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  • Geochemistry, zircon U-Pb age, Lu-Hf isotopes and tectonic setting of the Early Paleozoic gneissic granites from the Nyainrong microcontinent, Tibet Plateau
    HU Peiyuan, ZHAI Qingguo, TANG Yue, ZHU Zhicai, WANG Wei
    The geochemistry of the Cambrian gneissic granite from the Nyainrong microcontinent, central Tibet, and its zircon LA-ICP-MS U-Pb ages and Hf isotope data are reported.The granite was formed at 516±3 Ma(n=20, MSWD=1.60), and belongs to high-K calc-alkaline I-type.It is characterized by negative Eu anomaly and right-dip chondrite-normalized rare-earth element patterns, enrichment of Rb, Th, and Pb, and depletion of Ba, Nb, Ta, Sr, P, and Ti.It exhibits negative zircon εHf(t) values of -3.0~-0.1 and relatively high Mg# values of 32~47, and yields old zircon Hf model age of 1472~1659 Ma, suggesting a magma source of Proterozoic sedimentary materials modified by mantle-derived magma.Based on the above study results and regional geological setting, it is suggested that the Cambrian gneissic granite from the Nyainrong microcontinent could be attributed to one part of the Andean-type magmatic arc along the northern margin of the Gondwana supercontinent.
    2021  .  40(8):    1203-1214    [摘要](53)    [PDF](54)
  • Discovery of the Paleoproterozoic metamorphic basement in the Tuomoerrite area of North Qaidam: Constraint on the location of Early Paleozoic suture
    FU Changlei, YAN Zhen, WANG Bingzhang, CHEN Lei, LI Junhui
    The Paleoproterozoic gneiss and amphibolite were recognized in the Tuomoerrite area, eastern part of the North Qaidam ultra-high pressure metamorphic belt.Their tectonic affinities have significance for the understanding of regional tectonic framework.In the Tuomoerrite area, there are abundant early Paleozoic volcano-sedimentary rocks, ophiolitic rocks and intrusives, with minor gneiss and amphibolite lens discovered in the northeast part.The protoliths of gneiss and amphibolite are granidiorite and basic rock respectively.LA-ICP-MS U-Pb dating of zircon from gneiss and amphibolite yields ages of 2413±28 Ma(upper intercept age, n=29, MSWD=2.3), 1966±46 Ma(weighted mean 207Pb/206Pb age, n=4, MSWD=1.7), and 1922±22 Ma(upper intercept age, n=29, MSWD=1.3) respectively.Combined with cathodoluminescence images of zircon, 2413±28 Ma represents the magmatic crystallization age of the protolith of the gneiss.1966±46 Ma and 1922±22 Ma represent the metamorphic ages of the gneiss and amphibolite respectively.The gneiss shows typical adakitic geochemical feature, with high Sr, low Y, MgO, Cr, and Ni.The magmatic zircons have εHf(t) values between -0.8 and +4.0 and yield two-stage Hf model ages ranging from 2704 Ma to 2965 Ma.They also have positive εNd(t) ranging from +0.8 to +1.6.These geochemical and isotopic characteristics indicate that the gneisses were likely formed by partial melting of thickened mafic lower-crust.The rock assemblage, geochemical features and ages of these metamorphic rocks are consistent with those of the Delingha Complex to the north, indicating that they belong to the northern Oulongbuluke block rather than the North Qaidam belt.This understanding further constrains that the suture of the North Qaidam belt lies between the Saibagou ophiolite and the newly recognized Paleoproterozoic metamorphic rocks.
