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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 [查看] 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.

Zircon U-Pb dating for the earliest Neoproterozoic mafic magmatism in the southern margin of the North China Block [查看] LIUYongqing、GAOLinzhi、LIUYanxue、SONGBiao、WANGZongxiu

Huge diabase occurs within the Pro-terozoic strata in the southern margin of the North China Block, geographically in the northern parts of Anhui and Jiangsu provinces, where no strata younger than Neoproterozoic have been found. Zir-cons from the diabase show three different structures from cathodoluminesecence imaging, but they are all of magmatic origin. SHRIMP U-Pb dating yields a range of 206Pb/238U ages from 2600 to 124 Ma, but two geological events can clearly be identified from both concordant and discordant ages. One gives two weighted mean 207Pb/206Pb ages of 976 ± 24 and 1038 ± 26 Ma which are interpreted as timing of dia-base crystallization, and the other has a weighted mean 207Pb/206Pb age of 2523 ± 52 Ma that repre-sents formation time of inherited zircon from the old crust. The dating results indicate that the mafic magmatic event occurred in a transition period from the latest Mesoproterozoic to the earliest Neopro-terozoic, consistent with global subduction of Grenvill ocean crust to result in the assembly of superconti-nent Rodinia. Therefore, the North China Block needs to be taken into account when reconstructing the position of China continents within the framework of Rodinia supercontinent.

Zircon SHRIMP U-Pb dating for olivine gabbro at Wangmuguan in the Beihuaiyang zone and its geological significanc [查看] LIUYican、LIShuguang、GUXiaofeng、HOUZhenhui

Zircon SHRIMP U-Pb dating was per-formed for olivine gabbro at Wangmuguan in the Beihuaiyang zone of the Dabie orogen and its coun-try rock (garnet-bearing epidote-mica-quartz schist). The results show that the gabbro was crystallized at 635 ± 5 Ma, in the late Neoproterozoic rather than in the late Paleozoic as previously suggested; its coun-try rocks formed at 464±7 Ma, younger than the en-closed gabbro. The U-Pb age for the gabbro is in good agreement with ages for tuff interbedded with sediments from the Doushantuo Formation in the South China Block and late-Neoproterozoic basic dyke swarms distributed on a large scale over areas of Suizhou to Zaoyang of Hubei Province in the northern margin of the South China Block. This sug-gests a large-scale magmatic activity occurred at the late Neoproterozoic in the South China Block, so that the gabbro at Wangmuguan in the western segment of the Beihuaiyang zone is geotectonic affinity to the northern margin of the South China Block. Since the olivine gabbro occurs within the schist of Ordovician protolith with tectonic contact between them but forming in different tectonic settings, it is concluded that the late-Neoproterozoic gabbro was detached from the Precambrian basement of the South China Block during the Triassic subduction of the South China Block, and tectonically thrusted over the metamorphosed rocks in the southern margin of the North China Block.

U-Pb zircon age for the Daohugou Biota at Ningcheng of Inner Mongolia and comments on related issues [查看] LIUYongqing、LIUYanxue、JIShu’an、YANGZhiqing

SHRIMP U-Pb zircon dating was carried out for the Daohugou Biota near Ningcheng of Inner Mongolia and for lavas overlying or underlying sala-mander-bearing strata at Reshuitang in Lingyuan of West Liaoning. The results suggest that the Dao-hugou Biota occurred at an interval from 168 Ma to 164-152 Ma. Both the Daohugou Biota and the salamander-bearing fossil assemblage are the same biota and thus developed from 168 to 152 Ma, i.e. from late Middle Jurassic to the early Late Jurassic. The Daohugou Biota-bearing rocks, resting on the Jiulongshan Formation in disconformity and being overlain in unconformity by Late Jurassic Tuchengzi Formation and Early Cretaceous rocks containing the Jehol Biota, are mainly composed of volcanic-sedi-mentary rocks in a normal sequence. It is recom-mended that the Daohugou Biota and the related stratigraphy should be correlated with the Tiaojishan Formation (Lanqi Formation in West Liaoning) or its synchronous rocks. It is suggested that the Dao-hugou Biota and the Jehol Biota would be neither taken into one biota nor considered as the earliest elements of the Jehol Biota. The Daohugou Biota and the related rocks and the Yixian Formation were re-spectively formed in different periods of vol-canic-sedimentary tectonics.

