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Zircon U-Pb age and Hf-O isotope evidence for Paleoproterozoic metamorphic event in South China [查看] Shao-BingZhang、Yong-FeiZheng、Yuan-BaoWu、Zi-FuZhaoa、ShanGaob、Fu-YuanWu

To understand the connection between continental cratonization and global tectonothermal event is essential for recognizing the formation and evolution of continental crust. Paleoproterozoic is an important era with occurrence of megascale tectonomagmatism in the world, but it has been intriguing whether they also influenced the oldest continent in South China. In order to decipher the nature of Paleoproterozoic event in South China, a combined study of zircon U-Pb dating, Hf and O isotope analyses was carried out for metasediments and amphibolite from the Kongling terrane, the only Archean microcontinent outcropped in South China. U-Pb ages of 1.97±0.03 Ga were obtained with low Th/U ratios of 0.01–0.14, indicating that the ages are a record of Paleoproterozoic metamorphic event.18O values of ∼11‰ and ∼8‰ were measured for quartz from the metasediments and garnet from the amphibolite, respectively, suggesting that their sources experienced supracrustal recycling. εHf(t) values of about −6.5 and model Hf ages of about 3.0 Ga were acquired for zircons from the metapelites, suggesting an Archean source. Thus a response to the Paleoproterozoic global tectonothermal event in South China is reworking of Archean continental nucleus. Compared with Archean rocks at Kongling, abrupt changes in K2O/Na2O, REE and other trace elements are observed in the Paleoproterozoic metasedimentary rocks. This is interpreted to reflect a change in upper crustal composition at the Archean–Proterozoic boundary.A survey of Paleoproterozoic ages throughout the Yangtze Block suggests that metamorphic event and subsequent magmatic activity occurred in the north, but only magmatic activity in the south. Both metamorphic and magmatic activities are associated with formation of a unified basement responsible for cratonization of the Yangtze Block. This provides a geodynamic connection between the formation of this craton and the global tectonomagmatism in the Paleoproterozoic, marking continental accretion by arc-continent collision orogeny during assembly of the supercontinent Columbia.

Mesoproterozoic Continental Arc Type Granite in the Central Tianshan Mountains Zircon SHRIMP U-Pb Dating and Geochemical Analyses [查看] YANGTiannan、LIJinyi、SUNGuihua、WANGYanbin

Abstract: The Central Tianshan belt in northwestern China is a small Precambrian block located in the southern part of the Central Asia Orogenic Belt (CAOB), which is considered as "the most voluminous block of young continental crust in the world" that comprises numerous small continental blocks separated by Paleozoic magmatic arcs. The Precambrian basement of the central Tianshan Mountains is composed of volcanic rocks and associated volcano-sedimentary rocks that were intruded by granitic plutons. Geochemical analyses demonstrate that the granitic plutons and volcanic rocks were generated in the Andean-type active continental arc environment like today's Chile, and the zircon U-Pb SHRIMP dating indicates that they were developed at about 956 Ma, possibly corresponding to the subduction ofthe inferred Mozambique Ocean under the Baltic-African super-continent.

EPMA U-TH-Pb MONAZITE AND SHRIMP U-Pb ZIRCON GEOCHRONOLOGY OF HIGH-PRESSURE PELITIC GRANULITES IN THE JIAOBEI MASSIF OF THE NORTH CHINA CRATON [查看] XIWENZHOU、GUOCHUNZHAO、CHUNJINGWEI、YUANSHENGGENG、MINSUN

