Regional tectonic events are taken into consideration to identify the primary factors that controlled the development of strike-slip faults in the region.The Tazhong Uplift in the eastern part of Central Uplift Belt is believed to be a complex anticlinal uplift formed by compression-and-torsion acting on the Cambrian-Middle Ordovician platform during the middle-late Caledonian period.Multi-stage development of this north-trending fault-fold belt and the NE-trending strike-slip faults make it structurally complex.The NE-trending Paleozoic strike-slip faults were formed jointly by the differential thrusting of the Tazhong Uplift along Fault No.Ⅰand the NE-trending shear sliding of the basal fault blocks during the middle-late Caledonian-early Hercynian.Based on the distribution of effective source rocks,the tectonic setting during the period critical to hydrocarbon accumulation,and the distribution of conductive faults,the northern slope of the Tazhong Upliftd-especially its west-central part where the NE-trending faults are developed is not only located close to the northern hydrocarbon source rock area but also possesses conditions suitable for the development of carbonate reservoirs.In addition,the NE-trending faults provide passage for initial development of the reservoir and the subsequent migration of oil and gas from the Aman area to the uplifted zone,making the Tazhong Uplift an extremely favorable area for oil and gas accumulation with substantial potential for exploration.

Hydrothermal Origin of Early Permian Chert Nodules in the Central North China Craton Linked to Northern Margin Cratonic Activation

Sedimentary chert phases from the Archean to the present are widely used to trace sedimentary environments and tectonic settings.Recently,chert nodules occurring within carbonates have been the subject of possible hydrothermal or biogenic origin,in lieu of a diagenetic origin.However,chert nodules from a vast cratonic basin represent extremely rich silica accumulations but less noted is how they respond to submarine hydrothermal activity(and/or surface siliceous organism productivity).The links between the cratonic-type chert depositions and environmental changes regarding cratonic evolution need to be revisited at a large temporal-spatial scale.The chert nodules are widespread throughout the Lower Permian Taiyuan Formation in the North China Craton(NCC).Several Taiyuan chert-rich successions across the NCC have been selected to study possible links between chert deposition and cratonic evolution in scenario of partial cratonic activation of the northern NCC margin during the Late Paleozoic.Based on stratigraphic correlation,the chert nodules are ubiquitously,evenly distributed throughout the Taiyuan Formation at a large craton-basin scale from the northern to southern interior NCC.Petrological results,elemental abundances,together with silicon and oxygen isotopic compositions of chert samples infer significant hydrothermal contributions for the silica accumulations.Therefore,the cratonic-scale chert depositions of hydrothermal origin infer a giant and remote silica reservoir,linking to the large igneous province and magmatism in the NCC northern margins.The Taiyuan chert nodules could be unique marine sedimentary archives recording the Late Paleozoic NCC partial activation,which also generated continental records of igneous rocks,bauxites and tuffs.The strong tectonics of the northern margin,south-dipping topography and northward transgression of the Early Permian NCC facilitated the chert deposition on the shallow marine platform in the cratonic interior.

Main controlling factors for hydrocarbon reservoir formation and petroleum distribution in Cratonic Area of Tarim Basin

The Cratonic Area of the Tarim Basin is lo-cated in the central part of the basin, developing primarilywith Cambrian marine source rocks and secondly Middle toUpper Ordovician marine and Carboniferous-Permiantransitional facies source rocks. The source rocks werematured in the changeable period and space, formingmultiple hydrocarbon generating centers during the periods.The Cratonic Area experienced multiple tectonic orogenies,forming several palaeouplifts. The matching conditionbetween effective hydrocarbon generating centers and thepalaeouplifts in various periods is the main control factor forthe formation and distribution of hydrocarbon reservoirs.The palaeouplifts have experienced multiple hydrocar-bon-filling phases, several periods of modifications and evenbreakdown. The palaeouplifts and the adjacent slopesaround the effective hydrocarbon generating center composethe most favorable places for hydrocarbon accumulation.The hydrocarbon phase is related with the evolution of thehydrocarbon generating center. In the Tarim Basin’sCratonic Area, reservoirs were mostly formed during lateHercynian. The originally formed hydrocarbon reservoirswhich are adjacent to source kitchens and in the goodpreservation condition are the most favorable prospectingtargets. Hydrocarbon is richly accumulated under theregional caprock, surrounding the faulted trends, and overand below the unconformity surfaces. Reservoirs in theCarboniferous sandstone, Ordovician karstic weatheredcrust and carbonate rock inside the buried hill compose themain intervals for hydrocarbon accumulation. Carboni-ferous and Silurian sandstone pinchout reservoirs andcarbonate lithologic reservoirs with rich fractures and poresare the main targets for further prospecting.

