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    Mafic dykes derived from Early Cretaceous depleted mantle beneath the Dabie orogenic belt_ implications for changing lithosphere mantle beneath eastern China [查看] LINGCHEN、CHANGQ.MA、JINY.ZHANG、ROGERMASON、CHAOZHANG
    SHRIMP zircon U–Pb geochronological, elemental and Sr–Nd isotopic data from Early Cretaceous mafic dykes in North Dabie orogenic belt elucidate a change of Mesozoic lithospheric mantle in eastern China. The dykes are predominantly dolerite with the major mineral assemblage clinopyroxeneþhornblendeþplagioclase and yield a SHRIMP zircon U–Pb age of 111.65.3 Ma. They have a narrow range of SiO2 from 46.16% to 49.78%, and relative low concentrations of K2O (1.072.62%), Na2O (2.453.54%), Al2O3 (13.0414.07%), and P2O5 (0.420.55%) but relatively high concentration of MgO (5.94–6.61%) with Mg# 52–54. All the samples are characterized by enrichment of large ion lithophile elements (LILE, e.g., Ba, Th) and high field strength elements (HFSE, e.g., Nb, Ti). (87Sr/86Sr)i ratios from 0.704 to 0.705, eNd values from 3.36 to 4.33 and mantle-depletion Nd model ages (T2DM) in the range 0.56–0.64 Ga indicate that the magma of the Baiyashan mafic dykes was derived from a young depleted mantle source. This finding is different from previous research on mafic dykes in the age range 120–138 Ma that revealed enrichment of LILE and depletion of HFSE, high initial Sr isotopic ratios and negative eNd, value which represents an old enriched mantle source. Ours is the first report of the existence of Early Cretaceous depleted mantle in eastern China and it implies that changing of enriched mantle to depleted mantle occurred at ca. 112 Ma, associated with back-arc extension which resulted from the subduction of the Palaeo-Pacific Plate towards the Asian Continent.
    Implications of subduction and subduction zone migration of the Paleo-Pacific Plate beneath eastern North China, based on distribution, geochronology, and geochemistry of Late Mesozoic volcanic rocks [查看] ChaoZhang、Chang-QianMa、Qun-AnLiao、Jin-YangZhang、Zhen-BingShe
    Several major volcanic zones are distributed across the eastern North China Craton, from northwest to southeast: the Greater Xing’an Range, Jibei-Liaoxi, Xishan,and Songliao Basins, and the Yanji, Huanghua, and Ludong volcanic zones. The Huanghua depression within the Bohai Bay Basin was filled by middle Late Mesozoic volcanic rocks and abundant Cenozoic alkaline basalts. Zircon LA-ICP-MS and SHRIMP U–Pb dating show that basic–intermediate volcanic rocks were extruded in the Early Cretaceous of 118.8 ± 1.0 Ma (weighted mean 206Pb/238U age), before Late Cretaceous acid lavas at 71.5 ± 2.6 Ma.An inherited zircon from andesite has a Paleoprotoerozoic core crystallization age of 2,424 ± 22 Ma (206Pb/207Pb age) indicating that the basement of the Bohai Bay Basin is part of the North China Craton. Early Cretaceous basic and intermediate lavas are characterized by strong enrichments in LREE and LILE and depletions in HREE and HFSE,indicating a volcanic arc origin related to oceanic subduction.Depletion in Zr only occurs in basic and intermediate volcanic rocks, while depletions in Sr and Ti exist only in acid samples, indicating that the acid series is not genetically related to the basic–intermediate series. Formation ages and geochemical features indicate that the Late Cretaceous acid lavas are products of crustal remelting in an extensional regime. Combined information from all these volcanic zones shows that subduction-related volcanic rocks were generated in the Jibei-Liaoxi and Xishan volcanic zones during the Early Jurassic, about 60 Ma earlier than their analogues extruded in the Huanghua and Ludong volcanic zones during the Early Cretaceous. This younging trend also exists in the youngest extension-related volcanism in each of these zones: Early Cretaceous asthenosphere-derived alkaline basalts in the northwest and Late Cretaceous in the southeast. A tectonic model of northwestward subduction and continuous oceanward retreat of the Paleo-Pacific Plate is proposed to explain the migration pattern of both arc-related and post-subduction extensionrelated volcanic rocks. As the subduction zone continuously migrated, active continental margin and backarc regimes successively played their roles in different parts of North China during the Late Mesozoic (J1–K2).