ABSTRACT. The Jiaobei massif constitutes the southern segment of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton, and contains high-pressure (HP) mafic and pelitic granulites. The HP pelitic granulites contain four distinct mineral assemblages: garnet kyanite biotite muscovite rutile (M1),garnet kyanite ternary-feldspar biotite muscovite rutile (M2), garnet sillimanite biotite perthite anti-perthite (M3), and biotite muscovite sillimanite plagioclase (M4), of which M2 is a typical HP pelitic assemblage. The origin of these HP granulites is important for understanding the tectonic evolution of the Jiao-Liao-Ji Belt, but the timing of the HP event still remains unknown. This study applied the EPMA U-Th-Pb monazite and SHRIMP U-Pb zircon dating techniques to determine the metamorphic ages of the HP pelitic granulites in the Jiaobei massif. CL images and Th/U ratios show that all monazite and zircon grains in the analyzed samples are of metamorphic origin. Eighty-two EPMA analyses on 16 monazite grains from two samples yielded three age groups at 1895 to 1882 Ma, 1814 to 1813 Ma and 1706 to 1692 Ma, and the ages of 16 SHRIMP analyses on 14 zircon grains from one sample range from 1864 to 1803 Ma. The oldest ages of 1895 to 1882 Ma were yielded from high-Y monazite grains enclosed in garnet and sillimanite pseudomorph after kyanite, and thus interpreted as the time of the HP (M2) granulite-facies metamorphism.The EPMA monazite ages of 1814 to 1813 Ma and SHRIMP zircon ages of 1864 to 1803 Ma are interpreted as the approximate ages of the post-HP granulite-facies metamorphism (M3), most probably resulting from the exhumation of the HP granulites to the medium-pressure granulite-facies levels. The youngest age group of 1706 to 1692 Ma obtained from low-Y monazite grains in the matrix can be interpreted as the age of late cooling and retrograde metamorphism (M4) that occurred when the HP pelitic granulites were exhumed to the upper crust. The presence of Paleoproterozoic HP mafic and pelitic granulites in the southern segment of the Jiao-Liao-Ji Belt suggests that the evolution of the Jiao-Liao-Ji Belt in the Eastern Block of the North China Craton must have been involved in subduction- or collision-related tectonic processes.

Early cretaceous collapse of the zircon granitoids and their implications for the Dabie orogen, eastern China SHRIMP U-Pb dating and geochemistry [查看] HaijinXu、ChangqianMa、KaiYe

Two stages of early Cretaceous post-orogenic granitoids are recognized in the Dabie orogen,eastern China,which recorded processes of extensional collapse of the orogen.The early stage granitoids(~132Ma)are foliated hornblende quartz monzonites and porphyritic monzogranites.They are of high-K calc-alkaline series and mataluminous to weakly peraluminous,with high K2O and Low MgO contents(Mg# values:32.0-46.0),they contain high Sr,low Y and heavy rare earth elements(HREE),and have high Sr/Y and (La/Yb)N ratios,without clear negative Eu,Sr and Ti noomalies.The early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite-rich plagioclase,and thus indicate the existence of an over-chickened(〉50km)crustal root beneath the orogen at ~132Ma.The later stage granitoids(~128Ma)are undeformed fine-grained monzogranites,fine-grained K-feldspar granites and coarse-grained K-feldspar granite-porphyry.They belong to a peraluminous and high-K calc-alkaline to shoshonite series,the display a flat HREE pattern and have strong negative Eu,Sr and Ti anomalies,with Low Sr/Y and (La/Yb)Nratios.The late stage grantoids are equilibrated with residues rich in anortbite-rich plagioclase,hornblende.ilmeite/titanite and poor in garnet,indicating that the crust of the Dabie orogen became thinner(〈35km)at ~128Ma.SHRIMP zircon U-Pb ages and changing compositional trends for these two stages of granitoids indicate that the over-thickened crust formed by the Triassic continental subduction/collision under the Dabie orogen remained until the early Cretaceous,and collapsed quickly in a few million year during the early Cretaceous.

