Geochemical characteristics and formation age of Keke Pluton in South Qilian Mountain, Qinghai Province

Lying in the middle of Kaketumengke Mountain, the Keke pluton which was formed in Middle Ordovician is located in the southeastern part of South Qilian massif. Geochemical and geochronological studies were conducted to investigate the tectonic at-tribute of the Keke pluton. The pluton is mainly composed of syenogranite and is characterized by high content of SiO₂ (68.41%~73.10%) and high content of Al₂O₃ (13.03%~14.35%). Geochemical data also show depletion of Nb, P, Ti and HREE with Eu nega-tive anomaly. The REE patterns show the characteristics of crustal anatectic granite, and the genetic classification diagram shows that the granite was formed in a syn-collisional setting. LA-ICP-MS zircon U-Pb dating shows that gneissic granite was formed at 461±2.7Ma. Combined with regional geology, the authors hold that Keke pluton was probably derived from the source of the crust and re-sulted from syn-collision, which supports the argument of compressive regime of the Qilian block during the Mid-Ordovician and is of great significance for the further study of the evolution of the Qilian block.