The variations in source rocks and melting conditions of granites can provide essential clues for the crustal magmatic response in orogenic process.Based on geochronology,whole-rock and mineral chemistry,this paper re...The variations in source rocks and melting conditions of granites can provide essential clues for the crustal magmatic response in orogenic process.Based on geochronology,whole-rock and mineral chemistry,this paper reveals two different granites in the Northern Qinling migmatite complex,which reveal obvious differences in source region and melting condition.The older granodiorite(402±0.8 Ma)displays typical Na-rich adakite affinity,i.e.,high Na_(2)O/K_(2)O(2.04 to 2.64)and Sr/Y(96 to 117)ratios,they have relative evolved isotopic compositions(ε_(Nd)(t)=-0.52 to-0.04;zirconε_(Hf)(t)=-0.06 to+7.78).The younger leucogranite(371±2 Ma)displays higher SiO_(2)(72.32 to 73.45 wt%),lower(TFeO+MgO+CaO+TiO_(2))contents(<2 wt%)and depleted Sr-Nd-Hf isotopic compositions(i.e.,ε_(Nd)(t)=+2.6 to+3.0;zirconε_(Hf)(t)=+5.94 to+14.12),as well as high 10000×Ga/Al and TFe O/Mg O ratios,indicating that they represent highly fractionated I-type granites that derived from melting of juvenile crust.The variations in source rocks and melting condition of the two granites indicating a tectonic switch from compression to extension in 400 to 370 Ma,this switch is later than that in the eastern section of the North Qinling,indicating a scissor collision process between the South Qinling and North China Craton(NCC)in Devonian era.展开更多
The Indosinian post-collisional Wulong pluton intruded into the Mesoproterozoic Fuping Group, South Qinling, central China. In the southern part of the pluton, some mafic enclaves have sharp or gradational contact rel...The Indosinian post-collisional Wulong pluton intruded into the Mesoproterozoic Fuping Group, South Qinling, central China. In the southern part of the pluton, some mafic enclaves have sharp or gradational contact relationships with the host biotite granodiorite. Geochemistry, zircon LA-ICP MS (laser ablation inductively-coupled plasma mass spectrometry) U-Pb chronology and Sr- Nd-Pb isotope geochemistry of the pluton are reported in this paper. The biotite granodiorite shows close compositional similarities to high-silica adakite. Its chondrite-normalized REE patterns are characterized by strong HREE depletion (Yb = 0.33--0.96 10-6 and Y = 4.77-11.19 ×10^-6), enrichment of Ba (775-1386 x 10-6) and Sr (643-1115 × 10^-6) and high Sr/Y (57.83-159.99) and Y/Yb (10.99-14.32) ratios, as well as insignificant Eu anomalies (6Eu = 0.70-0.83), suggesting a feldspar-poor, garnet±amphibole-rich residual mineral assemblage. The mafic enclaves have higher MgO (4.15- 8.13%), Cr (14.79-371.31 × 10-6), Ni (20.00-224.24× 10^-6) and Nb/Ta (15.42-21.91) than the host granodiorite, implying that they are mantle-derived and might represent underplated mafic magma. Zircon LA-ICP MS dating of the granodiorite yields a ^206pb/^238U weighted mean age of 208±2 Ma (MSWD=0.50, 1σ), which is the age of emplacement of the host biotite granodiorite. This age indicates that the Wulong pluton formed during the late-orogenic or post-collisional stage (〈242±21 Ma) of the South Qinling belt. The host biotite granodiorite displays ^87Sr/^86Sr = 0.7059-0.7062, Isr = 0.7044-- 0.7050,^143Nd/^144Nd = 0.51236-0.51238, εNd(t)= -2.26 to -2.66 to ^206Pb/^204pb = 18.099-18.209, ^207pb/^204pb = 15.873-15.979 and ^208pb/^204pb = 38.973-39.430. Those ratios are similar to those of the Mesoproterozoic Yaolinghe Group in the South Qinling. Furthermore, its Nd isotopic model age (-1.02 Ga) is consistent with the age (-1.1 Ga) of the Yaolinghe Group. Based on the integrated geological and geochemical studies, coupled with previous studies, the authors suggest that the Wulong adakitic biotite granodiorite was probably generated by dehydration melting of the Yaolinghe Group-like thickened mafic crust, triggered by underplating of mafic magma at the boundary of the thickened mafic crust and hot lithospheric mantle, and that the Wulong adakitic biotite granodiorite may have resulted from thinning and delamination of the lower crust or breakoff of the subducting slab of the Mianlue ocean during the Indosinian post-collisional orogenic stage of the Qinling orogenic belt.展开更多