An island arc origin of plagiogranites at Oytag, western Kunlun orogen, northwest China SHRIMP zircon U-Pb chronology, elemental and Sr –Nd– Hf isotopic geochemistry and Paleozoic tectonic implications [查看] Yao-HuiJiang、Shi-YongLiao、Wan-ZhiYang、Wei-ZhouShen

This paper presents detailed SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotope geochemistry of three intrusive plagiogranite bodies formed in an island arc setting within the Oytag ophiolite suite of the Oytag–Kuda Suture Zone. SHRIMP zircon U–Pb dating of two of these intrusions shows that they were emplaced into the Oytag volcanic rocks at 337.5±4.1 Ma and 327.7±4.9 Ma, and that the younger intrusion contains inherited zircons ranging from 435.3 to 480.8 Ma. Both intrusions are composed of low-Al trondhjemite and tonalite. The rocks have low K2O and Sr and high Y contents with low Sr/Y ratios. They have relatively low rare earth element (REE) concentrations and are depleted in light REE (LREE) and relatively enriched in heavy REE (HREE), with marked negative Eu anomalies in the chondrite-normalized REE patterns. They have initial 87Sr/86Sr ratios of 0.7048 to 0.7068 and εNd (T) values of 6.2 to 7.6, which are lower than those of typical MORB. In situ Hf isotopic compositions of zircons with Early Carboniferous age from both intrusions are characterized by positive initial εHf values, ranging from +12.5 to +19.5, similar to typical MORB. The inherited Ordovician zircons in the younger intrusion have initial εHf values of −1.9 to +3.1, similar to zircons from Ordovician granitoids in the region. Detailed elemental and isotopic data suggest that the Oytag plagiogranites were formed in an oceanic island arc setting, by differentiation of arc tholeiitic magmas that formed in response to the intra-oceanic subduction. The inherited zircons are interpreted as the xenocrysts captured from the incipient oceanic crust that contains relicts of discrete fragments of continental crust with Ordovician zircons, through which the plagiogranite was emplaced. The Oytag ophiolite suite might be the remnants of an ancient island arc, the basement of which was the incipient oceanic crust. The Oytag–Kuda Suture Zone is suggested to be a multiple suture zone that represents the remnants of both the Proto-Tethyan and Paleo-Tethyan oceans. The western Kunlun orogen was therefore not a long-lived active continental margin during the early and late Paleozoic times. It most likely underwent at least two orogenic cycles and thus might represent a multiple orogenic belt.

SHRIMP Zircon U-Pb Dating of Gabbro and Granite from the Huashan Ophiolite,Qinling Orogenic Belt,China Neoproterozoic Suture on the Northern Margin of the Yangtze Craton [查看] SHIYuruo、LIUDunyi、ZHANGZongqing、MIAOLaicheng、ZHANGFuqin、XUEHongmei

Abstract: The recently identified Huashan ophiolitic melange was considered as the eastern part of the Mianliie suture in the Qinling orogenic belt. SHRIMP zircon U-Pb geochronology on gabbro from the Huashan ophiolite and granite intruding basic volcanic rocks indicates crystallization ages of 947±14 Ma and 876±17 Ma respectively. These ages do not support a recently proposed Hercynian Huashan Ocean, but rather favor that a Neoproterozoic suture assemblage (ophiolite) is incorporated into the younger (Phanerozoic) Qinling orogenic belt.

High-pressure granulite from Western Kunlun,northwestern China Its metamorphic evolution,zircon SHRIMP U-Pb ages and tectonic implication [查看] QUJunFeng、ZHANGLiFei、AIYongLiang、LÜZeng、WANGJianPing、ZHOUHui、WANGShiYan

High-pressure mafic granulites occurring as lenticular bodies within garnet-amphibolites in Kangxiwar Fault have been first reported in this paper. The P-T conditions of two metamorphic stages were obtained using calibrated geothermal barometers and ThermoCalc Program. The peak metamorphic condition of these high-pressure granulites is about 760―820℃, 1.0―1.2 GPa and the retrograde metamorphic condition is about 620―720℃, 0.7―0.8 GPa. The petrological studies show that they have a near-isobaric cooling P-T path which suggests that the Western Kunlun underwent initial crustal thickening, subsequent exhumation and cooling. The SHRIMP zircon U-Pb dating gives two groups of ages for high-pressure granulites. One is 177±6 Ma which is obtained from the rim of the zircon. We consider this age should be the metamorphic age. And the other is 456±30 Ma which is obtained from the core of the zircon and should be the protolith age. The formation of these high-pressure granulites in western Kunlun is closely correlated with the evolution of the Paleo-Tethys and has important implications for the research on Tethys and Paleo-Asian tectonic zone.