Geochemistry and depositional environment of fuchsite quartzites from Sargur Group,western Dharwar Craton,India

Meso-Neoarchean fuchsite quartzites are present in different stratigraphic positions of Dharwar Craton including the oldest(~3.3 Ga)Sargur Group of western Dharwar Craton.The present study deals with the petro-graphic and geochemical characteristics of the fuchsite quartzites from the Ghattihosahalli belt to evaluate their genesis,depositional setting and the enigma involved in the ancient sedimentation history.Their major mineral assemblages include quartz,fuchsite,and feldspars along with accessory kyanite and rutile.The geochemical com-positions are characterized by high SiO_(2),Al_(2)O_(3),low MgO,CaO,strongly enriched Cr(1326–6899 ppm),Ba(1165–3653 ppm),Sr(46–210 ppm),V(107–868 ppm)and Zn(11–158 ppm)contents compared to the upper continental crust(UCC).The UCC normalized rare earth element(REE)patterns are characterized by depleted light REE[(La/Sm)UCC=0.33–0.95]compared to heavy REE[(Gd/Yb)_(UCC)=0.42–1.65]with conspicuous positive Eu-anomalies(Eu/Eu^(*)=1.35–18.27)characteristic of hydrothermal solutions evidenced through the interlayered barites.The overall major and trace element systematics reflect a combined mafic-felsic provenance and suggest their deposition at a passive continental margin environ-ment.The comprehensivefield,petrographic,and geo-chemical studies indicate that these quartzites are infiltrated by Cr-richfluids released during high-grade metamorphism of associated ultramafic rocks.The Sargur and the subse-quent Dharwar orogeny amalgamated diverse lithounits from different tectonic settings,possibly leading to the release of Cr-richfluids and the formation of fuchsite quartzite during or after the orogeny.Thesefindings sug-gest a pre-existing stable crust prior to the Sargur Group and the link between orogenic events and various mineral deposits in the Dharwar Craton.

Magmatic-tectonic response of the South China Craton to the Paleo-Pacific subduction during the Triassic:a new viewpoint based on Well NK-1

The Nansha Block(NB)is one of the blocks separated from the southern margin of the South China Craton(SCC)by the western Pacific subduction,which contains rich information of geodynamic and tectonic transformation.To reveal the essence of western Paleo-Pacific subduction during the Triassic period,Well NK-1 in this block was selected for petrographic study,and published research data from other cooperative teams were compared.A double-cycle pattern of basic to neutral magmatic volcanism was established,and 36 lithological rhythmic layers and representative cryptoexplosive breccia facies and welded tuff bands were identified.Combined with a reanalysis of published geochronological data,geochemical elements,and isotope geochemistry,we found that the rock assemblages could be divided into an intermediate-acid dacite(DA)series(SiO_(2)>65%)and basaltic(BA)series(Co<40μg/g),which was formed during the early Late Triassic((218.6±3.2)–(217.9±3.5)Ma).BA exhibits obvious calc-alkaline island-arc magmatic properties:(^(87)Sr/^(86)Sr)_i ratio ranging 0.70377–0.71118(average:0.70645),^(147)Sm/^(144)Nd ratio ranging 0.119–0.193(average:0.168),and chondrite-normalized rare earth element(REE)curves being flat,while DA exhibits remarkable characteristics of subducted island-arc andesitic magma:(^(87)Sr/^(86)Sr)_i ratio(0.70939–0.71129;average:0.71035),εNd(t)value(-6.2–-4.8;average:-5.6)andε_(Hf)(t)value(-2.9–-1.7,average:-2.2)show obvious crust-mantle mixing characteristics.BA and DA reveal typical characteristics of island-arc magma systems and typeⅡenriched mantle(EM-Ⅱ)magma.BA magma was likely resulted from the process whereby the continental crust frontal accretionary wedge was driven by the Paleo-Pacific slab subduction into the deep and began to melt,resulting in a large amount of melt(fluid)joined the asthenosphere on the side of the continental margin.In contrast,DA magma was likely resulted from the process whereby the plate front was forced to bend with increasing subduction distance,which triggered the upwelling of the asthenosphere near the continent and subsequently led to the partial melting of the lithospheric mantle and lower crust due to continuous underplating.The lithospheric thinning environment in the study area at the end of Triassic created suitable conditions for the separation between the NB and SCC,which provided an opportunity for the formation of the early intracontinental rift during the later expansion of the South China Sea(SCS).