The ultrahasic rocks in the Mianxian-Lueyang ophiolite belt are mainly harzburgite and dunite, and exhibit two types of REE distribution pattern: (1) LREE depletion, with remarkable Eu enrichment; (2) slight enrichmen...The ultrahasic rocks in the Mianxian-Lueyang ophiolite belt are mainly harzburgite and dunite, and exhibit two types of REE distribution pattern: (1) LREE depletion, with remarkable Eu enrichment; (2) slight enrichment of LREE, with Eu deficiency. Both gabbros (accumulative gabbros) and diabasic dike swarms show a LREE enrichment but a slight Eu anomaly. The ratios of w (Ti) /w (V), w (Th) /w (Ta). w (Th) /w (Yb) and w (Ta) /w (Yb) indicate that the mid-ocean ridge basalts in this region originated from depleted mantle of asthenosphere, implying an association of MORB-type ophiolite and an ancient ocean basin between Qinling and Yangtze plates during Middle Paleozoic-Early Mesozoic era. The island arc volcanic rocks can be divided into tholeiitic and cafe-alkaline associations, which originated from the upper mantle wedge above slab by partial melting.展开更多
U-Pb zircon age, geochemical, and Sr-Nd-Pb isotopic data of mafic dykes from eastern Shandong Province, eastern China is reported herein. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb ...U-Pb zircon age, geochemical, and Sr-Nd-Pb isotopic data of mafic dykes from eastern Shandong Province, eastern China is reported herein. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon analyses of two samples from the investigated mafic dykes yield consistent ages ranging from 121.9 Ma ± 0.47 Ma to 122.9 Ma ± 0.61 Ma. The mafic dykes are characterized by high (87Sr/86Sr) i ranging from 0.7087 to 0.7089, low εNd(t) values ranging from -16.9 to -17.8, 206Pb/204Pb = 17.15 to 17.17, 207Pb/204Pb = 15.45 to 15.47, and 208Pb/204Pb = 37.59 to 37.68. Results from the current study suggest that the mafic dykes are derived from partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dykes may have been generated through subsequent insignificant crystal fractionation and very minor crustal contamination during magma ascent. Combined with previous studies, the current findings provide new evidence that the intense lithospheric thinning beneath the eastern Shandong Province of eastern China occurred at ~120 Ma, and that this condition was caused by the removal of the lower lithosphere (mantle and lower crust).展开更多
Whole-rock major and trace element and Sr-Nd isotopic data, together with zircon LA ICP- MS in-situ U-Pb and Hf isotopic data of the syenites and granites in the Tengchong Block are reported in order to understand the...Whole-rock major and trace element and Sr-Nd isotopic data, together with zircon LA ICP- MS in-situ U-Pb and Hf isotopic data of the syenites and granites in the Tengchong Block are reported in order to understand their petrogenesis and tectonic implications. Zircon U-Pb data gives the emplacement ages of ca. 115.3±0.9 Ma for syenites and 115.7±0.8 Ma for granites, respectively. The syenites are characterized by low SiO2 content (62.01-63.03 wt%) and notably high Na20 content (7.04- 7.24 wt%) and Na20/K20 ratios (2.02-2.10), low MgO, Fe2OaT and TiO2, enrichment of LILEs(large-ion lithophile element) such as Rb, Th, U, K, and Pb) and obvious depletion HFSE(high field strength element; e.g. Nb, Ta, P, and Ti) with clearly negative Eu anomalies (dEu=0.53-0.56). They also display significant negative whole-rock eNd(t) values of-6.8 and zircon eHf(t) values(-9.11 to -0.27, but one is +5.30) and high initial S7Sr/86Sr=0.713013. Based on the data obtained in this study, we suggest that the ca. l15.3Ma syenites were possibly derived from a sodium-rich continental crustal source, and the fractionation of some ferro-magnesian mineral and plagioclase might occur during the evolution of magma. The granites have high SiO2 content (71.35-74.47 wt%), metaluminous to peraluminous, low Rb/Ba, Rb/Sr, and AI2Oa/(MgO+FeOT+TiO2) ratios and moderate (AI2Oa+MgO+FeOT+TiO2) content. They show low initial a7Sr/86Sr (0.703408 to 0.704241) and eNd(t) values (-3.8 to -3.5), plotted into the evolutionary trend between basalts and lower crust. Hence, we suggest that the granites were derived from the melting of mixing sources in the ancient continental crust involving some metabasaltic materials and predominated metasedimentary greywackes. Together with data in the literatures, we infer that the Early Cretaceous magmatism in the Tengchong block was dominated by magmas generated by the partial melting of ancient crustal material, which represent the products that associated to the closure of Bangong-Nujiang Meso-Tethys.展开更多
