A New SHRIMP Age of the Xiamaling Formation in the North China Plate and Its Geological Significance

Setting up the hypostratotype of late Precambrian is the main aim of the research on the Meso- and Neoproterozoic in North China. The chronostratigraphic position is the key in this study. However, many key horizons have not been calibrated with the high-quality isotopic ages. Using the reported new U-Pb age with the Sensitive High-Resolution Ion Microprobe (SHRIMP II), a zircon U-Pb age was obtained of the ash bed in the Xiamaling Formation in North China Plate, yielding a weighted mean 206Pb/238U age of 1368±12 Ma. It is the first SHRIMP U-Pb age from the Xiamaling Formation in the North China Plate, and represents the depositing time of the middle part of the Xiamaling Formation. The zircon age plays an important role to understanding geological evolution of the North China Plate during Meso- and Neoproterozoic.     

Reclassification of the Meso- and Neoproterozoic Chronostratigraphy of North China by SHRIMP Zircon Ages

Abstract: High-quality zircon U-Pb ages acquired from Meso- and Neoproterozoic strata in North China in recent years has provided a high-resolution chronostratigraphic framework for dating. A basis of this high-level chronostratigraphic system provides the foundation for a global Precambrian study and stratigraphic correlation and so recent geological studies have focused attention on systemic SHRIMP zircon dating. A chronology of Meso- and Neoproterozoic strata and the time of origin of the overlying Changcheng System is given on the basis of new SHRIMP zircon dating from the Qianxi Complex and diabase of the Chuanlinggou Formation. A new tectonostratigraphy for a Neoproterozoic chronostratigraphic framework in the southeastern margin of the North China continent is underpinned by the new SHRIMP zircon dating of a Neoproterozoic mafic magma diabase in the Jiao-Liao-Xu-Huai Sub-Province.     

Neoproterozoic Tectonic Setting of Southeast China: New Constraints from SHRIMP U-Pb Zircon Ages and Petrographic Studies on the Mamianshan Group

<正>Precambrian tectonic history of Zhejiang,Fujian,and Jiangxi provinces of south China is important for understanding the tectonic evolution of South China but its magmatic activity, petrogenesis,stratigraphic sequence of the Mamianshan Group is still strongly controversial.Here we present new sensitive high resolution ion micro-probe(SHRIMP) U-Pb zircon geochronological data for the Mamianshan Group and petrographical data to constrain the tectonic framework of the regions.Our results showed that the SHRIMP U-Pb zircon age of green schists of the Dongyan Formation is 796.5±9.3 Ma,the Daling Formation is 756.2±7.2 Ma,and mica-quartz schist of the Longbeixi Formation is 825.5±9.8 Ma.These data indicate that the Mamianshan Group was formed not in the Mesoproterozoic,but in the Neoproterozoic and its stratigraphic sequences should be composed of Longbeixi,Dongyan,and Daling Formations from the bottom to the top.Rocks from this Group, from Zhejiang,Fujian and Jiangxi provinces,constituted the upper basement of the Cathaysia Block that overlay the lower basement of the Mayuan Group.Detailed petrographic studies demonstrate that the amphibole schists of the Dongyan Formation in the Mamianshan Group were formed within an intra-arc rift setting rather than a continental rift as previously suggested.Rather,this island-arc type formation was developed by collision and/or subduction between various blocks resulting from the breakup of the supercontinent Rodinia at c.850-750 Ma.The Zhuzhou conglomerate,distributed near Dikou Town,Jian'ou City,Fujian Province and previously considered as evidence of the Mesoproterozoic Dikou movement,is shown here not to be the basal conglomerate above the angular unconformity between the upper and lower basements.Our conclusions have important implications for understanding the Precambrian tectonics of South China.     

Neoproterozoic ana texts of the yunkai metamorphic Complex; Evidence from U-Pb geochronology of zircon in migniatite北大核心CSCD

In order to determine the migmatization time of the Preeambrian metamorphic basement(the Yunkai metamorphic complex) of the Yunkai terrain, a study of LA-MC-1CP-MS U-Pb dating of zircon in matrixes and felsic veins of mig-matites from the Tiantangshan Group. Results show that the inherited magmatic cores of zircon grains from the matrix of migmatite have oscillatory zoning and yield a U-Pb age of 1086 ±24 Ma, representing the pmtolith age, whereas the metamprohic rims of zircon grains from the felsic vein of migmatite yield an age of 852. 6±8. 8 Ma, representing the timing of the anatex is (migmatization). These results imply that there is a Mesoproterozoic metamorphic basement with a Neoproterozoic anatexis event in the Yunkai area. The timing of this anatexis event is consistent with that of the Sibao Movement. Therefore, we conclude that the timing of the anataxis could be an important geological record for the amalgamation of the Yangtze and Cathaysian blocks.     

