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Zircon SHRIMP U-Pb dating on plagiogranite from Kuerti ophiolite in Altay,North Xinjiang
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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.
SHRIMP U-Pb zircon geochronology of granulites at Rimana(Southern Tibet)in the central segment of Himalayan Orogen
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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.
SHRIMP U-Pb zircon geochronology and its implications on the Xilin Gol Complex,Inner Mongolia,China
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SHIGuanghai
、
LIUDunyi
、
ZHANGFuqin
、
JIANPing
、
MIAOLaicheng
、
SHIYuruo
、
Abstract The Xilin Gol Complex, consisting of deformed and metamorphosed rocks, was exposed as a large geological unit within the Central Asian Orogenic Belt, but its forming and subsequent deformed and metamorphic time has been an issue of little consensus. Petrographic analyses and SHRIMP dating on biotite-plagioclase gneiss, one of the major rocks within the Xilin Gol Complex, in southeast Xilinhot City, Inner Mongolia, China, where the Xilin Gol Complex was identified and named, yield its lower limit age of 437 ± 3 Ma (2s ) by its magmatic zircon SHRIMP U-Pb dating, and an upper limit age of 316 ± 3Ma (2s ), which was constrained by SHRIMP dating of magmatic zircons from adjacent undeformed garnet-bearing granite which intruded the Complex.The Complex was thus determined to be formed and subsequently deformed/metamorphosed from the late Ordovician-early Silurian to the mid-Carboniferous. Consequently, it is not the Precambrian terrane as previously considered by most geologists. More or less, the major rock —— biotiteplagioclase gneiss within the Complex is more likely to be Paleozoic fore-arc turbidite formation before metamorphism and intensive deformation, in which the detrital zircons gave sporadic Precambrian ages as old as up to 3.1 Ga. The source of the turbidite formation is multiple, which may be derived either from the North China Craton, or from the South-Mongolia Micro-continent, or probably came from a potential and undiscovered in situ terranes aged 600—800 Ma or even up to ca 3.1 Ga near the Complex.
SHRIMP dating of volcanic rocks from Ningwu area and its geological implications
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ZHANGQI
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JIANPING
、
LIUDunyi
、
WANGYanglong
、
QIANQing
、
WANGYan
、
XUEHuaimin
Abstract The SHRIMP U-Pb ages are reported for the volacnic rocks from the Longwangshan and the Dawangshan formations respectively,Ningwu area,Jiangsu Province,The Dawagnshan formation(NB-01) is dated at(127±3)Ma,and the Longwangshan formation(NL-01),(131±4)Ma,besides,a few Archean zircons are also found in the Longwangshan formation,which suggests a possible Archean basement in the region.
Zircon SHRIMP dating of sodium alkaline rocks from Maomaogou area of Huili country in Panxi,SW China and its geological implications
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LIUHongying
、
XIABin
、
ZHANGYuquan
Abstract Alkali-feldspar syenites from the Maomaogou area of Huili County in Panxi of Sichuan Province, SW China are sodium alkaline rocks, the minerals of which are mainly composed of calcic ferroaugite, ferroamphibole and biotite. Most of the zircons (>80%) selected for SHRIMP U-Pb dating are magmatic compound zircons and they have relict inner cores and growing outer shells. The age of zircon SHRIMP U-Pb dating is 224±8 Ma, suggesting that the Maomaogou alkali-feldspar syenite was formed in the late Triassic period. At the same time, two old age periods of relict cores of zircon are determined, they are the Archean age(2692—2818 Ma) and Neoproterozoic age (622—691 Ma),respectively. These periods indicate that the cores derived from the crystal basement. Furthermore, the Archean age of zircon provides the first reliable SHRIMP U-Pb zircon dating evidence of >2.8 Ga basement in the Xikang-Yunnan Axis of southwest China.
SHRIMP U-Pb zircon dating on Nyainquetanglha granite in central Lhasa block
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LIUQisheng
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WUZhenhan
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HUDaogong
、
YEPeisheng
、
JIANGWan
、
WANGYanbin
、
ZHANGHancheng
Abstract The Nyainqentanglha granite, a batholith with an area of 1500 km2 in the central Lhasa block, comprises mainly medium-fine-grained biotite monzonitic granite and medium-coarse-grained biotite monzonitic granite. Their contact relationship suggests that the medium-fine-grained biotite monzonitic granite was crystallized earlier than the medium-coarse-grained monzonitic granite. A SHRIMP U-Pb dating on zircons from representative samples of the batholith shows that the crystallization age of the medium-fine-grained biotite monzonitic granite is 18.3 Ma and that of the medium-coarse-grained monzonitic granite is 11.01 Ma. The emplacement and crystallization of the Nyainqentanglha granite, the youngest granite batholith ever known inside the Lhasa block, have a close relationship with the collisional orogeny, crustal thickening, plateau uplifting,E-W crustal extension and have a dynamic genetic relationship with the crustal partial melting.