The Late Triassic igneous rocks in the Yidun terrane can provide vital insights into the evolution of Plaeo-Tethys in western China. We present new zircon U-Pb, whole-rock geochemistry, and Sr-Nd-Pb-Hf isotopic data f...The Late Triassic igneous rocks in the Yidun terrane can provide vital insights into the evolution of Plaeo-Tethys in western China. We present new zircon U-Pb, whole-rock geochemistry, and Sr-Nd-Pb-Hf isotopic data for the Litang biotite monzogranites, Yidun terrane. The biotite monzogranites have a zircon U-Pb age of 206.1±1.0 Ma(MSWD=1.9,n=30), which indicates Late Triassic magmatism. The biotite monzogranites display I-type affinity, high Na_2O(3.38-3.60 wt%) contente,medii SiO_2(67.12-69.13 wt%), and low P_2 O_5 contents(0.10~0.12 wt%). They enriched in Rb,and Ba and depleted in Nb and Ta, with negative Eu anomalies(Eu/Eu*=0.74—0.81). They have evolved Sr-Nd-Pb-Hf isotopic composition, i.e.,(^(87) Sr/^(86 )Sr)i=0.714225 to 0.714763, negative ?_(Nd(t)) values of -2.0 to-2.6 with two-stage Nd model ages ranging from 1.01 to 1.05 Ga, negative ?_(Ht)(t)) values o f-3.4 to-4.1 with two-stage Hf model ages of 1.85 to1.88 Ga, suggesting a matured crustal sources. Their low Al_2O_3/TiO_2 ratios and medium Cao/Na_2O ratios, medium Mg~# and SiO_2 contents, low [molar Al_2O_3/(MgO+FeO^T)] values, and high [molar Cao/(MgO+FeO^T)] values indicate that the Litang biotite monzogranite was formed by partial melting of metabasaltic rocks. Based on the previous studies, we propose that the Litang biotite monzogranite derived from the westward subduction and closure of the Ganzi-Litang ocean during the Late Triassic-The mantle wedge-derived mafic melts provided sufficient heat for partial melting of ancient metabasalt protolith within the middle-lower crust.展开更多
Western China locates in the eastern section of the Tethys domain, granitic rocks in this region with variable formation ages and geochemistry record key information about the crust-mantle structure and thermal evolut...Western China locates in the eastern section of the Tethys domain, granitic rocks in this region with variable formation ages and geochemistry record key information about the crust-mantle structure and thermal evolution during the convergent process of Tethys. In this study, we focus on some crucial granitic magmatism in the western Yangtze, Qinling orogen, and western Sanjiang tectonic belt, where magma sequence in the convergent orogenic belt can provide important information about the crust-mantle structure, thermal condition and melting regime that related to the evolution processes from Pre-to Neo-Tethys. At first, we show some features of Pre-Tethyan magmatism, such as Neoproterozoic magmatism(ca. 870–740 Ma) in the western margin of the Yangtze Block were induced by the assembly and breakup of the Rodinia supercontinent. The complication of voluminous Neoproterozoic igneous rocks indicated that the western Yangtze Block underwent the thermodynamic evolution from hot mantle-cold crust stage(ca. 870–850 Ma) to hot mantle and crust stage(ca. 850–740 Ma). The Neoproterozoic mantle sources beneath the western Yangtze Block were progressively metasomatized by subduction-related compositions from slab fluids(initial at ca. 870 Ma), sediment melts(initial at ca. 850 Ma), to oceanic slab melts(initial at ca. 825–820 Ma) during the persistent subduction process. Secondly, the early Paleozoic magmatism can be well related to three distinctive stages(variable interaction of mantle-crust to crustal melting to variable sources) from an Andeans-type continental margin to collision to extension in response to the evolution of ProtoTethys and final assembly of Gondwana continent. Thirdly, the Paleo-Tethys magmatism, Triassic granites in the Qinling orogenic display identical formation ages and Lu-Hf isotopic compositions with the related mafic enclaves, indicate a coeval melting event of lower continental crust and mantle lithosphere in the Triassic convergent process and a continued hot mantle and crust thermal condition through the interaction of subducted continental crust and upwelling asthenosphere. Finally, the Meso-and Neo-Tethyan magmatism: Early Cretaceous magmatism in the Tengchong Block are well responding to the subduction and closure of Bangong-Nujiang Meso-Tethys, recycled sediments