Ultra-high pressure(UHP)eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation
Ultra-high pressure（UHP）eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhum...Ultra-high pressure（UHP）eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation（Hermann et al.,2000;Guillot et al.,2001）.Under normal subduction geotherms,serpentinites break down just before UHP conditions are reached and therefore most of the exhumed eclogites representing subducted oceanic crust formed under fore-arc conditions.We investigated eclogite blocks enclosed into serpentinites that occur in the southwestern Tianshan oceanic subduction,China.A previous study proved that the serpentinites derive from altered oceanic crust and experienced UHP metamorphism at low temperatures of 510-530°C（Shen et al.,2015）.Three relatively fresh eclogite samples were studied in detail.Sample 129-7 shows the retrograde mineral assemblage of amphibole+biotite+albite+chlorite+minor titanite and peak metamorphic relics of omphacite+garnet±chlorite.Sample C107-23 is mainly composed of amphibole+albite+chlorite+zoisite+muscovite+minor titanite as a retrograde assemblage and garnet+phengite as the peak metamorphic relics with omphacite only found as inclusions in garnet.Similar to sample C107-23,sample C11066 preserves large-grained euhedral to subhedral garnet relics with omphacite inclusions,and epidote,diopside,amphibole,muscovite,chlorite,albite and biotite are in the matrix belong to the retrograde assemblage.These three retrograde eclogite samples were modelled using thermodynamic calculations in the Mn NCKFMSHO（Mn O-Na;O-Ca O-K;O-FeO-Mg O-Al;O;-SiO;-H;O-Fe;O;）system.Based on the peak assemblage of omphacite+garnet and the crossing of the grossular and pyrope isopleths in garnet,peak P-T conditions of;60-470oC,28-29 kbar（129-7）,450-500oC,28-35 kbar（C107-23）,;75-505oC,26-29 kbar（C11066）were calculated.The retrograde assemblages indicate near isothermal decompression resulting in a clockwise P-T evolution of these eclogites.The peak metamorphic pressures at 500°C are well within UHP conditions（coesite stability field）and are within error the same as peak conditions of the host serpentinites（Shen et al.,2015）.This provides evidence that eclogites and serpentinites shared the same evolution.We infer that the subducted low-density serpentinites were assembled with the high-density eclogites during subdution and helped the latter to exhume back to the surface.The studied eclogites thus represent rare examples of relics of oceanic crust that was subducted to sub-arc depth.展开更多
In the southwestern Tianshan,the geologic ages of many strata and their depositional environments are still poorly constrained because of complex structures.The Mada＇er area is located in the Kuokesaleling belt,South...In the southwestern Tianshan,the geologic ages of many strata and their depositional environments are still poorly constrained because of complex structures.The Mada＇er area is located in the Kuokesaleling belt,Southwestern Tianshan.The cherts from the former Wupata＇erkan Group contain abundant radiolarian fossils,including 10 species which are identified as late Devonian to early Carboniferous in age.Eleven chert samples have SiO_2 contents ranging from 88.80 wt%to 93.28 wt%, and 2.02 wt%to 3.72 wt%for Al_2O_3.The SiO_2/Al_2O_3 ratios of all samples vary from 23.84 to 46.11, much lower than those of the pure cherts（80-1400）.These values suggest that the cherts contain high ratios of terrigenous materials.The Al_2O_3/（Al_2O_3＋Fe_2O_3）ratios vary between 0.64 and 0.77,whereas V and Cu concentrations range from 10.92 ppm to 26.7 ppm and from 2.15 ppm to 34.1 ppm respectively. The Ti/V ratios vary from 25.53 to 44.93.The total REE concentrations of the cherts are between 30.78 ppm and 59.26 ppm,averaging 45.46 ppm.The（La/Ce）_N ratios range from 0.81 to 1.12,and 0.88-1.33 for（La/Yb）_N,averaging 1.09,which suggests a continental margin environment.Consequently,it is inferred that the cherts formed in a residual sea environment during the late Devonian to early Carboniferous time,which suggests that the collision between the Karakum-Tarim and Kazakhstan-Junggar plates did not occur at the time.In addition,the regional geological information indicates that the study area experienced a post-collision stage during the early Permian,and thus it is likely that the collision between the two plates took place in the late Carboniferous.展开更多
