Gneisses with anatectic characteristics from the Liansan island in the Sulu UHPM(ultra-high pressure metamorphic)belt were studied for petrography,titanite U-Pb dating and mineral geochemistry.Three origins of garnets...Gneisses with anatectic characteristics from the Liansan island in the Sulu UHPM(ultra-high pressure metamorphic)belt were studied for petrography,titanite U-Pb dating and mineral geochemistry.Three origins of garnets are distinguished:metamorphic garnet,peritectic garnet and anatectic garnet,which are formed in the stages of peak metamorphism,retrograde anatexis and melt crystallization,respectively.The euhedral titanite has a high content of REE and high Th/U ratios,which is interpreted as indicating that it was newly-formed from an anatectic melt.The LA-ICP-MS titanite U-Pb dating yields 214-217 Ma ages for the titanite(melt)crystallization.The distribution of trace elements varies in response to the different host minerals at different stages.At the peak metamorphic stage,Y and HREE are mainly hosted by garnet,Ba and Rb by phengite,Sr,Nb,Ta,Pb,Th,U and LREE by allanite and Y,U and HREE by zircon.During partial melting,Y,Pb,Th,U and REE are released into the melt,which causes a dramatic decline of these element contents in the retrograde minerals.Finally,titanite absorbs most of the Nb,U,LREE and HREE from the melt.Therefore,the different stages of metamorphism have different mineral assemblages,which host different trace elements.展开更多
The present-day observed crustal-scale tectonic style of ultrahigh-pressure metamorphic (UH-PM) and high-pressure metamorphic (HPM ) belts in the Dabie-Sulu region was dominantly formedby extensional processes, postda...The present-day observed crustal-scale tectonic style of ultrahigh-pressure metamorphic (UH-PM) and high-pressure metamorphic (HPM ) belts in the Dabie-Sulu region was dominantly formedby extensional processes, postdating the Triassic collision between the Sino-Korean and Yangtze cratons. The extensional structures overprinting the previous structures related to contraction that produced the thickened continental crust of the UHPM and HPM belts, in particular display the typical features of a Cordilleran-type metamorphic core complex, in which at least four regionalscale, low-angle ductile shear zones that constitute a detachment system, are recognized in the Dabie region. In the Sulu region, the extensionaI structures show in the form of small-scale domes or a regional-scale SE-dipping pseudo-monocline. The geometry and the kinematics of tbe detachment zones are briefly described and their significance for the exhumation of UHPM and HPM rocks is discussed. It is iniliated that the subhorizontal crustal-scale extensional flow in the middle-lower crust, under amphibolite- to greenschist-facies conditions, was an important tectonothermal process at 200- 170 Ma and the exhumation or the UHPM and HPM rocks was achieved at least in part along multi-layered detachment zones. The regional detachment system has been the main factor enabling UHPM and HPM rocks to be brought from middle-lower crustal levels to middle-upper crustal levels.展开更多
The formation depth of metamorphic rocks in the Dabie ultrahigh pressure metamorphic (UHPM) zone influences not only our understanding of formation mechanism and evolution processes of collision orogenic belt, but...The formation depth of metamorphic rocks in the Dabie ultrahigh pressure metamorphic (UHPM) zone influences not only our understanding of formation mechanism and evolution processes of collision orogenic belt, but also the studies on earth's interior and geodynamic processes. In this study, the isotopic data of metamorphic rocks in the Dabie UHPM zone are discussed to give constraints on the formation depth in the Dabie UHPM zone. The ε Sr of eclogite in the Dabie UHPM zone varies from 18 to 42, and the ε Nd varies from -6.1 to -17, both of them show the characters of isotopic disequilibrium. The oxygen isotope studies indicate that the protoliths of these UHPM rocks have experienced oxygen isotope exchange with meteoric water (or sea water) before metamorphism and no significant changes in the processes of metamorphism on their oxygen isotope composition have been recorded in these rocks. Except for one sample from Bixiling, all samples of eclogite from Dabie UHPM zone show the 3He 4He ratios from 0.79×10 -7 to 9.35×10 -7 , indicating the important contribution of He from continental crust. All Sr, Nd, O and He isotopic studies indicate that the UHPM rocks retain the isotopic characteristics of their protoliths of crust origin. No significant influence of mantle materials has been found in these metamorphic rocks. Trying to explain above isotopic characteristics, some researchers assume that the speeds of dipping thrust and uplifting of rocks were both very high. In this condition, there will not be enough time for isotopic exchange between crust protolith and mantle materials. Therefore, we can not see the tracer of mantle materials in these UHPM rocks. However, this assumption can not be justified with available knowledge. Firstly, it was estimated that the whole process of UHPM took at least 15 Ma. During such a long period, and at the metamorphic temperature of ≥700 ℃, the protolith of crust origin can not escape from isotopic exchange with mantle materials if the UHPM have happened in the mantle depth of ≥100 km. In contrast, all problems will be dismissed if we assume that the UHPM have happened at the depth still in crust.展开更多
