内蒙古月牙山一带基性火山岩的地质特征、形成时代及归属

A discussion on the geological characteristics,formation age and attribution of basic volcanic rocks in Yueyashan area,Inner Mongolia

内蒙古月牙山一带出露一条基性火山岩带，呈北西西向带状展布，由层状玄武岩、枕状玄武岩和蚀变玄武岩组成，前人对该套火山岩的归属意见不同。该套火山岩分布于月牙山蛇绿质构造混杂岩带中部，空间上与月牙山蛇绿岩套紧密共生，枕状构造较发育，显示海底火山喷发的特征；岩石化学、地球化学研究表明，该套火山岩具有典型的正常洋中脊玄武岩（NMORB）特征。因此，月牙山基性火山岩属于月牙山蛇绿岩套的组成部分。在侵入该套火山岩的辉长闪长岩中获得了530Ma左右的同位素年龄，在辉长岩、斜长花岗岩中获得了527~530Ma的同位素年龄，相当于早寒武世，说明该套基性火山岩的形成时代为早寒武世。

A NWW-trending basic volcanic rock belt is exposed from east to west in the Yueyashan area. The basic volcanic rockbelt is composed of layered basalt, pillow basalt and altered basalt. Previous views concerning the attribution of volcanic rocks are con?troversial. The volcanic rocks are distributed at the center of Yueyashan ophiolite tectonic melange belt and are spatially closely associ?ated with the ophiolite. Volcanic pillow structure is well developed in this set of volcanic rocks, showing the characteristics of subma?rine volcanic eruption. Petrochemical and geochemical studies show that this set of volcanic rock has typical normal mid-ocean ridgebasalt (N- MORB) characteristics. Therefore, the Yueyashan basic volcanic rock belt should belong to the part of the Yueyashanophiolite suite. The authors obtained isotopic age of about 530Ma for gabbro diorite which invaded this volcanic rock belt and alsoobtained isotopic ages of about 527Ma to 530Ma for gabbro and plagioclase granite. These age values are equivalent to the age of Ear?ly Cambrian, indicatiang that the age of the basic volcanic rocks is Early Cambrian.