U–Pb zircon geochronology and geochemistry of Neoproterozoic volcanic rocks in the Tarim Block of northwest China: implications for the breakup of Rodinia supercontinent and Neoproterozoic glaciations |
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| Institution: | 1. Key Lab of Submarine Geosciences and Prospecting Techniques, Ministry of Education, and College of Marine Geosciences, Ocean University of China, Qingdao 266100, China;2. Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China;3. UCD School of Earth Sciences, University College Dublin, Belfield, Dublin 4, Ireland;1. Nanjing Institute of Geology and Mineral Resources, Nanjing 210016, China;2. School of Earth Science and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China;3. Department of Geology and Geography, Auburn University, Auburn 36849-5305, USA;4. Tianjin Institute of Geology and Mineral Resources, Tianjin 300170, China;1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. Nanjing Institute of Geology and Mineral Resources, China Geological Survey, Nanjing 210016, China |
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| Abstract: | As the lowest volcanics-bearing unit of the Neoproterozoic succession, the Beiyixi Formation is the key to understanding the early response to the breakup of the Roninia supercontinent in the Tarim Block. The SHRIMP analyses of zircons from the volcanic rocks at the bottom of the Beiyixi Formation yield a weighted mean 206Pb/238U age of 755 ± 15 Ma. This is interpreted as the eruption age of the Beiyixi volcanic rocks. The Beiyixi volcanic rocks consist of bimodal basalt and dacite-rhyolite with a SiO2 gap between 55% and 65%. The mafic rocks display negative ?Nd (755 Ma) values (?9.9 to ?10.8), moderate enrichment in LILE and variable depletion in Nb, Ta and P, resembling those of the tholeiitic basalts in continental rift. Geochemical and Nd isotopic characteristics suggest that the mafic rocks were derived from partial melting of an enriched lithospheric mantle reservoir. The felsic rocks show negative ?Nd (755 Ma) values (?7.9 to ?9.2), negative Nb, Ta, P and Ti anomalies, very high LaN/YbN (62–92) ratios and LILE abundances, and may be generated by melting of eclogites or garnet amphibolites in the lower crust, as a result of basalt emplacement into continental crust during continental rifting. The age of 755 ± 15 Ma indicates that the Beiyixi glaciation took place later than 755 Ma and it could be correlated with the Chang’an glaciation in the Yangtze Block and the Sturtian–Rapitan glaciation in other Rodinia Blocks. The geochemical characteristics of the Beiyixi volcanic rocks resemble those of the rift-related magmatism in other Rodinia Blocks, suggesting that the Beiyixi volcanism was a part of global magmatism during the breakup of Rodinia supercontinent. The age and geochemical features of the Beiyixi volcanic rocks also reveal that the mantle plume activity spread to the northwestern margin of the Rodinia supercontinent and probably resulted in the breakup between Australia and Tarim Blocks. |
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