The Heiyingshan granite and the Laohutai granite plutons exposed in the Southwest Tianshan resemble A-type granites geochemically. Analysis shows that the both are ferron calc-alkalic peraluminous or ferron alkali-cal...The Heiyingshan granite and the Laohutai granite plutons exposed in the Southwest Tianshan resemble A-type granites geochemically. Analysis shows that the both are ferron calc-alkalic peraluminous or ferron alkali-calcic peraluminous with a relatively high concentration of SiO2 （〉70%）, high alkali contents （Na20 ＋ K20 = 7.14%-8.56%; K20〉N20; A/CNK = 0.99-1.20）, and pronounced negative anomales in Eu, Ba, St, P and Ti. A SHRIMP zircon U-Pb age of 285±4 Ma was obtained for the Heiyingshan hornblende biotite granite intrusion. The geochemical and age dating data reported in this paper indicate that these granites were formed during the post-collisional crustal extension of the Southwest Tianshan orogenic belt, in agreement with the published data for the granites in the South Tianshan.展开更多
The Baleigong granites, located in the western part of the southwestern Tianshan Orogen(Kokshanyan region, China), records late Paleozoic magmatism during the late stages of convergence between the Tarim Block and the...The Baleigong granites, located in the western part of the southwestern Tianshan Orogen(Kokshanyan region, China), records late Paleozoic magmatism during the late stages of convergence between the Tarim Block and the Central Tianshan Arc Terrane. We performed a detailed geochronological and geochemical study of the Baleigong granites to better constrain the nature of collisional processes in the Southwest Tianshan Orogen. The LA-ICP-MS U-Pb zircon isotopic analyses indicate that magmatism commenced in the early Permian(~282 Ma). The granite samples, which are characterized by high contents of SiO2(67.68-69.77 wt%) and Al2O3(13.93-14.76 wt%), are alkali-rich and Mg-poor, corresponding to the high-K calc-alkaline series. The aluminum saturation index(A/CNK) ranges from 0.93 to 1.02, indicating a metaluminous to slightly peraluminous composition. Trace element geochemistry shows depletions in Nb, Ta, and Ti, a moderately negative Eu anomaly(δEu=0.40-0.56), enrichment in LREE, and depletion in HREE((La/Yb)N=7.46-11.78). These geochemical signatures are characteristic of an I-type granite generated from partial melting of a magmatic arc. The I-type nature of the Baleigong granites is also supported by the main mafic minerals being Fe-rich calcic hornblende and biotite. We suggest that the high-K, calc-alkaline I-type granitic magmatism was generated by partial melting of the continental crust, possibly triggered by underplating by basaltic magma. These conditions were likely achieved in a collisional tectonic setting, thus supporting the suggestion that closure of the South Tianshan Ocean was completed prior to the Permian and was followed(in the late Paleozoic) by collision between the Tarim Block and the Central Tianshan Arc Terrane.展开更多
The geological characteristics of ultrahigh-pressure (UHP) metamorphic belts formed by deep subduction of oceanic crust are summarized in this paper. Oceanic-type UHP metamorphic belt is characterized by its protolith...The geological characteristics of ultrahigh-pressure (UHP) metamorphic belts formed by deep subduction of oceanic crust are summarized in this paper. Oceanic-type UHP metamorphic belt is characterized by its protolithic assemblage of typical oceanic crust, the peak metamorphic temperature <600℃, P-T path undergoing blueschist facies during prograde and retrograde metamorphic evolution, respectively, with low geothermal gradient of cold subduction. The further study of oceanic-type UHP metamorphic belt is very significant for constructing metamorphic reaction series of cold subduction zone, for understanding how aqueous fluids were transported into deep mantle and for classifying the types of UHP metamorphism in cold subduction zone. The uplift and exhumation mechanism of oceanic UHP metamorphic rocks is one of the most challenging problems in the study of UHP metamorphism, which is very important for understanding the geodynamic mechanism of solid Earth. As a traveler subducted into the mantle depth and then uplifted to the surface, oceanic-type UHP metamorphic belts witness the bulk process from the subduction to exhumation and is an ideal target to study the geochemical behavior and cycling of elements in subduction zones. The tectonic evolution of one convergent orogenic belt can be usually divided into two stages of oceanic subduction and followed continental subduction and collision, and the two best-established examples of orogenic belts are Alps and Himalaya. Therefore, the study of oceanic-type UHP metamorphic belt is the frontier of the current plate tectonic theory. As two case studies, the current status and existing problems of oceanic-type UHP metamorphic belts in Southwest Tianshan and North Qaidam, NW China, are reviewed in this paper.展开更多
