Age Determination of Gaolan Terrain and Its Geological Significance

As lack of fossil evidence and the influence of regional magma events on isotopic age determination,the geological age of the Gaolan terrain,a group of grade metamorphic rocks as the fold basement of the central Qilian block,has not been determined for a long time.During the work of the regional geological surveying of 1/250000 Lanzhou city,the authors got a large number of field geological evidence and obtained the dependable Sm-Nd isotope age of 1180.3 Ma,which is consistent with the micro-fossils of the ancient plants from the Jixianian.The age of Gaolan terrain that is in the eastern section of central Qilian block fold basement belongs to mesoproterozoic Jixianian.It is significant to study the Central Qilian block structural properties and regional tectonic evolution.