    2021  .  40(8):    1215-1230    [摘要](34)    [PDF](36)
  • Late Jurassic magmatism in the Neo-Tethys Ocean: Evidence from zircon U-Pb ages and geochemistry of dolerites in the Bainang Terrane, southern Tibet
    YANG Shengbiao, YANG Jingsui, LI Yuan, BO Rongzhong, ZHANG Jian, LI Ruibao, LIU Chengjun, LIU Fei, XIONG Fahui, CHEN Xiaojian
    The Yarlung Zangbo suture zone(YZSZ) in southern Tibet marks the collision between the Eurasia plate and the Indian subcontinent, and the tectonic affinity of the ocean island-type(OIB-type) basaltic rocks within this suture remains controversial.The Bainang terrane in the middle segment of the YZSZ is a NE-SW-oriented tectonic slice, which is composed of radiolarian chert, siliceous mudstone, siliceous mudstone, shale, tuff, micritic limestone, basalt, dolerite and gabbro.The dolerite and gabbro are dikes or sills intruding into strata.The dolerites formed in the Late Jurassic, with a concordant zircon 206Pb/238U age of 150.3±0.8 Ma(n=39, MSWD=1.8).They are chemically characterized by LREE enrichment, variable enrichment of large ion lithophile elements(LILEs, e.g.Rb, Sr and Pb) and high field strength elements(HFSEs, Th, Nb, Ta, Zr, Hf, and Ti), indicating OIB affinity with little or no continental crust contamination.These features are similar to Late Jurassic-Early Cretaceous OIB in the YZSZ.It is suggested that these OIB rocks were derived from the intraplate environment within the Neo-Tethys and represented the remnants of seamounts.In combination with regional geological material, it is proposed that the Jurassic mantle plume in Neo-Tethys might drive the oceanic lithosphere northward subduction under the Lhasa terrane, and formed the Andean continental margin.During the Early Cretaceous, the Neo-Tethys oceanic lithosphere was decoupled from the overlying plate, and retreated southward to induce the Gangdese forearc extension to form the present ophiolites preserved in the YZSZ.
    2021  .  40(8):    1231-1246    [摘要](32)    [PDF](34)
  • Spreading ridge subduction of Bangong-Nujiang Ocean Evidence from geochemistry and Sr-Nd isotope of Middle Jurassic gabbro dikes in the Zongbai accretionary complex
    LIU Fei, LI Guanlong, BO Rongzhong, YANG Jingsui
    The Dingqing ophiolite, outcropped in the eastern segment of the Bangong-Nujiang suture zone, is divided into eastern part covering 400 km2 and western part covering 150 km2 by the Zongbai accretionary complex.The Zongbai accretionary complex is composed of the allogeneic Yazong mélange and the overlying autochthonous epicontinental clastic sedimentary rocks.The Yazong mélange consists of low metamorphic rock zone, conglomerate zone, basaltic tuff interlayered with thin layers of argillaceous siliceous rock, and argillaceous shale interbedded with thin layers of greywacke.The argillaceous rock is intruded by Middle Jurassic gabbro dikes with bilateral chill margins and striking 90°~110°.Compositions of major, rare earth and trace elements of all gabbro samples are between E-MORB and OIB.Combined with Nd isotope data of εNd(t) (1.88~2.41) and Sr data of (87Sr/86Sr) t(0.70912~0.70919), it is suggested that the gabbroic magmas are the product of the mixture of OIB-type melts rich in volatile and incompatible elements at the top of the seismic low-velocity zone(LVZ) and N-MORB-type melts originated from the lower part of LVZ.The gabbros were formed in a slab window setting during the interacting of the forearc spreading ridge within a subduction zone.
