The geochemical characteristics of metabasites with a pseudo-pillow structure from Ganghe,Dabie Orogen,China |
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| Institution: | 1. CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China;2. Gold Geological Institute of CAPF, Langfang, Hebei 065000, China;1. School of Economics and Management, Southwest Jiaotong University, Chengdu 610031, China;2. School of Information Science and Technology, Southwest Jiaotong University, Chengdu 610031, China;1. Department of Earth Sciences, Faculty of Geosciences, Utrecht University, 3584CD Utrecht, The Netherlands;2. State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, China;1. Department of Multimedia Technology, Faculty of Engineering and Information Technology, The Arab American University, Palestine;2. University of Lyon, Campus de la Doua, France;1. Institut für Geowissenschaften, Goethe-Universität Frankfurt, Altenhöferallee 1, D-60438 Frankfurt am Main, Germany;2. Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa;1. Key Laboratory of Cloud Computing and Intelligent Information Processing of Changzhou City, Jiangsu University of Technology, Changzhou 213001, China;2. School of Computer Engineering, Jiangsu University of Technology, Changzhou 213001, China;3. School of Computer Engineering and Science, Shanghai University, Shanghai 200444, China;4. Department of Mathematics and Computer Science, Gannan Normal University, Ganzhou 341000, China |
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| Abstract: | Several metabasite lenses in Ganghe, Central Dabie, that were previously described as pillow lavas are studied by elemental, Sr–Nd–Pb isotopic, and mineral oxygen isotopic analysis as well as zircon SHRIMP U–Pb dating. Zircon U–Pb geochronology results indicate that the protolith emplacement age of these metabasites is approximately 717 ± 38 Ma, consistent with the age of the volcanoclastic rocks in the same unit, and that they experienced the Triassic HP eclogite-facies retrograde metamorphism at 221 ± 2 Ma during exhumation after subduction to mantle depth and peak ultra-high pressure metamorphism. The low δ18O values of −5.5‰ to −2.0‰ indicate that the protoliths underwent high temperature meteoric-hydrothermal alteration before subduction but had no seawater interaction. These metabasites had similar formation processes, water–rock interactions and metamorphisms as other eclogite-facies rocks cropped out in the Central Dabie terrain. They showed negative abnormalities in Nb, Sr, and Ti content and positive abnormalities in Ba, Th, and Pb content; they also showed LREE enrichment. The insusceptible Sm–Nd isotopes during metamorphism yielded εNd (t) = −12 to −10 and TDM = 2.2–2.8 Ga for samples from lenses #1 to #3 and −7 to −6 and 2.1–2.2 Ga for lens #4; the samples also showed low radiogenic Pb isotope compositions of (206Pb/204Pb)i = 15.34–16.50, (207Pb/204Pb)i = 15.23–15.32, and (208Pb/204Pb)i = 35.93–37.04. The data suggest that the protolith sources of the metabasites were contaminated to variable degrees by old crustal materials during formation. Unlike the Maowu layered intrusions, which were contaminated by upper crust, the magmas of the metabasites were contaminated by lower crust in the magma chamber and during eruption. It can be concluded that the protoliths of these metabasites were derived from old crustal-contaminated mantle sources and initially emplaced in the crust at the Neoproterozoic and that they were altered by meteoric water at high temperatures. In this respect, they might be similar to the Neoproterozoic mafic intrusions in the North Huaiyang terrain. However, the studied metabasites experienced the Permo-Triassic subduction and metamorphism, whereas the North Huaiyang Neoproterozoic mafic intrusions did not. |
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