metasomatized mantle by subduction since 130 Ma and subsequently upwelling asthenosphere since ca. 122 Ma that causes melting of heterogeneous continental crust until the final convergence, this process well recorded the changing thermal condition from hot mantlecold crust to hot mantle and crust;The Late Cretaceous to Early Cenozoic magmatism well recorded the processes from Neo-Tethyan ocean slab flat subduction, steep subduction, to initial collision of India-Asia, it resulted in a series of continental arc magmatism with enriched mantle to crustal materials at Late Cretaceous, increasing depleted and/or juvenile materials at the beginning of early Cenozoic, and increasing evolved crustal materials in the final stage, implying a continued hot mantle and crust condition during that time. Then we can better understand the magmatic processes and variable melting from the mantle to crust during the evolution of Tethys, from Pre-, Paleo-, Meso-, to Neo-, both they show notably intensive interaction of crust-mantle and extensive melting of the heterogeneous continent during the final closure of Tethys and convergence of blocks, and thermal perturbation by a dynamic process in the depth could be the first mechanism to control the thermal condition of mantle and crust and associated composition of magmatism.展开更多
Objective The Cenozoic Indo-Asian collision caused significant crustal shortening and plateau uplift in the central Tibet. The extrusion tectonic model has been widely accepted to explain the strike-slip faults around...Objective The Cenozoic Indo-Asian collision caused significant crustal shortening and plateau uplift in the central Tibet. The extrusion tectonic model has been widely accepted to explain the strike-slip faults around the Tibetan Plateau. Previous studies indicate that the lower crust flow is the main drive force of the extrusion tectonics. Whether mantle extrusion process occurred during the Cenozoic uplift is a major problem to be addressed, which is significant for understanding the uplift mechanism and tectonic evolution of the Tibetan Plateau.展开更多
An in-situ microanalysis of Pb isotopic compositions in sulfide minerals is carried out by using femtosecond laser-ablation multi-collector inductively coupled plasma mass spectrometry(fs LA-MC-ICP-MS).High-temperatur...An in-situ microanalysis of Pb isotopic compositions in sulfide minerals is carried out by using femtosecond laser-ablation multi-collector inductively coupled plasma mass spectrometry(fs LA-MC-ICP-MS).High-temperature-activated carbon was used to filter Hg contained in the carrier gas,which reduced the Hg background signal by 48%and also lowered the detection limit of the analysis.Fractionation and mass discrimination effects existing in the ICP-MS analytical processes were corrected using an internal reference Tl in conjunction with an external reference NIST SRM 610.The proposed method was used to analyze the Pb isotopic compositions of chalcopyrite,pyrite,and sphalerite from the Dulong Sn-Zn-In polymetallic ore district.The results showed that in this ore district,the sulfide minerals and different grains of the same sulfide mineral show a large variation in Pb content up to 1000-fold.The studied pyrites show relatively higher Pb contents and homogeneous Pb isotopic compositions,whereas the sphalerites have low Pb contents but most variable Pb isotopic compositions.It is suggested that the large variation of Pb isotopic composition may reflect a late hydrothermal superimposition on the primary sulfide formation.In addition,radiogenic Pb accumulated by radioactive decay of trace amounts of U over time in the host minerals may also be one of the causes for the large variation range of Pb content and Pb isotopic composition of those low-Pb sphalerites.Chalcopyrite and sphalerite grains with Pb content greater than 10 ppm presented a consistent Pb isotopic distribution,whereas all the sulfide grains with Pb content greater than 100 ppm had consistent Pb isotopic composition within 2s measurement uncertainties.The in-situ analysis of Pb isotopic composition agreed well with the results obtained by conventional chemical methods within2s measurement uncertainties,indicating that the data obtained by fs LA-MC-ICP-MS are reliable.Additionally,this study indicates that the Pb isotopic composition could truthfully record the source of ore-forming minerals only for sulfide minerals with high Pb content.On the contrary,the Pb isotopic composition of low-Pb sulfide minerals may be affected by trace amounts of U in the host minerals that may lead to a highly radiogenic Pb isotope ratio.Alternatively,it is also possible that late fluid metasomatic overprinting may alter the Pb isotopic compositions.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.41102037,41421002)the Foundation for the Author of National Excellent Doctoral Dissertation of China(Grant No.201324)research grant of State Key Laboratory of Continental Dynamics(Grant No.SKLCD-04)。
