The Ailao Shan-Red River metamorphic belt consists largely of high-grade metamorphic rocks of the Ailao Group, which were previously thought to be the Paleoproterozoic basement rocks of the Yangtze Craton. We present new results of zircon U-Pb dating on four samples from the representative lithologies of the Ailao Shan metamorphic belt to explore their possible regional implications. Two groups of ages are identified in granitic gneiss sample 11AL17-1 of the Ailao Shan high metamorphic rock series. The older magmatic group yields an age of 700 ±6Ma ( MSWD = 1.4, n = 14) and the clearly metamorphic younger group yields an age of 27. 4rn±1.2Ma (MSWD = 1.9, n =3). A similar situation exists in granitic gneiss sample 11AL09-1, with magmatic zircons clustered around concordia at 220 ± 3Ma ( MSWD = 3. 1, n = 14) and metamorphic zircons at 31. 2 ± 2. 3Ma ( MSWD = 6, n = 5 ). The quartzite samplellAL08-l is also collected from the Ailao Shan high metamorphic rock series. All zircons extracted from quartzite sample 11AL08-1 have distinct core-rim structure. A total of 92 spots analyses on cores define six age populations at 493 ~528Ma (n =42) , 635~640Ma (n=2), 701 ~784Ma (n=44), 976~980Ma (n=2), 1839Ma (n = l) and 2487 Ma (n = l). All the 92 cores have high Th/U ratios ( > 0. 23) and were probably derived from igneous rocks. Forty-two of the youngest spots have a weighted mean age of 509Ma, representing the maximum depositional age of the protolith of the quartzite 11AL08-1. Seven spots on metamorphic rims yield much younger ages ranging from 26Ma to 75Ma, which is interpreted to reflect subsequent metamorphic event. A weighted mean 206Pb/238U age of 750±4Ma (MSWD =0.6,n =22) was obtained from a granite sample 11AL12-1 adjacent to the Ailao Shan high metamorphic belt and is considered to represent the intrusion age. The data described above demonstrate that the Ailao Group is not Paleoproterozoic basement but younger metamorphic complex, the protolith of which consists of at least three units including thern~700Ma and 220 ~240Ma igneous rocks, as well as ~509Ma sedimentary rocks. They have undergone metamorphism as a result of the Indo-Asian collision at 26 ~ 31 Ma, and acquired the current morphological features that is hard to be distinguished from the basement rocks. The zircon age spectra of the Ailao Shan-Red River metamorphic belt are roughly consistent with those of the western Yangtze Craton, suggesting a Yangtze Craton affinity of the Ailao Shan-Red River metamorphic belt.%哀牢山-红河构造带是滇西地区最著名的带状变质带之一,其主体由哀牢山深变质岩系(哀牢山岩群)组成,一直被认为是扬子陆块古元古代结晶基底.本文选取哀牢山深变质岩系内的花岗片麻岩(11 ALl7-1和11AL09-1)和石英岩(11AL08-1),以及邻区的花岗岩(11ALl2-1)进行LA-ICP-MS锆石U-Pb定年.结果显示,花岗片麻岩11 ALl7-1有岩浆和变质两类锆石,两者的206Pb/238U年龄加权平均值分别为700±6Ma(MSWD=1.4,n=14)和27.4±1.2Ma(MSWD=1.9,n=3),代表原岩形成时代和变质年龄.花岗片麻岩llAL09-1岩浆锆石206 pb/238U年龄为220±3Ma(MSWD=3.1,n=14),变质锆石年龄为31.2±2.3Ma(MSWD =6,n=5),分别代表原岩结晶时代和后期变质年龄.石英岩11AL08-1中所有锆石具有核-边结构,92颗锆石核部年龄集中分布在6组,分别为493～528Ma(n=42)、635 ～ 640Ma(n=2)、701～784Ma(n=44)、976 ～980Ma(n=2)、1839Ma(n=1)和2487Ma(n=1).92个核部分析点具有高的Th/U比值(＞0.23),指示岩浆来源.最年轻一组的42个核部年龄加权平均值为509Ma,代表石英岩原岩的最大沉积时代.7颗锆石变质边年龄为26～ 75 Ma内,代表变质年龄.花岗岩11 ALl2-1锆石206pb/238U年龄加权平均值为750±4Ma(MSWD =0.6),代表岩石形成时代.这些年龄表明哀牢山变质岩系是一个原岩复杂的变质杂岩带,它的原始物质至少包含新元古代～ 700Ma岩浆岩、～509 Ma沉积地层及220 ～ 240Ma的岩浆岩和地层,而不是以往认为的古元古代结晶基底.现今所见的哀牢山岩群“古老”岩石面貌主要是由地质历史上的浅变质或未变质的地层和岩浆岩在新生代26～31Ma发生变质变形作用改造的结果.哀牢山变质带的源区物质特征和主要岩浆事件与扬子陆块西缘十分相似,具有亲扬子的构造属性.
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