海南岛晚古生代-中生代古地磁新结果及其对南海北缘大地构造的意义

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

为了探讨南海西北缘海南岛琼中、三亚等主要地块自晚古生代以来的运动特征,在海南岛18个采点上采集了143个古地磁岩石标本,测试分析结果表明,大多数样品可分离出特征剩磁,其中早—中石炭世特征剩磁方向的磁偏角D=34.0°,磁倾角I=46.9°,精度参数κ=55.9,95%置信圆的角半径α95=5.2°;中三叠世特征剩磁方向D=359.8°,I=31.1°,κ=154.2,α95=3.3°;早白垩世特征剩磁方向D=3.0°,I=43.7°,κ=34.7,α95=4.4°,方向均区别于现代地磁场方向(D=358.6°,I=26.1°)。海南岛早—中石炭世古纬度28.1°N,中三叠世古纬度16.8°N,早白垩世古纬度25.5°N。结合前人的古地磁资料,琼中地块早—中石炭世古纬度28.1°N,早三叠世古纬度16.5°N,早白垩世古纬度28.6°N;三亚地块中三叠世古纬度16.8°N,早白垩世古纬度26.4°N。通过与华南地块早白垩世参考古地磁极对比的检验,证明海南岛中生代花岗岩形成以来未发生过显著倾斜运动,其古地磁数据反映了原生的剩磁特征。根据古地磁反映的地块运动过程,结合区域地质背景,可以推断琼中地块和三亚地块在中三叠世开始碰撞缝合,形成琼南缝合带。

In order to study the movement of the Qiongzhong and Sanya blocks of Hainan Island since Late Paleozoic, 143 drilling core samples were collected from 18 sites. Samples were tested with Schonstedt TSD-1 demagnetizer, Model-600 alternating demagnetizer and 2G-755R rock superconducting magnetome- ter in the paleomagnetic laboratory of South China Sea Institute of Oceanology, Chinese Academy of Sci- ences. 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°,k= 55.9,ags = 5.2°, while the Middle Triassic direction was D=359.8°,I=31.1°,k=154.2,a95 =3.3°, and the Early Cretaceous direction D= 3.0°, I=43.7° ,k=34.7 ,a95 =4.4°, which are different from the modern geomag- netic field direction（D=358.6°,I=26.1°）. The paleo-latitude of Hainan Island was 28. I°N for Early-Mid- dle Carboniferous, 16.8°N for Middle Triassic and 25.5°N for Early Cretaceous. Referring to precious pal- eomagnetic 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 mag- netic 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-Trias- sic, which resulted in the formation of the ＂Qiongnan suture zone＂.