40Ar-39Ar ages of the Louzidian-Dachengzi ductile shear zone near Chifeng, Inner Mongolia and their tectonic significance

The Louzidian normal fault occurs as the eastern detachment fault of the Kalaqin metamorphic core complex. Field observations and microstructural analyses reveal that the Louzidian-Dachengzi ductile shear zone developed in its lower-plate was genetically related to sinistral strike-slips and extensional faulting. Two samples from this ductile shear zone yield ⁴⁰Ar-³⁹Ar plateau ages of 133 Ma (Bi) and 126 Ma (Kp), which are concordant with their isochron ages. The plateau age of 133 Ma (Bi) records the formation age of the ductile shear zone. The inconsistent relationship between the earlier strike-slip ductile shear zone and the later normal fault makes the Kalaqin Quasi-metamorphic core complex distinctive from Cordilleran metamorphic core complex. These ages provide important geochronological data for putting constraints on the formation age and genesis of such ductile shear zones.     

^40Ar／^39Ar geochronological constraints on syn-orogenic strike-slip movement of Tan-Lu fault zone

Two phases of sinistral strike-slip ductile shear belts occur on the eastern margin of the Dabie orogenic belt. A muscovite ~(40)Ar/~(39)Ar plateau age of 128 Ma was obtained from mylonite in the later ductile shear zone. Three muscovite samples separated from mylonites of 3 localities in the earlier ductile shear belts yield ~(40)Ar/~(39)Ar plateau ages of 192.5±0.7 Ma, 189.7±0.6 Ma and 188.7±0.7 Ma, respectively. They are interpreted as cooling ages of the earlier sinistral strike-slip deformation. It is suggested that left-lateral displacement of the Tan-Lu fault zone started in a late stage of the collision orogeny in the Dabie-Sulu orogenic belt between the North and South China plates. Therefore, the earlier Tan-Lu fault zone was syn-orogenic strike-slip tectonics. The fault zone was used again for sinistral displacement during tectonic activities of peri-Pacific regime in Early Cretaceous. It is proposed that the fault zone occurred as a transform fault during the orogenic process.     

40Ar-39Ar thermochronology of two ductile shear zones from Yiwulüshan, West Liaoning Region: Age constraints on the Mesozoic tectonic events

Two ductile shear zones trending EW and NNE respectively not only controlled the tectonic framework of the northern North China, but also constrained the geodynamic background for gold mineralization in this region. Field observations and microstructural analyses reveal that the EW trending ductile shear zones are mainly contributed to dextral compressional deformation resulting from top-to- the-southeast oblique thrust shearing, whereas the NNE trending ones are genetically related to sinistral strike-slip and extensional faulting. One sample from the former yielded an 40Ar-39Ar plateau age of (219±4) Ma (Bi) and two samples from the latter gave 40Ar-39Ar plateau ages of (116±2) Ma (Bi) and (127±3) Ma (Bi). These ages provide constraints on the top-to-the-southeast oblique thrusting event occurring in Late Triassic and the sinistral extensional and strike-slip faulting event which occurred in Early Cretaceous.     

Deformation characteristics and formation mechanism of the Yunmengshan metamorphic core complex

The Yunmengshan metamorphic core complex in the middle part of the Yanshan Fold and Thrust Belt records crust extension processes of the eastern North China Craton during its peak destruction.Development of the metamorphic core complex was controlled by the generally NNE-striking Dashuiyu Shear Zone.The shear zone dips SE and becomes shallower NE-wards,leading to exposures of a ductile shear zone in the southern and middle parts and brittle faults in the northern part.Exposure structures,microstructures,and quartz C-axis fabrics indicate that the ductile shear zone belongs to an extensional shear zone with a top-to-the-SE shear sense.Deformation temperatures of 300–520°C suggest a midcrustal origin for the ductile shear zone.A ductile deformation belt in the footwall of the shear zone is only as wide as 1–3 km,indicating no widespread mid-crustal ductile flow in the region during the deformation.Zircon U–Pb dating of dykes and plutons as well as hornblende and biotite40Ar/39Ar dating demonstrate that the metamorphic core complex originated at 135 Ma and experienced intense shearing of the Dashuiyu Shear Zone,development of the supradetachment basins,and synkinematic intrusion during 135–125 Ma.The metamorphic core complex was subjected to rapid exhumation during 125–114 Ma when the Dashuiyu Shear Zone suffered continuous activity and passive doming.The shear zone and its hanging wall were cut or replaced by a series of brittle faults when they wereuplifted to a brittle regime,showing that exhumation took place in continuous extensional activities.The metamorphic core complex turned into slow exhumation in an extensional regime in the following latest Early Cretaceous.The evolution history suggests that the Yunmengshan metamorphic core complex was developed by the rolling-hinge model,a common formation mechanism for intraplate metamorphic core complexes in the North China Craton,under the continuous NW–SE extension during the Early Cretaceous(135–100 Ma).     

