STRUCTURAL AND GEOCHRONOLOGICAL STUDIES OF TWO CONTRASTING TECTONIC DOMAINS IN THE CHINESE ALTAI AND THEIR SIGNIFICANCE IN UNRAVELLING THE COMPLICATED ACCRETIONARY PROCESS OF THE WESTERN CENTRAL ASIA OROGENIC BELT

The Chinese Altai, a key component of the western Central Asia Orogenic Belt, is considered to be formed through multiple accretions of several terranes. However, the deformational histories of each terrane (tectonic domain), i.e. structural records before and after the accretion, are rarely studied, which greatly hindered our understanding of the accretionary processes. To fill the gap, a systematic macro- and microscopic structural analysis was carried out on two contrasting litho-tectonic units, i.e. the Early Paleozoic low-grade Alegedayi Ophiolitic Complex (AOC) juxtaposed to the high grade Tarlang Granitic Massif (TGM). Selected samples were analyzed for zircon U-Pb isotopic compositions to constrain timing of deformation of the polyphase fabrics. Our structural and geochronological data suggest that the two litho-tectonic units experienced distinct deformations under different P-T conditions prior to their juxtaposition. They were separated but both underwent a NWW-SEE-directed shortening deformational event (D₁) between 460 and 407 Ma, then juxtaposed with each other at ~ 407 Ma and jointly experienced a subsequent NE-SW-directed shortening deformation (D₂) that after ~ 383 Ma and ended before the commencement of the crustal-scale sinistral strike-slip shearing deformation (i.e. Erqis fault zone) at 290 - 240 Ma. Results of this study provide solid field-based evidence that the Chinese Altai initially underwent a NW-SE-trending subduction-accretion in the middle Paleozoic, before it was reoriented to present-day E-W-trending orogenic fabric.

This study was financially supported by the Major Basic Research Project of the Ministry of Science and Technology of China (Grant: 2014CB44801), Hong Kong Research Grant Council (HKU705311P and HKU704712P), National Science Foundation of China (NSFC 41273048), NSFC Major Project (41190075), a HKU CRCG grant and PROCORE France/Hong Kong Joint Research Scheme. The work is a contribution to IGCP 592 by the Joint Laboratory of Chemical Geodynamics between HKU and CAS (Guangzhou Institute of Geochemistry).