    2021  .  40(8):    1247-1264    [摘要](32)    [PDF](45)
  • Petrogenesis of anorthosite in the Laguoco ophiolite western part of the Bangong-Nujiang suture zone and its constraint to the evolution of the Meso-Tethys Ocean
    TANG Yue, ZHAI Qingguo, HU Peiyuan, WANG Wei
    The Laguoco ophiolite is distributed in the Gaize area, north Tibetan plateau, and is one of the most complete and well-preserved ophiolites in the Shiquanhe-Yongzhu belt. The petrological, zircon U-Pb geochronology and geochemistry of anorthosite intruding gabbros are studied to manifest the genesis and tectonic affinities of the Laguoco ophiolite. The anorthosite was emplaced within gabbros mainly as irregular dikes or lenses, with zircon U-Pb ages of 162±1 Ma, slightly later than mafic rocks of the ophiolite. All zircons yield remarkable positive εHf(t) values (+15.8~+19.7), indicating that the magma was derived from a long-term depleted mantle source. All anorthosites are characterized by low SiO2, TiO2 and high CaO contents, MORB-like REE patterns with low trace element concentrations, enrichment of Rb, Ba, Th and Sr, and depletion of Nb, Ta and Zr. Moreover, these samples show high Nb/La but low Th/Nb and Ba/Nb ratios, which suggests their magma was affected by enriched mantle with variable inputting of subduction materials. The comprehensive analysis suggests that the Laguoco anorthosite was derived from high degree partial melting of depleted mantle source with re-enrichment of plume. The primary magma deriving from (enriched) mantle wedge rapidly ascended into oceanic crust in company with the crystallization differentiation of Mg-Fe oxides. Then, residual Ca, Al-bearing magma injected into the gabbros and formed anorthosite with the decrease of pressure. Combining with regional geology, it is suggested that the Laguoco ophiolite was formed in a subduction-related setting during the Jurassic.
    2021  .  40(8):    1265-1278    [摘要](28)    [PDF](47)
  • Geochemical analysis and amphibole 40Ar-39Ar dating for meta-basalts from Tongka ophiolitic mélange, eastern Nujiang belt
    LI Huaqi, LI Tianfu, JI Fengbao
    The nature and early evolution of the ancient ocean basin represented by the Bangongco-Nujiang suture zone in the central Qinghai-Tibetan Plateau are still controversial.To identify the nature of the Early Jurassic tectono-magmatic events and micro-continental tectonics and orogenic processes along the mid-eastern Nujiang suture, geochemical study on meta-basaltic rocks and 40Ar-39Ar dating of amphibole from the Tongka ophiolitic mélange, eastern Nujiang belt, were conducted.The meta-basalts in the Tongka ophiolitic mélange witnessed strong metamorphic deformation, indicating the mineralogical composition of the epidote-amphibolite facies.Rare earth element(REE) patterns of the meta-basaltic rocks on the chondrite-normalized REE diagram display LREE depletions indicative of N-MORB, whereas on the N-MORB-normalized spider diagram, the samples indicate Rb, Ba, Th, U, Pb and Sr enrichments and Nb and Ta depletions, which suggest a mixing characteristic of arc and N-MORB, and an origin from back-arc basin spreading center, indicating a SSZ affinity.The 40Ar-39Ar dating of the amphibole from a meta-basalt sample yields a plateau age of 161 ±2 Ma, analogous to 40Ar-39Ar cooling ages(165~167 Ma) of biotite from the Tongka and Amdo gneisses, which provides direct evidence from ophiolite emplacement for local closure of the eastern Nujiang Ocean and Early Jurassic collision along the Tongka-Exue suture between the Jiayuqiao and Tongka micro-blocks.On the basis of comprehensive analyses of the early and late Jurassic magmatic rocks north of the Bangong-Nujiang belt, the Bangong-Nujiang belt is assumed to have distinct evolutionary histories, subduction polarities and collisional timing at different segments and ophiolitic branches.The multi-island-accretive orogeny model suggests that the orogeny in the hinterland of the Qinghai-Tibet Plateau might be the result of varying level convergences by diverse-sized terranes/blocks with consumption of different oceanic basins, rather than the re-opening process of the Meso-Tethyan Ocean after the closure of the Paleo-Tethyan Ocean.
