Tanggula Magmatic Belt located in the central uplift of the Qinghai Tibet Plateau and separated the North and South Qiangtang basin, and was necessary to do further studies for its special tectonic location. Two samples were selected from the Xiamari granites in the western part of Tanggula magmatic belt, and obtained zircon U-Pb ages 231.0±1.4Ma, 227.2±1.6Ma repectively by LA-ICP-MS in its oscillatory zones, which indicated that the magma emplaced and crystallized in Late Triassic.Two samples all showed negative Hf isotopic initial ratio, limited variation and very old two stage crust model ages, indicated that its primitive magma were old than Proterozoic. Inherited zircon had large range in its U-Pb ages, complex Hf isotopic composition and elder ages of Hf model ages (elder than 1.2Ga, up to 3.0Ga max), implied there were Prechambrian metamomorphic basement in study area. Geochemical data indicated that Xiamari granite chould be strongly peraluminous S type granitoids which had the A/CNK ranging from 1.15 to 1.82 (＞1.1) and only contained C member in CIPW composition.All samples showed rightly deviation from LREE to HREE, enriched in LREE,Ta and Pb, but deficited in Nb, Ba, Ce and Sr, espically in K and Ti. Late Triassic Xiamari granite should be strongly peraluminous S type granitoids, and its primitive magma was formed by partial melting by greywacke member in Precambrian crustal clastic rocks, in the condition of relative high temperature (above 800℃) and fluid enriched, as the magmatic response to Longmucuo Shuanghu Tethys evolution.