Zircon SHRIMP U-Pb dating was per- formed for olivine gabbro at Wangmuguan in the Beihuaiyang zone of the Dabie orogen and its coun- try rock (garnet-bearing epidote-mica-quartz schist). The results show that the gabb...Zircon SHRIMP U-Pb dating was per- formed for olivine gabbro at Wangmuguan in the Beihuaiyang zone of the Dabie orogen and its coun- try rock (garnet-bearing epidote-mica-quartz schist). The results show that the gabbro was crystallized at 635 ± 5 Ma, in the late Neoproterozoic rather than in the late Paleozoic as previously suggested; its coun- try rocks formed at 464±7 Ma, younger than the en- closed gabbro. The U-Pb age for the gabbro is in good agreement with ages for tuff interbedded with sediments from the Doushantuo Formation in the South China Block and late-Neoproterozoic basic dyke swarms distributed on a large scale over areas of Suizhou to Zaoyang of Hubei Province in the northern margin of the South China Block. This sug- gests a large-scale magmatic activity occurred at the late Neoproterozoic in the South China Block, so that the gabbro at Wangmuguan in the western segment of the Beihuaiyang zone is geotectonic affinity to the northern margin of the South China Block. Since the olivine gabbro occurs within the schist of Ordovician protolith with tectonic contact between them but forming in different tectonic settings, it is concluded that the late-Neoproterozoic gabbro was detached from the Precambrian basement of the South China Block during the Triassic subduction of the South China Block, and tectonically thrusted over the metamorphosed rocks in the southern margin of the North China Block.展开更多
Based on detailed field investigations and petrographic observations,we discovered Neoproterozoic-emplaced granite from the metavolcanics of the Dingyuan Formation in the western Beihuaiyang zone,the Dabie orogen. Thi...Based on detailed field investigations and petrographic observations,we discovered Neoproterozoic-emplaced granite from the metavolcanics of the Dingyuan Formation in the western Beihuaiyang zone,the Dabie orogen. This study reports the results of zircon U-Pb dating and preliminary petrographic observations on two metagranite samples. The studied rocks experienced epidote-amphibolite facies metamorphism and strong structural deformation. Their U-Pb ages are 726 ± 6 and 758 ± 12 Ma,respectively,similar to those for the Luzhenguan complex in the eastern segment of the Beihuaiyang zone. In combination with previously determined 635 ± 5 Ma low-grade metagabbro,this study suggests the occurrence of at least two types of Neoproterozoic intrusive rocks in the Beihuaiyang zone,the northern margin of the South China Block (SCB):726-758 Ma metagranite and 635 Ma metagabbro. These rocks occur within the metamorphosed Ordovician volcanic zone (originally named the Dingyuan Formation) and are in tectonic contact to each other,but they formed in different tectonic settings. The protolith ages for the Neoproterozoic low-grade metaigneous rocks are in good agreement not only with ages for two episodes of mid-and late-Neoproterozoic mafic and felsic magmatism in the Suizhou and Zaoyang area,Hubei Province,but also agree with protolith ages of ultrahigh-pressure metaigneous rocks in the Dabie-Sulu orogenic belt. In view of their tectonic relationships to country rocks,it appears that these Neoproterozoic low-grade rocks are exotic and they may have been detached and scraped from subducting SCB crust in the early Triassic during the initial continental subduction,and thrusted over Paleozoic metamorphosed rocks in the southern margin of the North China Block during continental collision.展开更多
The transitional pressure of quartz coesite under the differential stress and highly strained conditions is far from the pressure of the stable field under the static pressure. Therefore, the effect of the different...The transitional pressure of quartz coesite under the differential stress and highly strained conditions is far from the pressure of the stable field under the static pressure. Therefore, the effect of the differential stress should be considered when the depth of petrogenesis is estimated about ultrahigh pressure metamorphic (UHPM) rocks. The rheological strength of typical ultrahigh pressure rocks in continental subduction zone was derived from the results of the laboratory experiments. The results indicate the following three points. (1) The rheological strength of gabbro, similar to that of eclogite, is smaller than that of clinopyroxenite on the same condition. (2) The calculated strength of rocks (gabbro, eclogite and clinopyroxenite) related to UHPM decreases by nearly one order of magnitude with the temperature rising by 100 ℃ in the range between 600 and 900 ℃. The calculated strength is far greater than the faulting strength of rocks at 600 ℃, and is in several hundred to more than one thousand mega pascals at 700-800 ℃, which suggests that those rocks are located in the brittle deformation region at 600 ℃, but are in the semi brittle to plastic deformation region at 700-800 ℃. Obviously, the 700 ℃ is a brittle plastic transition boundary. (3) The calculated rheological strength in the localized deformation zone on a higher strain rate condition (1.6×10 -12 s -l ) is 2-5 times more than that in the distributed deformation zone on a lower strain rate condition (1.6×10 -14 s -1 ). The average rheological stress (1 600 MPa) at the strain rate of 10 -12 s -1 stands for the ultimate differential stress of UHPM rocks in the semi brittle flow field, and the average rheological stress (550-950 MPa) at the strain rate of l0 -14 - 10 -13 s -l stands for the ultimate differential stress of UHPM rocks in the plastic flow field, suggesting that the depth for the formation of UHPM rocks is more than 20-60 km below the depth estimated under static pre展开更多
基金This work was financially supported by Chinese National Key Project for Basic Research （Grant No. 2003CB716500） the National Natural Science Foundation of China （Grant Nos. 40572035 , 40373009）.
