SHRIMP U-Pb geochronology of the zircons from the Precambrian basement of the Qilian Block and its geological significances

Origin and tectonic evolution of the Qilian Precambrian basement on NW China were investigated using zircon U-Pb ages with collaborating stratigraphic and paleontological evidence. Zircon grains were separated from two schists, two granitic gneisses and one mylonized gneiss and dated with SHRIMP. Seventy percent of sixty-one detrital zircon ages from two schists ranges from 0.88 Ga to 3.09 Ga, mostly within 1.0 Ga to 1.8 Ga with a peak at 1.6 Ga to 1.8 Ga, and twenty percent varies from 2.0 Ga to 2.5 Ga. A few falls in the Archean and Neoproterozoic periods. The two granitic gneisses were dated 930±8 Ma and 918±14 Ma, whereas the mylonized granitic gneiss was dated 790±12 Ma. These ages represent two periods of magmatisms, which can be correlated with the early and late stages of magmatisms associated with the Jinningian movement on the Yangtze Blocks. The results from this and previous studies indicate that the ages of the Precambrian detrital zircons from the Qilian Block are widely distributed in the Proterozoic era, distinct from the North China Block which was stable in the Neo-Mesoproterozoic era. By contrast, the age histograms of the detrital zircons from the Qilian Block is similar to those from Precambrian basement of the Yangtze Craton. Therefore, it is suggested that the Qilian Block had a strong affinity toward the Yangtze Craton and might belong to the supercontinent Gondwana in the Neoproterozoic time. This inference is supported by Nd model age (TDM), stratigraphic, and paleontological evidence. It is further considered that the Qilian Block was rifted from the supercontinent Gondwana during late Sinian to form an isolated continent in the Proto-Tethyan Ocean, moving towards the Alaxa Block in the North China Craton. The part of Proto-Tethyan Ocean between the Qilian and Alaxa Blocks should correspond to the so-called Paleo-Qilian Ocean. Following the closure of the Paleo-Qilian Ocean in the early Paleozoic, the Qilian Block collided with the Alaxa Block to form the North Qilian Orogenic Belt. Based on this tectonic explanation, the North Qilian ophiolites should represent parts of lithosphere from the Proto-Tethyan Ocean. Lithological and geochronological evidence also indicates that the Qilian Block underwent continental reactivation possibly induced by the deep northward subduction of the North Qaidam Block in early Paleozoic time.     

U-Pb dating of detrital zircons from the Wudangshan Group in the South Qinling and its geological significance

U-Pb dating was made by the LA-ICP-MS method for detrital zircons from the Wudangshan volcanic-sedimentary succession in the South Qinling. Samples comprise quartz sandstones of the Wudangshan Group collected from the base of the Yangping Formation and an upper layer of the Shuangtai Formation overlain its volcanic sequence, and two river-sand collections from the drainage systems cutting across the two formations, respectively. The results show that the Yangping detrital zircons are dominated by 830–780 Ma grains with a minor population of ~1.0–0.84 Ga and sporadic grains of ~2.6, ~2.4 and 2.0 Ga, whereas the Shuangtai zircons yield an upper intercept age of 763±33 Ma, identical to the timing of the Wudangshan volcanism within error, with few concordant grains of 1.9 and 0.86 Ga. Age spectra for the two river-sand samples are similar to those of the Yangping and Shuangtai Formations, respectively. It thus suggests that the Wudangshan strata are less than 780 Ma, whereas their major detrital zircon populations of 1.0–0.85 Ga and 830–780 Ma are consistent with timing of the Hannan magmatic activities along the northwestern margin of the South China Block. This suggests a Hannan or adjacent area provenance for the Wudangshan strata. The Wudang area is characterized by rift-related igneous events at ~755 and ~680 Ma, respectively, pointing to a tectono- magmatic history different from the Hannan area. It is inferred that the ~755 Ma magmatism is likely to indicate a separation of the South China Block from the supercontinent Rodinia, while the ~680 Ma event suggests a further split between the South Qinling and some unknown continent.     