The Weiquan Ag-polymetallic deposit is located on the southern margin of the Central Asian Orogenic Belt and in the western segment of the Aqishan-Yamansu arc belt in East Tianshan,northwestern China. Its orebodies, c...The Weiquan Ag-polymetallic deposit is located on the southern margin of the Central Asian Orogenic Belt and in the western segment of the Aqishan-Yamansu arc belt in East Tianshan,northwestern China. Its orebodies, controlled by faults, occur in the lower Carboniferous volcanosedimentary rocks of the Yamansu Formation as irregular veins and lenses. Four stages of mineralization have been recognized on the basis of mineral assemblages, ore fabrics, and crosscutting relationships among the ore veins. Stage I is the skarn stage(garnet + pyroxene), Stage Ⅱ is the retrograde alteration stage(epidote + chlorite + magnetite ± hematite 士 actinolite ± quartz),Stage Ⅲ is the sulfide stage(Ag and Bi minerals + pyrite + chalcopyrite + galena + sphalerite + quartz ± calcite ± tetrahedrite),and Stage IV is the carbonate stage(quartz + calcite ± pyrite). Skarnization,silicification, carbonatization,epidotization,chloritization, sericitization, and actinolitization are the principal types of hydrothermal alteration. LAICP-MS U-Pb dating yielded ages of 326.5±4.5 and 298.5±1.5 Ma for zircons from the tuff and diorite porphyry, respectively. Given that the tuff is wall rock and that the orebodies are cut by a late diorite porphyry dike, the ages of the tuff and the diorite porphyry provide lower and upper time limits on the age of ore formation. The δ13C values of the calcite samples range from-2.5‰ to 2.3‰, the δ18OH2 Oand δDVSMOWvalues of the sulfide stage(Stage Ⅲ) vary from 1.1‰ to 5.2‰ and-111.7‰ to-66.1‰, respectively,and the δ13C, δ18OH2 Oand δDV-SMOWvalues of calcite in one Stage IV sample are 1.5‰,-0.3‰, and-115.6‰, respectively. Carbon, hydrogen, and oxygen isotopic compositions indicate that the ore-forming fluids evolved gradually from magmatic to meteoric sources. The δ34SV-CDTvalues of the sulfides have a large range from-6.9‰ to 1.4‰, with an average of-2.2‰, indicating a magmatic source, possibly with sedimentary contributions. The206Pb/204Pb,207Pb/204Pb, and208Pb/204Pb ratios of the sulfides are 17.9848-18.2785,15.5188-15.6536, and 37.8125-38.4650, respectively, and one whole-rock sample at Weiquan yields206Pb/204Pb,207Pb/204Pb, and208Pb/204Pb ratios of 18.2060, 15.5674, and 38.0511,respectively. Lead isotopic systems suggest that the ore-forming materials of the Weiquan deposit were derived from a mixed source involving mantle and crustal components. Based on geological features, zircon U-Pb dating, and C-H-OS-Pb isotopic data, it can be concluded that the Weiquan polymetallic deposit is a skarn type that formed in a tectonic setting spanning a period from subduction to post-collision. The ore materials were sourced from magmatic ore-forming fluids that mixed with components derived from host rocks during their ascent, and a gradual mixing with meteoric water took place in the later stages.展开更多
Recent progress in the study of the UHP metamorphic belt in southwestern Tianshan,China,is summarized in this paper.This about 80-km-long and over 10-km-wide UHP belt has been recognized by the discovery of coesite,co...Recent progress in the study of the UHP metamorphic belt in southwestern Tianshan,China,is summarized in this paper.This about 80-km-long and over 10-km-wide UHP belt has been recognized by the discovery of coesite,coesite pseudomorphs and other UHP minerals.It is the largest oceanic-type UHP metamorphic belt reported so far.It has formed due to northward subduction of the Tianshan Paleo-Ocean.U-Pb dating of metamorphic rims of zircons from a coesite-bearing garnet-phengite schist yields a peak UHP metamorphic ages of 320±3.7 Ma.Combined with ages of 233–226 Ma obtained from rims of zircons from retrograded eclogites,a long retrograde metamorphic evolution(>70 Ma)has been revealed.According to phase equilibria modeling,the P-T paths of both coesite-bearing eclogites and garnet-phengite schists are characterized by thermal relaxation,i.e.,the metamorphic temperature peak lags behind the pressure peak,indicating that the UHP rocks experienced slow and long heating and decompression during exhumation in the subduction channel.On the basis of the field observation that a small amount of eclogite lenses is wrapped in large volumes of metapelites,and the similar P-T paths of both rock types,we propose that the exhumation of the UHP eclogites from southwestern Tianshan,China,may have resulted from the exhumation of large volumes of low-density metapelites,which carried the denser eclogites to the Earth’s surface.展开更多
