Zircon U-Pb geochronology of Baoyintu beds in the northwestern margin of the North China Craton

The Baoyintu Inlier is located at the northwestern margin of the North China Craton (NCC). Here we present LA-ICP-MS and SHRIMP U-Pb data for zircons extracted from samples of monzogranitic orthogneiss and units in the beds. The 1679±13 Ma date for the orthogneiss demonstrates that Late Palaeoproterozoic rocks do exist in the inlier. The ca. 1413 Ma minimum age of detrital zircons in metasedimentary rocks shows that the maximum age of the Baoyintu beds is Mesoproterozoic. The age distribution of detrital zircons in the structurally lowest exposed part of the beds form clusters at ca. 2680, 2450, 1800 and 1560 Ma, which partly correspond to tectonic-magmatic–metamorphic events previously recognised in the NCC. A structurally higher unit in the beds yields age peaks at ca. 1640 and 1500 Ma limiting the maximum age of the beds at ca. 1500 Ma. Amphibolite with a dolerite protolith from the upper unit also shows evidence for metamorphism during ca. 877-851 Ma.

宝音图残出体（Baoyintu Inlier）位于华北克拉通（North China Craton, NCC）西北缘。本文报道了采自二长花岗岩质正片麻岩及该套地层层位样品的锆石激光剥蚀电感耦合等离子体质谱法（LA-ICP-MS）与SHRIMP U-Pb定年数据。该正片麻岩的定年结果为1679±13 Ma，证实该残出体中确有古元古代晚期岩石产出。变沉积岩中碎屑锆石的最小年龄约为1413 Ma，表明宝音图地层序列的最大沉积年龄为中元古代。该套地层构造位置最低的出露部位的碎屑锆石年龄分布呈现约2680 Ma、2450 Ma、1800 Ma及1560 Ma的峰值群，这些年龄峰值与华北克拉通此前已识别的构造-岩浆-变质事件部分对应。该套地层中构造位置较高的单元则呈现约1640 Ma与1500 Ma的年龄峰值，将该套地层的最大沉积年龄限定为约1500 Ma。采自上部层位、原岩为辉绿岩的角闪岩也存在约877~851 Ma期间发生变质作用的证据。