It is demonstrated in this paper that the so called "nondiamondiferous kimberlite" in the northern Chaidam mountains is actually garnet peridotite.This is the first garnet peridotite ever recognized in the Tibetan plateau.Petrographical and mineralogical data indicate that the rocks are representative of materials from the mantle wedge overlying a subducted slab. The temperature and pressure of the final reequilibrium of the mineral assemblage are estimated to be around 25 x lO8 Pa and 850℃ respectively, displaying a geotherm characteristic of an orogenic belt. The discovery of eclogite and garnet peridotite suggests that the northern margin of the Chaidam mountains is a coherent highpressure metamorphic belt formed as a consequence of subduction of oceanic lithosphere and continental collision in Paleozoic era.