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  • The discovery of Nb-rich volcanic rock of the Qushenla Formation in Yema area of the western segment of Bangong Co-Nujiang suture in Tibet and its implications
    WU Jianliang, YIN Xianke, LIU Wen, LEI Chuanyang, WANG Bo, LI Wei, PEI Yalun and ZHANG Wei
    The volcanic rocks from Qushenla Formation in Yema area is located in the west segment of Bangong Co-Nujiang suture zone, in reverse fault contact with the diabase slice of Bangong Co-Nujiang opiolite melange belt in the north. In this paper, detailed LA-ICP-MS zircon U-Pb dating geochronological and element geochemical studies were carried out for the basalt and andesite in the Qushenla Formation. The U-Pb dating of zircon from andesite yielded a weighted average age of 108.5±1.5Ma, indicating that the andesites were formed at the middle-late stages of Early Cretaceous. Lithogeochemical characteristics show that the Nb-rich basalts in Yema area are characterized by obvious enrichment of Nb and Al2O3, with the values of Nb being 10.5×10-6~11.1×10-6 and Al2O3 being 17.63%~17.96%, averaging 17.74%, suggesting Nb-rich, high-alumina basalt of calc alkaline series. The andesites belong to high-K calc alkaline series. All the volcanic rocks in Yama area are characterized by enriched LREE and relatively depleted HREE. The total REE of basalts are 119.20×10-6~120.49×10-6, with (La/Yb)N from 5.17 to 5.53, and the ∑REE of andesites are 179.97×10-6~184.75×10-6 and their (La/Yb)N range from 13.83 to 15.12. On primitive mantle-normalized trace element diagrams, the basalts and andesite display different degrees of enrichment of LIFEs (e.g., Rb, U, K), relative depletion of HFSE (Nb, Ta, Ti) with high content of Sr, Nb, Zr but low content of Cr, Ni, similar to features of island arc rocks. The magma source region was mainly influenced by the subduction fluid and underwent different degrees of fractionation crystallization of femic minerals with uplifting. A comprehensive study shows that the basalt and andesite of Qushenla Formation in Yema area were probably formed in the backarc tectonic rocks, which might have been the direct magma response to slab break-off caused by asthenosphere upwelling back-arcextension during the southward subduction of Bangong Co-Nujiang Tethyan Ocean at the late stage of Early Cretaceous.
    2019  .  38(4):    471-483    [摘要](76)    [PDF](56)
  • The discovery of Qushenla Formation argillaceous cherts in the western part of the Bangong Co-Nujiang suture zone, Tibet and its significance
    LIU Wen, YIN Xianke, WU Jianliang, LEI Chuanyang, WANG Bo, YIN Tao, LI Wei, YUAN Huayun, ZHANG Wei and PEI Yalun
    The Qushenla Formation argillaceous cherts, which occur along two horizons, were found for the first time in the western part of the Bangong Co-Nujiang suture. In order to discuss depositional environment and origin of argillaceous cherts and infer the closure of the western part of Bangong Co-Nujiang Tethys Ocean, the authors made a systematic analysis of petrology and geochemistry of argillaceous cherts in this study. The ratios of Al2O3/(Al2O3+Fe2O3), Ce/Ce*,(La/Ce)N, V/(Ni+V), Ce/La, Ceanom, Euanom of argillaceous cherts in the first horizon are 0.60, 0.80, 1.24, 0.72, 1.84, -0.08, 0.01 respectively, while their ratios in the second horizon are 0.65, 0.83, 1.16, 0.77, 1.97, -0.07, 0.02 respectively. Considering the diagrams of Fe2O3/TiO2 vs. Al2O3/(Al2O3+ Fe2O3),(La/Ce)N vs. Al2O3/(Al2O3 + Fe2O3) and Hf/3-Th-Ta,the authors hold that these characteristics indicate that all of argillaceous cherts tended to occur in an active continental margin setting and an anoxic environment. The diagrams of U-Th, ZnNi-Co, La-Ce and La/Yb-REE and the characteristics of REE demonstrates that the argillaceous cherts of the two horizons originated from hydrothermal sediments, closely related to basalt magmatism event, and the activities of hot water might have been more pronounced in the second horizon. These data provide petrologic and geochemical evidence for the evolution of Bangong CoNujiang Tethys Ocean. In Shiquanhe area there might have existed a certain scale ocean basin in the Early Cretaceous, which was closed at a time later than 109Ma.