40Ar-39Ar thermochronology of two ductile shear zones from Yiwulüshan, West Liaoning Region: Age constraints on the Mesozoic tectonic events

Two ductile shear zones trending EW and NNE respectively not only controlled the tectonic framework of the northern North China, but also constrained the geodynamic background for gold mineralization in this region. Field observations and microstructural analyses reveal that the EW trending ductile shear zones are mainly contributed to dextral compressional deformation resulting from top-to-the-southeast oblique thrust shearing, whereas the NNE trending ones are genetically related to sinistral strike-slip and extensional faulting. One sample from the former yielded an ⁴⁰Ar–³⁹Ar plateau age of (219± 4) Ma (Bi) and two samples from the latter gave ⁴⁰Ar–³⁹Ar plateau ages of (116± 2) Ma (Bi) and (127±3) Ma (Bi). These ages provide constraints on the top-to-the-southeast oblique thrusting event occurring in Late Triassic and the sinistral extensional and strike-slip faulting event which occurred in Early Cretaceous.     

~(40)Ar/~(39)Ar age evidence for Altyn fault tectonic activities in western China

As the northern boundary of the Tibetan Plateau theAltyn fault is a huge NEE strike-slip fault belt in the inner Asian continent. Its formation and evolution are closelyrelated to the uplift of the whole plateau and the mass es-cape to the east. In recent years the Altyn fault has be-come a hot point of geological study in the Tibetan Pla-teau. The formation age of the Altyn fault was argued for a long time. Some researchers emphasized that the Altyn fault had commenced since early Paleo…     

辽南万福变质核杂岩韧性剪切带的应变与剪切作用类型

辽南万福变质核杂岩韧性剪切带为一走向近SN,倾向NE的低角度韧性剪切带,带内的主要岩石类型为花岗质糜棱岩。以长石为应变标志体的Rf/φ应变测量显示应变类型为压扁应变;以极莫尔圆与长短轴法的运动学涡度估算表面,剪切带的剪切作用类型为以简单剪切为主的一般剪切。结合区域构造演化与年代学资料,认为辽南万福变质核杂岩韧性剪切带形成于加厚地壳造山后垮塌,且在早白垩纪华北克拉通大规模地壳伸展的联合作用下由地壳的中、深层次被抬升至地表,并与各时代的岩石在同一露头上并置而成。     

Structural and chronological constraints on a Late Paleozoic shortening event in the Yanshan Tectonic Belt

Early deformation history of the Yanshan Tectonic Belt is an unsolved, important issue. We report early structures and their dating results in basement rocks south of the Kalaqin metamorphic core complex. The basement deformation zone is characterized by WNW-ESE steep gneissosity, WNW-ESE shallow mineral elongation lineation and many intrafolial or isoclinal folds. Many kinematic indicators show that it is a WNW-ESE steep, ductile sinistral deformation zone. LA-ICPMS zircon dating gives a crystallization age of 271 Ma for tatolly transposed, deformed monzonite granite dyke and crystallization ages of 251–250 Ma and 210 Ma for undeformed dyke, stock and pluton. The dating results demonstrate that the deformation related to regional shortening took place between 271 and 251 Ma, i.e. Middle-Late Permian. Combined with previous dating results for post-orogenic magmatism in the Yanshan Tectonic Belt, it is inferred that the shortening-related deformation happened in Middle Permian (271–260 Ma). Recent studies suggest that collision along the Solonker Suture took place in Middle Permian, and is then followed by post-collision extenion in Late Permian. We propose that the intense shortening deformation of the Middle Permian in the Yanshan Tectonic Belt belongs to foreland deformation along the norterhn margin of the North China Craton which is caused by continent-continent collision in the Xing-Meng Orogen. The belt was then oriprinted by shortening deformation of the Yanshanian Movement.     

广东南雄断裂带Ar-Ar年龄及其地质意义

 南雄断裂带是一条大型的拉伸剥离断层。在详细的野外调查和岩石样品镜下观察的基础上,对南雄断裂带糜棱岩中的同构造变形-变质矿物白云母进行了Ar-Ar法同位素年龄测定。年代学研究结果表明,南雄断裂带作为拉伸剥离正断层活动开始于早白垩世（117.3±2.7 Ma）,而其强烈的伸展变形韧性剪切发生在早白垩世末～晚白垩世初（Ar-Ar坪年龄为94.6±0.4 Ma）,随后在88.9±1.0 Ma和81.1±2.0 Ma还可能发生过2次较明显的热扰动事件。南雄断裂带Ar-Ar年龄数据为进一步判断华南白垩纪岩石圈伸展作用发生的时间提供了新的热年代学制约,即是说,早白垩世末～晚白垩世初是区域岩石圈伸展的重要时期。     