U-Pb zircon age from the base of the Ediacaran Doushantuo Formation in the Yangtze Gorges,South China： constraint on the age of Marinoan glaciation

The reported new U-Pb age by sensitive high-resolution ion microprobe (SHRIMP Ⅱ) on zircon was obtained from a tuff sample at the basal Doushantuo Formation in the Jiuqunao section, which situated at the western limb of the Huangling anticline in the Yangtze Gorges in Zigui, Hubei, South China. Eighteen spots of zircons were analyzed and they form two clusters: one includes three spots, with an inherited age of 784+ 15 Ma (MSWD=0.05); the other consists of 15 spots and gives a weighted mean ^206pb/^238U age of 628.3&#177;5.8 Ma (MSWD=0.86). It is the first SHRIMP U-Pb zircon age obtained nearly the base of the Doushantuo Formation of Ediacaran and represents a maximum age of the Doushantuo Formation It also forms an age constraint on the upper limit age of the Nantuo (Marinoantype) glaciation.     

The Jiangnan orogenic belt between the Yangtze and Cathaysia blocks for Neoproterozoic context

In recent years, more high quality zircon U-Pb ages have been obtained from Neoproterozoic strata in the South China continental block. A refined chronostratigraphic framework is being set up on these data. A basis of the high quality chronostratigraphic system is fundamental for global Precambrian study and strata correlation. Our recent geological studies focus on systematic SHRIMP zircon dating. We will refine the chronostratigraphic framework of the Neoproterozoic and the start time of the cover beds of the Shuangqiaoshan Group basing on the new SHRIMP zircon ages from the volcanic ash beds in the Hengyong and Anlelin formations. Accordingly, we will also discuss Neoproterozoic tectonostratigraphy in South China     

Ordovician Proto–basin in South China and its Tectonic Implications: Evidence from the Detrital Zircon U–Pb Ages of the Ordovician in Central Hunan, China

Caledonian orogeny is another important tectonic event in South China Block after the breakup of the Rodinia supercontinent. With a view to constrain the tectonic evolution and proto–basin in South China, this paper reports the geochemical and zircon U-Pb dating data of the Ordovician strata in central Hunan, South China. Geochemical features and paleocurrent directions suggest that the lower Ordovician deposited in a passive continental margin basin with a provenance of quartzose components and showing an affinity with the Yangtze Block. U-Pb age data for 260 detrital zircons from upper Ordovician identify three major age populations as: 900–1200 Ma, 1400–1800 Ma and 2400–2700 Ma. The detrital zircon age spectrum as well as the paleocurrent directions suggest that upper Ordovician deposited in a foreland basin and showing a close affinity with the Cathaysia Block. It is also suggest that the lower Ordovician continuously accepted the mineral from the Yangtze Block, whereas the provenance of the upper Ordovician sedimentary basin changed from the Yangtze Block to the Cathaysia Block. This change implies a tectonic movement, which caused the transformation of the proto–basin in the Hunan area in SCB from passive continental margin basin to foreland basin probably took place during late Ordovician. This fact also demonstrate that the Caledonian orogeny in South China Block began no later than 453 Ma, and a new crustal evolution model is proposed.     

