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    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.
    Geochronology, petrogenesis and tectonic significance of peraluminous granites from the Chinese Altai, NW China [查看] KedaCai、MinSun、ChaoYuan、GuochunZhao、WenjiaoXiao、XiaopingLong、FuyuanWu
    The Paleozoic granites in the Chinese Altai are important for the study of tectonic evolution and crustal growth in the Central Asian Orogenic Belt (CAOB). Four representative peraluminous granitic intrusions were selected for systematic studies of zircon U–Pb and Hf isotopic compositions and whole-rock geochemical and Nd–Sr isotopic analyses. These rocks have high ASI (Alumina Saturation Index, Al2O3/(CaO+Na2O+K2O)=1.01–1.46 molecular ratios), with 0.6–5.6 wt.% of normative corundum, and are characterized by moderately negative Eu anomalies (Eu/Eu*=0.38–0.98) and strong depletion in Ba, Nb and Sr elements. Our data suggest that these intrusions were emplaced from 419 to 393 Ma, consistent with a period of intensive magmatic activities and high temperaturemetamorphismin the Chinese Altai.While in situ zircon Hf isotopic analyses for these granites give predominantly positive εHf(t) values (+0.8 to+12.8), a few inherited zircons yield negative εHf(t) values from −12.5 to −1.53. The U–Pb age and Hf isotopic data of these inherited zircons are similar to that of the widespread metasediments. In addition, the peraluminous granitic rocks have near-zero or negative εNd(t) values(−3.3 to−0.5) and relatively high initial 87Sr/86Sr ratios (0.7079–0.7266), distinct fromthose of the I-type granites in the study region, but similar to the Early Paleozoic Habahe sediments. These isotopic compositions suggest that the newly accreted metasediments of Habahe Group may be the major source rock of the peraluminous granites. The geochemical compositions indicate that their precursor magmas were derived from a relatively shallow crustal level (P≤5 kbar) and zircon saturation temperatures suggest that these granitic intrusions were emplaced at 672–861 °C. The peraluminous granitic magmas were generated by dehydration melting of newly accreted materials, which were possibly brought to at least middle crustal depth by subduction-related processes in an active margin, and were subsequently molten by strikingly high ambient temperature probably caused by upwelling of the hot asthenosphere associated with ridge subduction in the Paleozoic.
    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 features of the Cathaysia block (South China) New evidence for the Neoproterozoic breakup of Rodinia [查看] Liang-ShuShu、MichelFaure、Jin-HaiYu、Bor-MingJahn
    The Cathaysia block is an important element for the reconstruction of the Proterozoic tectonic evolution of South China within the Rodinia supercontinent. The Pre-Devonian Cathaysia comprises two litho-tectonic units: a low-grade metamorphic unit and a basement unit; the former was a late Neoproterozoic–Ordovician sandy and muddy sedimentary sequence, the latter consists essentially of metamorphosed Neoproterozoic marine facies sedimentary and basaltic rocks, and a subordinate amount of Paleoproterozoic granites and amphibolites. This block has undergone several tectono-magmatic events. The first event occurred in the late Paleoproterozoic, at ca. 1.9–1.8 Ga, and the tectonic–magmatic event dated at 0.45–0.40 Ga was resulted from the early Paleozoic orogeny that made the Pre-Devonian rocks to undergo a regional lower greenschist to amphibolite facies metamorphism. The Neoproterozoic geodynamic event is poorly understood. In this paper, new U–Pb zircon age, whole-rock chemical and zircon Hf isotopic data for mafic and felsic igneous rocks are used to constrain the tectonic evolution of Cathaysia. Zircon SHRIMP U–Pb analyses on four mafic samples yielded rather similar Neoprotorozoic ages of 836±7Ma (gabbro), 841±12Ma(gabbro), 847±8Ma (gabbro) and 857±7Ma (basalt).Combined with the published isotopic age data, most of the mafic samples dated at 800–860Ma show geochemical characteristics of continental rift basalt. By contrast, rhyolitic samples with an age of 970Ma have a volcanic arc affinity. All mafic samples have LREE-enriched REE patterns, and non-ophiolitic trace element characteristics. However, the zircon Hf isotopic data of mafic samples show positive epsilon εHf(t) values (+4.1 to +10.5), suggesting that they were originated from a long-term depleted mantle source. All the available ages indicate that the Cathaysia block has registered two stages of Neoproterozoic magmatism.The younger stage corresponds to a continental rifting phase with emplacement of mafic rocks during the period of 860–800 Ma, whereas the older stage represents an eruption of volcanic arc rocks at about 970 Ma. These two magmatic stages correspond to two distinct tectonic settings within the framework of the geodynamic evolution of Cathaysia. Such a similar Neoproterozoic stratigraphy and magmatism between the Cathaysia, Yangtze and Australian blocks provide a significant line of evidence for placing the Cathaysia block within the Rodinia supercontinent.