    Geology and geochronology of the Xilamulun molybdenum metallogenic belt in eastern Inner Mongolia, China [查看] QingdongZeng、JianmingLiu、ZuolunZhang、WeijunChen、WeiqingZhang
    The Xilamulun molybdenum metallogenic belt,located in eastern Inner Mongolia, China, has great economic potential as a major producer of molybdenum. Four major types of Mo deposits have been recognized in the Xilamulun molybdenum metallogenic belt: porphyry,quartz vein, volcanic-hosted, and greisen. These Mesozoic Mo deposits are closely related to Si- and K-rich intrusives and are usually hosted by granite plutons or located at the endo- or exo-contact zones of the granite porphyry.SHRIMP zircon U–Pb dating gives the emplacement ages of the intrusions related to Mo mineralization as 245.1 ± 4.4, 152.4 ± 1.6, and 139.1 ± 2.3 Ma. Re–Os analysis of five molybdenite samples from the Chehugou porphyry Mo deposit yields an isochron age of 245 ± 5 Ma (2r), indicating that the mineralization age of the porphyry Mo deposit is about 245 Ma. Re–Os analyses of six molybdenite samples from the Nianzigou quartzvein-type Mo deposit yield an isochron age of 154.3 ± 3.6 Ma (2r), constraining the mineralization age of the quartz-vein Mo deposit to 154 Ma. Our results suggest that the Mo mineralization in the Xilamulun belt formed during at least three stages, i.e., the Triassic, Late Jurassic, and Early Cretaceous, and is coeval with the granitic magmatism. The corresponding geodynamic background covers the syncollision between the North China and Siberian plates during the Early to Middle Triassic,a compression setting related to the subduction of the Paleo-Pacific plate during the Jurassic and lithospheric thinning during the Early Cretaceous in eastern China.
    Geochronological and Geochemical study of Palaeoproterozoic gneissic granites and clinopyroxenite xenoliths from NW Fujian, SE China Implications for the crustal evolution of the Cathaysia Block [查看] Long-MingLi、MinSun、YuejunWang、GuangfuXing、GuochunZhao、KedaCai、YuzhiZhang
    U–Pb zircon dating of gneissic granite samples in northwestern Fujian Province, Southeast China, gave Neoarchean to Palaeoproterozoic ages (2.6–1.9 Ga) for the rounded zircon cores and Palaeoproterozoic upper intercept ages (1851 ± 21 Ma, 1857 ± 29 Ma, respectively) for zoned rims that have oscillatory growth zoning and yield eHf(t) values between 8.8 and +3.7 and TDM model ages between 2.1 and 2.6 Ga. These gneissic granites are peraluminous (A/CNK = 0.94–1.28), characterized by high SiO2 (68–72 wt.%), Al2O3 (14–15 wt.%) and low CaO, MgO, FeOt, TiO2 and P2O5 contents. They also possess relatively low REE contents and moderate LREE/HREE fractionation and display pronounced negative anomalies in Nb, Ta, Sr, P and Ti. All these features indicate that the precursor magmas were likely formed by partial melting of sedimentary rocks.Clinopyroxenite xenoliths with gneissosity structures are found in these gneissic granites and they have high MgO, Ni and Cr, but low contents of TiO2, and are characterized by high CaO/Al2O3 (4.1–5.3),low Al2O3/TiO2 ratios (8.7–9.5) and HREE depletion. These features are similar to the typical Al-depleted type komatiites. Their geochemical characteristics, such as high (Gd/Yb)N and low HREE, Y, Zr and Hf suggest partial melting of the upper mantle source with residual garnet. The negative Nb, Ta and Ti anomalies indicate a strong arc-related affinity.Our results and recently published data for granitic and mafic basement rocks in this region, reveal that 1.85 Ga was an important period for the evolution of the Cathaysia Block, possibly related to the Columbia supercontinent assembly. Large amount of granitic magma was emplaced, probably as a result of partial melting of old Neoarchean to Palaeoproterozoic materials.
    Geochronological and geochemical constraints on Aolunhua porphyry Mo–Cu deposit, northeast China, and its tectonic significance [查看] HuayingWu、LianchangZhang、BoWan、ZhiguangChen、XiaojingZhang、PengXiang
    The Aolunhua porphyry Mo–Cu deposit is located in the northern margin of the North China Craton (NCC), and belongs to the northern part of the Xilamulun metallogenic belt. More than 90% of the mineralization occurs within the Aolunhua monzogranite-porphyry; a small part is hosted within quartz veins that crosscut Late Permian strata. The syenogranite, occurring as dikes and cutting through the Aolunhua monzograniteporphyry,is radially distributed in the mining district. Zircon U–Pb ages show that the Aolunhua monzogranite-porphyry and the post-ore syenogranite have concordant 206Pb/238U ages of 138.7±1.2 Ma and 131.4±2.8 Ma, respectively. Based on analyses of major, trace elements and Hf-isotopes, the Aolunhua porphyry is characterized by high Sr low Y with high La/Yb and Sr/Y ratios typical of adakitic granites, whereas the post-ore syenogranite has lower Sr and higher Y values, showing apparently negative Eu anomalies (δEu=0.26 to 0.31). The Hf isotopic composition of the Aolunhua porphyry [εHf(t)=+3.6 to +9.2] and the post-ore syenogranite [εHf(t)=+3.6 to+8.7] indicates that both juvenile crustal sources and depleted mantle contributed to their origin. The regional geological setting together with the discrepancy of geochemistry between the Aolunhua porphyry and the post-ore syenogranite probably indicates that they formed in different tectonic regimes. The Aolunhua porphyry is derived from partial melting of the thickened crust due to underplating of the basaltic magma under the transformation tectonic regime, while the post-ore syenogranite comes from the crustal root melting during the lithospheric delamination stage under the lithosphere thinning regime of northeast China.
