恩格尔乌苏蛇绿岩带与红柳河—牛圈子—洗肠井蛇绿岩带的对比研究

A COMPARATIVE STUDY OF THE ENGER US OPHIOLITE BELT WITH HONGLIUHE-NIUJUANZI-XICHANGJING OPHIOLITE BELT

北山及阿拉善地区地处中亚造山带中段南缘,是研究古亚洲洋演化的关键位置。位于北山及阿拉善地区的红柳河—牛圈子—洗肠井蛇绿构造混杂岩带和恩格尔乌苏蛇绿构造混杂岩带被认为是具板块分割意义的重要缝合带,两者地理位置相近,似乎是一条贯穿东西、代表同一古洋盆残留的东西两段,但从蛇绿岩带年代学、成因类型、古洋盆俯冲极性和闭合时限进行研究发现:红柳河—牛圈子—洗肠井蛇绿岩带形成时代较早、兼具SSZ型和MORB型、岩浆源区组成复杂、所代表古洋盆双向俯冲且闭合时限较早,均不同于恩格尔乌苏蛇绿岩带,指示这两条蛇绿岩带并非同一洋盆的东西两段,无法对比连接。此外,恩格尔乌苏蛇绿岩带与北山及敦煌地区内出露的其余蛇绿岩带均无相关性,亦无法有效连接。因此,代表古亚洲主洋盆的恩格尔乌苏蛇绿岩带向西如何延伸仍有待于深入的探讨。

The Beishan orogenic belt and the Alxa block located on the southern margin of the middle part of the Central Asian orogenic belt(CAOB)are the key positions for the study of the evolution of the CAOBand the Paleo-Asian Ocean(PAO).The Beishan orogenic belt and the Alxa block are connected to the east and west of the Badain Jaran Desert,and several ophiolitic melange belts are exposed in the study area.Among them,the Hongliuhe-Niujuanzi-Xichangjing ophiolite(HNX)belt of the Beishan orogenic belt and the Enger Us ophiolite(EUO)belt on the northern margin of Alxa are important suture zones with plate segmentation significance,and geographically the two ophiolite belts are very close.Some scholars suggest that these two ophiolite belts extend through the east and west and represent the remains of the same paleo-oceanic basin.But their structural properties are obviously different:the Hongliuhe-Niujuanzi-Xichangjing ophiolitic melange belt is of SSZ type with complex geochemical source area characteristics,the northward-southward bidirection subduction,and earlier closing time of the paleo-ocean(late Silurian-early Devonian?).While the Enger Us ophiolite is of MORB type,and the geochemical composition of its magmatic source area is relatively uniform.The paleo-oceanic basin represented by the EUO belt is southward subduction only,and the paleo-oceanic basin closed relatively later(late Carboniferous-early Permian?).Combined with the research results of regional geological background,it is concluded that HNO and EUO belt are not the same structural belt,that is,they are not the product of the closure of the east and west segments of the same ocean basin.In addition,the Enger Us ophiolite belt has no correlation with other ophiolite belts in the Beishan orogenic belt and the ophiolite belts exposed in the Dunhuang orogenic belt.Therefore,the westward extension of the Engel(EUO)belt,which represents the paleo-Asian main oceanic basin,needs to be further discussed。