文摘The variations in source rocks and melting conditions of granites can provide essential clues for the crustal magmatic response in orogenic process.Based on geochronology,whole-rock and mineral chemistry,this paper reveals two different granites in the Northern Qinling migmatite complex,which reveal obvious differences in source region and melting condition.The older granodiorite(402±0.8 Ma)displays typical Na-rich adakite affinity,i.e.,high Na_(2)O/K_(2)O(2.04 to 2.64)and Sr/Y(96 to 117)ratios,they have relative evolved isotopic compositions(ε_(Nd)(t)=-0.52 to-0.04;zirconε_(Hf)(t)=-0.06 to+7.78).The younger leucogranite(371±2 Ma)displays higher SiO_(2)(72.32 to 73.45 wt%),lower(TFeO+MgO+CaO+TiO_(2))contents(<2 wt%)and depleted Sr-Nd-Hf isotopic compositions(i.e.,ε_(Nd)(t)=+2.6 to+3.0;zirconε_(Hf)(t)=+5.94 to+14.12),as well as high 10000×Ga/Al and TFe O/Mg O ratios,indicating that they represent highly fractionated I-type granites that derived from melting of juvenile crust.The variations in source rocks and melting condition of the two granites indicating a tectonic switch from compression to extension in 400 to 370 Ma,this switch is later than that in the eastern section of the North Qinling,indicating a scissor collision process between the South Qinling and North China Craton(NCC)in Devonian era.
文摘The Indosinian post-collisional Wulong pluton intruded into the Mesoproterozoic Fuping Group, South Qinling, central China. In the southern part of the pluton, some mafic enclaves have sharp or gradational contact relationships with the host biotite granodiorite. Geochemistry, zircon LA-ICP MS (laser ablation inductively-coupled plasma mass spectrometry) U-Pb chronology and Sr- Nd-Pb isotope geochemistry of the pluton are reported in this paper. The biotite granodiorite shows close compositional similarities to high-silica adakite. Its chondrite-normalized REE patterns are characterized by strong HREE depletion (Yb = 0.33--0.96 10-6 and Y = 4.77-11.19 ×10^-6), enrichment of Ba (775-1386 x 10-6) and Sr (643-1115 × 10^-6) and high Sr/Y (57.83-159.99) and Y/Yb (10.99-14.32) ratios, as well as insignificant Eu anomalies (6Eu = 0.70-0.83), suggesting a feldspar-poor, garnet±amphibole-rich residual mineral assemblage. The mafic enclaves have higher MgO (4.15- 8.13%), Cr (14.79-371.31 × 10-6), Ni (20.00-224.24× 10^-6) and Nb/Ta (15.42-21.91) than the host granodiorite, implying that they are mantle-derived and might represent underplated mafic magma. Zircon LA-ICP MS dating of the granodiorite yields a ^206pb/^238U weighted mean age of 208±2 Ma (MSWD=0.50, 1σ), which is the age of emplacement of the host biotite granodiorite. This age indicates that the Wulong pluton formed during the late-orogenic or post-collisional stage (〈242±21 Ma) of the South Qinling belt. The host biotite granodiorite displays ^87Sr/^86Sr = 0.7059-0.7062, Isr = 0.7044-- 0.7050,^143Nd/^144Nd = 0.51236-0.51238, εNd(t)= -2.26 to -2.66 to ^206Pb/^204pb = 18.099-18.209, ^207pb/^204pb = 15.873-15.979 and ^208pb/^204pb = 38.973-39.430. Those ratios are similar to those of the Mesoproterozoic Yaolinghe Group in the South Qinling. Furthermore, its Nd isotopic model age (-1.02 Ga) is consistent with the age (-1.1 Ga) of the Yaolinghe Group. Based on the integrated geological and geochemical studies, coupled with previous studies, the authors suggest that the Wulong adakitic biotite granodiorite was probably generated by dehydration melting of the Yaolinghe Group-like thickened mafic crust, triggered by underplating of mafic magma at the boundary of the thickened mafic crust and hot lithospheric mantle, and that the Wulong adakitic biotite granodiorite may have resulted from thinning and delamination of the lower crust or breakoff of the subducting slab of the Mianlue ocean during the Indosinian post-collisional orogenic stage of the Qinling orogenic belt.
