GEOLOGICAL FRAMEWORK AND PALEOZOIC TECTONIC HISTORY OF THE CHINESE ALTAY, NW CHINA: A REVIEW
Keda Caia, Min Suna, Chao Yuanb, Xiaoping Longb, and Wenjiao Xiaoc
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
Keywords: Chinese Altay, CAOB, subduction-accretion complex, accretionary orogenesis, tectonic evolution, ridge subduction
Pages: 1619-1633
Abstract
The Chinese Altay, 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 U-Pb zircon 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 metasedimentary rocks were predominantly deposited in the Early Paleozoic. Petrological evidence and geochemical compositions suggest that these metasedimentary rocks were probably deposited on an active, 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 ε 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 Altay 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 Altay is a huge subduction-accretion complex in the Paleozoic. The U-Pb zircon dating results for granitoids indicate that magmatism was active continuously from Early to Middle Paleozoic, and the strongest magmatic activity took place in the Devonian, coeval with a significant change in zircon Hf isotope compositions. These findings, together with the occurrence of chemically distinctive igneous rocks and the high-temperature metamorphism, can be collectively accounted for by ridge-trench interaction during the accretionary orogenic process.
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