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.

Reservoir characteristics and formation model of Upper Carboniferous bauxite series in eastern Ordos Basin,NW China

Through core observation,thin section identification,X-ray diffraction analysis,scanning electron microscopy,and low-temperature nitrogen adsorption and isothermal adsorption experiments,the lithology and pore characteristics of the Upper Carboniferous bauxite series in eastern Ordos Basin were analyzed to reveal the formation and evolution process of the bauxite reservoirs.A petrological nomenclature and classification scheme for bauxitic rocks based on three units(aluminum hydroxides,iron minerals and clay minerals)is proposed.It is found that bauxitic mudstone is in the form of dense massive and clastic structures,while the(clayey)bauxite is of dense massive,pisolite,oolite,porous soil and clastic structures.Both bauxitic mudstone and bauxite reservoirs develop dissolution pores,intercrystalline pores,and microfractures as the dominant gas storage space,with the porosity less than 10% and mesopores in dominance.The bauxite series in the North China Craton can be divided into five sections,i.e.,ferrilite(Shanxi-style iron ore,section A),bauxitic mudstone(section B),bauxite(section C),bauxite mudstone(debris-containing,section D)and dark mudstone-coal section(section E).The burrow/funnel filling,lenticular,layered/massive bauxite deposits occur separately in the karst platforms,gentle slopes and low-lying areas.The karst platforms and gentle slopes are conducive to surface water leaching,with strong karstification,well-developed pores,large reservoir thickness and good physical properties,but poor strata continuity.The low-lying areas have poor physical properties but relatively continuous and stable reservoirs.The gas enrichment in bauxites is jointly controlled by source rock,reservoir rock and fractures.This recognition provides geological basis for the exploration and development of natural gas in the Upper Carboniferous in the study area and similar bauxite systems.

Experimental study on reactions between alkaline basaltic melt and orthopyroxenes: constraints on the evolution of lithospheric mantle in the North China Craton

The experimental results of the reactions between an alkaline basaltic melt and mantle orthopyroxenes under high-temperature and high-pressure conditions of 1300–1400℃ and 2.0–3.0 GPa using a six-anvil apparatus are reported in this paper.The reactions are proposed to simulate the interactions between melts from the asthenospheric mantle and the lithospheric mantle.The starting melt in the experiments was made from the alkaline basalt occurring in Fuxin,Liaoning Province,and the orthopyroxenes were separated from the mantle xenoliths in Damaping,Hebei Province.The results show that clinopyroxenes were formed in all the reactions between the alkaline basaltic melt and orthopyroxenes under the studied P–T conditions.The formation of clinopyroxene in the reaction zone is mainly controlled by dissolution–crystallization,and the chemical compositions of the reacted melt are primarily infl uenced by the diff usion eff ect.Temperature is the most important parameter controlling the reactions between the melt and orthopyroxenes,which has a direct impact on the melting of orthopyroxenes and the diff usion of chemical components in the melt.Temperature also directly controls the chemical compositions of the newly formed clinopyroxenes in the reaction zone and the reacted melt.The formation of clinopyroxenes from the reactions between the alkaline basaltic melt and orthopyroxenes can result in an increase of CaO and Al_(2)O_(3) contents in the rocks containing this mineral.Therefore,the reactions between the alkaline basaltic melt from the asthenospheric mantle and orthopyroxenes from the lithospheric mantle can lead to the evolution of lithospheric mantle in the North China Craton from refractory to fertile with relatively high CaO and Al 2 O 3 contents.In addition,the reacted melts in some runs were transformed from the starting alkaline basaltic into tholeiitic after reactions,indicating that tholeiitic magma could be generated from alkaline basaltic one via reactions between the latter and orthopyroxene.