A New SHRIMP Age of the Xiamaling Formation in the North China Plate and Its Geological Significance [查看] GAOLinzhi、ZHANGChuanheng、SHIXiaoying、ZHOUHongrui、WANGZiqiang、SONGBiao

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±12Ma.It is the first SHRIMP U-Pb age from the Ciamaling 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.

An Important Spreading Event of the Neo-Tethys Ocean during the Late Jurassic and Early CretaceousEvidence from Zircon U-Pb SHRIMP Dating on Diabase in Nagarze,Southern Tibet [查看] JIANGSihong、NIEFengjun、HUPeng、LIUYan

Widely distributed in Gyangze-Chigu area,southern Tibet,NW-and nearly E-W-trending diabase(gabbro)-gabbro diorite dykes are regarded as the product of the large-scale spreading of the late Neo-Tethys Ocean.In order to constrain the emplacement age of these dykes,zircons of two samples from diabases in Nagarze were dated by using the U-Pb SHRIMP method.Two nearly the same weighted mean 206Pb/238Uages were obtained in this paper,which are 134.9±1.8Ma(MSWD=0.65)and 135.5±2.1(MSWD=1.4),respectively.They not only represent the crystallization age of the diabase,but also documented an important spreading event of the Neo-Tethys Ocean during the late Jurassic and early Cretaceous.This dating result is of great significance to reconstruct the temporal framework of the late Neo-Tethys Ocean in the Qinghai-Tibet Plateau.

U-Pb Zircon Dating of the Granitic Conglomerates of the Hutuo GroupAffinities to the Wutai Granitoids and Significance to the Tectonic Evolution of the Trans-North China Orogen [查看] JimZHANG、GuochunZHAO、LISanzhong、MinSUN、LIUShuwen、XiaopingXIA、YanhongHE

The Wutai Complex associated with the adjacent Fuping and Hengshan Complexes represents the best and classical cross-section in the middle segment of the Trans-North China Orogen, generally divided into Eastern and Western Blocks. Unconformably overlying the Wutai and Fuping Complexes is the Hutuo Group considered as the youngest lithostratigraphic unit in the region and important both for interpreting Precambrian history as well as the overall evolution of the Trans-North China Orogen. Lack of knowledge about provenance of the sedimentary rocks in this group has hindered understanding of the depositional environments and tectonic significance. LA-ICP-MS was applied to obtain U-Pb zircon ages for the granitic pebbles, the lowest lithostratigraphic rock of the Hutuo Group, which, combined with previous lithostratigraphic, geochronological, structural and metamorphic data, provides new constraints on the sedimentary provenance and tectonic evolution of the region. The sequence of the Hutuo Group ranges upward from lower basal conglomerates and volcaniclastic rocks (Doucun Subgroup), through clastic sediments, slates, dolomites and marbles (Dongye Subgroup), to sandstones and conglomerates at the top (Guojiazhai Subgroup). Zircons from granitic pebbles preserved in the Doucun Subgroup basal conglomerates give weighted mean 207Pb/206Pb ages between 2517 Ma and 2566 Ma, which are the same as those for the late Archean Wutai Cranitoids, indicating that the pebbles were derived from the Wutai granitic intrusions (2566-2515Ma). Based on the new data and previous studies, combined with an igneous zircon crystallization age of 2087±9 Ma obtained for volcanics in the Hutuo Group, the Hutuo Group was deposited in a subduction-related retro-arc foreland basin environment that developed behind the Wutai arc during the eastward-directed subduction of the Western Block beneath the western margin of the Eastern Block. This basin underwent long-lived sedimentation and finally closed during the -1850 Ma collisional event that resulted in the final amalgamation of the North China Craton.