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    Carboniferous to Lower Permian stratigraphy of the southern Tamworth Belt, southern New England Orogen, Australia boundary sequences of the Werrie and Rouchel blocks [查看] J.Roberts、R.Offler、M.Fanning
    Carboniferous to Lower Permian successions along the western border of the Tamworth Belt between Wallabadah and Muswellbrook were remapped to clarify the stratigraphy and establish a boundary between the Werrie and Rouchel blocks. The boundary, located at the Waverley Fault, separates Carboniferous sequences containing different formations and volcanic members. SHRIMP AS3 dating of volcanic members indicates that successions within the Rouchel and Gresford blocks were deposited, uplifted and eroded at different times. The lacustrine Woodton Formation in the Werrie block, previously considered Carboniferous, is earliest Permian (Asselian) from palaeobotanical and SHRIMP AS3 evidence. Stratigraphic and palaeoenvironmental differences between the Werrie and Rouchel blocks suggest that they were not directly juxtaposed during the greater part of the Carboniferous, supporting palaeomagnetic evidence that blocks within the Tamworth Belt are allochthonous. Superposition of the western extremity of the Waverley Fault by Lower Permian (Sakmarian) formations and intrusion of folded and faulted Devonian to Lower Permian successions by the Barrington Tops Granodiorite (ca 280Ma) indicate that the Werrie, Rouchel and Gresford blocks were subjected to tectonism before the Late Permian Hunter –Bowen Orogeny.
    Age constraints on felsic intrusions,metamorphism and gold mineralisation in the Palaeoproterozoic Rio Itapicuru greenstone belt, NE Bahia State, Brazil [查看] EdsonF.Mello、RobertoP.Xavier、NealJ.McNaughton、SteffenG.Hagemann、IanFletcher、LarrySnee
    U–Pb sensitive high resolution ion microprobe mass spectrometer (SHRIMP) ages of zircon, monazite and xenotime crystals from felsic intrusive rocks from the Rio Itapicuru greenstone belt show two development stages between 2,152 and 2,130 Ma, and between 2,130 and 2,080 Ma. The older intrusions yielded ages of 2,152±6 Ma in monazite crystals and 2,155±9 Ma in zircon crystals derived from the Trilhado granodiorite, and ages of 2,130±7 Ma and 2,128±8 Ma in zircon crystals derived from the Teofilândia tonalite. The emplacement age of the syntectonic Ambrósio dome as indicated by a 2,080±2-Ma xenotime age for a granite dyke probably marks the end of the felsic magmatism. This age shows good agreement with the Ar–Ar plateau age of 2,080±5 Ma obtained in hornblendes from an amphibolite and with a U–Pb SHRIMP age of 2,076±10 Ma in detrital zircon crystals from a quartzite, interpreted as the age of the peak of the metamorphism. The predominance of inherited zircons in the syntectonic Ambrósio dome suggests that the basement of the supracrustal rocks was composed of Archaean continental crust with components of 2,937±16, 3,111±13 and 3,162±13 Ma. Ar–Ar plateau ages of 2,050±4 Ma and 2,054±2 Ma on hydrothermal muscovite samples from the Fazenda Brasileiro gold deposit are interpreted as minimum ages for gold mineralisation and close to the true age of gold deposition. The Ar–Ar data indicate that the mineralisation must have occurred less than 30 million years after the peak of the metamorphism, or episodically between 2,080 Ma and 2,050 Ma, during uplift and exhumation of the orogen.