Geology and mineralization of the Dongping supergiant alkalic-hosted Au-Te deposit(>100 t Au)in Northern Hebei Province,China:A review

The Dongping deposit is the largest alkalic-hosted gold deposit in China containing>100 t of Au.This paper presents a new understanding for Dongping ore system,based on the previous studies.The mineralization originally occurred at 400-380 Ma,simultaneous with emplacement of the Shuiquangou alkaline complex,and was overprinted by the hydrothermal activity in the Yanshanian.Isotope compositions of ores indicate metals of the deposit are mainly provided by the Shuiquangou complex.Ore-forming fluids are characterized by increasing oxygen fugacity and decreasing sulfur fugacity,while tellurium fugacity increased in the Stage II-2 and decreased in Stage II-3.These systematic changes are closely related to the processes of mineral precipitation and fluid evolution.Sulfide precipitation from Stage Ⅰ to Stage Ⅱ was triggered by fluid boiling,which leads to the precipitation of Pb-Bi-Te,due to decrement of sulfur fugacity.Condensation of gas phase containing high concentration of H_2Te leads to precipitation of Te-Au-Ag minerals and native tellurium.Based on these hypotheses,this paper present a polyphase metallogenic model as follow.During the Devonian,fluids were released from alkaline magmas,which carried ore-forming materials form the surrounding rocks and precipitate the early ores.During the Jurassic-Cretaceous,fluorine-rich fluids exsolved from highly factionated Shangshuiquan granite,which extracted and concentrated Au from the Shuiquangou complex and the Sanggan Group metamorphic rocks,and finally formed the Dongping gold deposit.

Genesis and tectonic setting of Early Jurassic granitic rocks in Huashan Town,Xingcheng,western Liaoning

There are a large number of Mesozoic intrusive and volcanic rocks in western Liaoning of China,which is an ideal place to study the Mesozoic Paleo-Pacific subduction processes,and lithospheric destruc-tion of North China Craton.Detailed petrographic,zircon U-Pb dating and geochemical studies of the Early Jurassic granites in Huashan pluton,Xingcheng,western Liaoning,indicate that the Early Jurassic granites were formed at 184-174 Ma,mainly composed of syenite and monzogranite.The geochemical characteristics show high contents of SiO_(2),Al_(2)O_(3)and Na_(2)O+K_(2)O,low contents of Fe_(2)O_(3)and MgO,enrichment in LREEs and LILEs,and depletion in HREEs and HFSEs,and have a high content of Sr and low contents of Y and Yb,with weak negative Eu anomalies and slightly negative anomalies of Ce,indicating that they are a set of intermediate-acidic adakitic granites in high-K calc-alkaline series.All the facts significantly suggest that the Early Jurassic adakitic granites were formed at the active continental margin in the context of Paleo-Pacific plate subduction.

Metamorphic P-T conditions and zircon U-Pb ages of amphibolite in Laojinchang area,southern Jilin Province

The Longgang Block is one of the most important parts of the eastern North China Craton,characterized by extensive Late Neoarchean(~2.5 Ga)granulite facies metamorphism.However,it remains uncertain whether it was influenced by Paleoproterozoic magmatism-metamorphism.The authors provide a comprehensive analysis of amphibolite in Laojinchang area,southern Jilin Province,through petrographic,geochemical,mineralogical,and zircon dating.The main findings are as follows:The mineral assemblage of amphibolite is Hb+Pl+Cpx+Bi+Kf+Q,characteristic of amphibolite facies;zircon U-Pb dating indicates that the metamorphic age of amphibolite is 1834±33 Ma;the amphibolite has geochemical characteristics of calcium alkaline,with depletion of Nb,Ta,Ti and P.The plagioclase in the amphibolite is oligoclase,belonging to acidic plagioclase.It is speculated that the protolith of the amphibolite is diorite;using geothermobarometer,the peak metamorphic P-T conditions of amphibolite are determined to be 536–593℃/3.4–5.0 kbar,and the post-peak conditions are 429–566℃/1.3–3.1 kbar.The above results indicate that the Paleoproterozoic metamorphism has been superimposed on Longgang Block,linked to a new orogenic event on the northern edge of North China Craton.

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.

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.

Geochronological and geochemical constraints on the granitic gneiss in the Huozhou Complex: implications for the tectonic evolution of the Trans-North China Orogen