SHRIMP zircon U-Pb geochronology of Indosinian granites in Hunan Province and its petrogenetic implications

The structure, magmatism and sedimentation within the South China Block (SCB) related to the Indosinian Orogeny had attracted considerable attention since De-prat[1] and Fromagat[2] proposed the “Indosinian move-ment” based on two unconformities between Pre-Norian and Pre-Rhaetian during Triassic in Vietnam. However, Indosinian tectonic evolution of the SCB has been long debated[3―6]. Some researchers believed that the compli-cated structure-magmatism-sedimentation within the SCB w…     

SHRIMP U-Pb zircon geochronology of Yangfang aegiriteaugite syenite in Wuyi Mountains of South China and its tectonic implications

In addition to the wide spread peraluminous granites, some alkaline or alkali-rich intrusive rocks were recently observed in the Wuyi Mountains. The Yangfang aegiriteaugite syenites in the Wuyi Mountains are strongly enriched in light rare earth elements (LREE), K, Ba, Ga, Zr and Y. What is more, differentiations of REE are obvious. In addition, the Yangfang aegiriteaugite syenites have also some characteristics similar to A-type granite. Zircons are selected from the Yangfang aegiriteaugite syenites for Sensitive High-Resolution Ion Microprobe (SHRIMP-Ⅱ) U-Pb geochronology study. 15 analyses give concordant 206Pb/238U ages in a narrow range of 233—249 Ma, which correspond to a single age population with a weighted mean 206Pb/238U age of 242 4 Ma, which records the crystallization time of the aegiriteaugite syenites. It is suggested that: (1) there were early Triassic peralkaline intrusive rocks in South China; (2) the early Indosinian extension event was probably active in local area, South China. This discovery provides a new insight for farther understanding of 揑ndosinian orogeny in South China.     

U-Pb ages and Hf isotopes for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southwestern margin of the North China Craton

In situ U-Pb dating and Lu-Hf isotopic analysis were carried out for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southern margin of the North China Craton （NCC）. The results provide further constraints on the crustal formation and evolution history of NCC. Four ^207Pb/^206Pb age populations were obtained from 99 analyses, with clusters at -3.40 Ga, 2.77-2.80 Ga, -2.50 Ga and 2.34 Ga, respectively. The 3.40 Ga old zircons have similar Hf isotopic compositions to those from Archean rocks in the Jidong and Anshan areas of NCC. However, crustal remnants older than 3.6 Ga have been identified in the southern margin of NCC, the South China Craton, the northwestern part of the Qinling Orogen and its adjacent area. Thus, it is not easy to trace the source rock from which the 3.40 Ge detrital zircons were derived. It can be inferred that the crustal remnants older than 3.40 Ga might have been widely distributed in the North China Craton. The 2.77-2.80 Ga zircons make up a relatively small proportion and have the highest εHf（t） values （up to 6.1±1.6）, consistent with the Hf isotopic composition of the depleted mantle at 2.83 Ga. Their single-stage Hf model age of 2.83 Ga is close to their crystallized age, suggesting that their source rocks were extracted from the contemporaneous depleted mantle. The -2.50 Ga zircon grains constitute about 85% of the total grain population and their Hf isotopic compositions indicate major growth of juvenile crust at -2.50 Ga but minor reworking of ancient crust. The youngest zircon dated in this study gave an U-Pb age of 2337±2.3 Ma, which can be considered the maximum depositional age of the formation of the Songshan Group.     

Zircon SHRIMP U-Pb geochronology and petrogenesis of the plagiogranites from the Lagkor Lake ophiolite, Gerze, Tibet, China

Field relationship, along with petrographical, petrological, and geochemical characteristics, and zircon populations, indicates that the plagiogranites from the Lagkor Lake ophiolite in the Bangong-Nujiang suture zone (~ 20 km south of Gerze County, central Tibet, China), derived from anatexis of hydrated gabbros by ductile shearing during transport of the oceanic crust. Therefore, this kind of plagiogranite is coeval to or little later than the spreading of the oceanic crust. Zircon SHRIMP age of 166 ± 2.5 Ma for the plagiogranites indicates that an oceanic basin existed in the western part of the Bangong-Nujiang zone in the Middle Jurassic.     

Formation ages and source regions of the Palaeoproterozoic Gaofan,Hutuo and Dongjiao groups in the Wutai and Dongjiao areas of the North China Craton from SHRIMP U-Pb dating of detrital zircons: Resolution of debates over their stratigraphic relationships

This paper reports detrital zircon age distributions of meta-sedimentary rocks of the Gaofan, Hutuo and Dongjiao groups in the Wutai and Dongjiao areas of the North China Craton. Detrital zircons of a quartzite from the Gaofan Group are mainly ~2.5 Ga in age, with some ~2.7 Ga and older. A quartzite pebble from the basal conglomerate of the Hutuo Group is similar in detrital zircon age distribution to the quartzite of the Gaofan Group. For a meta-feldspar-quartz sandstone from the Dongjiao Group, the age of...     

