东喜马拉雅构造结墨脱地区晚三叠世深熔作用的锆石U-Pb年代限定

Zircon LA-ICP-MS U-Pb Dating of the Triassic Anatexis at Mutuo, the Eastern Himalayan Syntaxis

东喜马拉雅构造结是研究青藏高原构造演化的关键地区。本文对东构造结墨脱地区的混合岩化片麻岩中的锆石进行了LA-ICP-MS微区微量元素分析及U-Pb定年。阴极发光（CL）图像揭示多数锆石具有明显的核-边结构特征,核部为原岩残留核,外形表现出晶棱圆化、港湾状等特征；而边部是混合岩化深熔作用变质过程中形成的新生变质锆石,并且具有典型岩浆锆石的环带特征。微量元素分析显示,锆石不同微域的微量元素含量不同,锆石核部含量明显高于边部（如：元素 Th、U、Nb、Ta 及 Th/U 比值）,这一结果与深熔熔体和残留相之间的平衡关系有关。锆石 U-Pb 定年结果显示12个原岩锆石分析点给出的206Pb/238U年龄为516~1826 Ma,其中有6个分析点的年龄值相对集中在911~1330 Ma,表明该混合岩化片麻岩的原岩可能主要形成在这个年龄区间；14个变质锆石分析点给出了206Pb/238U 年龄加权平均结果为216.7±3.2 Ma（MSWD=3.9）,这一年龄代表拉萨地体在晚三叠世发生深熔作用,与区域上拉萨地体东南缘发生变质事件的时间一致。

The eastern Himalayan Syntaxis is a key region to investigate the tectonic evolution of the Qinghai-Tibet Plateau. The internal structures, trace elements characteristics and ages of zircons from migmatitic gneiss at Motuo area in the eastern Himalayan Syntaxis were studied using cathodoluminescence （CL） images and LA-ICP-MS methods, respectively. The CL images show that many of the zircons have core-rim structure, including an inherited magmatic core and a crystallized rim with oscillatory zoning. The trace elements analyses show that there are notable differences between the cores and the rims. The core domains are distinguishable from the rim domains because of their higher Th, U, Nb, Ta concentrations and high Th/U ratios. Although having similar chondrite-normalized REE patterns, the core and rim domains have significantly different REE contents. The differences in REE contents between the cores and rims maybe relate to the chemical equilibrium between the melt and the restite. The LA-ICP-MS U-Pb zircon analyses indicate that：（1） the zircon cores give multi-stage magmatic event ages ranging from 516 Ma to 1826 Ma, of which six ages are converged on the range of 1330 Ma to 911 Ma, it is considered that the migmatitic gneiss has been formed in this time, while （2） the zircon rims yield 206Pb/238U weighted mean ages of 216.7±3.2 Ma （MSWD=3.9）, which was interpreted to register the ages of the Triassic anatexis. The anatexis in Motuo is coeval with the regional metamorphism in the Lhasa terrane.