The Trans-North China Orogen is a major Neoarchean Paleoproterozoic collisional orogenic belt above the North China Craton, formed due to prolonged and complex processes. Even though many NeoarcheanPaleoproterozoic magmatic and metamorphic activities have been reported, due to the Huozhou Complex’s small outcropping range, little attention has been paid to the origin of various igneous rocks of the Huozhou Complex in the center of the Trans-North China Orogen. The Huozhou Complex, located south of the Luè liang, Wutai, and Hengshan complexes, is an important window into the Early Precambrian structure and evolution of the North China Craton. Its magma and metamorphism are crucial to understanding the development of the structural evolution of the Trans-North China Orogen. The Huozhou metamorphic complex area exposes a range of Precambrian metamorphic rocks, among which the most extensively dispersed is felsic biotite plagioclase gneiss. In this study comprehensive geological field survey, micropetrology,chronology, geochemistry, and Hf isotope analysis were carried out for the Qinggangping and Anziping gneiss in the north of the Huozhou Complex. The results show that the magmatic zircon age of the Qinggangping gneiss is2196 ± 14 Ma, and its protolith is I-type granite, formed by partial melting of igneous rocks in the absence of weathering. Its source is mainly the juvenile crust from depleted mantle dating 2431–2719 Ma, with a small amount of mantle-derived material. The Anziping gneiss has a metamorphic zircon age of 1931 ± 13 Ma with an S-type granite protolith belonging to peraluminous granite.The Anziping gneiss is formed by recycling pre-existing crustal components at 2613–2848 Ma. A minor quantity of mantle-derived magma is also introduced to the crust simultaneously. The samples of Qinggangping gneiss and Anziping gneiss show the characteristics of obvious negative Nb, Ti, and P elements in the spider diagram of primitive mantle standardization. This implies that the rocks have the characteristics of magmatic rocks in an island arc or subduction environment, which could have formed in the tectonic environment of the continental margin arc.

Geochronology,Mineralogy,and Geochemistry of the Tonsteins from the Permo–Carboniferous Benxi Formation,Ordos Basin,North China Craton

Tonstein layers are found worldwide in the Permo-Carboniferous coal-bearing strata.This study investigates the geochronology,mineralogy,and geochemistry of four tonstein samples from the Permo-Carboniferous Benxi Formation,Ordos Basin,North China Craton(NCC).The typical features of the studied tonsteins include thin beds,lateral continuity,angular quartz grains,and euhedral zircons with similar U-Pb ages,indicating a significant pyroclastic origin.In addition,the tonstein samples have low TiO_(2)/Al_(2)O_(3)ratios(<0.02)and rare earth elements and yttrium(REY)concentrations with obvious negative Eu anomalies,indicating that the tonsteins have a felsic magma origin.Moreover,compared with the mean composition of clay shale,the studied tonsteins are characterized by high concentrations of the elements Nb and Ta,which may affect the concentration of the corresponding elements in surrounding coal seams.The zircon U-Pb ages of the tonsteins(293.9-298.8 Ma)provide a precise chronological framework on the Benxi Formation in the Ordos Basin,constraining the Gzhelian-Aselian stages.The tonsteins were probably sourced from arc volcanism along the western margin of the NCC during the early Permian,implying that the Alxa Terrane had not amalgamated with the NCC at that time.

2.7-Ga-old mafic dike in the Trans-North China Orogen of the North China Craton and its tectonic significance

We studied an Archean mafic dike in the TransNorth China Orogen of the North China Craton, which has a magmatic age of 2701 ± 83 Ma and is currently the oldest mafic dike in the North China Craton. Such an old dike is extremely rare in the world. The presence of mafic dikes indicates that the North China Craton was in a tensional tectonic environment at 2.7 Ga. Geochemical characteristics reveal that this mafic dike belongs to continental tholeiitic basalt. Results from Hf isotope analysis reveal that the mafic dike originates from a depleted mantle. The plate assembly in the North China landmass was realized during the Archean era(2.7 Ga), and a thick and stable continental crust was formed. Therefore, the first cratonization of the North China Craton was completed before 2.7 Ga. The intrusion of the 2.7-Ga-old mafic dike from the deep lithospheric mantle of the continent indicates that the North China Craton has undergone a period of extensional tectonic activity. This event marks a significant extensional event that occurred after the cratonization of the North China Craton.