文摘The ultrahasic rocks in the Mianxian-Lueyang ophiolite belt are mainly harzburgite and dunite, and exhibit two types of REE distribution pattern: (1) LREE depletion, with remarkable Eu enrichment; (2) slight enrichment of LREE, with Eu deficiency. Both gabbros (accumulative gabbros) and diabasic dike swarms show a LREE enrichment but a slight Eu anomaly. The ratios of w (Ti) /w (V), w (Th) /w (Ta). w (Th) /w (Yb) and w (Ta) /w (Yb) indicate that the mid-ocean ridge basalts in this region originated from depleted mantle of asthenosphere, implying an association of MORB-type ophiolite and an ancient ocean basin between Qinling and Yangtze plates during Middle Paleozoic-Early Mesozoic era. The island arc volcanic rocks can be divided into tholeiitic and cafe-alkaline associations, which originated from the upper mantle wedge above slab by partial melting.
基金supported by Opening Project(201206)of the State Key Laboratory of Ore deposit Geochemistry,Chinese Academia of Sciencesthe National Nature Science Foundation of China(40773020,40972071,90714010,and 40634020)
文摘U-Pb zircon age, geochemical, and Sr-Nd-Pb isotopic data of mafic dykes from eastern Shandong Province, eastern China is reported herein. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon analyses of two samples from the investigated mafic dykes yield consistent ages ranging from 121.9 Ma ± 0.47 Ma to 122.9 Ma ± 0.61 Ma. The mafic dykes are characterized by high (87Sr/86Sr) i ranging from 0.7087 to 0.7089, low εNd(t) values ranging from -16.9 to -17.8, 206Pb/204Pb = 17.15 to 17.17, 207Pb/204Pb = 15.45 to 15.47, and 208Pb/204Pb = 37.59 to 37.68. Results from the current study suggest that the mafic dykes are derived from partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dykes may have been generated through subsequent insignificant crystal fractionation and very minor crustal contamination during magma ascent. Combined with previous studies, the current findings provide new evidence that the intense lithospheric thinning beneath the eastern Shandong Province of eastern China occurred at ~120 Ma, and that this condition was caused by the removal of the lower lithosphere (mantle and lower crust).
基金supported by the National Natural Science Foundation of China (Grant Nos. 41421002, 41190072, 41372067 and 41102037)the program for Changjiang Scholars and Innovative Research Team in University (Grant IRT1281)+2 种基金the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (201324)the MOST Special Fund from the State Key Laboratory of Continental Dynamics, Northwest UniversityProvince Key Laboratory Construction Item (08JZ62)
文摘Whole-rock major and trace element and Sr-Nd isotopic data, together with zircon LA ICP- MS in-situ U-Pb and Hf isotopic data of the syenites and granites in the Tengchong Block are reported in order to understand their petrogenesis and tectonic implications. Zircon U-Pb data gives the emplacement ages of ca. 115.3±0.9 Ma for syenites and 115.7±0.8 Ma for granites, respectively. The syenites are characterized by low SiO2 content (62.01-63.03 wt%) and notably high Na20 content (7.04- 7.24 wt%) and Na20/K20 ratios (2.02-2.10), low MgO, Fe2OaT and TiO2, enrichment of LILEs(large-ion lithophile element) such as Rb, Th, U, K, and Pb) and obvious depletion HFSE(high field strength element; e.g. Nb, Ta, P, and Ti) with clearly negative Eu anomalies (dEu=0.53-0.56). They also display significant negative whole-rock eNd(t) values of-6.8 and zircon eHf(t) values(-9.11 to -0.27, but one is +5.30) and high initial S7Sr/86Sr=0.713013. Based on the data obtained in this study, we suggest that the ca. l15.3Ma syenites were possibly derived from a sodium-rich continental crustal source, and the fractionation of some ferro-magnesian mineral and plagioclase might occur during the evolution of magma. The granites have high SiO2 content (71.35-74.47 wt%), metaluminous to peraluminous, low Rb/Ba, Rb/Sr, and AI2Oa/(MgO+FeOT+TiO2) ratios and moderate (AI2Oa+MgO+FeOT+TiO2) content. They show low initial a7Sr/86Sr (0.703408 to 0.704241) and eNd(t) values (-3.8 to -3.5), plotted into the evolutionary trend between basalts and lower crust. Hence, we suggest that the granites were derived from the melting of mixing sources in the ancient continental crust involving some metabasaltic materials and predominated metasedimentary greywackes. Together with data in the literatures, we infer that the Early Cretaceous magmatism in the Tengchong block was dominated by magmas generated by the partial melting of ancient crustal material, which represent the products that associated to the closure of Bangong-Nujiang Meso-Tethys.
