Located near Pantuoshan district in Ejina Banner of southwestern Mongolia, Guoqing K-feldspar granite body lies in southern Niujuanzi-Xichangjing suture zone. LA-ICP-MS U-Pb isotopic dating of zircon separated form the rock yields an average weighted 206Pb/238U age of 439.1±8 Ma. The results of geochemical study indicate that SiO2 are in the range of 73.31%～74.36%， A/CNK>1.1, and K2O/Na2O=1.25～1.40, the Rittmann index of k-feldspar granite varies from 2.06 to 2.22, suggesting a high-K calc-alkaline series and peraluminous nature. The results also indicate that Guoqing K-feldspar granite body is highly enriched in large ion lithophile elements and strongly depleted in high field strength elements. The chondrite-normalized REE and trace elements patterns demonstrate low fractionation level between LREE and HREE, (La/Yb) N=3.40～5.44, and negative Eu(δEu=0．65～0．77), which might have been related to the plagioclase residuals during the partial melt of the original rock. The spider diagrams are characterized by Nb, Ce, Zr, Hf, Sm, Y, Yb depletion and K, Rb, Ba, Th enrichment. Rb/Sr ratios are between 4.37 and 4.70, and such high Rb/Sr ratios (>2) suggest that Guoqing granite body was probably triggered by dehydration melting of mica during the geological process. High levels of Nb and Ta and characteristics of Ta>Nb indicate high-degree evolution of the granite body, which laid the foundation of tungsten elements enrichment. CaO/Na2O ratios range from 0.152 to 0.165, and all the values are lower than 0.3, suggesting the origination from re-melting of the early Precambrian crystalline basement. In addition, the inherited zircon in the granite body yields a large number of ages between 616 Ma and 1046 Ma. In Hongliuhe area, the zircon U-Pb age of granite that intruded into ophiolite is 404.8±5.2 Ma, implying that the south Tianshan oceanic crust was formed as early as Early Cambrian, and closed before Emsian. In Xichangjing area, the closure of South Tianshan Ocean was before 439.1±8 Ma, suggesting a scissors-type opening-closing tectonic pattern. The results of geochemical study show that the S-type K-feldspar granite, which was crust-derived, might have been formed by the closure of the Hongliuhe-Yueyashan Paleo-ocean during the syn-collision stage. There are Yingzuihongshan, Wangxushan and Guoqing tungsten ore deposits in Xichangjing area. These ore deposits which were formed in the contact zone or the fissure system surrounding the granite intrusion have a close relationship with Silurian-Devonian granite intrusions. Moreover，in the tectonic setting of transformation from collision to extension from Silurian to early-Devonian， the granites related to such tectonic settings deserve detailed studies, which can not only help understand the tectonic evolution but also provide a theoretic basis for the exploration of regional mineral resources.