Abstract: We discovered some garnet-acmite syenite rocks in Sanya terrain at the southern margin of Hainan Island．The mineralogical and geochemical studies demonstrate that these rocks are silica-saturated and peralkaline，enriched in LILE(Rb，Th，U，K，Pb)and LREE，with relatively distinct Nb，Ta，Ti，Sr，Ba，Eu，P negative anomalies．The HREE contents are relatively high，without obvious differentiation．Their Sr，Nd and Pb isotopic compositions are similar to those of EM-Ⅱ lithospheric mantle．11 zircon SHRIMP analytical data give the 206Pb／238U age ranges of 237—251 Ma．with a weighted average of 244±7 Ma．The above—mentioned information indicates that the Sanya syenite rocks were formed by decompression-dehydration melting of enriched lithospheric mantle at a shallower depth(<60 km，which was in tern caused by Triassic delamination and asthenospheric upwelling．It is emphasized that during the Triassic，the Hainan island Was under a post-orogenic extension environment，being the continuous evolution of the Hercynian orogeny．It is not reasonable to divide the Triassic activity as an independent Indosinian orogeny．It is also proposed that the Sanya terrain Was placed to present site adjacent to middle Hainan terrain through a Late Permian large scale right-lateral slideslip along the Jiusuo—Lingshui deep fault．There exists a NE-trending Triassic alkali-rich igneous belt in SE Hainan Island．This belt probably extends to South China continent through Luoding of Guangdong Province to Mingxi of Fujiang Province．It is considered that the South China Was probably also under a post-orogenic environment during the Triassic.