    2021  .  40(8):    1279-1290    [摘要](24)    [PDF](82)
  • Tectonic attribute of the Shiquanhe-Namco ophiolitic belt: Constraint from geochemistry of the island-arc basalts in the Asa mélange zone, central Tibet
    ZENG Xiaowen, WANG Ming, LI Hang, ZENG Xianjin, SHEN Di
    The tectonic setting of the Shiquanhe-Namco ophiolitic belt in central Tibet is a key to understanding the Mesozoic tectonic evolution of the Meso-Tethys Ocean.Based on the analysis of major and trace element compositions of island arc basalts in the Asa mélange zone, central Tibet, its tectonic significance was discussed.Field observation and microscopic study indicate that they were formed in submarine eruption environment.Whole-rock geochemical analyses suggest that these rocks belong to calc-alkaline rocks, rich in light rare earth and Ba, Th, Pb large-ion lithophile elements, and depleted in Nb and Ta, with clear differentiation of light and heavy rare earth elements and flat distribution of heavy rare earth elements.Geochemical analysis shows that the rocks were originated from partial melting of depleted mantle metasomatized by melt of subducting sediments and formed under the oceanic island arc environment.Combined with previous work, it is suggested that there are some island arc basalts widely preserved in the Shiquanhe-Namco ophiolitic belt.The existence of the island-arc magmatic rocks within the Shiquanhe-Namco ophiolitic belt reveals that the Shiquanhe-Namco ophiolitic belt cannot be interpreted as an ophiolite in a single back-arc setting but contains magmatic products under various tectonic settings.The results show that the Shiquanhe-Namco ophiolite belt does not only retain the remains of backarc basin, but also contains the magmatic products of various genesis in the subduction environment.
    2021  .  40(8):    1291-1301    [摘要](31)    [PDF](51)
  • Geochronology, magmatic genesis and tectonic setting of the Musidanjiao tufflava in the Asa area, Tibet
    LI Hang, WANG Ming, ZENG Xiaowen, ZENG Xianjin, SHEN Di
    In order to further explore the evolution history of Cretaceous magmatism in the Asa area, central Tibet, samples were collected from Early Cretaceous rhyolitic tufflava near Musidanjiao, south of the Asa area, to carry out the geochronological and geochemical studies.The mean zircon 206Pb/238U age of Musidanjiao tufflava yields 129.3±1.9 Ma(n=17, mswd=0.039).The geochemical test results show that the tufflava has extremely high SiO2 content(76.27%~78.70%), moderate Al2O3(11.23~12.21%), total alkali content of 5.60%~6.30%, very low total iron(1.16%~1.50%), CaO(0.62%~1.40%), MgO(0.19%~0.24%), TiO2(0.09%~0.15%), and P2O5(0.02%~0.02%) contents.Moreover, they show enrichment of large ion lithophile elements such as U, Th and Pb, and depletion of high field strength elements such as Nb, Ta and Ti, as well as enrichment of LREE, flat HREE and obvious negative Eu anomaly.These characteristics indicate that they might be originated from the partial melting of middle crust and experienced significant crystallization differentiation in the later period, which is geochemically similar to the typical I-type granitoids developed in the continental margin arc region.In combination with the reported continental margin island arc magmatism of~120 Ma and the extensional magmatism of~105 Ma in the Asa area, it is suggested that the Early Cretaceous rhyolitic taffflava magmatism of~130 Ma in the Asa area might be formed in the background of the southward subduction of Bangong-Nujiang Ocean.
    2021  .  40(8):    1302-1313    [摘要](28)    [PDF](34)
  • Geochronology, geochemistry and tectonic setting of the Dianzhong Formation volcanic rocks in the Riduo area, Eastern Gangdese belt, Tibet
    LU Lin, PAN Liang, ZHOU Bin, HAN Kui, QIAO Xinxing
    The Linzizong Group is a typical Paleogene volcanic assemblage in the Gangdise belt, which records the orogenic history of subduction and collision between India and Eurasia.Zircon U-Pb chronology and Lu-Hf isotope studies were conducted on the Dianzhong Formation basic and intermediate volcanic rocks at the bottom of the Linzizong Group in the Riduo region of the Eastern Gangdese belt.LA-ICP-MS zircon U-Pb dating results show that ages of the Dianzhong Formation basalt and andesite crystalline tuff are 57.9±1.2 Ma and 57.4±1.2 Ma respectively, indicating that they were formed in the Late Paleozoic.Geochemically, the volcanic rocks generally belong to high-K(calc-alkaline) series.The basic volcanic rocks have low SiO2(48.67%~49.34%), high Al2O3(15.25%~18.59%) and high MgO(3.76%~8.69%).They are characterized by LREE enrichment and positive Eu anomalies(δEu=1.15~1.37).The intermediate volcanic rocks have high SiO2(54.92%~64.16%), high Al2O3(15.85%~16.72%), high K2O(0.65%~2.14%) and low MgO(1.34%~3.67%).They are relatively enriched in LREE and weak negative Eu anomalies(δEu=0.77~0.92).Both volcanic rocks exhibit fractionated REE and rightward patterns, strong enrichment of LILE(Rb, Ba, Sr), and depletion of HFSE(Nb, Ta, Ti).The zircon εHf(t) values of the basic volcanic rocks range from 4.86 to 8.97, and the εHf(t) values of the intermediate volcanic rocks are from 0.26 to 6.37.Based on previous results, it is suggested that the Dianzhong Formation volcanic rocks in Riduo area were formed in the syn-collision tectonic setting of the collision between India and Eurasia.The basic volcanic rocks were mainly derived from the partial melting of overlying mantle wedge metasomatized by the fluids of subducting plate, while the intermediate volcanic rocks were originated from partial melting of the juvenile crust materials, both of which were influenced by the contamination of old crustal materials to various degrees.
