Geochronology,Petrogenesis and Tectonic Setting of the Late Jurassic I-type Granites in the North Qinling Orogenic Belt,Central China

The North Qinling Orogenic Belt(NQOB)is a composite orogenic belt in central China.It started evolving during the Meso-Neoproterozoic period and underwent multiple stages of plate subduction and collision before entering intra-continental orogeny in the Late Triassic.The Meso-Cenozoic intra-continental orogeny and tectonic evolution had different responses in various terranes of the belt,with the tectonic evolution of the middle part of the belt being particularly controversial.The granites distributed in the Dayu and Kuyu areas in the middle part of the NQOB can provide an important window for revealing the geodynamic mechanisms of the NQOB.The main lithology of Dayu and Kuyu granites is biotite monzogranite,and the zircon U-Pb dating yielded intrusive ages of 151.3±3.4 Ma and 147.7±1.5 Ma,respectively.The dates suggest that the biotite monzogranite were formed at the end of the Late Jurassic.The whole-rock geochemistry analysis shows that the granites in the study areas are characterized by slightly high SiO_(2)(64.50-68.88 wt%)and high Al_(2)O_(3)(15.12-16.24 wt%)and Na_(2)O(3.55-3.80 wt%)contents.They are also enriched in light rare earth elements,large ion lithophile elements(e.g.,Ba,K,La,Pb and Sr),and depleted in high field strength elements(HFSEs)(e.g.,Ta,Nb,P and Ti).Additionally,the granites have weakly negative-slightly positive Eu anomalies(δEu=0.91-1.19).Zircon Lu-Hf isotopic analysis showedε_(Hf)(t)=-6.1--3.8,and the two-stage model age is T_(2DM(crust))=1.5-1.6 Ga.The granites in the study areas are analyzed as weak peraluminous high-K calc-alkaline I-type granites.They formed by partial melting of the thickened ancient lower crust,accompanied by the addition of minor mantle-derived materials.During magma ascent,they experienced fractional crystallization,with residual garnet and amphibole for a certain proportion in the magma source region.Comprehensive the geotectonic data suggest that the end of the Late Jurassic granite magmatism in the Dayu and Kuyu areas represents a compression-extension transition regime.It may have been a response to multiple tectonic mechanisms,such as the late Mesozoic intra-continental southward subduction of the North China Craton and the remote effect of the Paleo-Pacific Plate subduction.

The sedimentary record of the Sanshui Basin:Implication to the Late Cretaceous tectonic evolution in the northern margin of South China Sea

Whether the South China continental margin had shifted from active subduction to passive extension in the Late Cretaceous remains controversial.Located in the northernmost of the South China Sea continental margin,Sanshui Basin developed continuous stratigraphy from Lower Cretaceous to Eocene and provides precious outcrops to study the regional tectonic evolution during the Cretaceous.Therefore,we conducted field observations,petrology,clay mineralogy,geochemistry,and detrital zircon chronology analyses of sedimentary rocks from the Upper Cretaceous Sanshui Formation in Sanshui Basin.Results suggest that the Sanshui Basin is characterized as an intermoutane basin with multiple provenances,strong hydrodynamic environment,and proximal accumulation in the Late Cretaceous.An angular unconformity at the boundary between the Lower and Upper Cretaceous was observed in the basin.The sedimentary facies of the northern basin changed from lacustrine sedimentary environment in the Early Cretaceous to alluvial facies in the Late Cretaceous.The zircon U-Pb ages of granitic gravelly sandstone from Sanshui Formation prominently range from 100 Ma to 300 Ma,which is close to the deposition age of Sanshui Formation.The major and trace elements of the Late Cretaceous sedimentary samples show characteristics of active continental margin,and are different from the Paleogene rifting sequences.Hence,we propose that the northern South China Sea margin underwent an intense tectonic uplift at the turn of the Early and Late Cretaceous(around 100 Ma).Afterward,the northern South China Sea margin entered a wide extension stage in the Late Cretaceous(~100 to~80 Ma).This extensional phase is related to the back-arc extension in the active continental margin environment,which is different from the later passive rifting in the Cenozoic.The transition from active subduction to passive extension in the northern South China Sea may occur between the late Late Cretaceous and the Paleogene.

Metallogenic Effect of Transition of Tectonic Dynamic System

Tectonic dynamic system transition, one of the main factors in metallogenesis, controls metallogenic fluid movement and ore body location in orefields and on an ore deposit scale (mainly in the continental tectonic setting), and even the formation and distribution of large-scale deposit clusters. Tectonic dynamic system transition can be classified as the spacious difference of the tectonic dynamic system in various geological units and the temporal alteration of different tectonic dynamic systems. The former results in outburst of mineralization, while the latter leads to the metallogenic diversity. Both of them are the main contents of metallogenic effect of tectonic dynamic system transition, that is, the alteration of dynamic system, the occurrence of mineralization, and the difference of regional tectonic dynamic system and metallogenic diversity. Generally speaking, the coupling of spatial difference of tectonic dynamic system and its successive alternation controlled the tempo-spatial evolution regularity of mineralization on a larger scale. In addition, the analysis of mineralization factors and processes of typical ore deposits proved that the changes of tectonic stress field, the direct appearance of tectonic dynamic system transition, may lead to the accident of mineralization physical-chemical field and the corresponding accidental interfaces were always located at ore bodies.