    2019  .  38(4):    484-493    [摘要](64)    [PDF](45)
  • Geochronology, geochemistry and geodynamic significance of Awengcuo composite pluton and its dark microgranular enclaves in the north of Tibet
    LEI Chuanyang, WU Jianliang, YIN Xianke, LIU Wen, WANG Bo, LI Wei, YUAN Huayun, ZHANG Wei, YIN Tao and PEI Yalun
    The petrology, whole rock geochemistry and U-Pb isotopic chronology of the Awengcuo composite pluton and its dark microgranular enclaves in the western part of Bangong Co-Nujiang suture zone were studied in this paper. The results show that Awengcuo composite pluton contains abundant mafic magmatic enclaves (MMEs) comprising mainly biotitic diorite. The MMEs display igneous texture and long prisms of apatite. Host rock and MMEs are transitional rock belonging to quasi-aluminum, highcalcium alkaline-potassium kaolin series, and the main oxides in the Harker diagram show good linear relationship. Rare earth element distribution curve and normalized spider web for trace element primitive mantle exhibit a high degree of consistency. It shows that there is a strong geophysical relationship between the two groups, which experienced similar magmatic evolution processes. A good linear relationship is shown between the host rock and the inclusions in the denominator covariant map for the major and trace elements. The original magma of the inclusions might have been derived from the mantle wedge by fluid of the subduction zone, whereas the host rock is a product of mantle-derived magma mixed with the molten lower crust magma, mainly molten lower crust magma. The LA-ICP-MS single-zircon U-Pb dating demonstrates that the magmatic age is 109.1±1.0Ma for the host rock and 107.4±0.7Ma for the enclave. The two ages are basically the same, suggesting that the mixing event took place during the period of Early Cretaceous. Combined with the evolution and tectonic discrimination of Bangong Co-Nujiang suture zone, it is concluded that the Awengcuo composite pluton was formed at the soft-collision stage in the Bangong Co-Nujiang Tethyan Ocean, being a product of mantle wedge ascending with the derived mixture of subduction zone fluid and magma.
    2019  .  38(4):    494-508    [摘要](120)    [PDF](42)
  • Zircon U-Pb geochronology and geological implication of Early Jurassic ore-bearing porphyry from the Tangbai area on the southern margin of Gangdise, Tibet
    BAI Yun, LANG Xinghai, WANG Xuhui, CUI Zhiwei, XIE Fuwei, DENG Yulin, LI Zhijun, LOU Yuming, HAN Peng, YIN Qing, WANG Zizheng, DONG Shuyi, ZHANG Zhong, ZHANG Jinshu and JIANG Kai
    The Tangbai ore district, located in the western segment of the Gangdise porphyry copper belt. Its south margin is Xigaze forearc basin. The surface exploration engineering of the mining area revealed that there are three orebodies (No.1, 2 and 3) in Early Jurassic hornblende quartz diorite porphyry. On the basis of field geological investigation, the authors carried out zircon LA-ICPMS U-Pb dating and whole-rock geochemical testing of Early Jurassic hornblende quartz diorite porphyry. The results of the study are as follows. The age of ore-bearing porphyry is 183.3±1.2Ma, suggesting Early Jurassic; geochemical characteristics of ore-bearing porphyry are similar to those of andesite from the oceanic island arc setting, indicating that ore-bearing porphyry in Tangbai area was formed in the oceanic island arc setting, and the Gangdise porphyry copper belt has huge potential in the search for porphyry deposits of subduction period. It is therefore held that in the future work emphasis should be placed on the subduction stage porphyry deposit exploration and evaluation, especially on the Early-Middle Jurassic porphyry metallogenic potential evaluation.