    2021  .  40(8):    1314-1329    [摘要](28)    [PDF](40)
  • Discovery of the Late Triassic-Early Jurassic volcanic rocks of Xionglai Formation and constraint on the tectonic evolution of Zhikong-Sumdo Paleo-Tethyan orogen in the Riduo area of Tibet
    ZHOU Bin, QIAO Xinxing, HAN Kui, PAN Liang, WANG Feng, FAN Peng
    Zircon U-Pb dating, geochemical and Hf isotope analyses were carried out on the volcanic rocks of Xionglai Formation newly discovered in Riduo area, the eastern part of the Gangdese magmatic belt.The volcanic rocks of Xionglai Formation in Riduo area in the eastern part of Lhasa massif are mainly intermediate basic lava and pyroclastic rock.Zircon U-Pb dating of andesite and basaltic andesite yields weighted mean ages of 201.3 ±6.0 Ma and 184.5 ±4.4 Ma respectively, indicating that the Xionglai Formation volcanic rocks were formed in the Late Triassic-Early Jurassic.Geochemically, the basic volcanic rocks are characterized by low SiO2(48.53%~49.92%), high MgO(10.68%~11.43%), Mg#(72.67), Cr(224.7×10-6~347.8×10-6), and Ni(90.1×10-6~109.7×10-6), enriched in large ion lithophile elements(Rb, Th, U, Sr) and light rare earth elements(LREEs), and depleted in high field strength elements(Nb, Ta, P, Ti).The intermediate volcanic rocks are characterized by high SiO2(53.93%~60.54%), Al2O3(14.74%~16.12%), low MgO(2.45%~8.12%).They are enriched in light rare earth elements(LREEs), large ion lithophile elements(LILEs, e.g., Rb, Th, U and Ba), but depleted in high-field strength elements(e.g., Nb, Ta, P, Ti).Isotope analyses reveal that most samples have low zircon εHf(t) (-12.25~-3.64), and the values of tDMC range from 1621 Ma to 2004 Ma.Combining with regional geology, geochronological, geochemical and zircon Hf isotopic data, it is proposed that the Late Triassic-Early Jurassic volcanic rocks of Xionglai Formation were formed in post-collision tectonic setting as a magmatic response to post-collision and extension of Zhikong-Sumdo Paleo-Tethyan orogen.The basic volcanic rocks were derived from partial melting of a depleted lithospheric mantle, while the intermediate volcanic rocks were derived from partial melting of ancient lower crustal material and contaminated by ancient lower crust during ascending.
