海南岛白垩纪古地磁结果及其构造地质意义

Cretaceous Paleomagnetic Results from Hainan Island and Its Tectonic Implications

对海南岛地区白垩系鹿母湾组和报万组碎屑岩219个独立定向岩芯样品(29个采点)的岩石磁学和古地磁学研究表明,白垩纪的碎屑岩以赤铁矿为主要载磁矿物。逐步热退磁分析表明,绝大多数样品可分离出特征剩磁分量。综合前人的结果,获早白垩世特征剩磁方向D=6.5°,I=42.7°,κ=73.4,α95=8.2°;晚白垩世特征剩磁方向D=6.7°,I=44.7°,κ=125.5,α95=5.4°。其早白垩世古纬度24.8°(+6.2°/-5.8°),晚白垩世古纬度26.3°(+4.6°/-4.0°);均位于现在地理位置以北约5°～6°。与华南板块东南缘白垩纪的古地磁数据对比表明,晚白垩世海南地块仍是华南板块的一部分。海南岛白沙断裂东西两侧早白垩世古地磁数据的差异,表明存在一个北东向的构造走滑带,白沙断裂可能是华南沿海北东向构造带的南延部分。海南岛白垩纪古地磁结果也表明,相对印支地块,海南岛在早白垩世时发生了25°左右局部顺时针旋转。推测此局部旋转很可能与晚侏罗世—晚白垩世早期,印度洋开始第一次海底扩张,印度板块向北运动有关。

Two hundred and nineteen oriented drill-core samples （29 sites） from clastic rocks of Cretaceous Lumuwan and Baowan formations in Hainan Island were collected for analysis o~ rock magnetism and paloemagnetism. The results show that hematite in the Cretaceous clastic rocks is the main magnetic carrier mineral. Stepwise thermal demagnetiszation experiments indicate that most samples could separate signature remanence component. It is suggested by previous study that the early Cretaceous yielded a direction of D=6.5°,I=42.7°,κ=73.4,α95=8.2°, and the Late Cretaceous direction of D=6.7°,I=44.7°,κ=125.5,α95=5.4°. The paleo-latitudes of the Early Cretaceous [-24.8°（＋6.2°/-5.8°）], and the Late Cretaceous [26.3°（＋4.6°/-4.0°）] are 5°-6° north of the present geographic position. Comparison with the Cretaceous paleomagnetic data from the southeastern margin of South China shows that Hainan Island was part of the South China Block in Late Cretaceous. Difference of Cretaceous paleomagentic data on both sides of the Baisha fault in Hannan Island suggests existence of a northeast-trending structural strike-slipping and it is very likely that the Baisha fault is the southern extension of northeastern-trending structural belt in the coast of South China. The Cretaceous paleomagentic results of Hainan island also indicates that Hainan Island rotated clockwise by about 25 relative to Indo-Chinese Block in Early Cretaceous. It is presumed that the regional rotation was probably related to movement of Indian Plate northwards in Late Jurassic-early Late Cretaceous when the Indian Ocean started the first searfloor spreading.