    2019  .  38(4):    509-521    [摘要](43)    [PDF](24)
  • Magnetic fabric study of the Late Jurassic-Early Cretaceous strata in Baoji area of Lhasa block
    ZHOU Ya'nan, SHAO Ruiqi, JIANG Nan, GAO Yang, YU Liang and WU Hanning
    The Lhasa and Qiangtang blocks are at the heart of the Tibetan Plateau. However, the time for the specific combination of the two blocks is still in heated controversy. To further find geological evidence that can constrain the time limit between the two blocks, the authors studied the characteristics of the magnetic fabric of the Late Jurassic-Early Cretaceous strata in Lhasa block. According to the results obtained, the magnetic structure of Late Jurassic strata encountered strong tectonic stress, and the principal axis k1 of the magnetic susceptibility of the Tukari Formation was oblique to the ground plane. After the stratigraphic correction, the k1 azimuth and regional fold direction consensus indicate that the stress azimuth was NNE-SSW; the k1 orientation of the Sabozhibule Formation was parallel to the plane (NNE-SSW) before the stratigraphic correction, indicating the direction perpendicular to the principal compressive stress and inferring the late tilting. The Late Jurassic formation magnetic structure recorded the same period of stress, and the stress direction was NNE-SSW. The Early Cretaceous Duoni Formation magnetic structure shows that the subsequent tectonic stress field intensity was weak, and was not in the same period as the Late Jurassic stress. Therefore, by comparing the magnetic fabric characteristics of the Late Jurassic and Early Cretaceous strata, the authors concluded that the magnetic fabric of the Late Jurassic in Baoji area recorded a strong tectonic stress in NNE-SSW, and that the stress in this period originated from the LhasaQiangtang block collision event, while the Duoni Formation was not affected by the stress field, only the stress field of the regional fold uplift was recorded.
    2019  .  38(4):    522-535    [摘要](29)    [PDF](28)
  • Geochemical characteristics and LA-ICP-MS zircon U-Pb dating of volcanic rocks in Shangqihan Formation along eastern orogenic belt in West Kunlun Mountains
    SANG Mingshuai, YANG Yousheng and CHEN Bangxue
    Through a study of LA-ICP-MS zircon U-Pb isotopes of basaltic andesite in Shangqihan Formation at the east segment in the northern belt of Western Kunlun Mountains, zircon 206Pb/238U isotopic age was obtained as 473.1±2.2Ma, which represents crystallization age of basaltic andesite. Shangqihan Formation volcanic rocks in the study area were rich in large ion lithophile elements such as Sr, Ba, U and Th, but were depleted in high field strength elements such as Nb, Zr, Ta and Ti, and these features are symbolic features of volcanic rocks in the subduction belt. Ratio of incompatible elements Th/Ta is within 9.5~20.89, relatively approximate to Th/Ta ratio (10) of the upper crust. Rocks have higher La/Nb ratios (2.73~4.89), obviously higher than the ratio (2.2) in continental crust, and this indicates hybridization of crust materials. In the meantime, Nb content in rocks is high (8.14×10-6~12.3×10-6)and Nb/Ta ratio is 15.2~17.38, which indicates magmatic source region was subjected to metasomatism from subducted residual sheet bar fluid or melt. According to comprehensive analysis of the major and trace element features, petrologic features and geotectonic background, it is believed that volcanic rocks in Shangqihan Formation were formed in an island arc environment and were products of Kudi-Qimanyute Ocean which pitched down in the northward direction in Early Paleozoic.
