LA-ICP-MS Zircon U-Pb Age of Gabbro from Yangkang Area in South Qilian and Its Geological Significance

Through carrying out the 1:50000 regional geology and mineral resources survey in South Qilian, the neoproterozoic gabbro mass was firstly discovered in Yangkang area. The LA-ICP-MS dating method obtains the gabbro zircon ²⁰⁶Pb/²³⁸U age of (730±3)Ma (MSWD=0.88). This gabbro is enriched in light rare earth elements and deleted in heavy rare earth elements, belonging to a basic series. The ΣREE value is 156.1×10^-6~173.34×10^-6, showing the characteristics of upper mantle-derived materials. Thus, it might be a product of the early neoproterozoic magmatic arc root zone, belonging to subduction-collision type granitic magmatic belt. It reflects the neoproterozoic continent split after a regional tension, and it was formed in active continental margin volcanic arc environment, which was located in the tectonic system by extrusion to tensile relaxation stage of transformation and was formed by mantle material upwelling emplacement. Research shows that the gabbro might be served as a rock record of Rodinia supercontinent cracking in South Qilian area, which provides a precise chronology basis and the magma activity record for the formation and evolution of the South Qilian structural belt. Therefore, this discovery has a great significance for understanding the Neoproterozoic tectonic evolution in South Qilian area.