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Determination of U-Pb age and rare earth element concentrations of zircons from Cenozoic intrusions in northeastern China by laser ablation ICP-MS
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YUANHonglin
、
WUFuyuan
、
GAOShan
、
LIUXiaoming
、
XUPing
、
SUNDeyou
Abstract Using the in situ zircon U-Pb dating method of LA-ICPMS, we analyzed the 31 Ma old SHRIMP U-Pb age of the Yongsheng nepheline syenite from southern Jilin Province under different spot sizes. The obtained ages are comparable with that of SHRIMP in both accuracy and precision. The age is also identical to that of the Yinmawanshan gabbro from the Liaodong Peninsula within error. Both the Yongsheng nepheline syenite and the Yinmawanshan gabbro represent the youngest known exposed intrusions in northeastern and even eastern China.The results indicate the Eocene mantle-derived magmatic underplating, and the rapid crustal uplifting of this region since 30 Ma. The analyses also document extremely high LREE concentrations and relatively flat REE patterns for the zircons from the Yongsheng nepheline syenite, which represent a new type of zircon REE pattern.
Silurian A-type granitoids in the southern margin of the Tongbai-DabieshanEvidence from SHRIMP zircon geochronology and geochemistry
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MAChangqian
、
SHEZhenbing
、
XUPin
、
WANGLingyan
Abstract SHRIMP U-Pb dating on magmatic zircons extracted from a riebeckite quartz syenite in the Huangyangshan pluton in Suizhou, the southern margin of Tongbai-Dabieshan yielded an age of 439±6Ma. According to the morphology and high Th/U ratios of the zircons, the age is interpreted as crystallization timing of the pluton. The Huangyangshan pluton is composed of peralkaline quartz syenite, alkaline granite and syenite. All of the rocks are characterized by high agpaitic index (A.I.=(Na+K)/Al, molar ratio) and Fe-number [FeOT/(FeOT+MgO)], low CaO and MgO contents, enrichment of high field strength elements (i.e. Nb, Zr, Ga, Y, Hf) and light REEs,evidently negative Eu anomalies and high Ga/Al ratios, which are consistent with anorogenic A-type granitoids. Being part of the South Qinling-South Dabieshan Paleozoic alkaline rock belt,the Huangyangshan A-type granitoid pluton results from the Paleozoic extension-initial rifting in the northern margin of the Yangtze Craton related to the opening of the eastern Paleo-Tethyan Ocean.
SHRIMP U–Pb zircon ages of granitoid rocks in the Luliang Complex Implications for the accretion and evolution of the Trans-North China Orogen
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GuochunZhaoa
、
SimonA.Wilde
、
MinSun
、
SanzhongLi
、
XupingLi
、
JianZhang
The lvliang Complex is situated in the central segment of the Trans-North China Orogen (TNCO), a continent-continent collisional belt along which the discrete Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The complex consists of supracrustal and granitoid rocks, of which the latter can be subdivided into pre-tectonic TTG gneisses, including the Yunzhongshan, Guojiazhuang and Chijianling–Guandishan gneisses; syn-tectonic gneissic granites represented by the Huijiazhuang gneissic granite; and post-tectonic granites,including the Luyashan charnockite, Luchaogou porphyritic granite and Tangershang/Guandishan massive granite. The pre-tectonic TTG gneisses are mostly calc-alkaline and considered to have formed in a magmatic arc environment. SHRIMP U–Pb analyses reveal that the Yunzhongshan gneisses were emplaced at ∼2499±9 Ma, representing the earliest arc-related magmatic event in the Luliang Complex. This was followed by the intrusion of the Guojiazhuang gneisses at 2375±10 Ma. The most widespread arc-related magmatic event in the region was the emplacement of the Chijianling–Guandishan granitoid gneisses, of which the tonalitic, granodioritic and monzogranitic gneisses were emplaced at 2199±11 Ma,2180±7Ma and 2173±7 Ma, respectively. Metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207Pb/206Pb age of 1872±7 Ma, consistent with the metamorphic age range of 1880–1820 Ma defined by metamorphic zircons from various high-grade rocks in the TNCO. Thus, the evolution of the lvliang Complex involved emplacement of the Yunzhongshan granitoids at ∼2499 Ma, the Guajiazhuang granitoids at ∼2375 Ma and the Chijianling–Guandishan granitoids at 2199–2173 Ma, with the final collision between the Eastern and Western Blocks in this area occurring at 1872±7 Ma, the whole series of magmatic events lasting nearly 650 Ma. This suggests that the Trans-North China Orogen represents a long-lived magmatic arc. The new SHRIMP zircon data, combined with structural and petrological studies, also reveal the existence of both syn- and post-collisional granites in the Luliang Complex. The former is represented by the Huijiazhuang gneissic granite that was emplaced at 1832±11 Ma, whereas the latter include the 1815±5Ma Luyashan charnockite, 1807±10 Ma Luchaogou porphyritic granite,1798±11 Ma Guandishan massive granite and 1790±14 Ma Tangershan massive granite.
U-Pb Age and Hf Isotope Study of Detrital Zircons from the Wanzi Supracrustals Constraints on the Tectonic Setting and Evolution of the Fuping Complex, Trans-North China Orogen
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XiaopingXIA
、
MinSUN
、
GuochunZHAO
、
WUFuyuan
、
XUPing
、
JianZHANG
、
YanhongHEandZHANGJiheng
Located in the middle segment of the Trans-North China Orogen, the Fuping Complex is considered as a critical area in understanding the evolution history of the North China Craton (NCC). The complex is composed of various high-grade and multiply deformed rocks, including gray gneiss, basic granulite, amphibolite, fine-grained gneiss and marble, metamorphosed to upper amphibolite or granulite facies. It can be divided into four rock units: the Fuping TTG gneisses, Longquanguan augen gneisses, Wanzi supracrustals, and Nanying granitic gneisses, U-Pb age and Hf isotope compositions of about 200 detrital zircons from the Wanzi supracrustals of the Fuping Complex have been analyzed. The data on metamorphic zircon rims give ages of 1.82-1.84 Ga, corresponding to the final amalgamation event of the NCC, whereas the data for igneous zircon cores yield two age populations at -2.10 and -2.51 Ga, with some inherited ages scattering between 2.5 and 2.9 Ga. These results suggest that the Wanzi supracrustals were derived from the Fuping TTG gneisses (-2.5 Ga) and the Nanying granitic gneisses (2.0-2.1 Ga) and deposited between 2.10 and 1.84 Ga. All zircons with -2.51 Ga age have positive initial EW values from +1.4 to +10.9, suggesting an important crustal growth event at -2.5 Ga through the addition of juvenile materials from the mantle. The Hf isotope data for the detrital zircons further imply that the 2.8 Ga rocks are important components in the lower crust, which is consistent with a suggestion from Nd isotope data for the Eastern Block. The zircons of 2.10 Ga population have initial values of 4.9 to +6.1, interpreted as mixing of crustal remelt with minor juvenile material contribution at 2.1 Ga. These results are distinct from that for the Western Block, supporting that the Fuping Complex was emplaced in a tectonic active environment at the western margin of the Eastern Block.
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