    2019  .  38(4):    536-545    [摘要](33)    [PDF](74)
  • Detrital zircon geochronology of Neoproterozoic-Early Paleozoic sedimentary rocks in Baoshan terrane and its tectonic significance
    MA Zeliang, CAI Zhihui, QI Xuexiang, HE Bizhu and CHEN Xijie
    The Luxi area of west Yun'nan which belongs to Baoshan terrane is located on the southeastern margin of the Tibetan Plateau. It is difficult to confirm the relationship between Baoshan terrane and Tibetan Plateau due to the strong deformation in Cenozoic. Based on the field study and LA-ICP-MS U-Pb dating, the authors hold that most detrital zircons of Neoproterozoic to early Paleozoic strata in Luxi area have high Th/U ratios (>0.1), which corresponds to the features of magmatic zircons. Zircon UPb ages are distributed from Archean to Early Paleozoic, with age peaks showing strong peaks of Cambrian (562Ma), Neoproterozoic (890Ma), Paleoproterozoic (2292Ma), and some weaker age peaks of Mesoproterozoic (1676Ma) and Neoproterozoic (2500Ma). The detrital zircon age distribution pattern in Baoshan terrane is similiar to that in South Qiangtang and Tethys Himalayan sedimentary strata, indicating that they were all derived from the Indian block located on the northern margin of Gondwanaland. From late Neoproterozoic to Early Paleozoic, Baoshen terrane was located on the northern margin of India block, adjacent to the north Qiangtang and Himalayan terrane. During the accretionary orogenic process related to the subduction, the late Newproterozoic to Early Paleozoic sedimentary strata were formed in the Baoshan terrane.
    2019  .  38(4):    546-561    [摘要](28)    [PDF](23)
  • The LA-ICP-MS zircon U-Pb age of the pegmatoidal alaskite and its relationship with uranium mineralization in Hongshiquan area, Gansu Province
    QI Jiawei, ZHANG Shuming, YANG Chunsi, LAN Dechu and WANG Liling
    Alaskite is the product of low proportion melting during continental collision,which is closely related to the origin of uranium and polymetallic deposits. The pegmatoidal alaskite is the ore-bearing host rock in Hongshiquan uranium deposit. The previous research on the formation age of pegmatoidal alaskite is relatively weak and limited in accuracy, which affects the deep understanding of the genesis and tectonic background of uranium deposits. This paper focuses on high precision LA-ICP-MS zircon U-Pb isotopic dating for the ore-bearing host rock-pegmatoidal alaskite. The analytical result shows that the ages of the pegmatoidal alaskite are 2455~2500Ma, suggesting late Neoarchean-early Paleoproterozoic. It is concluded that the pegmatoidal alaskite should be the product of the late assemblage of the North China Craton microcontinent block during the late Neoarchean to early Paleoproterozoic. It is also a response to the Ca.2.5Ga magma tectonic thermal event in the western block of the North China Craton. As a result, there is no positive genetic relationship between the pegmatoidal alaskite and uranium mineralization,with only pegmatoidal alaskite being the ore-bearing host rock in Hongshiquan uranium deposit.
    2019  .  38(4):    562-572    [摘要](28)    [PDF](19)
  • Geochemistry, LA-ICP-MS zircon U-Pb ages of the Meicaogou ophiolite in East Kunlun, and its geological significance
    WANG Guoliang, LIU Jiandong, YANG Xuesong, LIU Rong, CAO Jinshan, HAN Haichen and CHEN Lijuan
    This paper mainly introduces the rock assemblage and geochemical characteristics of the Meicaogou ophiolite and attempts to discuss the tectonic background of the ophiolite which is located in Golmud City of Qinghai Province and lies in the western part of the East kunlun composite orogenic belt. The rock association is composed of metamorphic basic basalt and a small amount of diabase, gabbro, metamorphic peridotite and augite peridotite. The characteristics of main elements and trace elements indicate that the ophiolite is not related to subduction but belongs to N-MORB. In the past geological survey carried out by previous researchers, it was considered that the ophiolite was formed in Late Ordovician. The LA-ICP-MS zircon U-Pb dating of gabbro and basalt in the Weitogou ophiolite yielded the exact age data of 488.2±2Ma and 501.1±2.2Ma. The isotope data have filled the gaps of the ophiolite data in this area, and reflected the existence of remains of the ancient ocean in this area. The comprehensive regional geological characteristics suggest that the early period of the oceanic basin was at the initial stage,but at the late stage basalt was formed in an oceanic ridge expanding center environment. The Early Cambrian period was the flourishing period of the development of the oceanic basin, the late oceanic crust was formed in the late Ordovician period, and the late Silurian basin was basically closed together with later greenschist facies metamorphism.
