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    SHRIMP U-Pb zircon dating of the Wulian complex Defining the boundary between the North and South China Cratons in the Sulu Orogenic Belt,China [查看] Jian-BoZhou、SimonA.Wilde、Guo-ChunZhao、Xing-ZhouZhang、Chang-QingZheng、WeiJin、HongCheng
    Neoproterozoic metasedimentary and metavolcanic rocks of the Wulian Group are associated with gneissic granitoids at the northern margin of the Sulu UHP Belt,adjacent to the Wulian-Yantai Fault,and are collectively referred to as the Wulian complex,SHRIMP U-Pb dating of detrital zircon from the metasedimentary rocks of the Wulian Group indicates a range in age from 3478 to 625Ma.Based on zircons＜10% discordant,they can be broadly divided into four populations:＞3000Ma，with the oldest age being 3339Ma;2700~2400Ma,with a peak at 2436Ma;2200-1800Ma,with a peak at ~2156Ma;and at ~770Ma.These results indicate that deposition of the Wulian Group was after ~770Ma and that some detrital material most likely came from Neoproterozoic sources in the South China Craton(SCC).SHRIMP U-Pb zircon dating of a metavolcanic rock defines a weighted mean 206Pb/238U age of 761±8Ma，which is consistent with other rocks of South China affinity and can be correlated with syn-rift magmatism during the breakup of the supercontinent Rodinia.The Wulian Group,together with Neoproterozoic granites of the Wulian complex,constrains the location of the Triassic suture zone betwwen the North China and South China Cratons,Based on our new data,the suturemust lie north of the Wulian complex and is marked by the Baichihe-Yantai Fault.
    SHRIMP U–Pb zircon dating of the Neoproterozoic Penglai Group and Archean gneisses from the Jiaobei Terrane,North China, and their tectonic implications [查看] Jian-BoZhou、SimonA.Wilde、Guo-ChunZhao、Chang-QingZheng、WeiJin、Xing-ZhouZhang、HongCheng
    Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541±5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga.The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866±4Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at ∼1865 Ma.
    SHRIMP U–Pb zircon ages of granitoid rocks in the Luliang Complex Implications for the accretion and evolution of the Trans-North China Orogen [查看] GuochunZhaoa、SimonA.Wilde、MinSun、SanzhongLi、XupingLi、JianZhang
    The lvliang Complex is situated in the central segment of the Trans-North China Orogen (TNCO), a continent-continent collisional belt along which the discrete Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The complex consists of supracrustal and granitoid rocks, of which the latter can be subdivided into pre-tectonic TTG gneisses, including the Yunzhongshan, Guojiazhuang and Chijianling–Guandishan gneisses; syn-tectonic gneissic granites represented by the Huijiazhuang gneissic granite; and post-tectonic granites,including the Luyashan charnockite, Luchaogou porphyritic granite and Tangershang/Guandishan massive granite. The pre-tectonic TTG gneisses are mostly calc-alkaline and considered to have formed in a magmatic arc environment. SHRIMP U–Pb analyses reveal that the Yunzhongshan gneisses were emplaced at ∼2499±9 Ma, representing the earliest arc-related magmatic event in the Luliang Complex. This was followed by the intrusion of the Guojiazhuang gneisses at 2375±10 Ma. The most widespread arc-related magmatic event in the region was the emplacement of the Chijianling–Guandishan granitoid gneisses, of which the tonalitic, granodioritic and monzogranitic gneisses were emplaced at 2199±11 Ma,2180±7Ma and 2173±7 Ma, respectively. Metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207Pb/206Pb age of 1872±7 Ma, consistent with the metamorphic age range of 1880–1820 Ma defined by metamorphic zircons from various high-grade rocks in the TNCO. Thus, the evolution of the lvliang Complex involved emplacement of the Yunzhongshan granitoids at ∼2499 Ma, the Guajiazhuang granitoids at ∼2375 Ma and the Chijianling–Guandishan granitoids at 2199–2173 Ma, with the final collision between the Eastern and Western Blocks in this area occurring at 1872±7 Ma, the whole series of magmatic events lasting nearly 650 Ma. This suggests that the Trans-North China Orogen represents a long-lived magmatic arc. The new SHRIMP zircon data, combined with structural and petrological studies, also reveal the existence of both syn- and post-collisional granites in the Luliang Complex. The former is represented by the Huijiazhuang gneissic granite that was emplaced at 1832±11 Ma, whereas the latter include the 1815±5Ma Luyashan charnockite, 1807±10 Ma Luchaogou porphyritic granite,1798±11 Ma Guandishan massive granite and 1790±14 Ma Tangershan massive granite.
