This paper presents new geochronological, petrological, geochemical and Hf isotopic data of the intrusions from the Zongnaishan area in northern Alxa, Inner Mongolia, and discusses the tectonic settings and boundary of southernmost Central Asian Orogenic Belt (CAOB). Zircon U-Pb dating indicates that the Zongnaishan batholith includes the Middle Permian K-feldspar granite (272±1 Ma), Early Triassic diorite (249±1 Ma) and Early Triassic granodiorite (247±1 Ma). According to their geochemistry, the Middle Permian K-feldspar granite is weakly peraluminous and shoshonitic, belonging to the highly fractionated I-type granites; the Early Triassic diorite is calc-alkaline; and the Early Triassic granodiorite is metaluminous to slightly peraluminous and calc-alkaline. The REE patterns of all the three intrusions are high in LREEs and low in HREEs with week to moderate Eu negative anomalies. The primitive mantle normalized trace element diagrams are characterized by the enrichment of the LILE (K, Rb, Ba and Sr) and depletion of HFSE (Ta, Nb, P and Ti). Based on the zircon Hf isotopes, the authors hold that the intrusive rocks were mainly derived from juvenile material with high εHf(t) values (+0.3-+11.6) and young two-stage Hf model ages (1 275～533 Ma). The juvenile features of Zongnaishan-Shalazhashan terrane are similar to the most prominent features of the CAOB, but different from those of cratons or Precambrian blocks, such as the North China Craton and the Alxa Block, implying that the Zongnaishan-Shalazhashan terrane is part of the CAOB, and the southernmost boundary of CAOB here is to the south of the terrane. The recognition of Late Carboniferous typical adakite magmatism in this region provides evidence for the subduction of the oceanic crust of the CAOB. The Middle Permian magmatic activities (272～264 Ma) display a bimodal association with high-K calc-alkaline features and are interpreted as forming in a post-collision setting. These studies, in combination with regional geology, provide new constraints on the tectonic evolution of southern CAOB during the Late Paleozoic and the location of the southernmost margin of the CAOB.