    2019  .  38(4):    573-591    [摘要](34)    [PDF](31)
  • Evidence of zircon U-Pb dating and rock geochemical characteristics of Badangshan volcanic rocks and metamorphic quartz diorite on the northern margin of North China craton
    WU Wenbin, WANG Yuping, YU Haifeng, LI Chao, LI Haiyang, LIU Yongjun, ZHANG Yan, CHEN Lifang and WANG Xiaoliang
    Metamorphic quartz diorite intruded into the the Badangshan volcanic rocks in Langjunhala area, eastern Chifeng City. LAICP-MS zircon U-Pb dating technique constrained the age of schistosity rhyolite and quartz diorite. The results show that the ages of schistosity rhyolite and quartz diorite are 387.6±2.8Ma and 387.1±2.3Ma, aindicating the Middle Devonian magmatic activity. According to the geochemical characteristics of foliated rhyolite with high silicon, alkali, aluminum, iron, magnesium, calcium and poor peraluminous characteristics, the rocks belong to the calc alkaline series. The light and heavy REE fractionation is weak and the Eu has weak negative anomaly. The foliated rhyolite has lower Ti/Y and Zr/Y ratios, implying the source of lithospheric mantle. Metamorphic quartz diorite is characterized by high aluminium, iron, magnesium, calcium and peraluminous, belonging to calcalkaline series magma. The fractionation of light and heavy rare earth elements is weak, with weak negative Eu anomaly. The metamorphic quartz diorite is enriched in large ion lithophile elements and depleted in high field strength elements. It has obvious negative anomalies of Ta, Nb and Y, implying mixed source characteristics of crust and mantle. Tectonic environment discriminations show that it formed on the active continental margin and was the product of subduction of the oceanic plate, probably related to the extensional environment after the collision between the Bainaimiao island arc and the northern margin of the North China craton.
    2019  .  38(4):    592-602    [摘要](31)    [PDF](24)
  • Geochemistry and zircon U-Pb-Hf isotopes of Paleoproterozoic granitic rocks in Wangjiapuzi area, eastern Liaoning Province, and their geological significance
    YANG Zhongjie, WANG Wei, ZHAO Yan, ZHOU Yongheng, ZHANG Jing, SUN Shouliang and LIU Changchun
    Paleoproterozoic granites are widely distributed in the Wangjiapuzi area of eastern Liaoning Province. In the past, some granitic plutons were considered as granitic melange in this area. In this paper, the authors divided the granitic melange into two types, i.e., striate biotite monzogranites and gneissic biotite monzogranites, on the basis of comprehensive studies of field observation, petrography, geochemistry and isotope chronology. The results of geochemical analysis show that the two components are homogeneous, having uniform geochemical characteristics. They all show high SiO2\, rich K2O and poor Al2O3 features, with K2O/Na2O being 0.64~2.14. Their TiO2 values are between 0.16% and 0.3%, with lower MnO, MgO, CaO and P2O5 content. The saturation index A/CNK is between 1.06 and 1.1, and A/NK is between 1.50 and 1.62, which suggests that the granodiorites should belong to the peraluminum calcium alkaline series. Some trace elements show