    SHRIMP U-Pb zircon age of the Jinbaoshan ultramafic intrusion, Yunnan Province, SW China [查看] TAOYan、MAYanSheng、MIAOLaiCheng、ZHUFeiLin
    SHRIMP U-Pb zircon dating gives ages of 260.6 ± 3.5 Ma and 260.7 ± 5.6 Ma for serpentinised wehrlite and plag-hornblendite in the Jinbaoshan ultramafic intrusion, respectively. The results indicate that the Jinbaoshan intrusion was emplaced at ca.260 Ma and contemporaneous with the Emeishan continental flood basalts (ECFB), similar to other mafic-ultramafic intrusions of the Emeishan large igneous province(LIP). The new ages provide a geochronological constraint on the origin of the Jinbaoshan ultramafic intrusion. It confirms that the Jinbaoshan ultramafic intrusion belongs to the Emeishan LIP that formed at ca. 260 Ma.
    SHRIMP U–Pb dating, trace elements and the Lu–Hf isotope system of coesite-bearing zircon from amphibolite in the SW Sulu UHP terrane, eastern China [查看] FulaiLiu、AxelGerdes、LingsenZeng、HuaiminXue
    In this study, we link mineral inclusion data, trace element analyses, U–Pb age and Hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi-stage metamorphic evolution of amphibolites from the Sulu ultrahigh-pressure (UHP) terrane, eastern China. Zircon grains separated from amphibolites from the CCSD-MH drill hole (G12) and Niushan outcrop (G13) were subdivided into two main types based on cathodoluminescence (CL) and Laser Raman spectroscopy: big dusty zircons with inherited cores and UHP metamorphic rims and small clear zircons.Weakly zoned, grey-white luminescent inherited cores preserve mineral inclusions of Cpx + Pl + Ap ± Qtz indicative of a mafic igneous protolith. Dark grey luminescent overgrowth rims contain the coesite eclogite-facies mineral inclusion assemblage Coe + Grt + Omp + Phe + Ap, and formed at T = 732–839 C and P = 3.0–4.0 GPa. In contrast, white luminescent small clear zircons preserve mineral inclusions formed during retrograde HP quartz eclogite to LP amphibolite-facies metamorphism (T = 612–698 C and P = 0.70–1.05 GPa). Inherited zircons from both samples yield SHRIMP 206Pb/238U ages of 695–520 Ma with an upper intercept age of 800 ± 31 Ma. The UHP rims yield consistent Triassic ages around 236–225 and 239–225 Ma for G12 and G13 with weighted means of 229 ± 3 and 231 ± 3 Ma, respectively. Small clear zircons from both samples give 206Pb/238U ages around 219–210 Ma with a weighted mean of 214 ± 3 Ma, interpreted as the age of retrograde quartz eclogite-facies metamorphism. Matrix amphibole from both samples indicate Ar–Ar ages of 209 ± 0.7 and 207 ± 0.7 Ma, respectively, probably dating late amphibolite-facies retrogression. The data suggest subduction of Neoproterozoic mafic igneous rocks to UHP conditions in Middle Triassic (~230 Ma) times and subsequent exhumation to an early HP(~214 Ma) and a late LP stage (~208 Ma) over a period of ~16 and 6 Myr, respectively. Thus, early exhumation from a mantle depth of 120–100 km to about 60 km occurred at an average rate of 0.3 cm/y, while subsequent exhumation to a middle crustal level took place at approximately 0.54 cm/y. These exhumation rates are considerably slower than those obtained for UHP rocks in the Dora Maira and Kokchetav massifs (2–3 cm/y). Based on similar P–T estimates and trace element and Hf isotope compositions, Sulu amphibolites can be identified as retrograde UHP eclogites. The eHf(800) of +8 implies a significant input from the depleted mantle to the Sulu–Dabie terrane during the middle Neoproterozoic. Overgrown rims are characterized by a distinct trace element composition with low Lu/Hf and Th/U and significantly higher 176Hf/177Hf ratios than inherited cores, consistent with formation during/after garnet (re-)crystallization and fractionation of the Lu–Hf system during UHP metamorphism. The combined dataset suggests homogenization of the 176Hf/177Hf ratio within the metamorphic mineral assemblage and during protolith formation. Observed variations are explained by mixing of material from both domains during laser ablation, e.g., due to partial recrystallization of inherited cores.
