LA-ICP-MS zircon U-Pb ages of Heishishan granite bodies in north Qilian and their geological significance

The Haoquangou, Baimawa, Heishishangou and Fangfuya granite bodies are exposed from the south to the north in the basic-acidic volcanic rocks of Heishishan area of north Qilian, and the Haoquangou gold deposit was found in the Haoquangou plagiogranite body. Based on LA-ICP-MS zircon U-Pb geochronologic study, the authors obtained the age of （440.2±2.4）Ma (MSWD=0.052) for Baimawa tonalite and the age of （431.8±2.4）Ma (MSWD=0.042) for Haoquangou plagiogranite, suggesting that these intrusive bodies were formed in late Ordovician-early Silurian period. Based on the whole rock Rb-Sr and Sm-Nd isotope composition of Baimawa tonalite, the authors calculated the (87Sr/86Sr)i value and the εNd(t) value, which are 0.70535 and 1.11 respectively. These data show that Baimawa granite body might have originated in the thickened lower crust. This study has revealed that the north Qilian probably experienced the tectonic evolution of the closure of the Qilian Ocean, the collision of the central Qilian block with the north Qilian Ordovician island arc, the thickening of the crust, and then the partial remelting of basaltic rocks in the lower crust during the late Early Paleozoic; as a result, the granite bodies of Heishishan area were formed.