文摘Ultra-high pressure(UHP)eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation
文摘Ultra-high pressure（UHP）eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation（Hermann et al.,2000;Guillot et al.,2001）.Under normal subduction geotherms,serpentinites break down just before UHP conditions are reached and therefore most of the exhumed eclogites representing subducted oceanic crust formed under fore-arc conditions.We investigated eclogite blocks enclosed into serpentinites that occur in the southwestern Tianshan oceanic subduction,China.A previous study proved that the serpentinites derive from altered oceanic crust and experienced UHP metamorphism at low temperatures of 510-530°C（Shen et al.,2015）.Three relatively fresh eclogite samples were studied in detail.Sample 129-7 shows the retrograde mineral assemblage of amphibole+biotite+albite+chlorite+minor titanite and peak metamorphic relics of omphacite+garnet±chlorite.Sample C107-23 is mainly composed of amphibole+albite+chlorite+zoisite+muscovite+minor titanite as a retrograde assemblage and garnet+phengite as the peak metamorphic relics with omphacite only found as inclusions in garnet.Similar to sample C107-23,sample C11066 preserves large-grained euhedral to subhedral garnet relics with omphacite inclusions,and epidote,diopside,amphibole,muscovite,chlorite,albite and biotite are in the matrix belong to the retrograde assemblage.These three retrograde eclogite samples were modelled using thermodynamic calculations in the Mn NCKFMSHO（Mn O-Na;O-Ca O-K;O-FeO-Mg O-Al;O;-SiO;-H;O-Fe;O;）system.Based on the peak assemblage of omphacite+garnet and the crossing of the grossular and pyrope isopleths in garnet,peak P-T conditions of;60-470oC,28-29 kbar（129-7）,450-500oC,28-35 kbar（C107-23）,;75-505oC,26-29 kbar（C11066）were calculated.The retrograde assemblages indicate near isothermal decompression resulting in a clockwise P-T evolution of these eclogites.The peak metamorphic pressures at 500°C are well within UHP conditions（coesite stability field）and are within error the same as peak conditions of the host serpentinites（Shen et al.,2015）.This provides evidence that eclogites and serpentinites shared the same evolution.We infer that the subducted low-density serpentinites were assembled with the high-density eclogites during subdution and helped the latter to exhume back to the surface.The studied eclogites thus represent rare examples of relics of oceanic crust that was subducted to sub-arc depth.
基金supported by National 305 Project(2007BAB25B05,2011BAB06B02- 04)NSFC grant(40925006,40772045,40572047)111 Project(B07011)
文摘In the southwestern Tianshan,the geologic ages of many strata and their depositional environments are still poorly constrained because of complex structures.The Mada＇er area is located in the Kuokesaleling belt,Southwestern Tianshan.The cherts from the former Wupata＇erkan Group contain abundant radiolarian fossils,including 10 species which are identified as late Devonian to early Carboniferous in age.Eleven chert samples have SiO_2 contents ranging from 88.80 wt%to 93.28 wt%, and 2.02 wt%to 3.72 wt%for Al_2O_3.The SiO_2/Al_2O_3 ratios of all samples vary from 23.84 to 46.11, much lower than those of the pure cherts（80-1400）.These values suggest that the cherts contain high ratios of terrigenous materials.The Al_2O_3/（Al_2O_3＋Fe_2O_3）ratios vary between 0.64 and 0.77,whereas V and Cu concentrations range from 10.92 ppm to 26.7 ppm and from 2.15 ppm to 34.1 ppm respectively. The Ti/V ratios vary from 25.53 to 44.93.The total REE concentrations of the cherts are between 30.78 ppm and 59.26 ppm,averaging 45.46 ppm.The（La/Ce）_N ratios range from 0.81 to 1.12,and 0.88-1.33 for（La/Yb）_N,averaging 1.09,which suggests a continental margin environment.Consequently,it is inferred that the cherts formed in a residual sea environment during the late Devonian to early Carboniferous time,which suggests that the collision between the Karakum-Tarim and Kazakhstan-Junggar plates did not occur at the time.In addition,the regional geological information indicates that the study area experienced a post-collision stage during the early Permian,and thus it is likely that the collision between the two plates took place in the late Carboniferous.
