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    Precambrian crystalline basement in southern Mongolia asrevealed by SHRIMP zircon dating [查看] AntoineDemoux、AlfredKroner、DunyiLiu、GombosurenBadarch
    Single zircon ages determined by ion microprobe (SHRIMP II) for granitoid gneisses from the southern slope of the Baga Bogd massif (Gobi-Altai,southern Mongolia) reveal several episodes of zircon growth, ranging from late Palaeoproterozoic to late Cambrian.The oldest events are documented by a zircon crystallization age for a gneiss protolith at 1519 ± 11 Ma and by a xenocrystic zircon from a dark grey augen-gneiss yielding an age of c. 1701 Ma. Discrete igneous events are recorded in granite-gneisses with protolith emplacement ages of 983 ± 6, 956 ± 3 and 954 ± 8 Ma. These ages provide the first record of early Neoproterozoic magmatic activity in this region. A much younger and discrete magmatic event is recorded by several dioritic to granitic orthogneisses which are tectonically interlayered with the older gneisses and have protolith emplacement ages between 502 and 498 Ma. These late Cambrian granitoids of calc-alkaline affinity are likely to have been emplaced along an active continental margin and suggest that the Baga Bogd Precambrian crustal fragment was either docked against the southward (present-day coordinates) growing margin of the CAOB or was a large enough crustal entity to develop an arc along its margin. We speculate that the Precambrian gneisses of this massif may be part of a crustal fragment rifted off the Tarim Craton.
    Provenance of Meso- to Neoproterozoic cover sediments at the Ming Tombs, Beijing,North China Craton An integrated study of U–Pb dating and Hf isotopic measurement of detrital zircons and whole-rock geochemistry [查看] YushengWan、DunyiLiu、WeiWang、TianruiSong、AlfredKröner、ChunyanDong、HongyinZhou、XiaoyanYin
    The North China Craton (NCC)was subjected to an extensional regime after the Lüliangmovement at ~1.8 Ga and then was covered by an extensive Meso- to Neoproterozoic sedimentary succession, namely the Changcheng,Jixian and Qingbaikou Groups in ascending order. We report age spectra for detrital zircons and monazites, Hf isotopic systematics of detrital zircons, and whole-rock chemical and Nd isotopic compositions for sediments from the succession in the Ming Tombs area, Beijing, one of the typicalMeso- to Neoproterozoic areas in the NCC.Detrital zircons of six sedimentary samples have two distinct age peaks at ~2.52 Ga and ~1.85 Ga. There are some detrital zircons at 2.4–2.0 Ga but none at 2.3 Ga and only a few N2.7 Ga. The detrital zircon age spectra change with time. Sediments in the lower succession (Changcheng Group) and in the upper successions (Jixian and Qinbaikou Groups) are dominated by significant detrital zircon populations of late Neoarchean and late Paleoproterozoic ages, respectively. The ~2.5 Ga detrital zircons of the ChangchengGroup have εHf(2.5 Ga) values and tDM(Hf) model ages mainly ranging from −2 to +7 and 2.8 to 2.7 Ga, respectively. Detrital monazites of a sample fromthe JixianGroup exhibit amajor age peak between 1.95 and 1.80 Ga with somedata between 2.0 and 1.95 Ga. The sedimentary rocks of the Changcheng Group are characterized by high K2O contents (mostly 7.09–15.20%) and insignificant Eu anomalies (Eu/Eu*=0.71–1.16). They have tDM(Nd) model ages ranging from 2.70 to 2.43 Ga, being older than the tDM(Nd) ages (2.11 and 1.99 Ga) of sedimentary samples from the Qingbaikou Group. Based on a comparison with ages for the early Precambrian (N1.8 Ga) basement of the NCC, it can be concluded that (1) the sediments of theMeso- to Neoproterozoic coverwere undoubtedly derived fromthe NCC itself or once neighboring terranes; (2) variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late Paleoproterozoic rockswere exposed at the surface after the upper crustal late Neoarchean rocks had already been eroded.
    Devonian A-type granitic magmatism on the northern margin of the North China craton,SHRIMP U-Pb dating and Hf isotopes of the Hongshan granite at chifeng,inner mongolia,China [查看] YuruoShi、DunyiLiu、LaichengMiao、FuqinZhang、PingJian、WeiZhang、KejunHou、JunyuXu
    We report first data for the Devonian Hongshan A-type granite along the northern margin of the Precambrian North China Craton. The granite is rich in silica (SiO2=76.07–77.62 wt.%) and alkalis (Na2O+K2O=8.45–8.78 wt.%), but poor in calcium (CaO=0.28–0.70 wt.%) and magnesium (MgO=0.08–0.20 wt.%). Chondritenormalized REE patterns for the samples fromtheHongshan granite show significant negative Eu anomalies (Eu/Eu*=0.13–0.19). They contain high Rb, Ga, Y, Nb, and Zn, but low Ba and Sr contents, and in the multi-element spider diagrams normalized to primitive mantle, they show a similar pattern to that of A-type granites, such as negative Ba, Nb, Ta, Sr, P and Ti anomalies. Geochemical plots also show an affinity with A-type granites and suggest a post-orogenic setting (A2 type granite). SHRIMP U–Pb zircon dating yielded a concordant weighted mean 206Pb/238Uage of 387±4 Ma,which defines a Devonianmagmatic event. The zircons have negative εHf(t)of −8.4 to−11.5 and give old Hf model ages of TDM2=2548–2820 Ma (mean 2667Ma). Most 176Hf/177Hf and εHf values display Hf-isotope evolutional line of the lower continental crust. These results suggest that the Hongshan granite originated from melting of Archaean lower crust beneath the North China Craton in a post-orogenic extensional setting during the Devonian.
