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U-Pb zircon geochronology, geochemistry and Nd isotopic study of Neoproterozoic bimodal volcanic rocks in the Kangdian Rift of South China implications for the initial rifting of Rodinia [View] Xian-huaLi、Zheng-XiangLi、HanwenZhou、YingLiu、PeterD.Kinny

SHRIMP U–Pb zircon age, geochemical and Nd isotopic data are reported for the Neoproterozoic Suxiong volcanic rocks in the Kangdian Rift, western South China. These volcanic rocks are bimodal, consisting mainly of mildly alkaline basalts and trachydacites to rhyolites. SHRIMP U–Pb zircon age determination indicates that they were erupted at 803±12 Ma. Most basaltic rocks are characterized by high positiveεNd(T) values (+5 to +6),pronounced enrichment in Th, Ta, Nb, LREEs, Sr, P, Zr, Hf, Ti, smooth LREE-enriched patterns and generally‘humped’ trace element spidergrams. They resemble the alkali basalts of the Hawaiian oceanic island basalts (OIB) and the Ethiopian continental flood basalts (CFB). These features suggest that the basaltic rocks were most probably derived from an OIB-like mantle source without appreciable crustal/lithospheric contamination. Differentiated basalt and trachyandesite samples show relatively lowεNd(T) values (+1.7 to +2.4) and Nb-Ta depletion due to contamination by the mafic lithosphere and/or crustal materials. The rhyolite and dacite samples have small positiveεNd(T) values (+1.1 to +2.6), general enrichment in most incompatible trace elements (K, Rb, Th, Zr, Hf and REEs) but significant depletion in Nb, Ta, Sr, P, Eu and Ti. They share geochemical characters of A2-type granites, and are likely generated by shallow (P≤4 kbar) dehydration melting of hornblende-bearing granitoids. Geochemical and Nd isotopic characters and high-volcanicity of the Suxiong bimodal volcanic successions are consistent with their formation in a continental rift environment, such as the Ethiopian rift. The Kangdian Rift is considered as part of a wider continental rift system produced by a starting mantle plume beneath South China during the Neoproterozoic breakup of Rodinia.

SHRIMP U-Pb zircon geochronology of the Fuping Complex implications for formation and assembly of the North China Craton [View] HongGuan、MinSun、SimonA.Wilde、XinhuaZhou、MingguoZhai

The Fuping Complex, located within the central zone of the North China Craton, is composed of amphibolite to granulite facies orthogneisses, interleaved with minor supracrustal rocks at similar metamorphic grade. The oldest components recognised are hornblende gneiss enclaves within the predominant biotite orthogneiss which have a SHRIMP U–Pb zircon age of 27088 Ma. We consider these enclaves to represent fragments of ~2.7 Ga continental materials incorporated in the biotite gneiss. The biotite gneiss has a SHRIMP U-Pb zircon age of 2513±12 Ma, interpreted to be time of magmatic crystallisation of the igneous precursor, based on the igneous characteristics of the zircons. This indicates a major magmatic episode at 2.52 Ga in the Fuping Complex, identical to the age of felsic volcanism within the low-grade Wutai Complex which crops out immediately to the west. A gneissic granite that intrudes the biotite gneisses has a poorly-defined 207Pb/206Pb age of 2045±64 Ma. This is within error of the age of 2097±46 Ma obtained from a fine-grained gneiss interlayered with amphibolite of the Wanzi Supracrustal Suite (WSS), interpreted to be volcanic in origin. Zircons from both these samples have strong oscillatory zoning and provide the first indication of a Palaeoproterozoic magmatic event in the area, again similar in age to magmatic events recently recognised in the adjacent Wutai Complex. These data indicate a comparable history for the Fuping and Wutai Complexes and support geochemical evidence that they had a common origin and formed part of a Late Archaean arc, affected by later Palaeoproterozoic re-activation. Low-uranium zircons without oscillatory zoning, separated from a sample of biotite gneiss, yield data clustered at 181726 Ma, which is interpreted to reflect a period of new zircon growth during a major metamorphic event. These data support the recently-proposed tectonic model that amalgamation of the North China Craton occurred due to collision of the Eastern and Western Blocks along the central zone at 1.8 Ga ago.

