Ages and Hf isotopes of detrital zircons from Paleozoic strata in the Chagan Obo Temple area,Inner Mongolia: Implications for the evolution of the Central Asian Orogenic Belt |
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| Institution: | 1. Department of Geology, Northwest University, Xi’an, 710069, China;2. Department of Earth Sciences, The University of Hong Kong, Pokfulam, Road, Hong Kong, China;3. Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh, Pennsylvania, USA;1. College of Earth Sciences, Jilin University, Changchun 130061, China;2. Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China;1. Hebei GEO University, Shijiazhuang, Hebei 050031, China;2. Tianjin Institute of Geology and Mineral Resources, Tianjin 300170, China;3. School of Earth Sciences and Resources, China University of Geosciences Beijing, Beijing 100083, China;4. Department of Earth Science, University of Adelaide, Adelaide 5005, Australia |
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| Abstract: | The Central Asian Orogenic Belt (CAOB), as one of the largest accretionary orogens in the world, was built up through protracted accretion and collision of a variety of terranes due to the subduction and closure of the Paleo-Asian Ocean in the Neoproterozoic to Early Mesozoic. Located in the Uliastai continental margin of the southeastern CAOB, the Chagan Obo Temple area is essential for understanding the tectonic evolution of the southeastern part of the CAOB and its relation with the “Hegenshan Ocean”. In this study, detrital zircon U-Pb geochronology coupled with Hf isotopic analysis was performed on Paleozoic sedimentary strata in this area. Most detrital zircons from the studied samples possess oscillatory zoning and have Th/U ratios of 0.4-1.73, indicative of an igneous origin. Detrital zircons from the Ordovician to Devonian sedimentary strata yield a predominant age group at 511-490 Ma and subordinate age groups at 982-891 Ma, 834-790 Ma and ~ 574 Ma, and have a large spread of εHf(t) values (-20.77 to + 16.94). Carboniferous and Early Permian samples yield zircon U-Pb ages peaking at ~ 410 Ma and ~ 336 Ma, and have dominantly positive εHf(t) values (+ 1.30 to + 14.86). Such age populations and Hf isotopic signatures match those of magmatic rocks in the Northern Accretionary Orogen and the Mongolian arcs. A marked shift of provenance terranes from multiple sources to a single source and Hf isotope compositions from mixed to positive values occurred at some time in the Carboniferous. Such a shift implies that the Northern Accretionary Orogen was no longer a contributor of detritus in the Carboniferous to Early Permian, due to the opening of the “Hegenshan Ocean” possibly induced by the slab rollback of the subducting Paleo-Asian Ocean. |
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