    2021  .  40(8):    1330-1343    [摘要](24)    [PDF](65)
  • Petrogenesis and tectonic significance of the Yari granites in the west-middle segment of the Lhasa Terrane Tibet Evidences from geochronology geochemistry and Hf isotope
    TAO Gang, YANG Yuanjiang, YANG Wenguang, ZHU Lidong, QI Xin, LU Zhiyou, HE Bi
    More geological researchers are focusing on the Mesozoic magmatic activities of Lhasa terrane, which is significant for the evolution of the Lhasa terrane. But the petrogenesis and magmatic origin of the Late Jurassic Yari Granites(YRG) remain poorly constrained. This study presents petrology, zircon U-Pb ages, geochemistry, and Lu-Hf isotopic data of YRG. The zircon U-Pb age of the pluton is 152.1±1.5 Ma, indicating that it was formed in the Late Jurassic. The YRG are mainly adamellite characterized by high silicon (SiO2=73.55%~74.19%), rich alkali, peraluminous and poor MgO, MnO and P2O5(0.12%~0.14%). It belongs to the series of high potassium calc-alkaline with high degree of differentiation. It is characterized by ΣREE of 104.12×10-6~247.22×10-6, (La/Yb) N of 32.14~51.03, enrichment of light rare earth elements(ΣLREE), obvious negative Eu anomaly (δEu=0.54~0.68) and right-dipping distribution pattern of rare earth elements. It is enriched in Rb, Th, U and other large ion lithophile elements, and depleted in Nb, Ti. It belongs to highly differentiated S-type granite. Its petrology, geochronology and geochemistry indicate that the double subduction of the Neo-Tethys' northward subduction and Bangong-Nujiang Ocean' southward subduction led to the collision between the Gandese micro-continent and Chayu magmatic arc. The YRG were highly fractionated S-type adamellite and the source area might be related to the partial melting of mudstone with greywacke under the tectonic decompression settings. It is proposed that during the Late Jurassic, the Lhasa terrane area experienced a tectonic transformation from subduction to collision.
    2021  .  40(8):    1344-1356    [摘要](29)    [PDF](29)
  • Late Cretaceous Dongco granodiorite porphyry, Tibet: Product of lithospheric delamination
    CHE Xu, LIU Yiming, FAN Jianjun, YU Yunpeng, GUO Runhua, QUAN Licheng, XIE Chaoming, WANG Ming
    The Bangong-Nujiang suture zone is an important part of Tethyan tectonic domain, which is significant for revealing the evolution of Tethys Ocean, Tibet Plateau.The Late Cretaceous granodiorite porphyry in Dongco area from Southern Qiangtang terrane was selected to study petrology, zircon U-Pb age, Lu-Hf isotopic and whole-rock geochemistry.Zircon U-Pb dating and Lu-Hf isotopic analysis yields an age of 74.1±1.1 Ma, with positive εHf(t) values of 4.5~6.9.Geochemical data reveal that the granodiorite porphyry belong to high-K calc-alkaline series, enriched in light rare earth elements with weak negative Eu anomaly and strong fractionation between light and heavy rare earth elements.In spider diagram, it is enriched in Rb, Th, U, K and Pb, and depleted in Ba, Nb, Ta, La, Ce, P and Ti.It is inferred that the granodiorite porphyry was derived from partial melting of thickened juvenile lower crust.Combined with previous research, it is concluded that the Dongco granodiorite porphyry was emplaced in post-collision tectonic setting as the product of lithosphere delamination, postdating the closure of Bangong-Nujiang Ocean.
    2021  .  40(8):    1357-1368    [摘要](26)    [PDF](28)
  • Petrogenesis of the Zhagadang O-type adakite in Naidong County of Tibet: Constraints from its geochemistry and Sr-Nd-Pb isotopic compositions
    NIU Decao, ZHANG Zhiping, DONG Han, WANG Hongtao, WU Xuemei
    Adakite, as a set of intermediate-acid igneous rock assemblages with special geochemical characteristics, has a variety of genetic models and special tectonic significance.The Zedang ophiolite is of great significance to the study of the evolution of the Neo-Tethys Ocean.The trondhjemite outcropped in the Zedang ophiolite was selected to study its petrology, petrochemistry and Sr-Nd-Pb isotopes.The results show that the Zhagadang trondhjemite is a high SiO2 and Al2O3, rich Na2O(Na2O/K2O>2), low MgO, and para-aluminous calc-alkaline series.It is characterized by high Sr and Sr/Y, low Y and Yb.These geochemical characteristics are comparable with those of O-type adakite.The Zhagadang trondhjemite is rich in large ion lithophile elements(e.g., Rb, K and Ba), poor in high field-strength elements(e.g., Nb, Ti and P) and rare earth elements, and exhibits a weak positive Eu anomaly.The Sr-Nd-Pb isotopic compositions indicate that the Zhagadang trondhjemite is characterized by high Sr, Sm, Nd and Pb contents, with ISr of 0.704251~0.704329, INd of 0.512776~0.512807, positive εNd(t) values(6.61~7.22) and relatively uniform lead isotopic composition(206Pb/204Pb=18.378~18.495, 207Pb/204Pb=15.542~15.554 and 208Pb/204Pb=38.336~38.406).The study indicates that the Zhagadang trondhjemite bearing mantle characteristics was formed by partial melting of the subducting oceanic slab at a certain depth.The result confirms the existence of the Zedang intra-oceanic.