Eclogites and omphacite-bearing blueschists have been newly found in the eastern segment of the southwest Tianshan orogenic belt,Xinjiang,northwest China.After detailed petrological study,three samples including one f...Eclogites and omphacite-bearing blueschists have been newly found in the eastern segment of the southwest Tianshan orogenic belt,Xinjiang,northwest China.After detailed petrological study,three samples including one fresh eclogite TK003,one blueschist sample TK026-8 and one retrograded eclogite TK027,were selected for phase equilibrium modeling under NC(K)MnFMASHO(N2O-CaO-K2O-MnO-FeO-MgO-Al2O3-SiO2-H2O-O)system,by thermocalc 3.33 software.Composition analyses of garnets in these three samples show typical growth zoning with Xpy and Xgrs increasing,Xspss decreasing from core to rim.Pseudosection modeling of the garnet zonation reflects that the eclogites and blueschist experienced a similar P-T evolution trajectory,with a near iso-baric heating in the early stage,and reached eclogite facies metamorphic field with peak P-T regime of 480–515°C,2.00–2.30 GPa.Subsequently the rocks experienced an early iso-thermal decompression retrograde stage with P-T conditions of 515–519°C,1.78–1.93 GPa.Variations of mineralogy and modes of these rocks are probably due to different retrograde paths as a consequence of different bulk-rock composition,as well as a variation in fluid activity during exhumation.P-T calculation and a peak geothermal gradient of 6–7°C/km indicate HP rocks in the Kekesu Valley experienced cold subducted eclogite facies metamorphism.Thus a huge oceanic subduction eclogite facies metamorphic belt in southwest Tianshan has been recognized,extending from the Kekesu Valley in the east to the Muzhaerte Valley in the west for nearly200 km.However,UHP evidence has not been found in the Kekesu terrane,perhaps because the slab in east part of southwest Tianshan did not subduct into such a great depth.展开更多
文摘The Heiyingshan granite and the Laohutai granite plutons exposed in the Southwest Tianshan resemble A-type granites geochemically. Analysis shows that the both are ferron calc-alkalic peraluminous or ferron alkali-calcic peraluminous with a relatively high concentration of SiO2 （〉70%）, high alkali contents （Na20 ＋ K20 = 7.14%-8.56%; K20〉N20; A/CNK = 0.99-1.20）, and pronounced negative anomales in Eu, Ba, St, P and Ti. A SHRIMP zircon U-Pb age of 285±4 Ma was obtained for the Heiyingshan hornblende biotite granite intrusion. The geochemical and age dating data reported in this paper indicate that these granites were formed during the post-collisional crustal extension of the Southwest Tianshan orogenic belt, in agreement with the published data for the granites in the South Tianshan.
基金financially supported by the National Natural Science Foundation of China (Grant Nos.U1403292, 41472196, 41502085, and 41902214)the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2015BAB05B04, 2018YFC0604005)the China Geological Survey Bureau (JYYWF20183702, JYYWF20180602)
文摘The Baleigong granites, located in the western part of the southwestern Tianshan Orogen(Kokshanyan region, China), records late Paleozoic magmatism during the late stages of convergence between the Tarim Block and the Central Tianshan Arc Terrane. We performed a detailed geochronological and geochemical study of the Baleigong granites to better constrain the nature of collisional processes in the Southwest Tianshan Orogen. The LA-ICP-MS U-Pb zircon isotopic analyses indicate that magmatism commenced in the early Permian(~282 Ma). The granite samples, which are characterized by high contents of SiO2(67.68-69.77 wt%) and Al2O3(13.93-14.76 wt%), are alkali-rich and Mg-poor, corresponding to the high-K calc-alkaline series. The aluminum saturation index(A/CNK) ranges from 0.93 to 1.02, indicating a metaluminous to slightly peraluminous composition. Trace element geochemistry shows depletions in Nb, Ta, and Ti, a moderately negative Eu anomaly(δEu=0.40-0.56), enrichment in LREE, and depletion in HREE((La/Yb)N=7.46-11.78). These geochemical signatures are characteristic of an I-type granite generated from partial melting of a magmatic arc. The I-type nature of the Baleigong granites is also supported by the main mafic minerals being Fe-rich calcic hornblende and biotite. We suggest that the high-K, calc-alkaline I-type granitic magmatism was generated by partial melting of the continental crust, possibly triggered by underplating by basaltic magma. These conditions were likely achieved in a collisional tectonic setting, thus supporting the suggestion that closure of the South Tianshan Ocean was completed prior to the Permian and was followed(in the late Paleozoic) by collision between the Tarim Block and the Central Tianshan Arc Terrane.
