DETRITAL ZIRCON U-PB AND HF ISOTOPE ANALYSES OF EARLY PALEOZOIC SEQUENCE ACROSS THE AILAOSHAN BELT AND THEIR CONSTRAINS ON THE BOUNDARY BETWEEN THE YANGTZE AND SIMAO BLOCKS

The modern day Southeast Asia was formed during the Phanerozoic as a result of a series of complex assembly (collision or accretion) processes by several allochthonous continental blocks including the South China (including the Yangtze and Cathaysia), Simao-Indochina, Sibumasu blocks and several microcontinental fragments and silvers (Faure et al., 2014; Metcalfe, 2006). Among these blocks, the Yangtze and Simao blocks were separated by the Ailaoshan ocean,which was a branch of the Paleotethys. Uncertainty remains on this ocean including the exact location of the boundary between the Simao-Indochina and Yangtze blocks. Various locations have been suggested. From east to west, these includes the Cenozoic Ailaoshan fault (Sone and Metcalfe, 2008), the Anding-Jiujia fault (Wang et al., 2013) or the Amojiang-Lixianjiang fault (Chung et al., 1997). In order to test these models, we have carried out a comprehensive analysis of U-Pb age and Hf isotopes on detrital zircons from the Cambrian to Silurian sedimentary rocks collected from the Ailaoshan tectonic zone and its adjacent western margin of Yangtze block. Our results indicate distinguished provenances for the samples from two sides of the Ailaoshan-Santaipo fault. The samples from east of this fault includes that collected from western margin of the Yangtze block and the Jinping terrane within the Ailaoshan belt have a diagnostic detrital zircon age peak of 730–900 Ma which are characterized by positive εHf(t) values ranging from +1 to +15 wheresas the samples from west of this fault display major age populations of 400-500 and 950-1000 Ma, both of which being characterized by both positive and negative εHf(t) values. Therefore we sugget that the location of the boundary between the Yangtze and Simon blocks may line along the Ailaoshan-Santaipo fault. Our data also indicate that the Ailaoshan ocean may have started its early continental rift and formed a deep ocean basin in Silurian. This point is further supported by the presence of Silurian alkaline continental rift basalt.