Revised Chronostratigraphic Framework of the Metamorphic Strata in the Jiangnan Orogenic Belt,South China and its Tectonic Implications

In a re-study of regional geology by the China Geological Survey (CGS), the key problem is in the stratigraphical division and correlation. According to the new isotopic dating of the Meso- and Neoproterozoic in China, there have been great changes in the strata correlation and tectonic explanation. The authors obtained four zircon sensitive high resolution ion micro-probe (SHRIMP) U-Pb datings from the bentonite of the Lengjiaxi Group (822±10 Ma, 823±12 Ma and 834±11 Ma) and Banxi Group (802.6±7.6 Ma) in north Hunan Province, which is considered to be the middle part of the Jiangnan Orogenic Belt. On the basis of the zircon dating mentioned above, the end of the Wuling orogen is first limited in the period from 822 Ma to 802 Ma in one continued outcrop (Lucheng section) in Linxiang city, Hunan Province. Combining a series of new zircon U-Pb datings in the Yangtze and Cathaysia blocks, several Neoproterozoic volcanic events and distribution of the metamorphic rocks in the Jiangnan Orogenic Belt have been distinguished. In the context of the global geodynamics, it is useful to set up a practical and high precision chronological framework and basic and unified late Precambrian section in South China.     

Detrital Zircon Dating of Meso- and Neoproterozoic Rocks in North China and Its Implications

New dates from Meso- and Neoproterozoic strata contribute to the recently defined Precambrian stratigraphical timescale of China agreed by the Subcommission on the Precambrian System, and the National Commission on Stratigraphy of China on Nov. 24, 2009. First, the age range of the Changcheng System, including the Changzhougou, Chuanlinggou, Tuanshanzi and Dahongyu formations has been constrained to 1.8–1.6 Ga. Second, the Jixian System including the Gaoyuzhuang, Yangzhuang, Wumishan, Hongshuizhuang and Tieling formations has been constrained to 1.6–1.4 Ga. Third, an as-yet unnamed (undefined) system (1.4–1.0 Ga) is only developed in the Xiamaling Formation at the Jixian section, Tianjing. Fourth, the Qingbaikou System, including the Luotuoling and Jing’eryu formations has been constrained to 1.0–0.78 Ga. Fifth, the Nanhuan System ranges between 780–635 Ma, and the Sinian System is within 635–542 Ma. However, according to a series of SHRIMP U-Pb dates from the late Precambrian in the Jiangnan Orogen Belt in South China Platform, the constrained strata will be redefined as in the upper part of the Qingbaikou System. To aid global geodynamics, it is useful to denote a late Precambrian section with unified, precise and high-precision chronological dating; this is here defined in North China Block and Jiaoliao-Korean Block. However, the Neoproterozoic Qingbaikou study in North China will be influence in whole Meso- and Neoproterozoic in the Jiangnan Orogenic Belt in between the Yangtze Block and the Cathaysia Block in South China.     

Chronology and cooling history of the Tianmenshan pluton in South Jiangxi Province and their geological significance

Geochronological studies on the crust-derived Tianmenshan pluton were undertaken by SHRIMP zircon U-Pb dating and Ar-Ar dating of biotite, muscovite and K-feldspar, giving a petrogenetic age of 167 Ma. Owing to the closure systems in different minerals, the cooling history of the pluton can be determined with an age-temperature diagram. The late hydrothermal event has been recognized, which is related probably with mineralization. In terms of the comparative geochronological and petrologic records, it is concluded that there are some constrains on tectonic evolution and that the formation of the Tianmenshan pluton proceeded in a transition period from Indosinian post-orogeny extension to strong compressive tectonics. And the timing of the hydrothermal event matches the compressive climax of the Yanshanian orogeny. The temporal gap between granite emplacement and wolframite mineralization could last 10-20 Ma owing to the low cooling rate of the pluton.     

北京延庆高于庄组凝灰岩的锆石U-Pb定年研究及其对华北北部中元古界划分新方案的进一步约束

作者最近在北京延庆高于庄组张家峪亚组上部发现了凝灰岩,并测得了该凝灰岩中锆石1559±12Ma的SHRIMPU-Pb年龄和1560±5Ma的LA-MC-ICPMS U-Pb年龄。这一新的高精度定年结果表明,华北北部高于庄组形成于中元古代初期的盖层纪(Calymmian Period,1600~1400Ma)早期。结合早先大红峪组火山岩的锆石U-Pb年龄(1622~1625Ma),现在可以确切地将高于庄组的底界年龄限定在1600Ma左右。结合最近在铁岭组斑脱岩获得的锆石U-Pb年龄(~1440Ma),本文再次建议,应将华北中元古界蓟县系的底界下拉到高于庄组底界,自该组底部(1600Ma)到铁岭组顶部(1400Ma)的巨厚碳酸盐岩序列都属于新定义的蓟县系,并对应于国际中元古界的盖层系,高于庄组与大红峪组之间的界线则可作为蓟县系与长城系的分界标志。高于庄组凝灰岩锆石的精确定年,为华北北部中元古界年代地层划分等研究,提供了直接的年代学约束。     