that high field strength elements such as Nb, Ti, Ta are strongly depleted, with an obvious anomaly of negative Eu. All the geochemical characteristics suggest that they should belong to Atype granites.The most zircon grains of striate biotite monzogranites show clear oscillating zoning structures, and LA-ICP-MS zircon U-Pb weighted mean age is 2188±13Ma(MSWD=0.49), with the age of magma crystallization representing the age of the rock. The most zircons of gneissic biotite monzogranites show obvious metamorphic edge, zircons from the part of the nucleus show clear oscillating zoning structures. LA-ICP-MS nucleus zircon U-Pb weighted mean age is 2214±16Ma(MSWD=1.01), with the age of magma crystallization representing the age of the rock. LA-ICP-MS edge zircon U-Pb weighted mean age is 1905±13Ma(MSWD=4.5), representing the age of metamorphic rock. The model ages of these two types of granites obtained from Hf isotope are respectively 2387~2584Ma and 2474~2641Ma,the average of the crustal pattern is 2495~2808Ma and 2633~2868Ma,older than the age of rock formation. The source area of Paleoproterozoic granite in the study area was mainly Archean basement mixed with a small amount of Paleozoic Neoproterozoic crust. The formation environment of the dike granite diorite is reported by some geologists. It is shown that the early rising of the Jiao-Liao-Ji orogenic/activity belts experienced a tensile cracking process at about 2.2~2.15Ga, and then there was a process of subduction and compression around 2.0Ga.
    2019  .  38(4):    603-618    [摘要](30)    [PDF](16)
  • Zircon U-Pb dating and petrogenesis of Early Cretaceous alkaline rhyolites in Tuquan Basin of middle Da Hinggan Mountains
    SONG Weimin, DU Jiyu, NA Fuchao, PANG Xuejiao, YANG Jialin, LIU Yingcai, SUN Wei, TAO Nan and GE Jintao
    The LA-ICP-MS zircon 206Pb/238U age estimates for the alkeline rhyolites yielded weighted average ages of 135±1Ma and 133±1Ma, indicating Early Cretaceous. The whole-rock geochemistry of the alkeline rhyolites shows an A-type magmatic affinity, with a typical enrichment of SiO2, K2O and a high ratio of TFeO/MgO, and low content of Al2O3, CaO and MgO, belonging to Khigh calc-alkaline series rocks; high ∑REE and fractionated REE patterns show strong negative Eu anomalies, relative enrichment of Rb, U, Zr, Hf, but depletion of Sr, Ti, Nb, Ta. In the discrimination diagrams of granites, all of the samples show the characteristics of A-type granites; in the Nb-Y-3Ga diagram, the samples show the characteristics of A2-type granites. These elements show that the alkeline rhyolites were derived from partial melting of crust materials. Viewed from regional correlations with the volcanic rocks as well as A-type granites, it is held that the alkeline rhyolites were associated with an extensional tectonic setting, being the product of a regional extensional environment associated with the evolution of Mongol-Khotsk suture. Zircon Hf isotope analyses show that dacite of Cretaceous have εHf(t) values of +4.2~+10.7 with Hf modal ages being 0.74~0.76Ga,indicating that there existed similar crustal processes between Xing'an block and Songnen block.
