Mafic rocks, especially those with low crystallization degree such as basalt and diabase, generally contain very rare zircon. It is thus not very easy to determine their rock-forming epochs by in-situ zircon U/Pb analysis. However, this kind of magmatic rock is likely to entrap numerous zircon xenocrysts in the rising process of magma, whose geochronology may provide valuable constrains on the tectonic evolution of the penetrated geological units. Such zircon-bearing mafic rocks play a role just like the role of lithosphere probe. This paper reports a case study of such zircon xenocrysts. The Yushu tectonic mélange in north-central Tibet is a Late Paleozoic one consisting of metamorphosed and deformed continent-derived clastics, metabasalt, diabase dykes, and numerous gabbroic plutons. Its tectonic affinity and tectonic implications remain in controversy mainly due to the absence of precise geochronological data. In order to get the precise time of the mélange, the authors sampled a diabase dyke for zircon separation, which cuts across the basalts with pillow structures. Approximately 130 zircon grains were collected from the diabase sample, 12 and 60 of which were analyzed for U, Th, and Pb isotopic content by employing SHRIMP and LA-ICP-MS techniques, respectively. The results reveal that the zircon U/Pb ages are in a large range of 400 to 2 900 Ma, obviously older than the time of magmatism, suggesting that all the analyzed zircons are xenocrystals entrapped from the country rocks of the diabase. Based on the field relationship and the petrographic nature of the diabase, the authors hold that these xenocrystic zircons came from a crystalline basement at the earlier stage of the magma rising process instead of being captured from the meta-clastics of the Yushu mélange. Thus, the mafic rocks of the Yushu mélange were developed on a continental crystalline basement, probably in a rifting basin. Furthermore, the geochronology of the xenocrystic zircons provides valuable constraint on the thermo-tectonic evolution of the penetrated crystalline basement. The 72 analyses form an age distribution spectrum that exhibits three major age-peaks: 450 to 750 Ma, 850 to 1 400 Ma, and 1 500 to 1?800 Ma, respectively, which is quite comparable with the age distribution spectrum of the Yangtze block. For the zircon grains of each age-peak, internal zoning structure in the zircons with older ages was commonly clearer than those with younger ages. This fact implies that the thermo-tectonic events of the penetrated crystalline basement were all composed of magmatism and associated metamorphism. Although further studies are required, the new data presented here demonstrate that the Yushu mélange was a continent marginal one along the northern margin of the eastern Qiangtang block of Cathaysian affinity instead of a remnant oceanic slice.