中国西秦岭碎屑锆石U-Pb年龄及其构造意义

U-Pb ages of zircons in Western Qinling Mountain,China,and their tectonic implications

西秦岭是北接华北克拉通、西接祁连与柴达木、南接松潘一甘孜地块的东秦岭造山带的西延。文中研究了该区从前寒武纪到三叠纪的碎屑沉积岩。这些碎屑沉积岩中分离出的锆石由LA-ICPMS（激光剥蚀等离子体质谱）进行了U-Pb定年。全岩Nd亏损地慢模式年龄类似于扬子克拉通年龄，主要分布于1.55～1.98Ga，峰值为1.81Ga，而与华北克拉通主要为古元古代与太古宙的模式年龄形成明显的对比。泥盆系中的碎屑锆石930～730Ma的u—Pb年龄指示其与扬子克拉通具亲缘性。930～730Ma是源区地壳的强烈增长阶段。二叠系～三叠系的碎屑沉积岩主体以含老于1600Ma的碎屑锆石为特征。碎屑锆石U-Pb年龄与Sm-Nd同位素组成指示此时华北克拉通南缘的基底岩石成为二叠系-三叠系碎屑沉积岩的重要物源。扬子克拉通在三叠纪时与华北克拉通拼接。西秦岭二叠系-三叠系碎屑沉积岩含有高达50％的华北克拉通南缘的基氟岩石。

The Western Qinling Mountain is the westward extension of the Eastern Qinling Mountain, which connects the North China craton, the Qilian and Qaidam, and the Songpan-Ganzi block on the north, on the west, and on the south, respectively. Clastic sedimentary rocks of Precambrian to Triassic age from this region are studied in this article. Detrital zircons were extracted from the sedimentary rocks for U-Pb dating through LA-ICPMS. Nd depleted mantle model ages resemble those of the Yangtze craton, with a statistical population of 1.55-1.98 Ga, peaking approximately at 1.81 Ga, in contrast to Paleoproterozoic and Archean population of the North China craton. Devonian detrital zircon U-Pb ages of 930 to 730 Ma indicate their affinity to the Yangtze craton, which was the intensive crustal growth stage in the provenance region. The majority of the Permo- Triassic clastic sedimentary rocks are characterized by ages older than 1600 Ma. Detrital zircon U-Pb ages and Sm-Nd isotopic compositions indicate that the basement rocks, on the southern margin of the North China craton, became an important component of the Permo-Triassic clastic sedimentary rocks. The Yangtze craton conjoined the North China craton during the Triassic time period. Up to 50% of the Permo-Triassic clastic sedimentary rocks of the west Qinling segment contain material from the basement rocks on the southern margin of the North China craton.