Detrital zircon age and Hf isotopic studies for metasedimentary rocks from the Chinese Altai: Implications for the Early Paleozoic tectonic evolution of the Central Asian Orogenic Belt

摘要

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 ε Hf(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. Copyright 2007 by the American Geophysical Union.