SHRIMP U-Pb zircon geochronology of granitoids fromDongping area, Hebei Province, China constraints on tectonicevolution and geodynamic setting for gold metallogeny [View] LaichengMiao、YuminQiu、NealMcNaughton、ZhenkuanLuo、DavidGroves、YushengZhai、WeimingFan、MingguoZhai、KangGuan

The Dongping area in the central part of the northern marginal zone of the North China Craton (NCC) is one of the most important gold-producing areas in China, and has been intruded by a number of alkaline and granitic batholiths or plutons spatially associated with gold deposits. Previous dates on these intrusions yield contradictory data, depending on the isotopic methodology used, and have led to ambiguity in understanding magmatic evolution and gold mineralization of the region.Detailed SHRIMP U–Pb zircon geochronology of the alkaline and granitic rocks in the study area now reveals details of its tectono-magmatic history. Zircons from a granitic gneiss hosting the Xiaoyingpan gold deposit yielded concordant ages of ca.2.5 Ga. This constrains regional metamorphism of the Archaean basement to late Archaean time. A second magmatic event at ca. 1.8 Ga is revealed by xenocrystic/inherited zircons with this age. These two Precambrian events are coeval with those in other parts of the NCC. The largest igneous intrusion in the region, the Shuiquangou syenite–monzonite complex, was emplaced at ca. 390 Ma, as indicated by the magmatic zircon populations of the two samples from the central and eastern parts of the complex. The accretion of a volcanic arc onto the northern margin of the NCC is considered as a plausible cause of this alkaline magmatism. The emplacement of the Guzuizi porphyritic granite pluton at 236F2 Ma represents another major magmatic event in the region, probably a response to postcollisional orogeny following amalgamation of the North China and Angara cratons. Latest magmatism is marked by the emplacement of the Shangshuiquan monzogranite at 142.2F1.3 Ma, which is most likely related to an underplating event that took place in the Dongping and its adjacent areas at ca. 140 Ma. More importantly, these results suggest that gold mineralization in the area, essentially a single event as indicated by similar deposit characteristics, is broadly coeval with this latest granite magmatism. Hence, this mineralization is about 250 Ma later than the emplacement of the Shuiquangou alkaline complex and, thus, is interpreted to preclude the widely accepted model that suggests gold deposits have genetic relationships with alkaline magmatism. The late timing of gold mineralization with respect to felsic magmatism in the area is similar to those observed in other major gold-producing provinces in the NCC. In all these major gold provinces, we suggest that gold mineralization is coeval with the latest granite magmatic event at about 140–120 Ma. This farther suggests that these mesothermal gold deposits were formed from similar tectono-magmatic environments

SHRIMP U-Pb zircon geochronological and geochemicalevidence for Neoproterozoic arc-magmatism along thewestern margin of the Yangtze Block, South China [View] Mei-FuZhou、Dan-PingYan、AllenK.Kennedy、YunqianLi、JunDing

The magmatic and tectonic history of the Yangtze Block and its possible affinity with other Neoproterozoic arc terranes are important in the reconstruction of Neoproterozoic plate tectonics. In the Panxi Belt, adjacent to the eastern margin of the Tibetan Plateau, there are many metamorphic complexes associated with Neoproterozoic granites. These are granitic gneisses of upper greenschist to amphibolite metamorphic facies, which have traditionally been considered the Archean basement of the Yangtze Block, although their origin and age of formation were poorly understood. This study provides the first reliable, SHRIMP U^Pb zircon dating results for the gneissic complexes and the Neoproterozoic granites. Three samples of the Kangding gneissic complex yielded identical ages of 797þ10, 795 þ13 and 796þ14 Ma.The Gongcai gneissic complex has zircons dated to be 824þ 14 Ma with metamorphic rims of 177 þ3 Ma, whereas the Gezong granite has an older age of 864 þ8 Ma. Other gneissic complexes include the Miyi complex that has a younger age of 764 þ9 Ma. Geochemical data show that the Kangding gneissic complex has arc signatures, representing metamorphic products of Neoproterozoic, arc-related acidic plutons. This scenario suggests subduction of oceanic lithosphere eastward (present-day orientation) underneath the Yangtze Block. There is a well-defined arc assemblage with an identical Neoproterozoic age along the eastern margin of the Yangtze Block. Thus, during Neoproterozoic time, both the western and eastern margins of the block were active arcs separated by the Trans-Yangtze basin. The Yangtze Block must, therefore, have been an isolated continent, although it was presumably located near the Rodinian supercontinent.

