Geological framework and Paleozoic tectonic history of the Chinese Altai, NW China: a review |
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Authors: | Keda Cai Min Sun Chao Yuan Xiaoping Long Wenjiao Xiao |
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Affiliation: | aDepartment of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China;bKey Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;cState Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China |
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Abstract: | The Chinese Altai, as a key portion of the Central Asian Orogenic Belt (CAOB), is dominated by variably deformed and metamorphosed sedimentary rocks, volcanic rocks and granitic intrusions. Its Early Paleozoic tectonic setting has been variously considered as a passive continental margin, a subduction-accretion complex, or a Precambrian microcontinent, and two representative competing tectonic models have been proposed, i.e., open-closure versus subduction-accretion. Recent studies demonstrate that the high-grade metamorphic rocks previously considered as fragments of a Precambrian basement have zircon U-Pb ages (predominantly 528 to 466 Ma) similar to those of the widely distributed low-grade metasedimentary rocks named as Habahe Group in the region, and all these meta-sedimentary rocks were dominantly deposited in the Early Paleozoic. Petrological evidence and geochemical compositions further suggest that these meta-sedimentary rocks were probably deposited in an active margin, not a passive continental margin as previously proposed. The detrital zircons of sediments and igneous zircons from granitoids including the inherited ones (mainly 543–421 Ma) mostly give positive ?Hf(t) values, suggesting significant contributions from mantle-derived juvenile materials to the lower crust. A modeling calculation based on zircon Hf isotopic compositions suggests that as much as 84% of the Chinese Altai is possibly made up of “juvenile” Paleozoic materials. Thus, available data do not support the existence of a Precambrian basement, but rather indicate that the Chinese Altai represented a huge subduction-accretion complex in the Paleozoic. Zircon U-Pb dating results for granitoids indicate that magmatism was active continuously from the Early to Middle Paleozoic, and the strongest magmatic activity took place in the Devonian, coeval with a significant change in zircon Hf isotopic composition. These findings, together with the occurrence of chemically distinctive igneous rocks and the high-T metamorphism, can be collectively accounted for by ridge-trench interaction during the accretionary orogenic process. |
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Keywords: | Chinese Altai CAOB subduction-accretion complex accretionary orogensis tectonic evolution ridge subduction |
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