    2019  .  38(4):    619-631    [摘要](17)    [PDF](16)
  • Zircon U-Pb age and geochemical characteristics of Ordovician pluton in Geleaobao, Inner Mongolia
    CAO Lei, SU Maorong, ZHOU Fei, LI Lei and YANG Baohong
    The Geleaobao area of Inner Mongolia is located in the Xingmeng orogenic belt. By studying the Ordovician magmatic rocks in this area, researchers can further understand the structure and evolution of the Early Paleozoic strata in this orogenic belt. The ages of the LA-ICP-MS zircons of syenite and mottled monzogranite samples studied in this work are 441.6±3.7Ma to 454.9±4.6Ma. The age of formation is Ordovician. Geochemical analysis shows that the pluton has high-K calc-alkaline characteristics. Aluminum saturation index shows that it is peraluminous granite. The characteristics of trace elements in rocks indicate that they are enriched in large ionic lithophilic elements (LILEs) Rb, Th, U, K, depleted in Ba, Sr; enriched in high field strength elements (HFSEs) La, Ce, Nd, Zr, and depleted in Nb, P, Ti. The total amount of rare earths in the rocks is 38.7×10 -6 to 120.23×10 -6, and the light rare earths exhibit obvious differences in heavier rare earths, with higher fractionation and weaker negative Eu anomalies. Based on regional geological data, it is believed that the source region of the Ordovician magma in the Geleaobao area is partially melted upper crustal materials, and the formation environment was a back-arc basin. After the subduction of the ancient Asian Ocean to the north, it experienced an island-arc magmatic process cycle, being product of extension stage.
    2019  .  38(4):    632-642    [摘要](29)    [PDF](19)
  • Geochemistry and petrogenesis of the volcanic rocks from the Abaga Formation in Sonid Left Banner, central Inner Mongolia
    ZHANG Xiangxin, GAO Yongfeng and LEI Shihe
    The volcanic rocks of the Abaga Formation in Sonid Left Banner area of central Inner Mongolia are mainly composed of andesites. These andesites have high alkali, potassium and aluminum, but low magnesium, and are obviously enriched in LILEs such as Rb, Ba, U, K and LREE, and depleted in HFSEs such as Nb, Ta, Ti with obvious negative anomalies. They have high ∑REE values, and show intense REE fractionation with LREE enrichment and weak negative Eu anomalies. Their geochemical characteristics suggest that they belong to potassic volcanic rocks, and were likely generated from a small degree of partial melting of phlogopite-bearing garnet lherzolite in an enriched sub-lithospheric mantle. The enriched sub-lithospheric mantle was metasomatized by fluids released from the stagnant Pacific slab in the mantle transition zone. Geochemical features also reveal that fractional crystallization with the removal of plagioclase and ferromagnesian minerals played an important role in the evolution, and the potassic magma was not affected by crustal contamination as it passed through the thick continental crust. These potassic rocks were formed in an intraplate extensional tectonic setting resulting from the rollback of the stagnant Pacific slab in the mantle transition zone.
    2019  .  38(4):    643-655    [摘要](23)    [PDF](20)
  • Detrital zircon U-Pb dating of Maihanhada Formation in Yin'E Basin of northern Suhongtu depression and its geological significance
    ZHANG Hui, WANG Nashen, MENG Xiangzhen and MENG Zhan
    The Babei Sag is a secondary tectonic unit in the north of Suhongtu Depression in Yingen-Ejinaqi Basin(or Yin'E Basin). Based on the sporopollen assemblages and U-Pb isotope tests of clastic zircons of the Maihanhada Formation, the stratigraphic age and provenance direction are studied. The characteristics of sporopollen assemblages show that except for the types existing in the Late Paleozoic and Mesozoic, the types appearing only in the Cretaceous are not found, and most of the types appearing in the Late Paleozoic and Triassic. The CL image and Th/U(>0.4)of zircons in the clastic rocks of the Maihanhada Formation in the Northern Suhongtu depression, Yin-E basin, indicate that most of them belong to the zircons of magma, and the age distribution of zircons is between 246Ma and 1010Ma,the concentrated distribution is the 246~390Ma, 400~560Ma. The youngest peak is 260Ma. The geological age of the Maihanhada Formation should be Permian. And the age distributions of the magmatic rocks in the area of the dagger and the Bayannuoer-Langshan-Yinshan are similar, so the two are the possible sedimentary material sources of the Maihanhada Formation.