Grenvillian continental collision in south China New SHRIMP U-Pb zircon results and implications for the configuration of Rodinia [View] Zheng-XiangLi、Xian-huaLi、HanwenZhou、PeterD.Kinny

The timing of continental collision between the Yangtze and Cathaysia blocks of south China is an issue that bears on the accretion of Asia, as well as on the assembly and configuration of the Neoproterozoic supercontinent Rodinia. We report in this paper SHRIMP (sensitive high-resolution ion microprobe) evidence that suggests a Grenvillian continental collision in south China, including (1) evidence for 1.3–1.0 Ga metamorphism on both sides of the Sibao orogen between the Yangtze and Cathaysia blocks and (2)sedimentary provenance of possible foreland-basin deposits on the Yangtze side of the orogen that were derived from the Cathaysia block and the Sibao orogen during the continental collision. The occurrence of ca. 1430 Ma granodiorites in southern Cathaysia, along with ca. 1800 Ma basement and Archean protoliths in northern Cathaysia, makes Cathaysia a possible western extension of the Mojave province in southwestern Laurentia.Together with regional data, we suggest that the Sibao orogen could be one of the Grenvillian sutures at the center of Rodinia assembly that brought Australia, Yangtze, and Cathaysia-Laurentia together by ca. 1000 Ma.

Zircon SHRIMP U-Pb dating on plagiogranite from Kuerti ophiolite in Altay,North Xinjiang [View] ZHANGHaixiang、NIUHecai、KentaroTerada、YUXueyuan、HiroakiSato、Jun’ichiIto

Abstract Field observation, petrological and geochemical characteristics of plagiogranite from Kuerti ophiolite indicate a similar origin to those in shearing zones. It is derived from partial melting of amphibolite that is developed from gabbro within the ocean layer 3 shear zone by the low-angle shearing deformation during the oceanic crust migrating process. Zircon SHRIMP age of 372±19 Ma for the plagioganite from Kuerti ophiolite indicates that this ophiolite formed in the Devonian period and it also represented the time of extension of the Kuerti backarc basin that is relevant to the northwards subduction of the Paleo-Asian oceanic crust. Therefore, the northwards subduction of the Paleo-Asian Ocean beneath the Siberian Plate began in the early stage of the Late Paleozoic era.

Zircon SHRIMP U-Pb dating for the Cangshuipu volvanic rocks and its implications for the lower boundary age of the Nanhua strata in South China [View] WANGJian、LIXianhua、DUANTaizhong、LIUDunyi、SONGBiao、LIZhongxiong、GAOYonghua

Abstract The continental volcanic rocks and volcaniclastic sedimentary conglomerates of the Cangshuipu Formation occur well in Yiyang of Hunan Province, consisting of a wedge-shaped succession of Neoproterozoic strata that overlie with high-angle unconformity the flysch turbidites of the Lengjiaxi Group in the Upper Mesoproterozoic Eonothem.SHRIMP zircon U-Pb dating gives a weighted mean age of 814 ± 12 Ma for the dacitic volcanic agglomerates from the lowest part of the volcanic rocks in the Cangshuipu Formation.This age is younger than previously reported values of 921—933 Ma for the volcanic rocks from the Cangshuipu Formation. Our new dating represents the lower boundary age of the Neoproterozoic System in the studied area. The younger age for the Cangshuipu volcanic rocks is supported by the following observations: (1) Lower Neoproterozoic strata (814—1000 Ma) are missing in the studied area; (2) the Nanhua rift system was initiated at about 820 Ma; and(3) an age of ~820 Ma may be taken as the lower boundary timing of the Nanhua System (even Neoproterozoic) in South China.