基金National Natural Science Foundation of China (Grant No. 40730314)
文摘The geological characteristics of ultrahigh-pressure (UHP) metamorphic belts formed by deep subduction of oceanic crust are summarized in this paper. Oceanic-type UHP metamorphic belt is characterized by its protolithic assemblage of typical oceanic crust, the peak metamorphic temperature <600℃, P-T path undergoing blueschist facies during prograde and retrograde metamorphic evolution, respectively, with low geothermal gradient of cold subduction. The further study of oceanic-type UHP metamorphic belt is very significant for constructing metamorphic reaction series of cold subduction zone, for understanding how aqueous fluids were transported into deep mantle and for classifying the types of UHP metamorphism in cold subduction zone. The uplift and exhumation mechanism of oceanic UHP metamorphic rocks is one of the most challenging problems in the study of UHP metamorphism, which is very important for understanding the geodynamic mechanism of solid Earth. As a traveler subducted into the mantle depth and then uplifted to the surface, oceanic-type UHP metamorphic belts witness the bulk process from the subduction to exhumation and is an ideal target to study the geochemical behavior and cycling of elements in subduction zones. The tectonic evolution of one convergent orogenic belt can be usually divided into two stages of oceanic subduction and followed continental subduction and collision, and the two best-established examples of orogenic belts are Alps and Himalaya. Therefore, the study of oceanic-type UHP metamorphic belt is the frontier of the current plate tectonic theory. As two case studies, the current status and existing problems of oceanic-type UHP metamorphic belts in Southwest Tianshan and North Qaidam, NW China, are reviewed in this paper.
基金supported by the Major State Basic Research Development Program(Grant No.2009CB825007)the National Natural Science Foundation of China(Grant Nos.41121062&41272069)
文摘Eclogites and omphacite-bearing blueschists have been newly found in the eastern segment of the southwest Tianshan orogenic belt,Xinjiang,northwest China.After detailed petrological study,three samples including one fresh eclogite TK003,one blueschist sample TK026-8 and one retrograded eclogite TK027,were selected for phase equilibrium modeling under NC(K)MnFMASHO(N2O-CaO-K2O-MnO-FeO-MgO-Al2O3-SiO2-H2O-O)system,by thermocalc 3.33 software.Composition analyses of garnets in these three samples show typical growth zoning with Xpy and Xgrs increasing,Xspss decreasing from core to rim.Pseudosection modeling of the garnet zonation reflects that the eclogites and blueschist experienced a similar P-T evolution trajectory,with a near iso-baric heating in the early stage,and reached eclogite facies metamorphic field with peak P-T regime of 480–515°C,2.00–2.30 GPa.Subsequently the rocks experienced an early iso-thermal decompression retrograde stage with P-T conditions of 515–519°C,1.78–1.93 GPa.Variations of mineralogy and modes of these rocks are probably due to different retrograde paths as a consequence of different bulk-rock composition,as well as a variation in fluid activity during exhumation.P-T calculation and a peak geothermal gradient of 6–7°C/km indicate HP rocks in the Kekesu Valley experienced cold subducted eclogite facies metamorphism.Thus a huge oceanic subduction eclogite facies metamorphic belt in southwest Tianshan has been recognized,extending from the Kekesu Valley in the east to the Muzhaerte Valley in the west for nearly200 km.However,UHP evidence has not been found in the Kekesu terrane,perhaps because the slab in east part of southwest Tianshan did not subduct into such a great depth.
