柴达木盆地北缘达肯大坂片麻岩锆石离子探针U-Pb同位素测年

SHRIMP U-Pb Isotopic Dating of Zircons from Gneiss in Northwest Daken Daban Mountain on the North Margin of the Qaidam Basin

柴达木盆地北缘达肯大坂地区大面积出露角闪岩相—麻粒岩相变质岩,呈北西向构造岩片形式产出,局部发生过显著的韧性剪切变形。对达肯大坂西北侧片麻岩的锆石进行阴极发光(CL)、背散射电子成像(BSE)和离子探针U-Pb同位素测年,测出锆石206Pb/238U年龄为(417.7±9.9)^(459±10.0)Ma,平均年龄为(439.6±6.1)Ma;锆石呈自形—半自形柱状,环带清晰,Th/U值变化范围为0.45~0.74,指示原岩为中酸性侵入岩。说明达肯大坂西北出露加里东期正片麻岩,原岩为~440Ma侵位的中酸性侵入岩,其岩浆侵位及变质变形可能与早古生代大陆深俯冲存在动力学成因联系。

Metamorphic rocks of amphibolite-granulite facies are exposed widely in the Daken Daban Mountain on the north margin of the Qaidam basin as tectonic slices, with ductile shearing seen in some belts. Samples were collected and zircons were selected from gneiss in the northwest of the Daken Daban Mountain. Crystal features from CL image and BSE and Th/U ratio （0.45 - 0.74 ） of the zircons indicate that the sampled gneiss initially belonged to granite before metamorphism. SHRIMP U-Pb isotopic dating by ion micro-probe of the zircons give ages ranging from （417.7 ± 9.9） Ma to （459 ±10.0） Ma with an average of （439.6 ± 6.1） Ma, representing the period of zircon crystallization and the time of magma emplacement of the initial granite. These data further indicate that gneiss outcropped in northwest Buken Daban Mountain together with magma emplacement of the original granite, and that high-grade metamorphism and shearing deformation all occurred in Early Paleozoic as a kind of possible results of deep subduction of the continental plate along the South Qilian suture.