Paleozoic Accretion of Chinese Altai: Geochronological Constraints From Granitoids

The Central Asian Orogenic Belt (CAOB), also known as Altaids, is the largest and probably the least understood accretionary orogenic belt in the world. Chinese Altai, as one of typical regions of the CAOB, formed in response to the consumption of the Altay Ocean. The Chinese Altai is characterized by huge amount of granitoids, which cover about 40% exposed rock area in this region. However, lack of reliable ages for major strata and tectonothermal events has greatly retarded the effort for tectonic reconstruction of this area. The Habahe Group and Altai Formation, among others, are most debatable about their ages. Zircon U-Pb dating by SHRIMP and LA-ICP-MS for representative granitic intrusions in Chinese Altai have been conducted, in order to reveal the main geological framework and tectonothermal events of this area. Six granitic plutons have been investigated along two geo-traverses, which are perpendicular and parallel to the strike of the Altai orogen, respectively. Of these plutons, South Friendship Peak two-mica granite (SFP), Tielieketi biotite granite (TBG) and West Keketuohai granodiorite (WKG) intruded the Habahe Group, while the Habahe quartzdiorite (HG), Ashile granodiorite (AG) and Tarlang Intrusive Complex (TIC) intruded the Altai Formation. The SEP, TBG and WKG were dated to be 393±7 Ma, 410±6 Ma and 466±12 Ma, respectively. In addition, the SEP and TBG contain Cambrian (528±14 Ma) and late Archean (ca. 2.5 Ga) zircon grains that probably came from the Habahe Group. These ages well constrain the timing of the Habahe Group deposition to be pre-Mid Ordovician to Early Cambrian (466 - 528 Ma). Of the plutons intruding the Altai Formation, the HG and AG were dated to be 389±5 Ma and 318±6 Ma, respectively, while the TIC consists of multiple magma pulses with ages mainly between 382±5 Ma and 412±6 Ma. The latest pulse of the TIC was dated to be 357±7 Ma and contains both 469±12 Ma and Precambrian zircon xenocrysts captured probably from the Altai Formation. These results indicate that the Altai Formation was deposited most likely in between Late Silurian and Mid Ordovician (412 - 469 Ma), rather than in the Devonian or Paleoproterozoic as previously assumed. Moreover, there are 295 to 297 Ma zircon grains in the latest pulse of TIC, which is interpreted to reflect the gneissic to mylonitic deformation in the Early Permian. The above data suggest that the Devonian is a very important era for the emplacement of granitic intrusions in Chinese Altai. The erosion of the early Paleozoic intrusions and the consequent deposition of detrital materials are consistent with a Paleozoic southwestward accretionary continental margin. The study was co-supported by research grants from Major State Basic Research Development Program 2001CB409801, NSFC Project 40421303 and Hong Kong RGC (HKU 7040/04)