SHRIMP U-Pb zircon geochronology and its implications on the Xilin Gol Complex,Inner Mongolia,China

The Xilin Gol Complex was referred to the meta-morphic rocks exposed near Daqing Pasturage, east of Xilinhot, for the first time by the Hebei Geological Survey in 1958[1]. Similar metamorphic rocks have been increas-ingly identified in the Xilin Gol-Xiwuqi-Balinyouqi re-gion since then. They all are collectively grouped into the Xilin Gol Complex and considered as a part of the Bao- yintu Group (Pt1by)[1]. However, the timing of its deposi-tion and subsequent deformation and metamorphi…     

Initial movement of the Karakorum Fault in western Tibet： constraints from SHRIMP U-Pb dating of zircons

The Karakorum Fault zone(KFZ)plays an important role in understanding the formation,evolvement and deformation of the Tibetan Plateau.The high-T dextral shearing metamorphic rocks,e.g.,mylonites or mylonitized gneisses-granites,locally crop out along the southeastern part of the KFZ in the Ayila Ri’gyüRange area.The SHRIMP U-Pb dating of the syn-kinematic crystallized zircons indicates that the initial age of the KFZ is~27 Ma,~10 Ma older than previous results.The extensive high-T dextral shearing along the KFZ started at least at 27-20 Ma,accompanied by the syn-kinematic emplacement of leuco-granites.Deformation and concomitant fluid circulation during shearing most likely occurred as early as at 25-13 Ma.The KFZ probably grew from southeast to northwest along the fault as a result of continuous convergence between the India plate and Eurasia plate.     

SHRIMP U-Pb zircon age of tuff from the Kunyang Group in central Yunnan： Evidence for Grenvillian orogeny in South China

Whether or not Grenvillian orogeny occurred in South China still remains highly controversial because high-quality,discriminating data are lacking,and therefore,the key to resolve this matter is to find datable volcanic and/or sedimentary rocks related to Grenvillian orogeny. Such rocks are apparently present in the Fuliangpeng Member from the lower-middle part of Kunyang Group in central Yunnan; here the unit is more than 100 m thick and consists of andesitic ignimbrite,tuffite,terrigeous clastic rocks and carbonates. These volcanic rocks,developed south of the Sibao fold-thrust belts,represent the earliest calc-alkaline volcanic activity in late Precambrian time from central Yunnan and are coeval with both a change in sedimentary facies from detritus to carbonates and the beginning of seismite development elsewhere. Two samples for SHRIMP analysis were collected from this volcanic unit. Sample G3-29-2,from the bottom of Fuliangpeng Member,is an ignimbrite,and about 100 zircon crys-tals recovered from it have euhedral shapes and display relatively simple sector zonation under cathodoluminescent (CL) imaging,suggesting a magmatogenic origin. Twenty-five of the zircons were analyzed and a weighed-mean U-Pb age of 1032±9 Ma was obtained. Sample G3-29-3 from uppermost part of Fuliangpeng Member is a tuffite,and many rounded,evidently detrital zircons were recovered. Nine of these zircons were analyzed,and the oldest single-grain U-Pb zircon age is 1938±26 Ma,im-plying that Paleoproterozoic basement developed in Cathaysia. The dating result,combined with the geotectonic research on the Fuliangpeng Member,leads us to conclude that late Mesoproterozoic orogenic volcanic activity occurred in the western part of South China,and that the related collision of Yangtze and Cathaysian cratons was an integral part of the assembly of Rodinia.     

江西南部版石盆地火山岩SHRIMP锆石U-Pb年龄及其地质意义

应用SHRIMP锆石U-Pb定年方法,对江西南部三南-寻乌火山岩带版石盆地原鸡笼嶂组凝灰岩和原版石组流纹岩开展年代学研究.结果表明,凝灰岩SHRIMP锆石U-Pb年龄为(142.5±1.3)Ma,流纹岩SHRIMP锆石U-Pb年龄为(131.4±1.3)Ma.根据新的国际地层表,晚侏罗世与早白垩世的界线划在(145.5...     