文摘Zircon SHRIMP U-Pb dating was per- formed for olivine gabbro at Wangmuguan in the Beihuaiyang zone of the Dabie orogen and its coun- try rock (garnet-bearing epidote-mica-quartz schist). The results show that the gabbro was crystallized at 635 ± 5 Ma, in the late Neoproterozoic rather than in the late Paleozoic as previously suggested; its coun- try rocks formed at 464±7 Ma, younger than the en- closed gabbro. The U-Pb age for the gabbro is in good agreement with ages for tuff interbedded with sediments from the Doushantuo Formation in the South China Block and late-Neoproterozoic basic dyke swarms distributed on a large scale over areas of Suizhou to Zaoyang of Hubei Province in the northern margin of the South China Block. This sug- gests a large-scale magmatic activity occurred at the late Neoproterozoic in the South China Block, so that the gabbro at Wangmuguan in the western segment of the Beihuaiyang zone is geotectonic affinity to the northern margin of the South China Block. Since the olivine gabbro occurs within the schist of Ordovician protolith with tectonic contact between them but forming in different tectonic settings, it is concluded that the late-Neoproterozoic gabbro was detached from the Precambrian basement of the South China Block during the Triassic subduction of the South China Block, and tectonically thrusted over the metamorphosed rocks in the southern margin of the North China Block.
基金supported by the National Basic Research Program of China (2009CB825002)the National Natural Science Foundation of China (40921002,40973043,40634023 and 90814008)
文摘Based on detailed field investigations and petrographic observations,we discovered Neoproterozoic-emplaced granite from the metavolcanics of the Dingyuan Formation in the western Beihuaiyang zone,the Dabie orogen. This study reports the results of zircon U-Pb dating and preliminary petrographic observations on two metagranite samples. The studied rocks experienced epidote-amphibolite facies metamorphism and strong structural deformation. Their U-Pb ages are 726 ± 6 and 758 ± 12 Ma,respectively,similar to those for the Luzhenguan complex in the eastern segment of the Beihuaiyang zone. In combination with previously determined 635 ± 5 Ma low-grade metagabbro,this study suggests the occurrence of at least two types of Neoproterozoic intrusive rocks in the Beihuaiyang zone,the northern margin of the South China Block (SCB):726-758 Ma metagranite and 635 Ma metagabbro. These rocks occur within the metamorphosed Ordovician volcanic zone (originally named the Dingyuan Formation) and are in tectonic contact to each other,but they formed in different tectonic settings. The protolith ages for the Neoproterozoic low-grade metaigneous rocks are in good agreement not only with ages for two episodes of mid-and late-Neoproterozoic mafic and felsic magmatism in the Suizhou and Zaoyang area,Hubei Province,but also agree with protolith ages of ultrahigh-pressure metaigneous rocks in the Dabie-Sulu orogenic belt. In view of their tectonic relationships to country rocks,it appears that these Neoproterozoic low-grade rocks are exotic and they may have been detached and scraped from subducting SCB crust in the early Triassic during the initial continental subduction,and thrusted over Paleozoic metamorphosed rocks in the southern margin of the North China Block during continental collision.
文摘The transitional pressure of quartz coesite under the differential stress and highly strained conditions is far from the pressure of the stable field under the static pressure. Therefore, the effect of the differential stress should be considered when the depth of petrogenesis is estimated about ultrahigh pressure metamorphic (UHPM) rocks. The rheological strength of typical ultrahigh pressure rocks in continental subduction zone was derived from the results of the laboratory experiments. The results indicate the following three points. (1) The rheological strength of gabbro, similar to that of eclogite, is smaller than that of clinopyroxenite on the same condition. (2) The calculated strength of rocks (gabbro, eclogite and clinopyroxenite) related to UHPM decreases by nearly one order of magnitude with the temperature rising by 100 ℃ in the range between 600 and 900 ℃. The calculated strength is far greater than the faulting strength of rocks at 600 ℃, and is in several hundred to more than one thousand mega pascals at 700-800 ℃, which suggests that those rocks are located in the brittle deformation region at 600 ℃, but are in the semi brittle to plastic deformation region at 700-800 ℃. Obviously, the 700 ℃ is a brittle plastic transition boundary. (3) The calculated rheological strength in the localized deformation zone on a higher strain rate condition (1.6×10 -12 s -l ) is 2-5 times more than that in the distributed deformation zone on a lower strain rate condition (1.6×10 -14 s -1 ). The average rheological stress (1 600 MPa) at the strain rate of 10 -12 s -1 stands for the ultimate differential stress of UHPM rocks in the semi brittle flow field, and the average rheological stress (550-950 MPa) at the strain rate of l0 -14 - 10 -13 s -l stands for the ultimate differential stress of UHPM rocks in the plastic flow field, suggesting that the depth for the formation of UHPM rocks is more than 20-60 km below the depth estimated under static pre
