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The Chinese Altai, a typical region of the Central Asian Orogenic Belt (CAOB), has been envisaged as subduction‐accretion complex or Precambrian microcontinent. Thick metasedimentary rocks crop out extensively in the Central Altai and Qiongkuer domains, but their depositional age is not well constrained. Most workers have regarded these sedimentary rocks as passive continental margin sediments deposited on a Precambrian microcontinent. However, our studies of U‐Pb and Hf isotopes of detrital zircons separated from these rocks reveal that a predominant population has 206Pb/238U ages between 460 and 540 Ma and most grains of this population possess positive ɛHf(t) values. Zircons of the population have oscillatory zoning, possess high Th/U ratios, and are enhedral to subhedral crystals with sharp edges, showing short distance of transportation from an igneous provenance. The above results indicate that these metasedimentary rocks were deposited on an active continental margin not prior to the Middle Ordovician. Therefore the Chinese Altai orogen was an active continental margin in the Early Paleozoic, which is inconsistent with a Precambrian microcontinent model and reveals an arc accretionary history.

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Zircons were separated from granitoids, gneisses, and sedimentary rocks of the Chinese Altai. Those with igneous characteristics yielded U-Pb ages of 280-2800 Ma, recording a long history of magmatic activity in the region. Zircon Hf isotopic compositions show an abrupt change at ~420 Ma, indicating that prior to that time the magmas came from both ancient and juvenile sources, whereas younger magmas were derived mainly from juvenile material. This may imply that the lithosphere was significantly modified in composition by a rapid addition of melt from the mantle. We suggest that this dramatic change was due to the onset of ridge subduction, which can account not only for the formation of voluminous granitoids, mafic rocks with complex compositions, and the association of adakite + high-Mg andesite + boninite + Nb-enriched basalt, but also for the coeval high-T, low-P metamorphism.Paleozoic, ridge subduction, Hf isotope, granite, Altai Zircon is ideal for Hf isotopic study, because the crystal chemistry of zircon allows entrain of large amounts of Hf, but not Lu, thus the radiogenic daughter of 176 Hf from the decay of 176 Lu is extremely low in zircon, and the 176 Hf/ 177 Hf ratios of the zircon reflect those of the precursor magma [1] . Significant advances have been made on zircon Hf isotopic systematics, for example the Laboratory of Lithospheric Evolution in the Institute of Geology and Geophysics, Chinese Academy of Sciences has contributed greatly to the technological aspects of this work.For this study, samples were collected from granitoids, gneisses, and sedimentary rocks of different age in the Chinese Altai and analyzed in the above-mentioned Laboratory of Lithospheric Evolution. Our results show an abrupt change in zircon Hf isotopic compositions at ~420 Ma, which may be result of an important regional geological event. Based on an evaluation of the regional geology, geochemical data, and metamorphic development, we suggest that the observed change may mark the onset of ridge subduction. We plan to test this model with further research.

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