SHRIMP zircon U-Pb dating for subduction-related granitic rocks in the northern part of east Junggar, Xinjiang [查看] ZHANGZhaochong、YANShenghao、CHENBailin、ZHOUGang、HEYongkang、CHAIFengmei、HELixin、WANYusheng

SHRIMP U-Pb zircon dating on the Xileketehalasu granodiorite porphyry and Kalasayi monodiorite porphyry that intrude middle Devonian Beitashan Formation at the north part of east Junggar region shows that they were formed at 381±6 Ma and 376±10 Ma respectively. They are interpreted as subduction-related granitic rocks, which is the first report that the isotopic ages for the granitic rocks range from 350 to 390 Ma. Another determined age for the Kalasayi monodiorite porphyry is 408±9 Ma,representing the age of underlain Lower Devonian volcanic rocks. Thus, the U-Pb dates suggest that the northeastward subduction of Junggar ocean from southwest occurred at 408 to 376 Ma (the real inter-val may be larger). Because the ore bearing porphyry intruded following the formation of the volcanic rocks of middle Devonian Beitashan Formation, their tec-tonic setting is similar to the Andes Mountains that hosts world-class porphyry copper deposits, and the researched area could be regarded as a potential area for prospecting large porphyry copper deposits.

SHRIMP zircon U-Pb dating an d its geological significance of Chibaisong gabbro in To nghua area, Jilin Province, China [查看] PEIFuping、XUWenliang、YANGDebin、ZHAOQuanguo

SHRIMP zircon U-Pb dating for Chibaisong No.1 gabbro in the Tonghua area, Jilin Province, is discussed in the paper. The cathodolumines cence (CL) images of the zircons indicate that they can be divided into two major types, i.e. euhedral-subhedral one with striped absorption and round one with obvious oscillatory zoning rims. The dating results of the zircons suggest that Chibai-song gabbro in the Tonghua area was formed at 134±7Ma--the Early Cretaceous. The concordia ages of 2497 ±13Ma, 787 ±35Ma, 321 ±10Ma, 217 ±11Ma of zircons imply that the Tonghua area might geologically be located in the convergent region of the Yangtze Block (YB) and the North China Block (NCB) in the early Mesozoic and the zircons should be caught during magma intrusion. The existence of the Early Cretaceous basic dyke swarm in the Tonghua area shows that an intensive extensional environment occurred in the Early Cretaceous. It is suggested that the magmatism and lithospheric extension in the Tonghua area should be related to the subduction of the Pacific plate.

Polyphase tectonothermal history recorded in granulitized gneisses from the North Qaidam HPUHP metamorphic terrane,Western Chinaevidence from zircon U-Pb geochronology [查看] ZhangJianxin、ChristopherG.Mattinson、MengFancong、WanYusheng、Dongkuoan

Petrogenesis and significance of the Hongshan syenitic pluton,South Taihangzircon SHRIMP U-Pb age,chemical compositions and Sr-Nd isotopes [查看] ZHOULing、CHENBin