Petrogenesis of the Early Cretaceous Laojunshan monzogranite at the southern margin of the North China Craton: Constrains on the transition of the tectonic regime

1 Introduction Voluminous Mesozoic magmatic rocks containing abundant Au-Mo polymetallic mineralization resources are developed in the Xiaoqinling-Xiong’ershan district of the southern margin of the North China Craton(NCC).

A late Carboniferous transition from subduction to collision:Tectonosedimentary evidence from southern Junggar,NW China

The Carboniferous to Permian tectono-sedimentary evolution of the southern Junggar area brings new insights into understanding the subduction-collision processes in the northern Tianshan region.Integrating geophysics,geochemistry,and geochronology approaches,this study investigates the Carboniferous-Permian strata in the southern Junggar Basin.The results have revealed three distinct tectono-stratigraphic evolutionary stages,each marked by a distinctive volcano-sedimentary sequence.The Early Carboniferous strata suggest intense volcanic activities in the southern Junggar area.During the Late Carboniferous,the southern Junggar Basin was controlled by normal faulting in an extensional setting,receiving sedimentary inputs from the Junggar terrane.The Lower Permian,unconformably overlying the Upper Carboniferous,was shaped by an extensional regime and is comprised by volcano-clastic sequences that received detritus from the Yili-Central Tianshan block.These findings indicate that a Late Carboniferous forearc basin developed in the southern Junggar area,and it evolved into a post-collisional rift in the Early Permian.This period marked a dynamic shift from bidirectional subduction(rollback)to the detachment of the North Tianshan oceanic slab.We propose that the collision between the YiliCentral Tianshan block and the Junggar terrane,along with the closure of the North Tianshan Ocean,likely occurred in the Late Carboniferous(ca.306-303 Ma).

The Geological Significance of the Deformation and Geochronology of the Xiaotian–Mozitan Shear Zone in the Dabie Orogenic Belt(East-Central China)

The Xiaotian–Mozitan Shear Zone(XMSZ)is the boundary of the Dabie High-grade Metamorphic Complex(DHMC)and the North Huaiyang Tectonic Belt.It was deformed in ductile conditions with a top-to-NW/WNW movement.Geothermometers applied to mineral parageneses in mylonites of the shear zone give a temperature range of 623–691°C for the predeformation and 515–568°C for the syndeformation,respectively,which indicates a retrograde process of evolution.A few groups of zircon U-Pb ages were obtained from undeformed granitic veins and different types of deformed rocks in the zone.Zircons from the felsic ultramylonites are all magmatic,producing a weighted mean 206Pb/238U age of 754±8.1 Ma,which indicates the time of magmatic activities caused by rifting in the Neoproterozoic.Zircons from the granitic veins,cutting into the mylonites,are also of magmatic origin,producing a weighted mean 206 Pb/238 U age of 130±2.5 Ma,which represents the time of regional magmatic activity in the Cretaceous.Zircons from the mylonitic gneisses are of anatectic-metamorphic origins and are characterized by a core-mantle interior texture,which yielded several populations of ages including the Neoproterozoic ages with a weighted mean 206 Pb/238 U age of 762±18 Ma,similar to that of the felsic ultramylonites and the Early Cretaceous ages with a weighted mean 206Pb/238U age of 143±1.8 Ma,indicating the anatectic metamorphism in the Dabie Orogenic Belt(DOB).Based on integrated analysis of the structure,thermal conditions of ductile deformation and the contact relations of the dated rocks,the activation time of the Xiaotian–Mozitan Shear Zone is constrained between~143 Ma and 130 Ma,during which the DOB was undergoing a transition in tectonic regime from compression to extension.Therefore,the deformation and evolution of this shear zone plays an instrumental role in fully understanding this process.This research also inclines us to the interpretation of it as an extensional detachment,with regard to the tectonic properties of the shear zone.It may also be part of a continental scale extension in the background of the North China Block’s cratonic destruction,dominated by the subduction and roll-back of the Paleo-Pacific plate,but more detailed work is needed in order to unravel its complicated development.

Emplacement and deformation of Shigujian syntectonic granite in central part of the Dabie orogen:Implications for tectonic regime transformation