    SHRIMP U-Pb and CAMECA 1280 oxygen isotope results from ancient detrital zircons in the Caozhuang quartzite, Eastern Hebei, North China Craton Evidence for crustal reworking 3.8 Ga ago [查看] SIMONA.WILDE、JOHNW.VALLEY、NORIKOT.KITA、AARONJ.CAVOSIE、DUNYILIU
    ABSTRACT. The Caozhuang quartzite, located in Eastern Hebei Province in the eastern part of the North China Craton, has long been known to contain the oldest detrital zircon population in China. A suite of 30 zircons with preliminary ages >3.7 Ga were selected for detailed SHRIMP U-Pb and CAMECA 1280 oxygen isotope analyses.SHRIMP U-Pb dating reveals that ages range from 3860 to 3135 Ma, the former being the oldest 207Pb/206Pb zircon age so far obtained from the North China Craton. Values of 18O range from 5.4 to 7.5 permil, similar to Archean igneous zircons worldwide, but the average value for the 30 zircons is 6.6 permil, which is 0.3 permil higher than suite averages for detrital zircons from the Jack Hills and Beartooth Mountains and nearly 1 permil higher than magmatic zircons from igneous rocks of the Superior Province and Barberton. These elevated values of 18O suggest reworking of a significant amount of juvenile crust, older than 3.8 Ga, that underwent a low temperature supracrustal history and was distributed throughout the source region in northeast China.
    SHRIMP dating of the Permo-Carboniferous Jinshajiang ophiolite, southwestern China Geochronological constraints for the evolution of Paleo-Tethys [查看] PingJian、DunyiLiu、XiaomengSun
    SHRIMP U–Pb zircon dating of gabbro, anorthosite, trondhjemite and granodiorite from the Jinshajiang ophiolitic me´ lange of southwestern China provides geochronological constraints on the evolution of Paleo-Tethys. The ophiolitic me ´ lange is exposed for about 130 km along the Jinshajiang River where numerous blocks of serpentinite, ultramafic cumulate, gabbro, sheeted dikes, pillow lavas and radiolarian chert are set in a greenschist matrix. A cumulate gabbro-anorthosite association and an amphibole gabbro have ages of 338 ± 6 Ma, 329 ± 7 Ma and 320 ± 10 Ma, respectively, which constrain the time of formation of oceanic crust. An ophiolitic isotropic gabbro dated at 282–285 Ma has the same age as a trondhjemite vein (285 ± 6 Ma) cutting the gabbro. These ages probably reflect a late phase of sea-floor spreading above an intra-oceanic subduction zone. At the southern end of thenJinshajiang belt, a granitoid batholith(268 ± 6 Ma), a gabbro massif (264 ± 4 Ma), and a granodiorite (adakite) intrusion (263 ± 6 Ma) in the ophiolitic me´ lange constitute a Permian intra-oceanic plutonic arc complex. A trondhjemite dike intruded serpentinite in the me´ lange at 238 ± 10 Ma and postdates the arc evolution of the Jinshajiang segment of Paleo-Tethys.
    PALEOPROTEROZOIC CRUSTALLY DERIVED CARBONATE-RICH MAGMATIC ROCKS FROM THE DAQINSHAN AREA, NORTH CHINA CRATON GEOLOGICAL, PETROGRAPHICAL, GEOCHRONOLOGICAL AND GEOCHEMICAL (Hf, Nd, O AND C) EVIDENCE [查看] YUSHENGWAN、DUNYILIU、ZHONGYUANXU、CHUNYANDONG、ZHEJIUWANG、HONGYINGZHOU、ZHENSHENGYANG、ZHENGHONGLIU、JIASHANWU
    ABSTRACT. Most carbonate-rich magmatic rocks are mantle-derived, namely car-bonatite(s), with a minority of them being contaminated by crustal rocks. It is debated whether there are also carbonate-rich magmatic rocks derived solely from crustal sources. In this contribution, we report crustally derived carbonate-rich magmaticrocks, named herecrustal carbonatite(s) , in the Daqingshan area, Western Block of the North China Craton. The Daqingshan crustal carbonatites were previously considered to be metasedimentary marbles. However, they cut adjacent rocks and contain some enclaves that are irregular in shape and show unoriented distribution of lithologies that cannot be found in the local wall rocks. Zircons from the crustal carbonatites show clear oscillatory zoning, and contain calcite, quartz, feldspar, diopside and CO2 inclusions. The zircons have correlated U and Th abundances and give a U/Pb age of 