Petrogenesis of the~1.94 Ga Meta-gabbronorites in Liangcheng:Implications for Tectonic Evolution of the Khondalite Belt,North China Craton

We investigated the meta-gabbronorites in Liangcheng and used detailed petrography,geochemistry,zircon geochronological and in-situ Hf isotopic studies to clarify their formation and metamorphic ages,petrogenesis,tectonic setting and provide constraints on the tectonic evolution of Khondalite Belt(KB).The zircon U-Pb dating results show that the meta-gabbronorites crystallized at~1.94 Ga and were metamorphosed at~1.91–1.89 Ga.They can be subdivided into the low-Mg and high-Mg groups.The low-Mg meta-gabbronorites contain relatively lower MgO and higher SiO2 contents than high-Mg meta-gabbronorites.They are enriched in light rare earth elements and large ion lithophile elements,depleted in high field strength elements,and exhibit positive(high-Mg meta-gabbronorites)and negative(low-Mg metagabbronorites)Sr and Eu anomalies.The zircon in-situεHf(t)of meta-gabbronorites is 0.07–4.12,with Hf model ages(TDM)of 2169–2400 Ma.The meta-gabbronorites in Liangcheng originated from the asthenospheric mantle and experienced fractional crystallization of olivine,orthopyroxene,clinopyroxene,and plagioclase.They were contaminated by the crustal rocks(mainly khondalite series)during ascent,especially for low-Mg gabbronorites.The ridge subduction is the most plausible tectonic setting for meta-gabbronorites,indicating the eastern segment of KB was in a ridge subduction setting at~1.94 Ga following an orogenic thickening event during a prolonged orogenic process.

Tensile Fractures and in situ Stress Measurement Data Constraints on Cretaceous-Present Tectonic Stress Field Evolution of the Tanlu Fault Zone in Shandong Province,North China Craton

Tectonic stress fields are the key drivers of tectonic events and the evolution of regional structures.The tectonic stress field evolution of the Tanlu fault zone in Shandong Province,located in the east of the North China Craton(NCC),may have preserved records of the NCC’s tectonic history.Borehole television survey and hydraulic fracturing were conducted to analyze the paleo and present tectonic stress fields.Three groups of tensile fractures were identified via borehole television,their azimuths being NNW-SSE,NW-SE and NE-SW,representing multiple stages of tectonic events.Hydraulic fracturing data indicates that the study region is experiencing NEE-SWW-oriented compression and nearly-N-Soriented extension,in accordance with strike-slip and compression.Since the Cretaceous,the orientation of the extensional stress has evolved counterclockwise and sequentially from nearly-NW-SE-oriented to NE-SW-oriented and even nearly N-S-oriented,the stress state having transitioned from strike-slip-extension to strike-slip-compression,in association with the rotating and oblique subduction of the Pacific Plate beneath the NCC,with the participation of the Indian Plate.

Geochemistry and petrogenesis of the Paleoproterozoic orthogneisses and granitoids of the Banded Gneissic Complex,central Rajasthan,NW India:Implications for crustal reworking processes

Granitoids and orthogneisses(1.7 Ga)from the northern part of the Banded Gneissic Complex(BGC-II domain),Aravalli Craton(NW India)are geochemically analyzed to understand the geodynamic condition and crustal evolution processes.The samples are metaluminous to peraluminous(molar A/CNK:0.74±2.12),and characterized by Eu-anomaly ranging from 0.17 to 1.06,Fe_(2)O_(3)^(T)/(Fe_(2)O_(3)^(T)+MgO)from 0.8 to 0.9 and high magmatic zircon saturation temperature(>830°C)which are the features suggestive of A-type granite affinity.Tectonic discrimination diagrams classify the samples as post-collisional extensional A2-type granites.Geochemical characteristics along with trace element ratios[(Y/Nb)_(N)=0.15 to 4.33(avg.0.76),(Th/Nb)_(N)=4.63 to 255.47(avg.63.13),(Th/Ta)_(N)=1.37 to 9.84(avg.8.86),(Ce/Pb)_(N)=0.05 to 3.05(avg.1.43)]indicate that the rocks were derived from a plagioclase-rich and garnet-free crustal source under lowpressure conditions.Further,it is also proposed that tonalite-trondhjemite-granodiorite(TTG)rocks which occur dominantly in southern Rajasthan(BGC-I)are precursors and their partial melting led to the generation of the studied A-type granite samples.The studied samples also bear close geochemical similarity with A-type granites of similar age(1.7 Ga)near the Sakhun-Ladera region of northern BGC-II.The studied A-type granites are believed to be coeval to similar aged A-type granites of the Khetri and Alwar sub-basins of the North Delhi Fold Belt(NDFB).They are comparable in age and magmatic history to recorded A-type magmatism in North America and parts of the Chinese craton.The large geographical extents of synchronous A-type granites are proposed to be related to the Columbia Supercontinent assembly(ca.1.7 Ga;postcollisional granites).Thus,based on the studied extensional granites,we surmise that BGC-II was part of the Columbia Supercontinent.