SHRIMP U-Pb zircon ages for the UHP metamorphosed granitoid gneiss in Altyn Tagh and their geological singificance
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ZHANGAnda
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LIULiang
、
SUNYong
、
CHENDanling
、
WANGYan
、
LUOJinhai
Abstract Cathodoluminescence (CL) and mineral inclusions investigations show that zircons from Yinggelisayi UHP metamorphosed granitoid gneiss in Altyn Tagh exhibit core-mantle-rim internal texture with magmatic residual core, the metamorphic overgrowth mantle and the late stage rim. SHRIMP 206Pb/238U ages of 5 spots from the mantles vary from 484±11 to 491±12 Ma with Th/U ratios from 0.01 to 0.03, yielding a weighted average age of 487±10 Ma (2σ ) as the age of the UHP metamorphosed granitoid gneiss. The 206Pb/238U ages from the oscillatory zoned cores range from 809±19 to 885±21 Ma with Th/U ratios ranging from 0.42 to 0.83, which are interpreted as the crystallization age of the protolith of the granitoid gneiss. The metamorphic age (487±10 Ma) of the UHP metamorphosed granitoid gneiss,well consistent with the peak metamorphic age (about 500Ma) of the UHP eclogite with crystallization age of the protolith between 809±19 and 885±21 Ma at Jianggelisayi in western Altyn Tagh, indicates the existence of a UHPM belt along the south margin of the Altyn Tagh as a result of deep subduction during Early Paleozoic age of previously existing continental crust.
Emplacement age and tectonic implications of the Xilinhot A-type granite in Inner Mongolia,China
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SHIGuanghai
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MIAOLaicheng
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ZHANGFuqing
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JIANPing
、
FANWeiming
、
LIUDunyi
Abstract A new rock type of granite, approximate 45km2 in area and located about 10 km south of Xilinhot, Inner Mongolia, was found in the Sunitezuoqi (or called Suzuoqi)-Xilinhot tectonic belt and identified as an A-type miarolitic intrusion. The pluton has miarolitic structure and is composed chiefly of perthite, quartz, euhedral albite and potassium feldspar. Various types of textures occur in the pluton, such as perthitie, graphic and myrmekite textures.Only quartz is found in miarolitic cavity. This A-type granite with seagull-shaped REE patterns and obvious negative Eu anomaly (d Eu = 0.24—0.28) is high in SiO2 (76%—77%), K and Na (Na2O + K2O = 7.75%—8.15%) and low in Ca (CaO= 0.20%—0.22%), Fe and Mg. Both petrographical observations and chemical compositions indicate that it is an A-type granite. Zircon SHRIMP U-Pb analyses indicate that this A-type granite was emplaced at 276 ± 2 Ma and coeval with the same type of granites in the adjacent areas. Therefore, it suggests that this pluton was likely formed in a post-orogenic extensional setting and probably related to break-off of subducted slabs in Central Asian Orogenic Belt (CAOB), which indicate that the Sunitezuoqi-Xilinhot belt was tectonically evolved into post-orogenic stage since early Permian.
Zircon SHRIMP Dating for the Weiya Pluton,Eastern TianshanIts Geological Implications
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ZHANGZunzhong
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GULianxing
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WUChangzhi
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LIWeiqiang
、
XIAihua
、
WANGShuo
Abstract The timing of the emplacement of the Weiya pluton remains controversial due to the absence of systematic and precise dating. This paper reports zircon SHRIMP U-Pb dating of different lithologic phases in the Weiya pluton, and discusses the genesis and tectonic environment. The ages of gabbro, quartz syenite, diorite porphyrite and fine-grained granite are 236±6 Ma, 246±6 Ma, 233±8 Ma and 237±8 Ma, respectively. All these phases were formed in early-middle Indosinian(Triassic) in a post-orogenic environment. In addition to underplating, intraplating of mantle-derived magmas is also a substantial mechanism for magma generation and vertical accretion of the continental crust. Granitoid rocks are important products of vertical continental accretion as underplating evolves gradually to intraplating. The existence of post-orogenic Indosinian granites shows that the middle Tianshan orogenic belt underwent an important tectonic conversion from the Paleo-Asian ocean subduction-collision system to the Paleo-Tethys ocean regime.
SHRIMP U-Pb zircon dating for Qiashikansayi granodiorite,the Northern Altyn Tagh Mountains and its geological implications
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QIXuexiang
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LIHaibing
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WUCailai
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YANGJingsui
、
ZHANGJianxin
、
MENGFancong
、
SHIRendeng
、
CHENSongyong
Abstract The Qiashikansayi granodiorite is foliated resulting in a granodioritic gneiss. Its geochemical features,such as alumina saturation index(A/CNK) of 0.81—0.99,Na2O/K2O values>1, TiO2 contents <1.0%, LREE enrichment with high fractionation factors, weakly negative or no Eu anomalies, and significant Ba and Ti negative anomalies,suggest that it is similar to a typical island arc pluton. The trace elements of the Qiashikansayi granodiorite are plotted in the island arc field in the tectonic setting discrimination diagrams as well. Cathodoluminescence images demonstrated that the zircons have clear rhythmic crystallized zoning, without any remnant core and new crystallized rim,suggesting the zircons be magmatic ones. Their U and Th contents vary in the range of 574—870 μg/g, and 279—556 μg/g respectively, with the Th/U ratio in the range of 0.52—0.68. SHRIMP zircon U-Pb dating yielded the 481.5±5.3 Ma age for the intrusion of the granodiorite, which is coeval with the island arc volcanic rocks in the northern Qilian Mountain, and confirms that there is an early Paleozoic island arc in the Hongliugou- Lapeiquan area.
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