The age of the base of the paleoproterozoic Hutuo Group in the Wutai Mountains area,North China Craton: SHRIMP zircon U-Pb dating of basaltic andesite

Basalt and basaltic andesite metamorphosed at greenschist facies occurs with conglomerate layers at the bottom of the Paleopro-terozoic Hutuo Group in the Wutai Mountains area, North China Craton. Detailed geological surveying confirms that these volcanic rocks are conformable within the neighboring sedimentary rocks. The SHRIMP results on basaltic andesite are divided into two groups. In one group the ²⁰⁷Pb/²⁰⁶Pb ages are from 2433 to 2558 Ma, which is consistent with the basement crustal age in Fuping and Wutai areas. In the other group, 13 grains yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2140±14 Ma. The latter is interpreted as the eruption age of the basaltic andesite, and gives the age of the base of the Hutuo Group. This result further suggests that the Hutuo Group formed in the middle Paleoproterozoic, not the early Palaeoproterozoic or late Archaean, as thought before, and is related to a 2.2–2.1 Ga rifting event in the Central North China Craton.     

赣南苗云岩体U-Pb年代学、地球化学特征及其成因

位于武夷—云开造山带中部的广东始兴—江西全南一带,是研究华南早古生代构造属性的重要区域;以盆地内苗云岩体二云母花岗岩为研究对象,通过系统的年代学和地球化学研究,分析其成因,对早古生代赣南地区构造属性特征进行补充。结果表明:苗云岩体二云母花岗岩具有高硅、高碱、富铝等特征,且相对富钾贫钠,属于高钾钙碱性系列过铝质S型花岗岩类;岩石富集大离子亲石元素Rb、K和Th,亏损Ba、Nb、Ta、Sr等元素,具有岛弧岩浆岩的特征;稀土元素总量较低,富集轻稀土,轻重稀土分馏明显,具有中等Eu负异常,暗示其主要来自于地壳熔融;构造判别图显示其形成于同碰撞的挤压环境,表明苗云岩体可能形成于陆陆碰撞所导致的陆壳加厚环境。SHRIMP锆石U-Pb年龄分析结果显示,其加权平均年龄为(442. 7±3. 2) Ma,为晚奥陶末期—早志留初期岩浆产物;结合区域地质特征,表明赣南地区在晚奥陶末期—早志留初期已由俯冲环境转换到陆陆碰撞环境。     

吐拉盆地侏罗系地层碎屑锆石U-Pb定年及其地质意义

文章依据吐拉盆地下侏罗统大煤沟组碎屑锆石激光剥蚀-电感耦合等离子体质谱仪(laser ablation inductively coupled plasma mass spectrometry,LA-ICP-MS)年代学结果,对该盆地早侏罗世时期的物源进行了分析;采集了2件砂岩样品,编号为TL14-27和TL14-31。样品TL14-27的年龄谱形成了0.45、1.08、1.32、1.68、2.58 Ga等年龄峰值,该频谱特征明显有别于现今盆地两侧的阿尔金块体与东昆仑块体,而与中祁连块体年龄谱信息(1.00～1.80 Ga之间分布广泛且频度较高)高度一致;样品TL14-31的年龄集中于0.40～0.50 Ga之间,与祁连块体结晶岩年龄信息一致而明显有别于东昆仑及阿尔金块体。研究结果表明,吐拉盆地在早侏罗世时期与柴达木盆地北缘(简称"柴北缘")的物源一致,皆来自柴达木盆地东北侧的祁连造山带。该文首次证实了吐拉盆地在中生代时期确实为柴北缘的一部分,现今的吐拉盆地是新生代时期柴达木盆地强烈改造后的剥离残片。研究成果对深入认识青藏高原北缘新生代时期的区域构造演化具有一定意义。     

Zircon SHRIMP geochronology of the Xinkailing-Kele complex in the northwestern Lesser Xing''''an Range, and its geological implications

Located in the eastern portion of the Xing'an-Mongolian Orogenic Belt (XMOB), the Xinkailing-Kele complex has previously been considered to be Precambrian metamorphic rocks, mainly according to its relatively high metamorphic grade. Our filed observation, however, revealed that the complex is composed mainly of metamorphic rocks (Kele complex), tectono-schists ("Xinkailing Group"), and granitoids (Xinkailing granitic complex). Dating on these rocks using advanced SHRIMP zircon U-Pb technique indicates that: (1) Biotite-plagioclase gneiss from the Kele complex has a protolith age of 337±7 Ma (2σ) and a metamorphic age of 216±3 Ma (2σ); (2) the tectono-schist of the "Xinkailing Group" gave a magmatic age of 292±6 Ma (2σ), indicative of felsic volcanic protolith of the schist formed in late Paleozoic time; and (3) the Menluhedingzi and Lengchuan granites of the Xinkailing granitic complex were emplaced at 167±4 (2σ) and 164±4 Ma (2σ), respectively. These results suggest that the Xinkailing-Kele c     