1951±5 Ma, being the same age as metamorphic zircon cores and rims from a diopsidite enclave (1954±27 Ma of core and 1944±40 Ma of rim). The zircon from the crustal carbonatite has tDM(Hf) and dHf(t) of 2353 to 2457 Ma and –3.2 to 0.7, whereas the tDM(Hf) and dHf(t) of the core and rim zircons from diopsidite range from 2228 to 2160 Ma and 3.0 to 4.8 and 2219 to 2057 Ma and 3.2 to 7.4, respectively. The δ18O(‰)Zircon-V-PDB of zircon from the crustal carbonatite ranges from –21.5 to –19.6, with O isotope equilibrium temperature being 555°C to 635 °C. The crustal carbonatite shows a large variation in chemical composition, with SiO2 δ10.2–37.3% and Total REE δ48 –267 ppm. The tDM(Nd) age and dNd(t) are 2.5 Ga and –2.9 to –2.4. Theδ18O(‰)Rock-V-PDB andδ13C(‰)Rock-V -PDB vary from –19.5 to –15.2 and from –5.2 to –2.4, being distributed between primary carbonatite field and the field of dolomitic marble from the study area inδ18O(‰) vs.δ13C(‰) diagram. Combined with previous studies, we drew the conclusion that some of the carbonate-rich rocks in the study area are magmatic in origin, by anatexis of impure marble plus common contamination by calc-silicates and other materials. This is consistent with the high-P-T experiments of CaO-CO2-H2O system (Wyllie and Tuttle, 1960) and MgO-CaO-CO2-H2O system (Fanelli and others, 1986), which indicate that partial melting of limestone will happen when temperature is> 700°C and when water is also present.
    NEW U-Pb AND Hf ISOTOPIC DATA CONFIRM ANSHAN AS THE OLDEST PRESERVED SEGMENT OF THE NORTH CHINA CRATON [查看] DUNYILIU、SIMONA.WILDE、YUSHENGWAN、JIASHANWU、HONGYINZHOU、CHUNYANDONG、XIAOYANYIN
    ABSTRACT. The Anshan area in the northeastern part of the North China Craton contains the oldest known rocks in China. Rocks with ages of 3.8 Ga are present in three trondhjemite-dominated gneiss complexes, referred to as the Baijiafen, Dongs-han and Shengoushi complexes. The oldest material identified in these complexes has weighted mean 207Pb/206Pb ages of 3800 5 Ma, 3794 4 Ma and 3777 13 Ma,respectively. In addition, younger geological events at 3680 19 Ma, 3620 23 Ma, and 3573 21 Ma have been identified, recording the emplacement of several generations of trondhjemite. A biotite schist of possible supracrustal origin contains a zircon population with a weighted mean 207Pb/206Pb age of 3723 17 Ma and is cut by a 3620 Ma trondhjemite, implying formation prior to3.62 Ga. TDM(Hf) model ages for the zircons range from 3256 to 4264 Ma, but are mostly 3.9 Ga, indicating that significantly older crust was not present in the source region of the trondhjemitic magmas.
    Further SHRIMP Geochronology on the Early Cambrian of South China [查看] W.COMPSTON、ZICHAOZHANG、J.A.COOPER、GUOGANMA、R.J.F.JENKINS
    ABSTRACT. Zircon 206Pb/238U ages are reported for bentonites from two Cambrian sections in South China, the Meishucun and the Fandian. The analyzed zircons vary widely in age because of the presence of detrital and altered grains. Spot ages were corrected for calibration drift during analysis, and the age of reference zircon SL13 was calibrated as 580 Ma against other standards using SIMS. Use of mixture modeling was essential to resolve the different age components and to identify the volcanic ages. Two age groups occur within the Meishucun Bed 5 bentonite, of which the larger at 518.7±2.5 Ma (σ) is interpreted as a post depositional zircon growth event and the other at 539.4±2.9 Ma as the volcanic age. Bentonites from the Meishucun Bed 9 and its Fandian equivalent have two age groups, 526.5±1.1 Ma and 540.2±1.0 Ma. We interpret the younger of these as the tuff magmatic age and the older as detrital. Small shelly fossils indicate Meischucunian Bed 5 is an equivalent of the late Nemakit Daldynian, and its 539.4±2.9 Ma age both closely times the base of the Cambrian and is a maximum for the basal Tommotian. The 526.5±1.1 Ma for Bed 9 and its equivalents approximates the close of the Tommotian.
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