藏北羌塘南部发现早古生代地层及意义

Discovery of Early Paleozoic Strata in South of Qiangtang,Northern Tibet and Its Significance

藏北羌塘南部地区沿龙木错-双湖缝合带南侧发育大面积的变质岩系,此前统称为阿木岗群.由于没有发现古生物化石,时代归属争议很大.有人依缝合带以北查桑地区有未变质的含古生物化石泥盆系出露为据,将该套变质岩系时代归属前泥盆纪,多数人是根据所测同位素年龄值,确定其时代为前震旦系.2003—2004年我们在玛依岗日一带变质岩系上部采获大量鹦鹉螺类和笔石类化石,经研究确定时代属中、晚奥陶世-志留纪.在羌塘南部地区发现有可靠化石依据的早古生代地层存在尚属首次.为该岩系的时代归属、地层划分及与邻区对比提供了古生物证据,也为研究该区岩相古地理、大地构造格局提供了可靠资料.

Due to the lack of fossil evidence, the age of Amugang Group, previously referring to the large area of metamorphic formations developed along the south of Longmucuo-Shuanghu suture in southern Qiangtang area, northern Tibet, has been under heated debate. Some geologists were convinced that the age of metamorphic formations was of Pre-Devonian according to the emergence of nonmetamorphic fossil-beating Devonian in Chasang area, north of the suture. Most geologists believed that its age was of PreSinian. From 2003 to 2004, we collected a large amount of Nautiloid and Graptolite fossils in upper metamorphic formations of Mayigangri area and determined the age to be middle and Late Ordovician-Silurian. It is the first time to discover the Early Paleozoic strata based on reliable fossils, which not only provides evidence to its age ascription, classification of the strata and its correlation to adjacent areas but also offers data to lithofacies, palaeogeography and tectonic pattern of studied area.