    Zircon as the best mineral for P-T-time history of UHP metamorphism A review on mineral inclusions and U-Pb SHRIMP ages of zircons from the Dabie-Sulu UHP rocks [查看] F.L.Liu、J.G.Liou
    Zircon is the best mineral to record the complex evolution history of ultrahigh-pressure (UHP) metamorphic rocks as mineralogical and geochemical tracers of UHP metamorphism are almost obliterated in matrix assemblages resulted from subsequent retrogression during exhumation. Zircons from Dabie–Sulu UHP rocks, including outcrop and core samples from drill holes ranging from 432 to 5158 m in depth contain abundant mineral inclusions of protolith, prograde, peak (UHP) and retrograde minerals in different domains; these minute inclusions were identified by laser Raman spectroscopy and/or electronic microprobe analysis. Systematic studies on inclusions in zircons from previous and present studies indicate that the Dabie–Sulu UHP terrane extends for >2000 km, is about 50 km wide, and has at least 10 km thick, probably the largest UHP terrane recognized in the world thus far. The internal structure of zircon revealed by cathodoluminescence (CL) imaging displays a distinct zonation, which comprises an inherited (magmatic or detrital) core, prograde, peak (UHP), and outmost retrograde domains, each with distinctive mineral inclusion assemblages. Low-pressure, igneous mineral inclusions are common in the inherited (magmatic or detrital) zircon cores. In contrast, quartz eclogite-facies inclusion assemblages occur in prograde domains, coesite eclogite-facies inclusion assemblages are preserved in UHP domains, and amphibolitefacies inclusion assemblages are enclosed in outmost retrograde rims. Parageneses and compositions of inclusion minerals preserved in distinct zircon domains were used to constrain the metamorphic P–T path of many Dabie–Sulu UHP rocks. The results indicate that Neoproterozoic supracrustal rocks together with minor mafic-ultramafic rocks were subjected to a prograde subduction-zone metamorphism at 570–690℃ and 1.7–2.1 GPa, and UHP metamorphism at 750–850 C and 3.4–4.0 GPa, following by rapid decompression to amphibolite-facies retrograde metamorphism at 550–650℃and 0.7–1.05 GPa. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb spot analyses of the zoned zircons show four discrete and meaningful ages of the Dabie–Sulu metamorphic evolution: (1) Neoproterozoic protolith ages(800–750 Ma); (2) 246–244 Ma for early-stage quartz eclogite-facies prograde metamorphism; (3) 235–225 Ma for UHP metamorphism; and (4) 215–208 Ma for late-stage amphibolite-facies retrogression. This indicates that Neoproterozoic voluminous igneous protoliths of orthogneiss in response to the breakup of Rodinia supercontinent, together with various sedimentary rocks, and minor mafic-ultramafic intrusive and extrusive rocks, were subjected to coeval Triassic subduction to mantle depths and exhumation during the collision between the South China Block and North China Block. The estimated subduction and exhumation rates for the Dabie–SuluUHPterranewould be up to 4.7–9.3 km Myr1 and 5.0–11.3 km Myr1, respectively.The zonal distribution of mineral inclusions and the preservation of index UHP minerals such as coesite imply that zircon is the best mineral container for each metamorphic stage, particular for supracrustal rocks as their metamorphic evolution and UHP evidence have been almost or completely obliterated.Similar conclusions have been documented elsewhere for other UHP terranes.
    U–Pb SHRIMP and 40Ar-39Ar ages constrain the deformation history of the Karakoram fault zone(KFZ),SW Tibet [查看] F.L.Liu、J.G.Liou
    Zircon is the best mineral to record the complex evolution history of ultrahigh-pressure (UHP) metamorphic rocks as mineralogical and geochemical tracers of UHP metamorphism are almost obliterated in matrix assemblages resulted from subsequent retrogression during exhumation. Zircons from Dabie–Sulu UHP rocks, including outcrop and core samples from drill holes ranging from 432 to 5158 m in depth contain abundant mineral inclusions of protolith, prograde, peak (UHP) and retrograde minerals in different domains; these minute inclusions were identified by laser Raman spectroscopy and/or electronic microprobe analysis. Systematic studies on inclusions in zircons from previous and present studies indicate that the Dabie–Sulu UHP terrane extends for >2000 km, is about 50 km wide, and has at least 10 km thick, probably the largest UHP terrane recognized in the world thus far. The internal structure of zircon revealed by cathodoluminescence (CL) imaging displays a distinct zonation, which comprises an inherited (magmatic or detrital) core, prograde, peak (UHP), and outmost retrograde domains, each with distinctive mineral inclusion assemblages. Low-pressure, igneous mineral inclusions are common in the inherited (magmatic or detrital) zircon cores. In contrast, quartz eclogite-facies inclusion assemblages occur in prograde domains, coesite eclogite-facies inclusion assemblages are preserved in UHP domains, and amphibolitefacies inclusion assemblages are enclosed in outmost retrograde rims. Parageneses and compositions of inclusion minerals preserved in distinct zircon domains were used to constrain the metamorphic P–T path of many Dabie–Sulu UHP rocks. The results indicate that Neoproterozoic supracrustal rocks together with minor mafic-ultramafic rocks were subjected to a prograde subduction-zone metamorphism at 570–690℃ and 1.7–2.1 GPa, and UHP metamorphism at 750–850 C and 3.4–4.0 GPa, following by rapid decompression to amphibolite-facies retrograde metamorphism at 550–650℃and 0.7–1.05 GPa. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb spot analyses of the zoned zircons show four discrete and meaningful ages of the Dabie–Sulu metamorphic evolution: (1) Neoproterozoic protolith ages(800–750 Ma); (2) 246–244 Ma for early-stage quartz eclogite-facies prograde metamorphism; (3) 235–225 Ma for UHP metamorphism; and (4) 215–208 Ma for late-stage amphibolite-facies retrogression. This indicates that Neoproterozoic voluminous igneous protoliths of orthogneiss in response to the breakup of Rodinia supercontinent, together with various sedimentary rocks, and minor mafic-ultramafic intrusive and extrusive rocks, were subjected to coeval Triassic subduction to mantle depths and exhumation during the collision between the South China Block and North China Block. The estimated subduction and exhumation rates for the Dabie–SuluUHPterranewould be up to 4.7–9.3 km Myr1 and 5.0–11.3 km Myr1, respectively.The zonal distribution of mineral inclusions and the preservation of index UHP minerals such as coesite imply that zircon is the best mineral container for each metamorphic stage, particular for supracrustal rocks as their metamorphic evolution and UHP evidence have been almost or completely obliterated.Similar conclusions have been documented elsewhere for other UHP terranes.
