Accurately determining the closure time of the Huoersen- Chaganchulu back- arc basin has important meaning for understanding the tectonic evolution history of the Alxa block and clarifying the spatial and temporal sequence of mineralizing events in the southern part of the Alxa continent. The back- arc basin disappeared in the Badanjilin fault zone. Although the eastern part of the Badanjilin fault zone was confirmed to close at about 275 Ma through the Chaganchulu Ophiolite Suite, the closure time of the western part of that fault zone is still unknown because of lack of ophiolite suite in the west. In this paper, the Tebai quartz diorite and the Guancaitaolugai granite porphyry in the western part of the Badanjilin fault zone were studied using petrology, geochemistry and zircon U- Pb dating. The results show that the Tebai quartz diorite belongs to the I- type granitoid, which is relatively enriched in LILE (like Rb, Th, U, K) and LREE, and relatively depleted in HFSE (like Ta, Nb, P, Zr, Hf, Ti) and HREEs, with weak Eu negative anomaly. Sr- Nd isotopic compositions of the Tebai quartz diorite, which are similar to that of OIB or EM I- type mantle source, suggest that the rock formed in the extrusion tectonic environment at 281. 7±1. 1 Ma when the Huoersen- Chaganchulu back arc basin was subducted to the Alxa continent. The Guancaitaolugai granite porphyry belongs to the A- type granitoid, which is relatively enriched in LILE (such as Rb, Th, U, K, Zr, Hf), LREEs, Zr and Hf, and relatively depleted in HFSE (such as Ta, Nb, P and Ti), Ba, Sr and Eu. Enrichment of LREE and depletion of HREE, distinct Eu anomaly, and Sr- Nd isotopic composition close to that of EM II- type enriched mantle source suggest that the Guancaitaolugai granite porphyry formed in the extensional tectonic environment at 272. 6±0. 8 Ma when the continent in the south of Huoersen- Chaganchulu back- arc basin collided with volcanic arc. The closure time of that basin in this paper should be no earlier than 272. 6±0. 8 Ma, but not later than 272. 6±0. 8 Ma. Combined with the understanding that the eastern part of Huoersen- Chaganchulu back- arc basin closed at ~275 Ma, this study argued that the Huoersen- Chaganchulu back arc basin closed at the time between 282 Ma and 272 Ma. Most of the mineralization events in the southern side of the Badanjilin fault zone are the response to the closure event of the Huoersen- Chaganchulu back- arc basin. Distributed along the Badanjilin fault zone, these deposits should be related to the Variscan magma activities, with the mineralization age close to 282~272 Ma.