Origin and tectonic evolution of the Qilian Precambrian basement on NW China were investigated using zircon U-Pb ages with collaborating stratigraphic and paleontological evidence. Zircon grains were separated from tw...Origin and tectonic evolution of the Qilian Precambrian basement on NW China were investigated using zircon U-Pb ages with collaborating stratigraphic and paleontological evidence. Zircon grains were separated from two schists, two granitic gneisses and one mylonized gneiss and dated with SHRIMP. Seventy percent of sixty-one detrital zircon ages from two schists ranges from 0.88 Ga to 3.09 Ga, mostly within 1.0 Ga to 1.8 Ga with a peak at 1.6 Ga to 1.8 Ga, and twenty percent varies from 2.0 Ga to 2.5 Ga. A few falls in the Archean and Neoproterozoic periods. The two granitic gneisses were dated 930±8 Ma and 918±14 Ma, whereas the mylonized granitic gneiss was dated 790±12 Ma. These ages represent two periods of magmatisms, which can be correlated with the early and late stages of magmatisms associated with the Jinningian movement on the Yangtze Blocks. The results from this and previous studies indicate that the ages of the Precambrian detrital zircons from the Qilian Block are widely distributed in the Proterozoic era, distinct from the North China Block which was stable in the Neo-Mesoproterozoic era. By contrast, the age histograms of the detrital zircons from the Qilian Block is similar to those from Precambrian basement of the Yangtze Craton. Therefore, it is suggested that the Qilian Block had a strong affinity toward the Yangtze Craton and might belong to the supercontinent Gondwana in the Neoproterozoic time. This inference is supported by Nd model age (TDM), stratigraphic, and paleontological evidence. It is further considered that the Qilian Block was rifted from the supercontinent Gondwana during late Sinian to form an isolated continent in the Proto-Tethyan Ocean, moving towards the Alaxa Block in the North China Craton. The part of Proto-Tethyan Ocean between the Qilian and Alaxa Blocks should correspond to the so-called Paleo-Qilian Ocean. Following the closure of the Paleo-Qilian Ocean in the early Paleozoic, the Qilian Block collided with the Alaxa Block to form the North Qilian Orogenic Belt. 展开更多
Quantitative analysis of Early Eocene radiolarian assemblages discovered in the sedimentary mélange (accretionary prism) of the Saga area,southern Tibet provides new information to constrain the timing of Tethys ...Quantitative analysis of Early Eocene radiolarian assemblages discovered in the sedimentary mélange (accretionary prism) of the Saga area,southern Tibet provides new information to constrain the timing of Tethys closure and the initial collision of India and Eurasia. The radiolarian species of Saga include Amphisphaera coronata (Ehrenberg),Buryella hannae Bak & Barwicz-Piskorz,Buryella clinata Foreman,Buryella tetradica Foreman,Calocycloma ampulla (Ehrenberg),Lamptonium fabaeforme constrictum Riedel and Sanfilippo,Lamptonium pennatum Foreman,Lithomespilus coronatus Squinabol and Lamptonium (?) colymbus Foreman. The adequate and reliable correlation of these radiolarians specimens indicates that the assemblage is of Early Eocene in age. The age and depositional envi-ronment of these radiolarians testify that deep ocean basins existed between India Plate and Asia Plate during the Early Eocene. The complete closure of Tethys must have taken place at least after the Early Eocene.展开更多
基金the Chinese Development Found and National Science Council (Grant Nos. 91-2116-M-006-16 and 92-2116-M-006-010)
文摘Origin and tectonic evolution of the Qilian Precambrian basement on NW China were investigated using zircon U-Pb ages with collaborating stratigraphic and paleontological evidence. Zircon grains were separated from two schists, two granitic gneisses and one mylonized gneiss and dated with SHRIMP. Seventy percent of sixty-one detrital zircon ages from two schists ranges from 0.88 Ga to 3.09 Ga, mostly within 1.0 Ga to 1.8 Ga with a peak at 1.6 Ga to 1.8 Ga, and twenty percent varies from 2.0 Ga to 2.5 Ga. A few falls in the Archean and Neoproterozoic periods. The two granitic gneisses were dated 930±8 Ma and 918±14 Ma, whereas the mylonized granitic gneiss was dated 790±12 Ma. These ages represent two periods of magmatisms, which can be correlated with the early and late stages of magmatisms associated with the Jinningian movement on the Yangtze Blocks. The results from this and previous studies indicate that the ages of the Precambrian detrital zircons from the Qilian Block are widely distributed in the Proterozoic era, distinct from the North China Block which was stable in the Neo-Mesoproterozoic era. By contrast, the age histograms of the detrital zircons from the Qilian Block is similar to those from Precambrian basement of the Yangtze Craton. Therefore, it is suggested that the Qilian Block had a strong affinity toward the Yangtze Craton and might belong to the supercontinent Gondwana in the Neoproterozoic time. This inference is supported by Nd model age (TDM), stratigraphic, and paleontological evidence. It is further considered that the Qilian Block was rifted from the supercontinent Gondwana during late Sinian to form an isolated continent in the Proto-Tethyan Ocean, moving towards the Alaxa Block in the North China Craton. The part of Proto-Tethyan Ocean between the Qilian and Alaxa Blocks should correspond to the so-called Paleo-Qilian Ocean. Following the closure of the Paleo-Qilian Ocean in the early Paleozoic, the Qilian Block collided with the Alaxa Block to form the North Qilian Orogenic Belt.
基金the National Natural Science Foundation of China (Grant No.40672086)the National Key Basic Research Program of China (Grant No.2006CB701400)
文摘Quantitative analysis of Early Eocene radiolarian assemblages discovered in the sedimentary mélange (accretionary prism) of the Saga area,southern Tibet provides new information to constrain the timing of Tethys closure and the initial collision of India and Eurasia. The radiolarian species of Saga include Amphisphaera coronata (Ehrenberg),Buryella hannae Bak & Barwicz-Piskorz,Buryella clinata Foreman,Buryella tetradica Foreman,Calocycloma ampulla (Ehrenberg),Lamptonium fabaeforme constrictum Riedel and Sanfilippo,Lamptonium pennatum Foreman,Lithomespilus coronatus Squinabol and Lamptonium (?) colymbus Foreman. The adequate and reliable correlation of these radiolarians specimens indicates that the assemblage is of Early Eocene in age. The age and depositional envi-ronment of these radiolarians testify that deep ocean basins existed between India Plate and Asia Plate during the Early Eocene. The complete closure of Tethys must have taken place at least after the Early Eocene.