    U–Pb ages and Hf–O isotopes of zircons from Late Paleozoic mafic–ultramafic units in the southern Central Asian Orogenic Belt Tectonic implications and evidence for an Early-Permian mantle plume [查看] Ben-XunSu、Ke-ZhangQin、PatrickAsamoahSakyi、Xian-HuaLi、Yue-HengYang、HeSun、Dong-MeiTang、Ping-PingLiu、Qing-HuaXiao、SanjeewaP.K.Malaviarachchi
    U–Pb age and Hf–O isotope data of zircons fromthe mafic–ultramafic complexes and their related rhyolites and granites in the Eastern Tianshan and Beishan regions were obtained by SIMS and LA-ICPMS. The Hf–O isotopic composition and Hf model age of the zircons are closely related to their tectonic setting and formation age. The zircons from the mafic–ultramafic complexes in the Bogeda–Haerlike and Jueluotage belts have higher εHf(t)(+8–+17) and lower δ18O (+4‰–+6‰), whereas most of the zircons in the gabbros, granites and rhyolite from theMiddle TianshanMassif and Beishan Rift display lower εHf(t) (0–+8) and higher δ18O (+5‰–+8‰).The positive εHf(t) and relatively lower δ18O values suggest that thesemafic–ultramafic complexes were derived from depleted mantle which was subjected to subduction-related modification processes. The Hf isotopic composition of zircons in the granites has revealed that the growth of juvenile crust was also very significant in the southern margin of the Central Asian Orogenic Belt during the Paleozoic. The Hf model ages of the analyzed zircons, together with the regional geology suggest that the Beishan area had a northward subduction, possibly fromca. 900 Ma to ca. 400 Ma,whereas the Eastern Tianshan had a south-directed subductionmost likely fromca.600 Ma to ca. 310 Ma. Additionally, zircons with ca. 280 Ma U–Pb ages display wider and more scattered εHf(t) and δ18O variations than the relatively older and younger ones, which further support the Early Permian mantle plume model.
    Triassic mineralization with Cretaceous overprint in the Dahu Au–Mo deposit,Xiaoqinling gold province Constraints from SHRIMP monazite U-Th-Pb geochronology [查看] NuoLi、Yan-JingChen、IanR.Fletcher、Qing-TaoZeng
    The Xiaoqinling terrane in central China is the second largest gold producing area in China. Previous application of age constraints has yielded ages for the gold mineralization ranging from Paleoproterozoic, through Triassic (Indosinian), to Jurassic and Cretaceous (Yanshanian). SHRIMP U–Th–Pb dating of hydrothermal monazite in a Mo-mineralized quartz vein from the Dahu Au–Mo deposit in the northern Xiaoqinling terrane identifies two mineralization episodes. Hydrothermal monazite intergrown with molybdenite yields an initial precipitation age of 216±5 Ma, which is identical to, but more precise than,the molybdenite Re–Os isochron age of 218±41 Ma. A younger generation of monazite is distinguished by data falling on a mixing array. It is inferred that the younger monazite was formed at the expense of the older ones during the Yanshanian, probably during a single event that occurred at ~125 Ma. Combining these results with available geochronological data and geological evidence, we conclude that molybdenum mineralization accompanied by monazite deposition took place in the Indosinian, followed by a Yanshanian tectono-hydrothermal event that resulted in large-scale gold mineralization in the Xiaoqinling terrane. Our observation demonstrates that monazite is sensitive to hydrothermal overprint, and thus is useful for dating fluid infiltration event.