SHRIMP U-Pb zircon Ages of Granitoids in the Wulasha Gold Deposit,Inner Mongolia,ChinaTiming of Mineralization and Tectonic Implications [View] LAICHENGMIAO、YUMINQIU、NEALMCNAUCHTON、WEIMINGFAN、DAVIDI.GROVES、MINCUOZHAI

Abstract:The wulashan gold deposit,situated along the northwestern margin of the North China Craton(NCC),is hosted by ductile-brittle faults with Archean metamophic volcano-sedimentary rocks of the Wulashan Group.This deposit is characterized by gold-bearing quartz-K feldspar and quartz veins.Both granitoid batholiths and pegmatite dikes intruded the metamorphic basement rocks,and are spatially associated with gold mineralization.Contrasting genetic medels have been proposed for the deposit due to lack of reliable age data.Our new SHRIMP U-Pb zircon ages for these intrusions now reveal important constraints on the mineraliztion time and tectonic evolution of this region.These intrusions contain inherited zircons of about 2.55Ga.probably from the Wulashan basement that was intruded by pegmatite dikes at about 1.84Ga:the latter probably are related to the major tectonic event leading to the final amalgamation for the NCC.The basement subsequently underwant at least three tectono-thermal events during Phanerozoic time(at353±7，169±7，and 132±2Ma）。 Combined with previous Ar-Ar and K-Ar ages,we suggest two gold mineralization episodes for the Wulashan gold deposit.The first episode occurred at about 350Ma .incicated by ages of a gold-related fuchsite and the Dahuabei granitoid batholith.This supports a previously proposed medel the relates gold mineralization to the Dahuabei granite the formed during collision of the Paleo-Mongolian block with the NCC.The second one occurred in the late Yanshannian period.as indicated by the mineralized K-feldspar-rich vein of 132±2Ma.This episode is simultaneous with those in the eastern NCC.indicative of a widespread late Yanshannian metallogenic event that was a response either to the subduction of lzanagi-Pacific plate beneath eastern China or to the removal of the Early Cretaceous lithosphere in the eastern NCC.

SHRIMP U-Pb zircon age, geochemistry and Nd isotope of the Guandaoshan pluton in SW Sichuan Petrogenesis and tectonic significance [View] LIXianhua、LIZhengxiang、ZHOUHanwen、LIUYing、LIANGXirong、LIWuxian

Abstract SHRIMP U-Pb zircon age, geochemical and Nd isotopic data are reported for the Neoproterozoic Guandaoshan pluton in the Yanbian region, SW Sichuan. This pluton is of typical I-type granite and emplaced at (857 ± 13) Ma. Geochemical and Nd isotopic characters suggest that the pluton was generated by partial melting of pre-existing, young (late Mesoproterozoic to early Neoproterozoic) low-K tholeiitic protolith within an intraplate anorogenic setting. The Guandaoshan pluton probably records the earliest magmatism induced by the proposed ca. 860–750 Ma mantle superplume beneath the supercontinent Rodinia.

SHRIMP U-Pb zircon geochronology of granulites at Rimana(Southern Tibet)in the central segment of Himalayan Orogen [View] LIDewei、LIAOQunan、YUANYanming、WANYusheng、LIUDemin、ZHANGXionghua、YIShunhua1、CAOShuzhao、XIEDefan

Abstract High-pressure mafic granulites, occurring as lenses within gneisses and quartzite in the central segment of the Himalayan orogen, were dated using SHRIMP U-Pb technique. 13 analyses out of a total of 15 are plotted along a concordia line and yield a mean 206Pb/238U age of 17.6 ± 0.3Ma. This age indicates adiabatic decompression and a metamorphic event associated with rapid uplift of granulites in a tectonic environment resulted from the collision between India and Eurasia, synchronous with large-scale thrusting, extension, detachment as well as emplacement of leucogranite.One analysis gives a 206Pb/238U age of 29.5 ± 0.4 Ma that is interpreted to represent the timing of the final closure of the Neo-Tethys. Another age is 1991 ± 26 Ma that represents the age of the protolith of the granulites. In summary,dating results show that granulites in this area underwent multiphase metamorphism and complex geological evolution.

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