基金funded by the China Geological Survey (No. 1212011220731)
文摘The Weiquan Ag-polymetallic deposit is located on the southern margin of the Central Asian Orogenic Belt and in the western segment of the Aqishan-Yamansu arc belt in East Tianshan,northwestern China. Its orebodies, controlled by faults, occur in the lower Carboniferous volcanosedimentary rocks of the Yamansu Formation as irregular veins and lenses. Four stages of mineralization have been recognized on the basis of mineral assemblages, ore fabrics, and crosscutting relationships among the ore veins. Stage I is the skarn stage(garnet + pyroxene), Stage Ⅱ is the retrograde alteration stage(epidote + chlorite + magnetite ± hematite 士 actinolite ± quartz),Stage Ⅲ is the sulfide stage(Ag and Bi minerals + pyrite + chalcopyrite + galena + sphalerite + quartz ± calcite ± tetrahedrite),and Stage IV is the carbonate stage(quartz + calcite ± pyrite). Skarnization,silicification, carbonatization,epidotization,chloritization, sericitization, and actinolitization are the principal types of hydrothermal alteration. LAICP-MS U-Pb dating yielded ages of 326.5±4.5 and 298.5±1.5 Ma for zircons from the tuff and diorite porphyry, respectively. Given that the tuff is wall rock and that the orebodies are cut by a late diorite porphyry dike, the ages of the tuff and the diorite porphyry provide lower and upper time limits on the age of ore formation. The δ13C values of the calcite samples range from-2.5‰ to 2.3‰, the δ18OH2 Oand δDVSMOWvalues of the sulfide stage(Stage Ⅲ) vary from 1.1‰ to 5.2‰ and-111.7‰ to-66.1‰, respectively,and the δ13C, δ18OH2 Oand δDV-SMOWvalues of calcite in one Stage IV sample are 1.5‰,-0.3‰, and-115.6‰, respectively. Carbon, hydrogen, and oxygen isotopic compositions indicate that the ore-forming fluids evolved gradually from magmatic to meteoric sources. The δ34SV-CDTvalues of the sulfides have a large range from-6.9‰ to 1.4‰, with an average of-2.2‰, indicating a magmatic source, possibly with sedimentary contributions. The206Pb/204Pb,207Pb/204Pb, and208Pb/204Pb ratios of the sulfides are 17.9848-18.2785,15.5188-15.6536, and 37.8125-38.4650, respectively, and one whole-rock sample at Weiquan yields206Pb/204Pb,207Pb/204Pb, and208Pb/204Pb ratios of 18.2060, 15.5674, and 38.0511,respectively. Lead isotopic systems suggest that the ore-forming materials of the Weiquan deposit were derived from a mixed source involving mantle and crustal components. Based on geological features, zircon U-Pb dating, and C-H-OS-Pb isotopic data, it can be concluded that the Weiquan polymetallic deposit is a skarn type that formed in a tectonic setting spanning a period from subduction to post-collision. The ore materials were sourced from magmatic ore-forming fluids that mixed with components derived from host rocks during their ascent, and a gradual mixing with meteoric water took place in the later stages.
基金supported by the National Basic Research Program of China(2009CB825007)the National Natural Science Foundation of China(41121062 and41272069)
文摘Recent progress in the study of the UHP metamorphic belt in southwestern Tianshan,China,is summarized in this paper.This about 80-km-long and over 10-km-wide UHP belt has been recognized by the discovery of coesite,coesite pseudomorphs and other UHP minerals.It is the largest oceanic-type UHP metamorphic belt reported so far.It has formed due to northward subduction of the Tianshan Paleo-Ocean.U-Pb dating of metamorphic rims of zircons from a coesite-bearing garnet-phengite schist yields a peak UHP metamorphic ages of 320±3.7 Ma.Combined with ages of 233–226 Ma obtained from rims of zircons from retrograded eclogites,a long retrograde metamorphic evolution(>70 Ma)has been revealed.According to phase equilibria modeling,the P-T paths of both coesite-bearing eclogites and garnet-phengite schists are characterized by thermal relaxation,i.e.,the metamorphic temperature peak lags behind the pressure peak,indicating that the UHP rocks experienced slow and long heating and decompression during exhumation in the subduction channel.On the basis of the field observation that a small amount of eclogite lenses is wrapped in large volumes of metapelites,and the similar P-T paths of both rock types,we propose that the exhumation of the UHP eclogites from southwestern Tianshan,China,may have resulted from the exhumation of large volumes of low-density metapelites,which carried the denser eclogites to the Earth’s surface.