    Geochronological and geochemical constraints on the petrogenesis of late Triassic aluminous A-type granites in southeast China [查看] YangSun、ChangqianMa、YuanyuanLiu、ZhenbingShe
    The late Permian–Triassic granites in the South China Block (SCB) have important tectonic significance for the evolution of East Asia. Most of the intrusive bodies in the central SCB consist of S-type granites. Here,we report the first unambiguous discovery of Wengshan aluminous A-type granites in southwest Zhejiang Province, southeast China. SHRIMP and LA-ICP-MS zircon U–Pb analyses yielded similar ages about 224 Ma for two samples from the Wengshan pluton, in concordance with peak time of late Indosinian (234–205 Ma) tectonic magmatic activity in the SCB. The Wengshan granites are peraluminous with A/CNK values ranging from 1.09 to 1.24, enriched in K, Rb, Th, REE and Zr + Nb + Ce + Y, have elevated FeOtot/MgO and Ga/Al ratios, and are depleted in Mg, Ca, Mn, Ba, Sr, P and Ti. In addition, they show high zircon saturation temperatures (820–878 C). Their REE patterns are highly fractionated, with (La/Yb)N ratios of 36.89–101.24 and strongly negative Eu anomalies (Eu/Eu= 0.28- 0.53). In-situ zircon Hf isotopic analyses indicate that the Wengshan granites have eHf(t) values ranging from 15.9 to 12.7 and two-stage depleted mantle Hf model ages from 2.05 to 2.26 Ga. Sr–Nd isotope systematics are characterized by a high initial 87Sr/86Sr ratios of 0.7092–0.7218 and a low eNd(t) values of 15.0 to 12.1, with two-stage depleted mantle Nd model ages of 1.99–2.22 Ga, consistent with those of Paleoproterozoic basement rocks in the area. It is inferred that the Wengshan granite magma formed by partial melting of Paleoproterozoic metasedimentary sources in Cathaysia Block. Meanwhile, the underplating of mafic magma in the lower crust may provide the heat source for the formation of the Wengshan granites.The data show that the Wengshan granites, unlike other S-type granites in the SCB, are late Triassic aluminous A-type granite. Therefore they may indicate distinct geodynamic significance. Our study of the Wengshan A-type granites (224 Ma), together with Jingju A-type granites (215 Ma) in Zhejiang Province and Gaoxi A-type granites (215 Ma), Xiaotao A-type granites (222 Ma), Tieshan (254 Ma) and Yangfang (242 Ma) alkaline syenites in Fujian Province, define important extensional events in Triassic. Considering these available data, we suggest that the late Permian–Triassic A-type granites and alkaline syenites in the coastal region of the SCB probably formed under an extensional regime related to oblique subduction of the paleo-Pacific Plate.
    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, zircon U–Pb dating and Sr-Nd-Hf isotopic constraints on the age and petrogenesis of an Early Cretaceous volcanic-intrusive complex at Xiangshan,Southeast China [查看] Shui-YuanYang、Shao-YongJiang、Yao-HuiJiang、Kui-DongZhao、Hong-HaiFan
    The Late Mesozoic geology of Southeast China is characterized by extensive Jurassic to Cretaceous magmatism consisting predominantly of granites and rhyolites and subordinate mafic rocks, forming a belt of volcanic-intrusive complexes. The Xiangshan volcanicintrusive complex is located in the NW region of the belt and mainly contains the following lithologies: rhyodacite and rhyodacitic porphyry, porphyritic lava, granite porphyry with mafic microgranular enclaves, quartz monzonitic porphyry, and lamprophyre dyke. Major and trace-element compositions, zircon U–Pb dating, and Sr–Nd–Hf isotopic compositions have been investigated for these rocks. The precise SHRIMP and LA–ICP–MS zircon U–Pb dating shows that the emplacement of various magmatic units at Xiangshan took place within a short time period of less than 2 Myrs. The stratigraphically oldest rhyodacite yielded a zircon U–Pb age of 135±1 Ma and the overlying rhyodacitic porphyry has an age of 135±1 Ma. Three porphyritic lava samples yielded zircon U–Pb ages of 136±1 Ma, 132±1 Ma, and 135±1 Ma, respectively. Two subvolcanic rocks (granite porphyry) yielded zircon U–Pb ages of 137±1 Ma and 137±1 Ma. A quartz monzonitic porphyry dyke, which represented the final stage of magmatism at Xiangshan, also yielded a zircon U–Pb age of 136±1 Ma. All these newly obtained precise U–Pb ages demonstrate that the entire magmatic activity at Xiangshan was rapid and possibly took place at the peak of extensional tectonics in SE China. The geochemical data indicate that all these samples from the volcanic-intrusive complex have an A-type affinity. Sr–Nd–Hf isotopic data suggest that the Xiangshan volcanic-intrusive complex derived mainly from remelting of Paleo-Mesoproterozoic crust without significant additions of mantle-derived magma. However, the quartz monzonitic porphyry, which has zircon Hf model ages older than the whole-rock Nd model ages, and which has εNd(T) value higher than the other rocks, may indicate involvement of a subordinate younger mantle-derived magma in its origin. Geochemical data indicate that the various rocks show variable REE patterns and negative anomalies of Ba, Nb, Sr, P, Eu and Ti in the trace element spidergrams, suggesting that these rocks may have undergone advanced fractional crystallization with separation of plagioclase, K-feldspar and accessory minerals such as allanite. We suggest that this Cretaceous volcanic-intrusive complex formed in an extensional environment, and the formation of the Xiangshan mafic microgranular enclaves can be explained by the injection of mafic magma from a deeper seated mantle magma chamber into a hypabyssal felsic magma chamber at the crustal emplacement levels.
    Geochemical Constrains on Petrogenesis and Tectonic Setting of Volcanic Rocks from the Baimianxia and Sanwan Formations in the Northern Margin of the Yangtze Plate [查看] XUXueyiCHENJunlu、LIXiangmin、MAZhongping、WANGHongliang、LIPingandLITing
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