摘要
中亚造山带中的古生代—中生代花岗岩普遍具有正εNd值,在世界上是十分独特的。美国西部加利福尼亚中生代—新生代花岗岩同样具有正εNd值,并且其地壳的速度分布特征与内蒙古中部十分相似。本文通过与美国加利福尼亚的地球物理研究成果进行对比,来研究内蒙古中部的地壳密度结构,特别是下地壳的组成,试图探讨产生正εNd值花岗岩的深部地质原因。研究结果表明,在大兴安岭-内蒙古造山带下地壳中可能存在与美国加里福尼亚类似的洋壳物质。此外,还对重、磁异常进行了处理,以确定蛇绿岩带的延展情况。由于在西拉木伦河附近存在切割至莫霍面的深断裂,结合地表出现的蛇绿岩带,故提出温都尔庙—西拉木伦河一线可能是内蒙古中部最重要的地质构造界线。
Paleozoic to Mesozoic granites have positive εNd values in the Central Asia orogenic belt, which is a very particular case in the world. Mesozoic to Cenozoic granites in California, western United States, also have positive εNd values and the velocity distribution of the crust below is similar to that in central Inner Mongolia. By comparing the results of geophysical research obtained in central Inner Mongolia and California, the authors have studied the density structure of the crust below central Inner Mongolia, and especially the composition of the lower crust in an attempt to understand the deep geological cause for producing granites with positive εNd values. The results of the study show that in the lower crust below the Hinggan-Mongolian orogenic belt there might exist oceanic material similar to that in California. In addition, the authors have also processed gravity and magnetic anomalies in order to ascertain the extension of an ophiolite belt in this area. According to the existence of a deep fault cutting the Moho near the Xar Moron River, combined with the appearance of the ophiolite belt on the surface, the authors suggest that the tectonic line from Ondor Sum to the Xar Moron River is probably the most important tectonic boundary in central Inner Mongolia.