    SHRIMP zircon U-Pb age constraints on Neoproterozoic Quruqtagh diamictites in NW China [查看] BeiXua、ShuhaiXiao、HaiboZou、YanChen、Zheng-XiangLi、BiaoSong、DunyiLiu、ChuanmingZhoug、XunlaiYuang
    The Neoproterozoic Quruqtagh Group in the Tarim Block, NW China, contains multiple diamictites in the Bayisi, Altungol, Tereeken, and Hankalchough formations. These diamictites may represent three or possibly four discrete glaciations, although evidence for a glacial origin of the Bayisi and Altungol diamictite is ambiguous. To constrain their age and duration, we dated three volcanic beds (V1, V2, and V3) in the Quruqtagh Group using the SHRIMP (sensitive high-resolution ion microprobe) zircon U–Pb method.Volcanic bed V1 near the base of the Bayisi diamictite yields a 740±7Ma age, volcanic bed V2 near the top of the Bayisi Formation gives a 725±10Ma age, and volcanic bed V3 between the Tereeken and Hankalchough diamictites yields a 615±6Ma age. V1 and V2 have overlapping ages, and together these dates suggest that the Bayisi diamictite was deposited at around 730 Ma. The Tereeken and Altungol diamictites were deposited between 725±10Ma and 615±6 Ma, and the Hankalchough diamictite between 615±6Ma and ∼542Ma (i.e., the Neoproterozoic–Cambrian transition). These dates and previously published chemostratigraphic data are consistent with (but doe not require) the correlation of the Tereeken and Hankalchough diamictites with the 635Ma Nantuo and 582Ma Gaskiers glaciations, respectively.However, the new dates are inconsistent with a single and globally synchronous Sturtian glaciation that occurred in the pre-Nantuo Neoproterozoic Era. Instead, currently available data necessitate that either multiple glaciations occurred, or a globally diachronous glacial event developed during a protracted period between ∼750Ma and ∼650 Ma.
    Combined U-Pb, hafnium and oxygen isotope analysis of zircons from meta-igneous rocks in the southern North China Craton reveal multiple events in the Late Mesoarchean-Early Neoarchean [查看] DunyiLiu、SimonA.Wilde、YushengWan、ShiyanWang、JohnW.Valley、NorikoKita、ChunyanDong、HangqiangXie、ChangxiuYang、YixinZhang、LinzhiGao
    We report the results of a comprehensive isotopic investigation of zircons from metamorphosed tonalites and amphibolites from Lushan in the far south of the Trans-North China Orogen of the North China Craton that reveal the oldest rocks yet dated from the orogen. A detailed investigation of the internal structures of zircons, combined with targeted SIMS U–Pb and oxygen isotope analyses and ICP-MS Hf zircon analyses, has resulted in the discovery of previously unknown geological events in the North China Craton. Magmatic zircon from the tonalite and amphibolite was found to be essentially coeval with a small spread of ages from 2829±18Ma to 2832±11 Ma in the tonalites and from2838±35 Ma to 2845±23Ma in amphibolite. Hafnium isotope data suggest derivation of both tonalite and amphibolite from a depleted mantle source with minor crustal contamination. Oxygen isotope ratios in zircons range from 5.1 to 7.3‰ VSMOW, with high δ18O results recorded only from metamorphic domains. The earliest stage of metamorphic zircon growth, present in all four samples analyzed in this study, occurred between 2772±17/22 in the tonalites and 2776±20 and 2792±12 Ma in the amphibolites. A younger metamorphic event at 2638±61 in the tonalite and between 2671±13 and 2651±13 Ma in the amphibolites is recognized in zircons from three of the four samples investigated. This is the first time that metamorphic events at either of these times have been recorded in the North China Craton. Importantly, there is no evidence for the 1.8 Ga high-grade metamorphic event that is manifest in the northern and central parts of the Trans-North China Orogen.
    Zircon SHRIMP U-Pb ages of the Gangdese Batholith and implications for Neotethyan subduction in southern Tibet [查看] Da-RenWen、DunyiLiu、Sun-LinChung、Mei-FeiChu、JianqingJi、QiZhang、BiaoSong、Tung-YiLee、Meng-WangYeh、Ching-HuaLo
    The Trans-Himalayan magmatism, which occurred extensively in the Lhasa terrane of southern Tibet, has long been related to the Neotethyan subduction before the India–Asia collision. To better delineate the magmatic duration, we report a geochronological study with 25 SHRIMP zircon U–Pb ages from the Gangdese Batholith that represents the largest Trans-Himalayan plutonic complex. The results suggest two distinct stages of plutonism in the Late Cretaceous (ca. 103–80 Ma) and early Paleogene (ca. 65–46 Ma),respectively. Our new data confirm if not refine the notion that a Gangdese magmatic gap or quiescent period existed between ca. 80 and 70 Ma. It is furthermore identified that the early stage ended with adakitic intrusion and the latter stage is marked by a peak activity at ca. 50 Ma.We attribute the cessation of the early stage, and following magmatic gap, to a flattening of the northward Neotethyan subduction, and the initiation of the latter stage to rollback of the subducted slab. The proposed scenarios can also account for the southward migration and intensification of Cretaceous to Paleogene volcanism in the Lhasa terrane that demonstrates a coeval, eruptive “flare-up” event around 50 Ma, interpreted as the result of detaching the Neotethyan oceanic slab from the adherent, more buoyant Indian continental lithosphere owing to the India–Asia collision. Our model is, moreover, in general accord with sedimentary and structural geologic records from southern Tibet where subduction-related orogenesis appears to have evolved through time before India started colliding Asia.
    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.
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