    Geochemistry and SHRIMP U-Pb zircon geochronology of the Korla mafic dykes Constrains on the Neoproterozoic continental breakup in the Tarim Block, northwest China [查看] WenbinZhu、BihaiZheng、LiangshuShu、DongshengMa、JinglinWan、DewenZheng、ZhiyongZhang、XiaoqingZhu
    The Kuruktag uplift is located to the north of the Tarim Block, NW China. At the western end of the Kuruktag uplift, mafic dykes crop out in the Korla area, which were rarely subjected to deformation and metamorphism in contrast to their metamorphic wall-rocks that were strongly deformed and metamorphosed to amphibolite facies. A SHRIMP U–Pb zircon age of 634 Ma was obtained for a spessartite dyke, documenting the youngest known igneous activity associated with rifting in the Tarim Block during the Neoproterozoic.Most samples of the Korla mafic dykes show clear enrichments in Th, La, and variable depletions in Nb, Ta and Ti, except for samples 08T-14 and 08T-19 that display incompatible element distribution patterns similar to those of OIB without obvious depletions in Nb, Ta and Ti. The geochemical signatures suggest that the Korla mafic dykes were formed in an intra-plate setting and their primary magama was possibly produced by partial melting of a sub-continental lithospheric mantle that has been metasomatized by previous subduction processes, and then heated by a rising mantle plume. In combination with previous geochronological data of Neoproterozoic igneous rocks throughout the Tarim Block, at least three pulses of magmatic activity, from ca. 830 to 800 Ma, from ca. 790 to 740 Ma and from ca.650 to 615 Ma, are recognized, which reveal that multiple episodes of rifting occurred within the Tarim Block, implying that the breakup of the Rodinia supercontinent in the Tarim Block may have been a longlasting process.
    Geochemical and Hf–Nd isotope data of Nanhua rift sedimentary and volcaniclastic rocks indicate a Neoproterozoic continental flood basalt provenance [查看] Xuan-CeWang、Zheng-XiangLi、Xian-HuaLi、Qiu-LiLi、Qi-RuiZhang
    Geochemical and Hf–Nd isotope studies of Neoproterozoic sedimentary and volcaniclastic rocks in the Nanhua Rift Basin, South China, demonstrate that their source provenances contained large proportions of mafic rocks and various amounts of granites. A significant proportion of the studied Neoproterozoic rift sedimentary and volcaniclastic rocks have initial εNd(t) values higher than those of Neoproterozoic granites, but fall within the range of ca. 825–750 Ma basaltic rocks. Their initial εNd(t) values correlate with ratios of La/Sc, La/Cr, La/V, La/Co and La/Ni. The Nd isotope and trace element data, in combination with existing in-situ U–Pb and hafnium–oxygen isotope data fromthe detrital zircon grains indicate a dominant ca. 825–800 Mamafic provenance. Furthermore,the studied samples formed a linear array and plot within the field defined by the remanent of ca. 825–810 Ma continental flood basalts in the Hf–Nd isotopic space. Thus, the inferred large proportions of mafic rocks in the source provenance of the Neoproterozoic rift sedimentary and volcaniclastic rocks likely signify an eroded continental flood basalt province, similar to that reported for the Neoproterozoic sedimentary rocks in Australia. This work thus provides further evidence for the possible once existence of a common large igneous province between South China and eastern Australia as adjacent parts of the supercontinent Rodinia.