    2019  .  38(4):    656-666    [摘要](19)    [PDF](15)
  • Detrital zircon U-Pb dating of the Late Cretaceous aeolian sandstones from the Tangbian Formation in the Yiyang area of Jiangxi Province and its provenance significance
    WANG Yujia, CHEN Liuqin, LI Wenhao and LI Pengcheng
    The Tangbian Formation of the Late Cretaceous Guifeng Group in the Xinjiang Basin of northeastern Jiangxi Province is characterized by large-scale cross-bedded sandstones. They are generally products of a desert depositional environment under arid climate. The sandstone samples are purple red, fine-to medium-grained feldspar quartz sandstones. Sand particles are mainly composed of quartz, feldspar, mica and rock fragments, which are subangular to subrounded and well sorted. In order to analyze the source rocks of the Tangbian Formation, LA-ICP-MS zircon U-Pb analysis was performed. The zircon grains are mostly larger than 100μm. The results show that the Th/U ratios range from 0.1 to 2.1, and most of the zircons have a clear internal shock band structure, indicating that most of them were magmatic zircons. The mean age of 206Pb/238U is 141.4±1.2Ma. The age of zircon U-Pb ages are mainly concentrated in five zones of 137~170Ma, 431~506Ma, 798~1172Ma, 1268~2493Ma, 2501~3488Ma, corresponding to the major magmatic events in Southeast China, namely Yanshanian, Caledonian, Jinning, and Lvliang movements, with the Yanshanian volcanic-intrusive complex providing the dominant sediments. In combination with the paleo-wind directions from the aeolian sandstones, the sediments are interpreted to be largely from the northwestern Jiuling mountain area and the northeastern areas.
    2019  .  38(4):    667-679    [摘要](39)    [PDF](18)
  • Phlogopite 40Ar/39Ar dating of the lamproite veins in Du'an County, central Guangxi and its tectonic significance
    WANG Xinyu, WU Xiangke, HE Chuan, SHI Yu, LIU Xijun, FU Wei and WANG Youchen
    The Youjiang Fold Belt is located in the connection region between Yangtze Block and Cathaysia Block. It is characterized by Late Yanshanian bimodal magmatic activities and consists of basic-ultrabasic dyke swarms and ultra-acid rock veins. In this study, the phlogopite 40Ar/39Ar dating of phenocryst and matrix from phlogopite lamproite veins, which are located in the Du' an County of central Guangxi, yielded two plateau ages of 93.6±0.6Ma, 92.9±0.6Ma and two isochron ages of 93.4±0.9Ma, 92.7±0.8Ma, respectively. These ages are interpreted as the emplacement age for phlogopite lamproite veins. The new results obtained by the authors in junction with the coeval magmatic events further reveal that the voluminous Late Cretaceous bimodal magmatic activities occurred in the Youjiang Fold Belt as well as its peripheral regions in response to the Late Yanshanian tectonic transition and subsequent lithospheric thinning and extension in South China.
    2019  .  38(4):    680-688    [摘要](17)    [PDF](23)
  • The new volcanics record in Sanshui Basin and its relationship with the spreading of the South China Sea
    YUAN Xiaobo and FANG Nianqiao
    Sanshui Basin is the only basin that has Cenozoic volcanic records in the northern adjacent land area of the South China Sea, and the age of the last volcanic eruption in this basin is 38Ma. This age is also the last eruption age before the South China Sea spreading in its northern land. Based on the K-Ar isotope dating method, the basalts (29.27±1.52Ma) and rhyolite (28.25±1.14Ma) were first found in Sanshui Basin. This bimodal volcanic rock that represents intraplate rifting tectonic setting postponed the volcanic records from Palaeocene-Middle Eocene to Mid-Oligocene. As we all know, the initial age of the South China Sea spreading is about 32Ma. So, it has great significance to answer if there is the relationship between the South China Sea spreading and its surrounding land volcanic activity. Meanwhile, the findings have important significance for establishing early spreading evolution model of the South China Sea.
    2019  .  38(4):    689-695    [摘要](16)    [PDF](24)

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