THE ACID-INTERMEDIATE INTRUSIVE ROCKS IN THE EAST SECTION OF MIDDLE QILIAN OROGENIC BELT: LA-ICP-MS Zircon U-Pb Age and Tectonic Implications

The acid-intermediate intrusions are developed in Shenjiaxia-Jiangou area in the east section of Middle Qilian Mountains, Qinghai Province. The field geological survey and petrographic study show that the intrusions occur in stock, mainly consisting of monzogranite and granodiorite which are in pulsation contact with each other. The LA-ICP-MS zircon U-Pb isotopic dating of 2 monzogranite samples gains the weighted average ages of 465.7±2.2 Ma (n=24, MSWD=0.031) and 459±2 Ma (n=21, MSWD=0.76) respectively, indicating that the intrusions were formed in Middle Ordovician. The lithogeochemical characteristics reveal that the intrusions belong to quasialuminous-peraluminous medium-high K calc-alkaline series with the (La/Yb)_N of 10.57-23.126 and δEu of 0.47-0.69. The REE patterns show right-dipping "V" shaped curves. The LILEs such as Rb, Ba, Th, K, La, Ce, Sm and Nd are strongly enriched, Zr and Hf weakly enriched, while the HSFEs (Nb, Ta, P and Ti) are strongly depleted. The rocks are characterized by island arc magmatite, which may be related to the southward subduction of North Qilian Ocean.