内蒙古武川西乌兰不浪地区早前寒武纪变质基底锆石SHRIMP定年及Hf同位素组成

SHRIMP dating and Hf isotope analysis of zircons from the Early Precambrian basement in the Xi Ulanbulang area, Wuchuan, Inner Mongolia.

本文报道了华北克拉通西部武川西乌兰不浪地区太古宙变质基底的锆石SHRIMP年龄和Hf同位素组成。一个片麻状奥长花岗岩样品的锆石具核边结构,核部岩浆锆石和边部变质锆石的207Pb/206Pb加权平均年龄分别为2692±17Ma和2528±16Ma。对9个样品进行了锆石Hf同位素分析。新太古代早期(2692～2697Ma)片麻状奥长花岗岩(2个样品)的岩浆锆石的εHf(t)、tDM1(Hf)和tDM2(Hf)分别为4.78～8.83、2646～2780Ma和2632～2845Ma;新太古代二辉麻粒岩(2个样品)中的捕获锆石的εHf(t)、tDM1(Hf)和tDM2(Hf)分别为-2.30～8.62、2543～2954Ma和2529～3189Ma;新太古代变质深成岩(4个样品)的岩浆锆石的εHf(t)、tDM1(Hf)和tDM2(Hf)分别为-2.60～8.09、2529～2880Ma和2538～3089Ma;古元古代蓝晶石榴长英质片麻岩(1个样品)的碎屑锆石的εHf(t)、tDM1(Hf)和tDM2(Hf)分别为1.52～6.59、2432～2774Ma和2498～2925Ma。结合前人研究结果,可得出如下结论和认识:1)该区存在新太古代早期片麻状奥长花岗岩,太古宙岩石在新太古代晚期普遍遭受高级变质作用影响;2)新太古代早期为该区地壳形成主要时期,新太古代晚期则主要表现为陆壳物质再循环;3)作为阴山地块的典型代表,固阳-武川地区与华北克拉通东部太古宙基底十分类似,可能表明华北克拉通在新太古代晚期已成为统一的整体。

This paper reports SHRIMP U-Pb ages and Hf isotopes of zircons from Archean basement in the Xi Ulanbulang area, Wuchuan, western North China Craton （NCC）. Zircons from a trondhjemite gneiss sample show core-rim textures, with magmatic cores and metamorphic rims having weighted mean 207Pb/206Pb ages of 2692±17Ma and 2528±16Ma, respectively. Hf isotope analyses have been carried out on zircons from 9 samples. Magmatic zircons of Early Neoarchean （2692～2697Ma） trondhjemite gneisses （2 samples） have εHf（t）, tDM1（Hf） and tDM2（Hf） being 4.78～8.83, 2646～2780Ma and 2632～2845Ma, respectively. Captured zircons of Neoarchean two-pyroxene granulite （2 samples） have εHf（t）, tDM1（Hf） and tDM2（Hf） of -2.30～8.62, 2543～2954Ma and 2529～3189Ma. Neoarchean meta-intrusive rocks （4 samples） have magmatic zircons with εHf（t） values of -2.60～8.09, tDM1（Hf） ages of 2529～2880Ma and tDM2（Hf） ages of 2538～3089Ma. Analyses on detrital zircons from a Paleoproterozoic kyanite-garnet felsic gneiss yielded εHf（t） values ranging from 1.52 to 6.59 and tDM1（Hf） and tDM2（Hf） model ages of 2432～2774Ma and 2498～2925Ma. Combined with early studies, we draw conclusions as follows： 1） in the study area, there are Early Neoarchean TTG rocks which underwent strong metamorphism at the end of Neoarchean together with Late Neoarchean rocks; 2） Early Neoarchean is an important period when continental crust was formed, however, in Late Neoarchean, reworking of older continental materials played an important role; 3） Guyang-Wuchuan, a typical area of the Archean Yinshan block, is similar to the Archean basement of the eastern NCC in many aspects, supporting an idea that the NCC had already been a single unit at the end of the Neoarchean.