Geological and chronological evidence of Indo-Chinese strike-slip movement in the Altyn Tagh fault zone

A set of granitic and amphibole mylonite are exposed in the Altyn Tagh fault zone. The preliminary study shows that these rocks are the product of the syntectonic anatexis in the process of the left-lat- eral strike-slip shear, and are the result of the ductile transpression. There are two types of zircon sorted from the mylonite formed with synshear anatexis. Among them, one is the anatectic long columnar zircon and another is the residual metamorphic sub-rounded columnar zircon. Two groups of age for single zircon measured by ion microprobe (SHRIMP) are obtained: one is 461-547 Ma for the sub-rounded columnar residual metamorphic zircon, and the other is 239-244 Ma for the long-columnar anatectic zircon. This type of zircon is directionally spread in rock, and the long axis direction of its crystal is identical to that of stretching lineation, representing the direction of tectonic stress in the process of the strike-slip. 40Ar-39Ar age of the directional growth hornblende in the same mylonite sample measured is 223-226 Ma. The above geological and chronological data prove that the syntectonic anatexis occurred during Indo-Chinese Epoch, showing that the strong strike-slip movement occurred in the Altyn Tagh fault zone at least as early as Indo-Chinese Epoch.     

Evolution of the Xiaotian-Mozitan fault and its implications for exhumation of Dabie HP-UHP rocks

The Xiaotian-Mozitan fault （XMF） located north of the Dabie orogenic belt separates the North Dabie complex to the south from the Beihuaiyang low-grade metamorphic rocks to the north. It comprises several NW-striking ductile shear zones and brittle faults. The brittle faults obviously overprinted on the ductile shear zones and promoted the development of the volcanic basins in early Cretaceous to the north, which suggests that the brittle faults were normal faults formed in early Cretaceous during doming of the Dabie orogenic belt. The ductile shear zone superposed on the north Dabie gray gneiss, and it is an important channel where the Dabie HP-UHP rocks exhumed. For obtaining new structural constraint on exhumation of the HP-UHP rocks, we present here experimental results on the microstructure, quartz C-axis fabrics and the microprobe analyses of phengite. The ductile shear zone was determined to be formed at a temperature of 600-650 ℃ and pressure of 1.1 GPa by the mineral deformation, microprobe analyses and geobarometry of Si-in-phengite of the mylonite, the results suggest that the mylonite now exposed on the surface experienced an upper amphibolite-facies metamorphism in the lower crust. The mineral stretching lineation varies from horizontal in the east segment to sub-dip in the west. Shear sense indicators from outcrop and thin sections of orientated specimen and quartz C-axis fabrics suggest that the XMF is a sinistral normal fault. The kinematics analysis of the ductile shear zone indicates that the exhumation of Dabie HP-UHP rocks is the results of a SE-directed extrusion and an anticlockwise rotation around its eastern pivot simultaneously.     

新疆若羌县赤石山变质核杂岩研究

新疆北山裂谷带西缘若羌县赤石山地区存在大型变质核杂岩构造。其下拆离盘为古元古界结晶基底杂岩 ,上拆离盘为中元古界白湖群变质岩系 ,滑脱拆离带为已变形的大型韧性正断层。核杂岩南、北侧发育上叠盆地 ,北侧形成“年轻”正断层及同构造期岩浆岩。变质核杂岩的形成与晚古生代北山陆内裂谷区域伸展有直接联系。该区变质核杂岩的首次厘定 ,对深化北山裂谷的研究具有重要意义     

The large-scale ductile shear zone in Kuda, western Kunlun

A set of metamorphic suites, previously known as the Precambrian metamorphic basement in Kuda, western Kunlun has been determined as a large nappe ductile shear zone which develops penetrative foliation, oriented stretching lineation, various but consistent kinematic indicators of thrusting from north to south. The microstructure and the supermicrostructure also display the ductile deformation characteristics, and the paleotectonic differential stress is obtained. The metamorphic age is 426–451 Ma dated by⁴⁰Ar/³⁹Ar method. According to tectonic background, it is a product of early Paleozoic accretionary wedge orogenesis in western Kunlun.     