The Shigujian pluton is a gneissic quartz monzonite located in Tiantangzhai area in central part of the Dabie orogen.Anisotropy of magnetic susceptibility(AMS) data show that most magnetic foliations dip steeply to southeast.About 85% of sampling points dip from 40° to 90°.Magnetic foliations are generally parallel to the foliations measured in the field.The pluton has NWW-SEE trending lineations in the southeast and NE-SW trending lineations in central part and north,but the lineations plunge to SW in central part and to NE in the north.All plunges are moderate.The anisotropy degree(P) is between 1.065 and 1.532 and the shape parameter(T) is between 0.005 and 0.694.A Flinn diagram of the magnetic fabrics shows that the value of K is less than 1.The analysis of AMS suggests that the pluton was emplaced and deformed under a SE-NW compressional stress regime.The analysis of quartz C-axis fabrics indicates that the pluton was deformed under compressional stress and deformation temperatures range from 400 to 500℃.Microstructures indicate that the pluton is deformed in near solidus conditions and the pluton is a synkinematic intrusion.The emplacement of the Shigujian granite is inferred to have taken place syntectonically.The zircon U-Pb dating of the granite suggests that the pluton was intruded at 141±2.3 Ma.By synthesizing all data,it seems that the Shigujian pluton was emplaced in a compressional environment and the transformation time of the Dabie orogen from compression to extension took place after 141 Ma.The structural evolution of the Dabie orogen was controlled by the Pacific tectonic domain when the Shigujian pluton was emplaced,whereas the adjacent Tiantangzhai complex massif is the result of an extensional environment.

Metallogenetic model of Jiaodong-type gold deposits,eastern China

The genesis of giant gold provinces is an international scientific frontier,in which the source of a huge amount of gold and the drive for mineralization are key challenges.The mineralization intensity of the Jiaodong gold province in eastern China is a rare occasion in the world,because it owns gold reserves of~5500 tons within an area of less than 10,000 km^(2).The Jiaodong gold province formed in the superimposed domain of diverse tectonic regimes in an intracontinental setting.Paleoproterozoic regional peak metamorphism and Triassic continental collision are followed by the tectonic transition and gold mineralization at ca.120 Ma with time intervals of 1.9 billion years and about 100 million years,respectively.The Jiaodong gold deposits are different from orogenic and intrusion-related gold deposits in terms of the tectonic background,types of host rocks,and oreforming mechanism.These gold deposits show close spatial-temporal and genetic relationships to mafic igneous rocks,implying the derivation of ore-forming fluids from the metasomatic mantle domains.Mafic dykes in Jiaodong have negativeε_(Hf)(t)values of-29.9 to-9.1,Os content of 0.002-0.16 ppb,heavyδ^(18)O up to 8.23‰,and high initial^(187)Os/^(188)Os ratios of 0.1352-0.8858.These indicate that the lithospheric mantle was metasomatized by ancient crust-derived components.The lithospheric mantle in the western Jiaodong shows generally more enriched isotope features than that in the eastern part,which is explained to be an important reason for its huge gold resources.The mafic dykes show lighter Mg isotope characteristics(averageδ^(26)Mg of-0.33‰,n=50)and high Ca O content(overall greater than 6.5 wt%),indicating that the lithospheric mantle would also record the metasomatism by the carbonate rocks from the subducting oceanic slab.Under the background of the rollback of the subducting paleo-Pacific slab and the destruction of the North China Craton,partial melting of the lower crust would produce granitic magmas that led to the enrichment of gold in the residual crust.The syn-mineralization asthenosphere upwelling would promote the recycling of the lower crust and the partial melting of the metasomatic mantle domains.Basic magmas,produced by the partial melting,and the mantle itself would degas to form ore fluids.The ore fluids would further leach gold in the lower crust to increase its fertility.The auriferous fluids were transported to the middle to upper crust along the detachment and strike-slip faults.Water-rock interaction and fluid immiscibility,which occurred in and above the ductile-brittle transition zone to induce gold precipitation,formed the Jiaodong gold deposits.Given the unique geological features and genetic model of the Jiaodong gold deposits,they can be defined as“Jiaodong-type”gold deposits.

The important turning points during evolution of Cenozoic basin offshore the Bohai Sea:Evidence and regional dynamics analysis

The Cenozoic basin offshore the Bohai Sea underwent a multicycle-rifting during its evolutionary process, which resulted in the multiple unconformities in the strata. The tectonic activities shown by these unconformities have different manifestations and influences on the basin evolution. The authors systematically analyze the tectonic evolution characteristics of the sags off-shore the Bohai Sea with a large set of hydrocarbon exploration data. The analysis reveals that two phases of tectonic activities during the late depositional stage of the third member of Shahejie Formation (about 38 Ma) and the late depositional stage of Dongying Formation (about 24 Ma) reflect the significant changes in the basin's features and structural framework before and after these tectonic activities. As a result, the two phases of tectonic activities are recognized as important turning points (i.e., tectonic transitions) of Cenozoic basin evolution. The regional dynamic backgrounds of the two phases of tectonic transitions are also discussed. It is suggested that the early tectonic transition occurred at about 38 Ma under such regional dynamic back- ground that a huge kinematics adjustment happened between Eurasian Plate and its neighboring plates, i.e., Pacific Plate and Indian Plate. Meanwhile, the Tan-Lu Fault's slip reversed from left lateral to right lateral. The late tectonic transition occurred in late Paleogene (about 24 Ma) and reflected the Himalayan orogeny's influence on the Chinese continent and even the Asian continent; at the same time, the stress field produced by the escape tectonics was related to the Himalayan orogeny, superposed on the pre-existing stress field, and then enhanced the right lateral slip activity of the Tan-Lu Fault.