    Timing of skarn deposit formation of the Tonglushan ore district, southeastern Hubei Province, Middle–Lower Yangtze River Valley metallogenic belt and its implications [查看] GuiqingXie、JingwenMao、HaijieZhao、KetaoWei、ShangguangJin、HuijunPan、YufuKe
    The Tonglushan ore district in theMiddle–Lower Yangtze River Valleymetallogenic belt includes the Tonglushan Cu–Fe, the Jiguanzui Au–Cu, and the Taohuazui Au–Cu skarn deposits. They are characterized by NE-striking ore bodies and hosted at the contact of Triassic carbonate rocks and Late Mesozoic granitoid deposits. New Sensitive High-Resolution Ion Microprobe (SHRIMP) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA–ICP-MS) zircon U–Pb,molybdenite Re–Os, and phlogopite 40Ar–39Ar ages indicate that these skarn deposits formed between 140.3±1.1 and 137.3±2.4 Ma. These dates are identical to the zircon U–Pb ages for host quartz diorites ranging from 140±2 to 139±1 Ma. These results confirm that both skarn mineralization and related intrusions were initiated during the Early Cretaceous. The high rhenium contents (261.4–1152 μg/g) of molybdenites indicate that ametasomaticmantle fluidwas involved in the ore-forming process of these skarn ore systems. This conclusion is consistent with previously published constraints from sulfur, deuterium, and oxygen isotope compositions, and the geochemical signatures, and Sr–Nd isotopic data of the mineralization-hosting intrusions. Geological and geochronological evidence demonstrates that there were two igneous events in the Tonglushan ore district. The first resulted in the emplacement of quartz diorite during the Early Cretaceous (140±2 to 139±1 Ma), and the second is characterized by the eruption of volcanic rocks during the mid-Early Cretaceous (130±2 to 124±2 Ma). The former is spatially, temporally and genetically associated with skarn gold-bearing mineralization (140.3±1.1 to 137.3±2.4 Ma). The recognition of these two igneous events invalidates previous models that proposed continuous magmatism and associated mineral deposits in the Middle–Lower Yangtze River Valley metallogenic belt.
    Timing of metamorphism in the Paleoproterozoic Jiao-Liao-Ji Belt New SHRIMP U-Pb zircon dating of granulites, gneisses and marbles of the Jiaobei massif in the North China Craton [查看] PuiYukTam、GuochunZhao、FulaiLiu、XiwenZhou、MinSun、SanzhongLi
    The Paleoproterozoic Jiao-Liao-Ji Belt lies in the Eastern Block of the North China Craton, with its southern segment extending across the Bohai Sea into the Jiaobei massif. High-pressure pelitic and mafic granulites have been recently recognized in the Paleoproterozoic Jingshan Group (Jiaobei massif). New SHRIMP U–Th–Pb geochronology combined with cathodoluminescence (CL) imaging of zircon has been applied to the determination of the timing of the metamorphism of the high-temperature and high-pressure granulites and associated gneisses and marbles. Metamorphic zircons in these high-pressure granulites, gneisses and marbles occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores. Metamorphic zircons are all characterized by nebulous zoning or being structureless, with high luminescence and relatively low Th/U values. Metamorphic zircons from two high-pressure mafic granulites yielded 207Pb/206Pb ages of 1956±41 Ma and 1884±24 Ma. One metamorphic zircon from a garnet–sillimanite gneiss also gave an apparent 207Pb/206Pb age of 1939±15 Ma. These results are consistent with interval of ages of c. 1.93–1.90 Ga already obtained by previous studies for the North and South Liaohe Groups and the Laoling Group in the northern segment of the Jiao-Liao-Ji Belt. Metamorphic zircons from a high-pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207Pb/206Pb ages of 1837±8 Ma, 1821±8 Ma and 1836±8 Ma respectively. Two diopside–olivine–phlogopite marbles yielded weighted mean 207Pb/206Pb ages of 1817±9 Ma and 1790±6 Ma. These Paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c. 1.85 Ga produced from the Ji'an Group in the northern segment of the Jiao-Liao-Ji Belt and c. 1.86–1.80 Ga obtained for the high-pressure pelitic granulites from the Jingshan Group in the southern segment. As this metamorphic event was coeval with the emplacement of A-type granites in the Jiao-Liao-Ji Belt and its adjacent areas, it is interpreted as having resulted from a post-orogenic or anorogenic extensional event.