The Hongshan syenitic pluton(South Taihang)is mainly made up of hornblemde syenite and minor granite.SHRIMP zircon dating yields a U-Pb age of 135±2.7Ma for the emplacement of the pluton,while biotite Rb-Sr isotopic data give an isochron of 120.3±2.4Ma.Hongshan syenites show low silica,high alkalis and LILE such as Sr,and exhibit negligible Eu anomalies or slightly positive anomalies in the REE patterns.The syenites show quite enriched isotopic compositions with Isr from 0.7052 to 0.7102 andεNd(t)from -7.5 to -11.1.Petrogenesis of the pluton can be that partial melting of an enriched lithospheric mantle gave birth to an alkali basaltic magma,which subsequently underplated in the lower crust and experienced a coupled fractionation of ferromagnesian phases like pytoxene and hornblende and minor(〈10%)contamination of lower continental crust,producing the Hongshan syenites that partially evolved into granite through combined fractionation of hornblende and feldspar.our data are not in agreement with a previous model that the syenites originated from melting of lower crust in a thickened crust circumstance,and thus do not support the speculation of "East China Plateau"in the Mesozoic

Magmatic Event at the End of the Archean in Eastern Hebei Province and Its Geological Implication [查看] GENGYuansheng、LIUFulaiandYANGChonghui

By using the SHRIMP U-Pb and single zircon stepwise evaporation methods, the authors have obtained some results for granitoids from eastern Hebei Province. The Yuhuzhai hyperthene tonalitic granite was formed 2550 Ma ago, the Qingyangshu gabbroic gneiss 2536 Ma, the Yinmahe granodioritic gneiss near Lücao, Lulong County, 2533 Ma, the gabbro-dioritic gneiss near Longwan, Qianxi County, 2518− 2515 Ma, the Qiuhuayu trondjemitic gneiss at Zunhua 2515 Ma, the Xiaoguanzhuang tonalitic gneiss at Zunhua 2495 Ma, and the Cuizhangzi gneiss in Qianxi County 2492 Ma. These geochronilogical data demonstrate that, though diverse in composition, type and origin, the granitic gneisses in eastern Hebei Province were emplaced and crystallized in a rather short period of magmatic activity. The formation of such a great amount of gneisses in this small time gap suggests that it was a critical crust accretion stage at the end of Ne oarchean. The fact that granitoids of various types occurred at the same time implies a large-scale underplating (mantle plume) activity, which was then responsible for the crust accretion.

Identification of Hercynian shoshonitic intrusive rocks in central Hainan Island and its geotectonic implications [查看] XIECaifu、ZHUJinchu、DINGShijiang、ZHANG

An identification has been made of some shoshonitic intrusive rocks in central Hainan Island recently. These rocks are K-rich (K2O=2.9%-5.1%, K2O/Na2O=0.95-2.12), distinctly enriched in LILE and LREE, strongly depleted in Nb, Ta, and moder-ately depleted in Sr and Ti, with (87Sr/86Sr)i = 0.70859-0.71425 andεNd(t)=(− 2.77―− 7.49). They were derived from an EMⅡ-type mantle source. The enrichment process is related to metasomatism of depleted mantle caused by a great amount of fluid-melt released from oceanic crust and terri-genous sediments at great depth (eclogite facies) during the subduction of the South China plate under the Indochina-South China Sea plate in the Carbon-iferous–Early Permian. A SHRIMP U-Pb zircon dating yields a crystallization age of 272±7 Ma for the sho-shonitic intrusions, which is coeval with the strongly peraluminous granites found in central Hainan Island. These two kinds of rocks generally possess syn-intrusion ductile deformation structures. Thus they are considered to have been generated during the early stage (syn-thrust phase) of a post-collisional event. The primary magma of shoshonitic rocks was produced at a depth > 80 km by decompression-dehydration melting of previously enriched litho-spheric mantle wedge, phlogopite-bearing garnet peridotite, which was in turn caused by the break-off of a descendent slab and upwelling of a hot as-thenosphere. The rising of melts was accompanied by crustal contamination and crystallization fractiona-tion (AFC). Combining with other related data, it is proposed that the southwards subduction and amal-gamation of the South China plate with the Indo-China-South China Sea plate took place at ca. 287－278 Ma, which was a part of the convergence proc-ess of the Pangea supercontinent. The suture zone was probably located along the line of Song Ma-Beibu Gulf-north margin of the Yunkai Moun-tains-Wuyi Mountains.

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