云南晋宁地区柳坝塘组凝灰岩SHRIMP锆石U-Pb年龄及其对晋宁运动的制约

云南晋宁地区柳坝塘组凝灰岩SHRIMP锆石U-Pb年龄(890±9Ma、834±34Ma)和Hf同位素分析结果,有利于云南晋宁地区青白口系的年代定位,为中国南、北方青白口纪地层对比提供了可靠的同位素年龄数据。通过同位素年代学研究探讨昆阳群与八街群的关系,对比华南块体青白口系区域地层和构造演化序列,特别是对晋宁运动底界年龄进行限定。对柳坝塘组凝灰岩样品LBT26-3和LBT26-4进行Hf同位素分析。结果显示,柳坝塘组凝灰岩主要来自于中元古代的岩浆物质,几乎不含地壳组分。样品LBT26-3的εHf(t)值较高,二阶段Hf模式年龄较小,说明其组分为更年轻的岩浆物质,显示在柳坝塘组形成时期可能发生了持续的陆壳增生事件,此区域可能更多为洋壳组成。上述年龄为云南青白口纪地层的对比增添了可靠的年代学依据。     

黔东南—桂北地区四堡群凝灰岩锆石SHRIMP U-Pb年龄及其地层学意义

首次获得黔桂地区晚前寒武纪四堡群凝灰岩(斑脱岩)锆石SHRIMP U-Pb年龄,结合新获得的侵位于四堡群的花岗岩锆石SHRIMP U-Pb年龄,将四堡群明确定位于新元古界。"江南造山带"普遍发育的新元古代四堡群(梵净山群、冷家溪群、双桥山群)和上覆的丹州群(下江群、板溪群、河上镇群)均为低变质绿片岩系,构成了江南古陆的变质基底。多年来四堡群一直划归中元古界,并且构成"四堡运动"的主体,其时代的定位一直影响着整个江南古陆变质基底的地层划分和对比,也制约着中国地质学家对江南造山带的地质背景和成矿条件解疑。通过对四堡群及其上覆地层中斑脱岩锆石的研究,精确地测定并获得了四堡群斑脱岩锆石的年龄841.7Ma±5.9Ma和黔东南摩天岭地区侵入四堡群的花岗岩锆石的年龄826.8Ma±5.9Ma。     

Geochronology of the Limu W–Sn–Nb–Ta‐Bearing Granite Pluton in South China

Limu W–Sn–Nb–Ta mining district is located in the Nanling Range W–Sn poly‐metallic mineralization belt in south China. The district includes a number of Sn–Nb–Ta and W–Sn ore occurrences; all of them are spatially associated with granite stocks of a largely‐unexposed pluton, the Limu granitic pluton. A granite sample collected from the Sn–Nb–Ta‐bearing Jinzhuyuan granite stock yields a zircon SHRIMP U–Pb age of 218.3 ± 2.4 Ma, a muscovite ⁴⁰Ar/³⁹Ar plateau age of 212.4 ± 1.4 Ma, and a muscovite ⁴⁰Ar/³⁹Ar isochron age of 213.2 ± 2.2 Ma. Another granite sample collected from the W–Sn‐bearing Sangehuangniu granite stock yields a zircon SHRIMP U–Pb age of 214 ± 5 Ma. The geochronological data provide new constraints on the age of the Limu granite pluton and the timing of the associated W–Sn–Nb–Ta mineralization—at least it sets a reasonable upper age limit for the mineralization of the W–Sn–Nb–Ta ores. The reported ages suggest an active Late Triassic granitic magmatism in Limu area which is part of a regional magmatic event near the end of the Indosinian orogeny in south China.     

