滇西哀牢山构造带变质岩系LA-ICP-MS锆石U-Pb年龄及其地质意义

为厘定哀牢山岩群的时代和大地构造归属,选取哀牢山岩群中的花岗质片麻岩(SM-15)、花岗闪长质片麻岩(SM-22)、同变形的花岗质岩脉(SM-18)及紧邻哀牢山岩群的变质砂岩(NO-1)为研究对象,运用LA-ICP-MS锆石U-Pb定年法对其进行定年。结果显示,样品SM-15中有岩浆锆石和变质锆石2类,岩浆锆石的年龄集中于722~740Ma之间,变质锆石的~(206)Pb/~(238)U年龄加权平均值为29.9±0.5Ma。样品SM-22中2组~(206)Pb/~(238)U年龄加权平均值,原岩年龄为232.3±4.3Ma;后期变质重结晶年龄为29.2±0.4Ma。样品SM-18中锆石~(206)Pb/~(238)U年龄加权平均值为27.19±0.36Ma,代表变质年龄。样品NO-1中的碎屑锆石年龄分4组:246~276Ma、600~800Ma、800~1000Ma和1000Ma以上。以上年龄数据表明,哀牢山岩群中至少包括元古宙岩浆岩(722~740Ma)、二叠纪末—三叠纪初岩浆岩或地层(235~255Ma),以及新近纪岩浆岩(27~32Ma),是一个复杂的变质岩带,而不是以往认为的全部属于元古宙扬子地台的结晶基底。哀牢山岩群的主变质时代集中在27~32Ma之间,哀牢山岩群现今的面貌主要在这一时期成型。

LA-ICP-MS zircon U-Pb dating of the metamorphic rocks in the Ailaoshan tectonic belt, western Yunnan, and its geological implications

In order to determine the age of Ailaoshan Group and its tectonic affinity, the authors chose granodioritic gneiss (SM-15 and SM-22), its synchronic deformational granite vein intrusion (SM-18) in the high-grade metamorphic rock series and its proximate meta-sandstone (NO-1) in the low-grade metamorphic rock series as research objects and present new results of zircon U-Pb dating of four samples so as to do the accurate chronology tests. Two groups of ages were identified in sample SM-15 and SM-22. In sample SM-15, the older magmatic group yielded an age of 722~740Ma and the metamorphic younger group yielded an age of 29.9±0.5Ma. In sample SM-22, the older is 232.3±4.3Ma and the younger is 29.2±0.4Ma. In addition, the zircons from sample SM-18 yielded a concordant U-Pb ages of 27.19±0.36Ma. A total of 30 spots analyses of sample NO-1 defined four age groups at 246~276Ma, 600~800Ma, 800~1000Ma and >1000Ma respectively. From these data, the authors have drawn some conclusions: Firstly, the Ailaoshan Group consists of at least three stages of tectonic and magmatic activities, i.e., the 722-740Ma and 235±10Ma igneous rocks and sedimentary rocks, and the 27-32Ma igneous rocks. The metamorphic rocks in the study area are unlikely to be of the Yangtze Plate; Secondly, the peak metamorphism deformation time of Ailaoshan Group was about 27-32Ma.