The plastic deformation of garnet in coesite bearing eclogite, quartz eclogite and garnet amphibolite of the UHPM complex in Yingshan County in the Dabie Mountains has been studied. The stress generated by the strong...The plastic deformation of garnet in coesite bearing eclogite, quartz eclogite and garnet amphibolite of the UHPM complex in Yingshan County in the Dabie Mountains has been studied. The stress generated by the strong tectonic movement was an important component of the total pressure that resulted in the formation of the eclogite in the Dabie UHPM zone. The three dimensional tectonic principal stresses and additional tectonic stress induced hydrostatic pressure [ p s=( σ 1+ σ 2+ σ 3)/3] are reconstructed according to the differential stress and the strain ratio ( α ) of the garnet in the minor coesite bearing eclogite of the Yingshan County. Then the gravity induced hydrostatic pressure ( p g) is calculated following the equation p minus p s, where p is estimated to be 2.8 GPa based on the quartz coesite geobarmeter. Therefore, the thickness of the rock column overlying the coesite bearing eclogite in the Ying shan County is determined ≥32 km. This estimation, significantly different from ≥100 km, the previous one obtained solely based on the weight/specific weight ratio (W/SW), offers a proper explanation for the puzzle that no tracer of the addition of mantle derived material has been found in the Dabie UHPM zone during the process of UHPM, although a number of researchers claim that this process took place at the depth of the mantle (≥100 km). It is concluded that attention should be paid to the additional tectonic stress induced hydrostatic pressure in the study of UHPM zones.展开更多
The knowledge that hydrostatic pressure is equal to the gravity value of the overlying rocks in studying a dynamic state of certain underground site is argued. It is suggested that the stress field T in the crust is a...The knowledge that hydrostatic pressure is equal to the gravity value of the overlying rocks in studying a dynamic state of certain underground site is argued. It is suggested that the stress field T in the crust is a combination or superposition of total hydrostatic pressure P with differential stress σ,and the total hydrostatic pressure P at any point in the crust comprises two parts: one is spherical stress tensor P<sub>G</sub> caused by the gravity, and the other is spherical stress tensor P<sub>s</sub> caused by tectonic stress; therefore P could not be attributed to the gravity of overlying rocks only. The results obtained by a finite-element simulation indicate that the tectono-original additional hydrostatic pressures P<sub>s</sub> decrease gradually from the compressive zone （p<sub>c</sub><sup>S</sup>） to the shear zone (P<sub>SH</sub><sup>s</sup>)and to the tensile zone （P<sub>T</sub><sup>s</sup>）, i.e.P<sub>c</sub><sup>s</sup>】P<sub>sH</sub><sup>s</sup>】P<sub>T</sub><sup>s</sup> in the same depth. On the basis of the above-mentioned research, the method of measurement and calculation of metallogenetic depth corrected by P<sub>s</sub> is展开更多
基金supported by funds from the National Natural Science Foundation of China(Grant Nos.42172067,41972064,U1906207)the SDUST Research Fund。
文摘Gneisses with anatectic characteristics from the Liansan island in the Sulu UHPM(ultra-high pressure metamorphic)belt were studied for petrography,titanite U-Pb dating and mineral geochemistry.Three origins of garnets are distinguished:metamorphic garnet,peritectic garnet and anatectic garnet,which are formed in the stages of peak metamorphism,retrograde anatexis and melt crystallization,respectively.The euhedral titanite has a high content of REE and high Th/U ratios,which is interpreted as indicating that it was newly-formed from an anatectic melt.The LA-ICP-MS titanite U-Pb dating yields 214-217 Ma ages for the titanite(melt)crystallization.The distribution of trace elements varies in response to the different host minerals at different stages.At the peak metamorphic stage,Y and HREE are mainly hosted by garnet,Ba and Rb by phengite,Sr,Nb,Ta,Pb,Th,U and LREE by allanite and Y,U and HREE by zircon.During partial melting,Y,Pb,Th,U and REE are released into the melt,which causes a dramatic decline of these element contents in the retrograde minerals.Finally,titanite absorbs most of the Nb,U,LREE and HREE from the melt.Therefore,the different stages of metamorphism have different mineral assemblages,which host different trace elements.
文摘The present-day observed crustal-scale tectonic style of ultrahigh-pressure metamorphic (UH-PM) and high-pressure metamorphic (HPM ) belts in the Dabie-Sulu region was dominantly formedby extensional processes, postdating the Triassic collision between the Sino-Korean and Yangtze cratons. The extensional structures overprinting the previous structures related to contraction that produced the thickened continental crust of the UHPM and HPM belts, in particular display the typical features of a Cordilleran-type metamorphic core complex, in which at least four regionalscale, low-angle ductile shear zones that constitute a detachment system, are recognized in the Dabie region. In the Sulu region, the extensionaI structures show in the form of small-scale domes or a regional-scale SE-dipping pseudo-monocline. The geometry and the kinematics of tbe detachment zones are briefly described and their significance for the exhumation of UHPM and HPM rocks is discussed. It is iniliated that the subhorizontal crustal-scale extensional flow in the middle-lower crust, under amphibolite- to greenschist-facies conditions, was an important tectonothermal process at 200- 170 Ma and the exhumation or the UHPM and HPM rocks was achieved at least in part along multi-layered detachment zones. The regional detachment system has been the main factor enabling UHPM and HPM rocks to be brought from middle-lower crustal levels to middle-upper crustal levels.