    Early Cretaceous volcanism in the northern Songliao Basin, NE China, and its geodynamic implication [查看] F.Q.Zhang、H.L.Chen、X.Yua、C.W.Dong、S.F.Yang、Y.M.Pang、G.E.Batt
    The Songliao Basin is characterized by episodic rifting and intense volcanism during its early development,and forms a key concealed part of the Late Mesozoic magmatic province of NE China. Few precise geochronological and geochemical data were previously available for the volcanic elements of this basin,restricting understanding of its geodynamic setting and evolution. We present new SHRIMP U–Pb zircon ages and geochemical data for the volcanic rocks from the northern Songliao Basin, which limit this volcanism to the Early Cretaceous period (115–109 Ma). Although dominated by rhyolite, the rocks cover a wide compositional spectrum encompassing trachyandesite, basaltic trachyandesite, trachyte and dacite.This suite exhibits a range of geochemical signatures characteristic of subduction-related genesis, falling into a high-K calc-alkaline series, with enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), and weak depletion in high field strength elements (HFSE) and heavy rare earth elements (HREE). The suite also shares a common isotopic composition, consistent with derivation from partial melting of a single depleted mantle source. This Early Cretaceous volcanism occurred in an extensional backarc setting associated with the subduction of the Paleo-Pacific plate, large scale upwelling of the asthenosphere, and intensive lithospheric thinning of the eastern continental margin of NE China which may have lasted until ca. 109 Ma.
    Early Cretaceous stratigraphy and SHRIMP U-Pb age constrain the Valanginian-Hauterivian boundary in southern Tibet [查看] XIAOQIAOWAN、ROBERTSCOTT、WENCHEN、LIANFENGGAOANDYIYIZHANG
    The Late Jurassic to the Early Cretaceous marine strata are extensively distributed in southern Tibet. In Gyangze, the strata are divided into the Weimei and Jiabula formations.In Nagarze, they are divided into the Weimei and Sangxiu formations. Previous work has reported diverse ammonite species of Haplophylloceras and Himalayites in the Weimei Formation, and a few species of Spiticeras in the lower Jiabula and Sangxiu formations.The present study has found the bivalve Inoceramus and nannofossil assemblages in the lower Jiabula and Sangxiu formations. The nannofossil assemblage of Nannoconus steimannii steinmannii, N. steinmannii minor and Watznaueria barnesae indicates Berriasian age, and the Calcicalathina oblongata–Speetonia colligata assemblage is Valanginian in age. Numerical ages for the Jiabula and Jiabula-goukou sections in Gyangze have been interpolated by comparing the fossil ranges with ages calibrated in other sections. The correlation experiment plots fossil ranges in the two sections to the CRET1 Database. The estimated rate of sediment accumulation of the lower Sangxiu Formation is 22.6 m⁄ myr. The Jurassic–Cretaceous (J/K) boundary is at the bottom of the Jiabula Formation in Gyangze, and the base of the Sangxiu Formation in Nagarze.The boundary is marked by the appearance of the ammonite Spiticeras and the nannofossil assemblage of Nannoconus st. steinmannii–N. st. minor–Watznaueria barnesae. The radiometric age in Tibet is the first to be integrated with upper Valanginian fossils. The volcanic rocks of the upper Sangxiu Formation are dated at 136 ± 3.0 Ma deduced from zircon SHRIMP age of rhyolite. By consideration of the rate of sediment accumulation of the underlying sedimentary deposits, the J/K boundary in the Gyangze–Nagarze area is approximately 145 Ma as suggested by the newly issued International Stratigraphic Chart, and the Valanginian ⁄Hauterivian boundary lies between 134 Ma and 136 Ma.
    Detrital Zircon Dating of Meso-and Neoproterozoic Rocks in North China and Its Implications [查看] GAOLinzhi、LIUPengju、YINChongyu、ZHANGChuanheng、DINGXiaozhong、LIUYanxue、SONGBiao
    New dates from Meso-and Neoproterozoic strata contribute to the recently defined Orecambrian 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 Chingcheng System,including the Changzhougou,Chuanlinggou,Tuanshanzi and Dahongyu formations has been constrained to 1.8-1.6Ga.second,the Jinxian System including the Gaoyuzhuang,Yangzhuang,Wumishan,HOngshuizhuang and Tieling formations has been constrained to 1.6-1.4Ga.Third,an as-yet unnamed(undefined)system(1.4-1.0Ga)is only developed in the Xiamaling Formation at JIxian section,Tianjing.OFrth,the Qingbaikou System,including the Luotuoling and jing'eryu formationshas been constrained to 1.0-0.78Ga.Fifth,the Nanhuan System ranges betwwen 780-635Ma，and the Sinian System is within 635-542Ma,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 Qingkaikou SYstem.To aid global geodynamics,it is useful to denote a late Precambrain section with unified,precise and high-precision chronological dating;this is here defined in North China Block and Jiaoliao-Korean Block. However,the Neoproterozoic Qingkaikou study in North China will be influence in whole Meso-and Neoproterozoic in the Jiangnan Orogenic Belt in betwwen the Yangtze Block and the Cathaysia Block in South China.
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