新疆西昆仑“依沙克群”中的放射虫组合及其形成时代探讨

THE ASSEMBLAGES OF THE RADIOLARIA FOSSILS FOUND IN “ YISAK GROUP” OF WEST KUNLUN OF XINJIANG, AND DISCUSSION ON ITS AGE

依沙克群是 80年代建立的一个岩石地层单元。由于缺乏可信的古生物学证据 ,因此关于该套地层的时代一直颇有争议。近期 ,笔者首次在该套地层上部的复理石中发现了早古生代和晚泥盆 -早石炭世 2个放射虫组合。两套含有化石的地层间为冲断接触。结合前人对依沙克群下部岩性段基性熔岩形成年龄的研究 ,笔者认为依沙克群已不能作为一个统一的岩石地层单元存在 。

The Yisak group is a newly built stratagraphical unit which was found in 1980's. Its outcrop lies in the 5th suture zone of Qinghai-Tibet plateau. It is also the chief component of the Kuda ophiolitic suite. From the geological profile of Yixikegou, the Yisak group is composed of three lithological parts: the lower part of basic lava, pillow lava and altered gabbro; the middle part of andesites and pyroclastic rocks; and the upper one of a set of deep-marine sediments with apparent turbidite characteristics. Since no definite evidence of bio fossils has ever been found in this group, it is very difficult to determine its age properly. However, the age of this group is judged as Late Proterozoic Era according to isotopic data of its related igneous rocks by former researchers. The authors have made a thorough study upon the geological section of Yixikegou, and found two assemblages of radiolaria fossils in the upper part of the Yisak group. By comparison with the radiolaria fossils found in other places of the world, they are assigned to two geological age:the Early Paleozoic and the Late Devonian to Early Carboniferous, respectively. Although the first radiolaria fossil assemblage occurs only about 150m lower than the second one in the profile section of Yixikegou, they are seperated by a thrust fault. Therefore, this turbidite set should at least be divided into two stratigraphical slices in varied geological times. Combined with the studies upon the age of the lower part of the Yisak group, we do not think it suitable for the Yisak group to be considered as one single stratigraphic units. It should be at least divided into two stratigrphic units:the Early Paleozoic one which include the lower and middle parts of the formly “Yisak group”, as well as the lower part of the turbidite set, and the D 3-C 1 one which constitutes of the upper part of the turbidite set in Yixikegou section.