文摘The formation depth of metamorphic rocks in the Dabie ultrahigh pressure metamorphic (UHPM) zone influences not only our understanding of formation mechanism and evolution processes of collision orogenic belt, but also the studies on earth's interior and geodynamic processes. In this study, the isotopic data of metamorphic rocks in the Dabie UHPM zone are discussed to give constraints on the formation depth in the Dabie UHPM zone. The ε Sr of eclogite in the Dabie UHPM zone varies from 18 to 42, and the ε Nd varies from -6.1 to -17, both of them show the characters of isotopic disequilibrium. The oxygen isotope studies indicate that the protoliths of these UHPM rocks have experienced oxygen isotope exchange with meteoric water (or sea water) before metamorphism and no significant changes in the processes of metamorphism on their oxygen isotope composition have been recorded in these rocks. Except for one sample from Bixiling, all samples of eclogite from Dabie UHPM zone show the 3He 4He ratios from 0.79×10 -7 to 9.35×10 -7 , indicating the important contribution of He from continental crust. All Sr, Nd, O and He isotopic studies indicate that the UHPM rocks retain the isotopic characteristics of their protoliths of crust origin. No significant influence of mantle materials has been found in these metamorphic rocks. Trying to explain above isotopic characteristics, some researchers assume that the speeds of dipping thrust and uplifting of rocks were both very high. In this condition, there will not be enough time for isotopic exchange between crust protolith and mantle materials. Therefore, we can not see the tracer of mantle materials in these UHPM rocks. However, this assumption can not be justified with available knowledge. Firstly, it was estimated that the whole process of UHPM took at least 15 Ma. During such a long period, and at the metamorphic temperature of ≥700 ℃, the protolith of crust origin can not escape from isotopic exchange with mantle materials if the UHPM have happened in the mantle depth of ≥100 km. In contrast, all problems will be dismissed if we assume that the UHPM have happened at the depth still in crust.
文摘The plastic deformation of garnet in coesite bearing eclogite, quartz eclogite and garnet amphibolite of the UHPM complex in Yingshan County in the Dabie Mountains has been studied. The stress generated by the strong tectonic movement was an important component of the total pressure that resulted in the formation of the eclogite in the Dabie UHPM zone. The three dimensional tectonic principal stresses and additional tectonic stress induced hydrostatic pressure [ p s=( σ 1+ σ 2+ σ 3)/3] are reconstructed according to the differential stress and the strain ratio ( α ) of the garnet in the minor coesite bearing eclogite of the Yingshan County. Then the gravity induced hydrostatic pressure ( p g) is calculated following the equation p minus p s, where p is estimated to be 2.8 GPa based on the quartz coesite geobarmeter. Therefore, the thickness of the rock column overlying the coesite bearing eclogite in the Ying shan County is determined ≥32 km. This estimation, significantly different from ≥100 km, the previous one obtained solely based on the weight/specific weight ratio (W/SW), offers a proper explanation for the puzzle that no tracer of the addition of mantle derived material has been found in the Dabie UHPM zone during the process of UHPM, although a number of researchers claim that this process took place at the depth of the mantle (≥100 km). It is concluded that attention should be paid to the additional tectonic stress induced hydrostatic pressure in the study of UHPM zones.
文摘The knowledge that hydrostatic pressure is equal to the gravity value of the overlying rocks in studying a dynamic state of certain underground site is argued. It is suggested that the stress field T in the crust is a combination or superposition of total hydrostatic pressure P with differential stress σ,and the total hydrostatic pressure P at any point in the crust comprises two parts: one is spherical stress tensor P<sub>G</sub> caused by the gravity, and the other is spherical stress tensor P<sub>s</sub> caused by tectonic stress; therefore P could not be attributed to the gravity of overlying rocks only. The results obtained by a finite-element simulation indicate that the tectono-original additional hydrostatic pressures P<sub>s</sub> decrease gradually from the compressive zone （p<sub>c</sub><sup>S</sup>） to the shear zone (P<sub>SH</sub><sup>s</sup>)and to the tensile zone （P<sub>T</sub><sup>s</sup>）, i.e.P<sub>c</sub><sup>s</sup>】P<sub>sH</sub><sup>s</sup>】P<sub>T</sub><sup>s</sup> in the same depth. On the basis of the above-mentioned research, the method of measurement and calculation of metallogenetic depth corrected by P<sub>s</sub> is
