塔里木盆地东南缘阿克塔什塔格地区新太古代陆壳增生:米兰岩群和TTG片麻岩的地球化学及年代学约束

The growth of Archean continental crust in Aqtashtagh Area of Southeast Tarim,China:Constraints from petrochemistry and chronology about Milan Group and TTG-gneiss

塔里木盆地东南缘的阿克塔什塔格地区,保存有较为完好的早前寒武纪基底变质岩——阿克塔什塔格杂岩,主要由米兰岩群、新太古代TTG花岗片麻岩和侵入其中的各类古元古代花岗片麻岩构成。其中米兰岩群和TTG片麻岩发育塑性流变褶皱和高角闪岩相-麻粒岩相变质,具有强烈的混合岩化,并遭受后期的角闪岩相变质改造。米兰岩群中的长英质片麻岩和TTG岩系的锆石SHRIMP U-Pb年龄分别为(2 567±32)Ma和(2 592±15)Ma,二者普遍低Si高Al、富Na贫K、富Sr贫Mg、富集LILE和LREE,亏损HSFE和HREE、轻重稀土分馏强烈、Eu异常不明显,具有类似于埃达克岩的岩石地球化学特征,表明它们形成于俯冲带的岛弧环境,为岛弧玄武岩俯冲至下地壳部分熔融的产物,指示了塔里木盆地东南缘新太古代晚期古老克拉通的大陆地壳水平增生。在此基础上,文章还探讨了塔里木盆地周缘早前寒武纪基底岩系的年代格架问题,认为塔里木盆地具有统一的早前寒武纪变质基底。

In Aqtashtagh, southeastern margin of Tarim Basin, there are relatively well-preserved early Precambrian basement metamorphic rocks-Aqtashtagh complex, consisting of Milan Group, Neoarchean TTG gneisses and paleoproterozoic granitic gneiss intruding into them. With rheomorphic folding, Milan Group and TTG underwent high-amphibolite to granulite facies metamorphism, intensive migmatization and later alteration of amphibolite facies metamorphism. Their geochemical characteristics- low Si and Na, high A1 and K content, enriched in LILEs and LREE, depleted in HFSE and HREE, heavy fractionation between LREE and HREE, and no obvious Eu anomaly, indicate that they are products of arc basalt melting after subducting beneath the lower continental crust. The zircon U Pb SHRIMP ages of them are 2567~32 Ma and 2592i 15 Ma respectively, suggesting that they are formed from the same magma stage, and hinting the assembly of Tarim Craton in the late Neoarchean and horizontal crust growth similar to the Japan arc model in southeastern margin of Tarim Craton in Wutai stage. Moreover, many xenocrystals of zircon with ages ranging from 2. 67 to 2.8 Ga and two generation metamorphic zircon with the ages of 2.35 - 2.4 Ga and 1.9 - 2. 0 Ga reflect the major magmatic event in the middle Neoarchean and the cratonization of Tarim platform in the Paleoproterozoic.