    Timing of initiation of extension in the Tianshan, based on structural, geochemical and geochronological analyses of bimodal volcanism and olistostrome in the Bogda Shan (NW China) [查看] LiangshuShu、BoWang、WenbinZhu、ZhaojieGuo、JacquesCharvet、YuanZhang
    This paper describes an olistostrome formation and accompanied bimodal volcanic rocks occurring in the Baiyanggou area, south of Bogda Shan. The main lithotectonic units consist of olistostrome, volcanic rocks and turbidite. The olistostrome is tectonically underlain by Upper Carboniferous limestone and sandstone along a NEE-trending detachment fault. Paleo-growth fault is locally observed. The olistostrome unit includes plenty of blocks of limestone, sandstone, rhyolite and volcaniclastic rocks, and a matrix of graywacke. Limestone blocks are dated as Pennsylvanian-Bashkirian in age by the coral and brachiopod fossils that are extensively recognized in the Upper Carboniferous strata. The volcanic unit consists of pillowed and massive basalt and rhyolite, the latter occur as an 8- to 10-meter-thick layer above the olistostrome unit.The turbidite unit is mainly composed of chert, siliceous mudstone and sandstone, within which the Bouma sequence can be locally recognized. Meter-wide gabbro and diabase dykes intrude these three units. Geochemically,rhyolites are characterized by high ACNK value of[1.1,depletion of Ba, Nb and Sm, and enrichment in Rb, Th and Zr. Basaltic rocks are rich in K2O, they show a LREE-enriched pattern and depletion in Ba, Nb and Zr, and enrichment in Ti, Ce and Hf, similar to continental rift-type tholeiite series. A gabbro porphyrite intruding the olistostrome was dated at 288 ± 3 Ma by a sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb method, and a rhyolite at 297 ± 2 Ma by a laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) zircon U–Pb method. The Baiyanggou olistostrome and accompanying bimodal volcanic series are linked to an extensional setting that developed in the south of the Bogda Shan.Several lines of evidence, e.g. occurrence of large-scale strike-slip shear zones, large number of mantle-derived magmatic rocks and available geochronological data,demonstrate a significant geodynamic change from convergence to extension in the Chinese Tianshan belt, even in the whole Central Asian Orogenic Belt. The extension in the Chinese Tianshan belt is initiated at ca. 300 Ma, i.e. around Carboniferous–Permian boundary times, and the peak period of intra-plate magmatism occurred in the interval of 300–250 Ma.
    The Dayingzi detachment fault system in Liaodong Peninsula and its regional tectonic significance [查看] SHENLiang、LIUJunLai、HULing、JIMo、GUANHuiMei、GregoryADAVIS
    Large scale lithosphere thinning is an important characteristic of the destruction of the North China Craton (NCC) during the late Mesozoic. A series of extensional structures were developed under extensional setting, among which is the Dayingzi detachment fault system (DFS). The DFS is constituted by three parts, volcano-sedimentary basins at the hanging wall, the Dayingzi-Huanghuadian detachment fault zone, and Paleoproterozoic metamorphic rock series and Mesozoic plutons at the footwall.In the section across the detachment fault zone, there is a sequence of tectonites including fault gouge, microbreccia, cataclastic-mylonites, mylonites, and gneissic biotite monzonite granite. Microstructural characteristics of tectonites and electron backscatter diffraction (EBSD) patterns of quartz indicate that the rocks from the footwall experienced a process from upper greenschist facies to lower greenschist facies. SHRIMP and LA-ICP MS U-Pb dating of zircons from the volcanic rocks in the basins, the tectonic evolution of the DFS is summarized as follows: 1) regional extension started at 135.0±1.2 Ma ago, when the detachment fault cut through the middle crust. Faulting induced the upwelling of magma and eruption of volcanic rocks and deformed a series of medium-acid volcanic rocks; 2) after 135.0±1.2 Ma, a large scale detachment faulting was active cross-cutting the mid-upper crust. The western margin of Jurassic and Triassic granite was ductilly and brittly sheared; besides,the Cretaceous volcano-sedimentary rocks were tilted when the master fault approached the surface; 3) at around 127±1 Ma,the detachment fault stopped its activity and was intruded by the unsheared Cretaceous granite near Chaoyang. Comparison with the Liaonan metamorphic core complex (MCC) and other extensional structures in Liaodong Peninsula led to a general trend of including three zones in the Peninsula: MCC zone, detachment fault systems (DFS) zone, and half graben zone. MCC commonly cuts through the mid-lower crust, DFS through the mid-upper crust, and half graben through the upper crust.Therefore, development of the extensional structures in Liaodong Peninsula indicates that they are the results of crustal extension and thinning at different crustal levels. They may provide a deep insight into the dynamic mechanism, history of destruction and lithosphere thinning of the North China Craton (NCC).
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