    2021  .  40(8):    1369-1379    [摘要](27)    [PDF](24)
  • Petrogenesis and tectonic setting of Middle Silurian A-type granite in Kaerqueka area, East Kunlun
    CHEN Haifu, HE Shuyue, ZHANG Aikui, SUN Jinlei, YAN Zhengping, LIU Jinlong, ZHANG Liang, QIAN Ye
    A lot of porphyritic monzogranites are outcropped in the Kaerqueka area, west of the East Kunlun orogenic belt.Zircon U-Pb age and geochemistry of the porphyritic monzonites were studied.Zircon U-Pb dating yields an age of 425±2.3 Ma, indicating Middle Silurian.The rocks are characterized by high contents of SiO2(69.0%~71.2%) and K2O(4.3%~5.54%), enrichment of large ion lithophile element(Rb, K, Th and U), depletion of high field strength element(Nb, Ta and Ti), and strong Eu negative anomaly(Eu/Eu*=0.40~0.51).Their lower content of Cr, Ni(Cr=13.81×10-6~32.9×10-6, Ni=5.48×10-6~12.05×10-6) and the lower Ce/Pb (2.01~4.19) and Nb/U(4.1~7.57) values indicate that they were originated from partial melting of the upper crust.The high ratio of 10000Ga/Al (3.22~3.42), high contents of Zr+Nb+Ce+Y(459×10-6~656×10-6) and zircon saturation temperature(827~881℃) imply they belong to A-type granite.Tectonic diagram and high Y/Nb ratio(2.93~3.15) indicate that they are A2 type granites, and were formed in the post-collision extension setting.Combined with the spatial and temporal distribution of magmatic rocks in the region, it is suggested that the East Kunlun Orogen entered the post-collision extension stage from the Middle Silurian period.
    2021  .  40(8):    1380-1393    [摘要](24)    [PDF](26)
  • Discussion on the development regularity of salt water in red beds in the southern margin of Sichuan Basin
    DONG Jianxing, SUN Dong, WEI Liangshuai, ZHANG Zhipeng, HUANG Rui, YANG Haijun
    Compared with the center of Sichuan Basin, the red beds distributed in the southern margin of it are characterized by many geomorphological types, large topographic slopes, well-developed structures and steep dipping, and there are few reports on the development regularity of the saline water in the red beds.Through investigation, hydrogeological drilling, sampling analysis and testing, it is found that the development of salt water is mainly controlled by lithology and groundwater circulation.Soluble salt in red sandstone and mudstone is the material basis for the development of salt water.Salt water is more likely to develop in mudstone-bearing strata.Groundwater type, slope structure, depth and fault mainly affect the cycling conditions of groundwater.Fresh water is strongly developed in cycling alternation, while salt water is developed in slow cycling alternation.Salt water with complex composition in the deep part of the fault zone will well up due to exploitation of shallow groundwater, resulting in salinization of shallow groundwater.The study result has reference significance for avoiding saline water layer in the process of water exploration in red beds in the southern margin of Sichuan Basin.
    2021  .  40(8):    1394-1401    [摘要](19)    [PDF](22)

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