基金supported by the National Natural Science Foundation of China (41421002, 41772052, 41372067)independent innovation project of graduate students of Northwest University (YZZ17192)
文摘The Late Triassic igneous rocks in the Yidun terrane can provide vital insights into the evolution of Plaeo-Tethys in western China. We present new zircon U-Pb, whole-rock geochemistry, and Sr-Nd-Pb-Hf isotopic data for the Litang biotite monzogranites, Yidun terrane. The biotite monzogranites have a zircon U-Pb age of 206.1±1.0 Ma(MSWD=1.9,n=30), which indicates Late Triassic magmatism. The biotite monzogranites display I-type affinity, high Na_2O(3.38-3.60 wt%) contente,medii SiO_2(67.12-69.13 wt%), and low P_2 O_5 contents(0.10~0.12 wt%). They enriched in Rb,and Ba and depleted in Nb and Ta, with negative Eu anomalies(Eu/Eu*=0.74—0.81). They have evolved Sr-Nd-Pb-Hf isotopic composition, i.e.,(^(87) Sr/^(86 )Sr)i=0.714225 to 0.714763, negative ?_(Nd(t)) values of -2.0 to-2.6 with two-stage Nd model ages ranging from 1.01 to 1.05 Ga, negative ?_(Ht)(t)) values o f-3.4 to-4.1 with two-stage Hf model ages of 1.85 to1.88 Ga, suggesting a matured crustal sources. Their low Al_2O_3/TiO_2 ratios and medium Cao/Na_2O ratios, medium Mg~# and SiO_2 contents, low [molar Al_2O_3/(MgO+FeO^T)] values, and high [molar Cao/(MgO+FeO^T)] values indicate that the Litang biotite monzogranite was formed by partial melting of metabasaltic rocks. Based on the previous studies, we propose that the Litang biotite monzogranite derived from the westward subduction and closure of the Ganzi-Litang ocean during the Late Triassic-The mantle wedge-derived mafic melts provided sufficient heat for partial melting of ancient metabasalt protolith within the middle-lower crust.
基金supported by the National Natural Science Foundation of China(Grant Nos.40872060,41102307,41372067,41772052,41802054,41190072,41421002,and 41902046)China Postdoctoral Science Special Foundation(Grant Nos.2019T120937 and 2018M643713)+1 种基金Natural Science Foundation of Shannxi(Grant No.2019JQ-719)Shannxi Postdoctoral Science Foundation。
文摘Western China locates in the eastern section of the Tethys domain, granitic rocks in this region with variable formation ages and geochemistry record key information about the crust-mantle structure and thermal evolution during the convergent process of Tethys. In this study, we focus on some crucial granitic magmatism in the western Yangtze, Qinling orogen, and western Sanjiang tectonic belt, where magma sequence in the convergent orogenic belt can provide important information about the crust-mantle structure, thermal condition and melting regime that related to the evolution processes from Pre-to Neo-Tethys. At first, we show some features of Pre-Tethyan magmatism, such as Neoproterozoic magmatism(ca. 870–740 Ma) in the western margin of the Yangtze Block were induced by the assembly and breakup of the Rodinia supercontinent. The complication of voluminous Neoproterozoic igneous rocks indicated that the western Yangtze Block underwent the thermodynamic evolution from hot mantle-cold crust stage(ca. 870–850 Ma) to hot mantle and crust stage(ca. 850–740 Ma). The Neoproterozoic mantle sources beneath the western Yangtze Block were progressively metasomatized by subduction-related compositions from slab fluids(initial at ca. 870 Ma), sediment melts(initial at ca. 850 Ma), to oceanic slab melts(initial at ca. 825–820 Ma) during the persistent subduction process. Secondly, the early Paleozoic magmatism can be well related to three distinctive stages(variable interaction of mantle-crust to crustal melting to variable sources) from an Andeans-type continental margin to collision to extension in response to the evolution of ProtoTethys and final assembly of Gondwana continent. Thirdly, the Paleo-Tethys magmatism, Triassic granites in the Qinling orogenic display identical formation ages and Lu-Hf isotopic compositions with the related mafic enclaves, indicate a coeval melting event of lower continental crust and mantle lithosphere in the Triassic convergent process and a continued hot mantle and crust thermal condition through the interaction of subducted continental crust and upwelling asthenosphere. Finally, the Meso-and Neo-Tethyan magmatism: Early Cretaceous magmatism in the Tengchong Block are well responding to the subduction and closure of Bangong-Nujiang Meso-Tethys, recycled sediments metasomatized mantle by subduction since 130 Ma and subsequently upwelling asthenosphere since ca. 122 Ma that causes melting of