Time constraints on the closure of the Paleo–South China Ocean and the Neoproterozoic assembly of the Yangtze and Cathaysia blocks: Insight from new detrital zircon analyses

The South China Block was built up by the assembly of the Yangtze and Cathaysia blocks along the Neoproterozoic Jiangnan Orogenic Belt. The timing of the Jiangnan Orogeny remains controversial. The widespread orogeny–related Neoproterozoic angular unconformity that separates the underlying folded Sibao (ca.1000–820 Ma) and overlying Danzhou (ca.800–720 Ma) Groups was investigated. Six sedimentary samples, below and above the unconformity in three distal localities (Fanjingshan, Madiyi, and Sibao) yield detrital zircon with UPb ages ranging from 779 ± 16 Ma to 3006 ± 36 Ma, with a prominent peak at ca. 852 Ma. The youngest ages of 832 ± 11 Ma and 779 ± 16 Ma are revealed for the underlying Sibao and overlying Danzhou Groups, respectively. The detrital zircon UPb age relative probability plot of the Jiangnan Orogen matches well with those of the Yangtze and Cathaysia blocks since ca. 865 Ma. Integrating geological, geochemical and geochronological results, we suggest that the Paleo–South China Ocean began to subduct under the Yangtze block at ca. 1000 Ma, and was partly closed at ca. 865 Ma. Afterwards, the Yangtze and Cathaysia blocks initially collide at 865 Ma, forming the Jiangnan Orogen. This collision resulted in not only the folding of the Sibao Group, but also sediment deposition in a syn-collisional setting, which makes the upper part of the Sibao Group. The youngest S-type granite dated at ca. 820 Ma that intruded in the Sibao Group marks the late stage of the Jiangnan Orogeny.     

滇中新元古代澄江组层型剖面锆石U-Pb年代学及其地质意义

澄江组是华南新元古代地层中的重要地层单元之一,其确切沉积时限对于建立和完善华南新元古代区域地层格架具有关键意义。对滇中澄江组层型剖面顶部的凝灰岩夹层进行高精度SHRIMP锆石U-Pb定年研究,获得3组有效的206Pb/238U加权平均年龄值,其分别为(819±14)Ma(MSWD=0.15)、(781±11)Ma(MSWD=0.24)和(725±11)Ma(MSWD=0.65)。其中,最年轻的一组年龄值(725±11)Ma被解释为澄江组顶部凝灰岩夹层的形成时间,可以代表滇中澄江组的顶界年龄,从而进一步确认澄江组的沉积时限为800～725 Ma。结合相关研究资料,证实澄江组与开建桥组、莲沱组的沉积时限基本相当,三者与下冰期长安组不存在对比关系,而应与冰期前板溪群的上部进行对比。此外,推测两组年龄值较老的锆石可能是与新元古代罗迪尼亚(Rodinia)超大陆裂解有关的幕式岩浆活动的记录。     

Australian provenance for Upper Permian to Cretaceous rocks forming accretionary complexes on the New Zealand sector of the Gondwanaland margin

U–Pb (SHRIMP) detrital zircon age patterns are reported for 12 samples of Permian to Cretaceous turbiditic quartzo‐feldspathic sandstone from the Torlesse and Waipapa suspect terranes of New Zealand. Their major Permian to Triassic, and minor Early Palaeozoic and Mesoproterozoic, age components indicate that most sediment was probably derived from the Carboniferous to Triassic New England Orogen in northeastern Australia. Rapid deposition of voluminous Torlesse/Waipapa turbidite fans during the Late Permian to Late Triassic appears to have been directly linked to uplift and exhumation of the magmatically active orogen during the 265–230 Ma Hunter‐Bowen event. This period of cordilleran‐type orogeny allowed transport of large volumes of quartzo‐feldspathic sediment across the convergent Gondwanaland margin. Post‐Triassic depocentres also received (recycled?) sediment from the relict orogen as well as from Jurassic and Cretaceous volcanic provinces now offshore from southern Queensland and northern New South Wales. The detailed provenance‐age fingerprints provided by the detrital zircon data are also consistent with progressive southward derivation of sediment: from northeastern Queensland during the Permian, southeastern Queensland during the Triassic, and northeastern New South Wales — Lord Howe Rise — Norfolk Ridge during the Jurassic to Cretaceous. Although the dextral sense of displacement is consistent with the tectonic regime during this period, detailed characterisation of source terranes at this scale is hindered by the scarcity of published zircon age data for igneous and sedimentary rocks in Queensland and northern New South Wales. Mesoproterozoic and Neoproterozoic age components cannot be adequately matched with likely source terranes in the Australian‐Antarctic Precambrian craton, and it is possible they originated in the Proterozoic cores of the Cathaysia and Yangtze Blocks of southeast China.