商南-商城断裂带中段大型韧性走滑剪切带研究

商南－商城断裂带中段的韧性剪切带是由多条糜棱岩带及其所夹的变质岩块体组成的大型韧性剪切带。它是以简单剪切为主要变形机制的、于绿片岩相温压条件下形成的韧性剪切带。剪切带内糜棱岩的塑性变形和恢复重结晶作用强烈。糜棱叶理产状多较陡倾，ａ线理近水平，表明为走滑断裂。大量不对称旋转构造指示该剪切带早期为左行剪切，后期在不同地段又叠加上程度不同的右行剪切     

西天山那拉提构造带及其找矿前景

那拉提构造带位于天山西段伊犁盆地与南天山之间 ,出露宽度约 2 5 km,由早元古代片岩、片麻岩及混合岩、混合花岗岩组成 ;深层次韧性剪切带及叠加中 -浅层次剪切带发育 ;那拉提构造带南缘断裂明显 ,南侧为古生代浅变质黑色岩系 ;构造带北缘韧 /脆性剪切带宽度大 ,两侧岩石劈理化后易于混淆 ,故界线不明显。那拉提构造带基本地质特征可与邻国中天山对比 ,存在寻找国外类似大型金矿床的条件和可能     

Whole-rock Ar-Ar dating for low-grade metavolcanics within the Dabie orogen and its geological significance

Greenschist-facies metasedimentary and metaigne- ous rocks are frequently found to occur continuously along convergent plate margins where high pressure (HP) or ultrahigh pressure (UHP) metamorphic rocks also crop out[1-7]. Geological investigations of co…     

Sm-Nd and Rb-Sr isotopical ages of magnetite-chlorite formation gold deposit in the volcanic rock area of Late Paleozoic Era, East Tianshan

Through studying geo-geochemical characteristics and determining Sm-Nd and Rb-Sr isotopical ages in the Kanggur magnetite-chlorite formation gold deposit, some conclusions can be achieved: Sm-Nd isochron age is (290.4 ± 7)Ma, Rb-Sr isochron age is (282.3 ± 5)Ma, mineralization is Late Hercynian and the formation of this gold deposit is related to the macroscopic Huangshan-Qiugemingtashi ductile shear zone which is produced as a result of collision between the Tarim plate and the Junggar plate.     

Post-collisional extensional and thrust-nappe structures in northern part of Dabie Mountains

Thrust-nappe structures and extensional structures simultaneonsly occur in the northern part of the Dabie Mountains. The systematic structural study reveals that extensional structures along the Mozitan-Xiaotian fault and thrust-nappe structures that take the Jinzhai-Shucheng fault as their frontal thrust share the same shear sense, and display a transitional relationship from the ductile extensional structure of deep level in the south to the ductile-brittle and brittle thrust-nappe structure of shallow level in the north. The extensional and thrnst-nappe structures in the region are explained to result from post-collisional processes by the continuous subduction of the Yangtze continental block and the extension induced by the uplifting of the core part of the Dabie Mountains, which are components of the exteasional structures produced in the exhumation process of the ultrahigh pressure metamorphic rocks in the Dabie Mountains. Because of the frontal blocking in the process of the north-westward spreading, the extension and detachment of the low-grade metamorphic rocks along the Mozitan-Xiaotian fault was transformed into the northwestward thrustiag, resulting in the thrust-nappe structures. They developed in the period of 200-170 Ma, maybe last till the late Jurassic.     

Age of Anzishan granulites in the Mianxian-Lueyang suture zone of Qingling orogen: With a discussion of the timing of final assembly of Yangtze and North China craton blocks

Many granulite blocks have been recently discovered in the Anzishan area, Mianxian County, Shaanxi Province, in the central segment of the Mianxian-Lueyang suture zone. Sm-Nd isochron and 40Ar/39Ar ages of mineral samples of granulites from this area are reported in the note. The granulite mineral samples give a Sm-Nd isochron age of t=(206 ( 55) Ma (2(), with INd = 0.51302(7(2() and MSWD = 1.6, and a biotite 40Ar/39Ar plateau age of (199.7(1.7) Ma (2(). The formation and uplift of the granulites were related to subduction of the Yangtze block beneath the South Qinling microcontinent and their collision and the final amalgamation of the Yangtze and Sino-Korean craton blocks in the Indosinian, and their age is consistent with or close to those of the Dabie ultrahigh-pressure and high-pressure metamorphic rocks, so have great tectonic significance.     

排山楼金矿床~(40)Ar-~(39)Ar年龄及其地质意义

采用Ar-Ar同位素测年法对辽宁排山楼金矿床矿化阶段形成的钾长石和似斑状花岗岩中的钾长石进行了测试,获得矿化阶段形成钾长石40Ar-39Ar坪年龄为(116.69±1.15)Ma,等时线年龄为(116.97±1.39)Ma,以及似斑状花岗岩中原生钾长石40Ar-39Ar坪年龄为(126.71±2.03)Ma,等时线年龄为(124.00±4.32)Ma的数据.前者代表了该矿床的形成年龄,即该矿床形成于早白垩世,后者表明金矿化不早于(126.71±2.03)Ma.这一成矿年龄与华北陆块北缘大部分金矿床的成矿年龄一致,表明华北陆块北缘绝大多数金矿床形成于中国东部中生代动力学体制发生转折的时期.