江西寻乌破火山口火山-侵入杂岩SHIRMP锆石U-Pb年龄及其地质意义

位于武夷隆起带和南岭构造带过渡部位、受NE向会昌—寻乌断裂带和EW向三南—寻乌断裂带控制的寻乌破火山口发育一套由碎斑熔岩和花岗斑岩组成的火山-侵入杂岩.SHIRMP锆石U-Pb定年结果显示,寻乌破火山口碎斑熔岩的年龄为(96.7±1.1)Ma,花岗斑岩的年龄为(97.3±0.9) Ma,根据国际地质年代表,早白垩世与晚...     

Zircon U-Pb SHRIMP ages of high-pressure granulite in Yushugou ophiolitic terrane in southern Tianshan and their tectonic implications

Zircons from two high-pressure granulite samples from the Yushugou ophiolitic terrane, southern Tian shan have been investigated by cathodoluminescence, LA ICPMS and ion microprobe (SHRIMP) for their internal textures, trace elemental compositions and in situ dating. The weighted mean ages of these two samples are 392±7 and 390±11 Ma, respectively, representing the granulite-facies meta morphic age of the ophiolitic terrane, and indicating that the southern Tianshan ocean initiated its northward subduction since the early Devonian.     

SHRIMP ages of detrital zircons from the Changcheng System in the Ming Tombs area, Beijing： Constraints on the protolith nature and maximum depositional age of the Mesoproterozoic cover of the North China Craton

The Mesoproterozoic Changcheng System is widely distributed in the North China Craton. Determining its time of deposition and sources is important to understand the Precambrian crustal evolution of the North China Craton. This paper suggests age distribution patterns for detrital zircons from clastic sediments of the Changcheng System in the Ming Tombs area, Beijing. Samples of feldspar-bearing sandstone (CHc-2) and pure sandstone (CHc-9) were collected from the Changzhougou Formation, which constitutes the basal part of the Changcheng System. Detrital zircons show an age range from 2.35 to 2.60 Ga. However, sample CHc-9 in the upper Changzhougou Formation also contains some zircons with ages of 1.9-1.8 Ga and 2.3-2.1 Ga. The age patterns lead to the following conclusions: (1) Most of the detrital material came from a source area composed predominantly of -2.5 Ga continental crust of the North China Craton; (2) 1.9--1.8 Ga reflects the age record of Palaeoproterozoic continent-continent collisional event in the North China Craton; and (3) the oldest age for deposition of the Changcheng System is 1.8 Ga.     

SHRIMP zircon U-Pb ages and tectonic implications for Indosinian granitoids of southern Zhuguangshan granitic composite,South China

The large southern Zhuguangshan granitic batholith composite consists of granites with ages varying from the Caledonian through Indosinian to Yanshanian. Based on K-Ar dating data, the ages of the major parts of this composite were previously regarded as Yanshanian. In this study, the SHRIMP zircon U-Pb dating method has been adopted for six plutons, Ledong, Longhuashan, Dawozi, Zhaidi, Baiyun and Jiangnan, in the southern Zhuguangshan composite, in which the four plutons other than Baiyun and Jiangnan were previously regarded as Yanshanian granites. Magmatic zircons from these six plutons, dated by this study, have yielded ages of 239±5 Ma (MSWD = 2.5), 239±5 Ma (MSWD = 2.5), 239±2 Ma (MSWD = 1.7), 239±4 Ma (MSWD = 3.2), 231±2 Ma (MSWD = 0.81) and 231±3 Ma (MSWD = 1.8), respectively. The results indicate that these plutons were formed by early Indosinian magmatism. Geochemical characteristics suggest that these granites were formed in an extensional tectonic environment. Therefore, the Indosinian period granites in the southern Zhuguangshan composite were formed by partial melting of the Paleo-Mesoproterozoic crustal components during the collapse of thickened lithosphere after the collision between the South China and Indosinian plates.     

扬子板块西缘盐边群及周边岩体地质年代学新格局

扬子板块西缘的盐边群及其周边岩体,作为前寒武纪构造岩浆活动带的核心,是观察和研究新元古代大陆壳生长的重要窗口.查明这一构造岩浆活动带内地质体的产出时代及构造岩浆活动的轨迹,是揭示新元古代大陆壳产生、发展地球动力学过程的重点环节.对盐边群及周边岩体的地质年代学研究表明:(1)区内大规模的岩浆事件集中在中元古代晚期至新元古...