LATE PALEOZOIC TO MESOZOIC PALEOMAGNETIC RESULTS FROM HAINAN ISLAND AND ITS TECTONIC IMPLICATIONS FOR THE NORTHERN MARGIN OF THE SOUTH CHINA SEA

In order to study the movement of the Qiongzhong and Sanya blocks of Hainan Island since Late Paleozoic,143drilling core samples were collected from 18sites.Samples were tested with Schonstedt TSD-1demagnetizer,Model-600alternating demagnetizer and 2G-755Rrock superconducting magnetometer in the paleomagnetic laboratory of South China Sea Institute of Oceanology,Chinese Academy of Sciences.For most of the samples,characteristic remanence component could be separated.Data Indicate that the Early-Middle Carboniferous geomagnetic field was in a direction of D=34.0,I=46.9°,κ=55.9,α95=5.2°,while the Middle Triassic direction was D=359.8°,I=31.1°,κ=154.2,α95=3.3°,and the Early Cretaceous direction D=3.0°,I=43.7°,κ=34.7,α95=4.4°,which are different from the modern geomagnetic field direction(D=358.6°,I=26.1°).The paleo-latitude of Hainan Island was 28.1°N for Early-Middle Carboniferous,16.8°N for Middle Triassic and 25.5°N for Early Cretaceous.Referring to precious paleomagnetic data,the paleo-latitude of the Qiongzhong block was at 28.1°N in Early-Middle Carboniferous,16.5°N in the Early Triassic,and 28.6°N in the Early Cretaceous;but the paleo-latitude of the Sanya block was at 16.8°N in Middle Triassic,and 26.4°N in Early Cretaceous.Comparing with the Cretaceous magnetic reference pole of the South China Block,the magnetic paleopole of Hainan Island calculated from our data shows no significant tilting movement since the Mesozoic granites of Hainan Island formed.According to the movement process of the blocks reflected by the paleomagnetism and the regional geological setting,it can be deduced that collision and suture between the Sanya and Qiongzhong blocks began in Mid-Triassic,which resulted in the formation of the"Qiongnan suture zone".