heterogeneous continental crust until the final convergence, this process well recorded the changing thermal condition from hot mantlecold crust to hot mantle and crust;The Late Cretaceous to Early Cenozoic magmatism well recorded the processes from Neo-Tethyan ocean slab flat subduction, steep subduction, to initial collision of India-Asia, it resulted in a series of continental arc magmatism with enriched mantle to crustal materials at Late Cretaceous, increasing depleted and/or juvenile materials at the beginning of early Cenozoic, and increasing evolved crustal materials in the final stage, implying a continued hot mantle and crust condition during that time. Then we can better understand the magmatic processes and variable melting from the mantle to crust during the evolution of Tethys, from Pre-, Paleo-, Meso-, to Neo-, both they show notably intensive interaction of crust-mantle and extensive melting of the heterogeneous continent during the final closure of Tethys and convergence of blocks, and thermal perturbation by a dynamic process in the depth could be the first mechanism to control the thermal condition of mantle and crust and associated composition of magmatism.
基金supported by the National Natural Science Foundation of China (grant No.41072052)
文摘Objective The Cenozoic Indo-Asian collision caused significant crustal shortening and plateau uplift in the central Tibet. The extrusion tectonic model has been widely accepted to explain the strike-slip faults around the Tibetan Plateau. Previous studies indicate that the lower crust flow is the main drive force of the extrusion tectonics. Whether mantle extrusion process occurred during the Cenozoic uplift is a major problem to be addressed, which is significant for understanding the uplift mechanism and tectonic evolution of the Tibetan Plateau.
基金supported by the National Natural Science Foundation of China(Grant Nos.41427804,41421002,41373004)Program for Changjiang Scholars and Innovative Research Team in University(Grant No.IRT1281)the MOST Research Foundation from the State Key Laboratory of Continental Dynamics(Grant No.BJ08132-1)
文摘An in-situ microanalysis of Pb isotopic compositions in sulfide minerals is carried out by using femtosecond laser-ablation multi-collector inductively coupled plasma mass spectrometry(fs LA-MC-ICP-MS).High-temperature-activated carbon was used to filter Hg contained in the carrier gas,which reduced the Hg background signal by 48%and also lowered the detection limit of the analysis.Fractionation and mass discrimination effects existing in the ICP-MS analytical processes were corrected using an internal reference Tl in conjunction with an external reference NIST SRM 610.The proposed method was used to analyze the Pb isotopic compositions of chalcopyrite,pyrite,and sphalerite from the Dulong Sn-Zn-In polymetallic ore district.The results showed that in this ore district,the sulfide minerals and different grains of the same sulfide mineral show a large variation in Pb content up to 1000-fold.The studied pyrites show relatively higher Pb contents and homogeneous Pb isotopic compositions,whereas the sphalerites have low Pb contents but most variable Pb isotopic compositions.It is suggested that the large variation of Pb isotopic composition may reflect a late hydrothermal superimposition on the primary sulfide formation.In addition,radiogenic Pb accumulated by radioactive decay of trace amounts of U over time in the host minerals may also be one of the causes for the large variation range of Pb content and Pb isotopic composition of those low-Pb sphalerites.Chalcopyrite and sphalerite grains with Pb content greater than 10 ppm presented a consistent Pb isotopic distribution,whereas all the sulfide grains with Pb content greater than 100 ppm had consistent Pb isotopic composition within 2s measurement uncertainties.The in-situ analysis of Pb isotopic composition agreed well with the results obtained by conventional chemical methods within2s measurement uncertainties,indicating that the data obtained by fs LA-MC-ICP-MS are reliable.Additionally,this study indicates that the Pb isotopic composition could truthfully record the source of ore-forming minerals only for sulfide minerals with high Pb content.On the contrary,the Pb isotopic composition of low-Pb sulfide minerals may be affected by trace amounts of U in the host minerals that may lead to a highly radiogenic Pb isotope ratio.Alternatively,it is also possible that late fluid metasomatic overprinting may alter the Pb isotopic compositions.