ISSN 1009-6248CN 61-1149/P 双月刊

主管单位:中国地质调查局

主办单位:中国地质调查局西安地质调查中心
中国地质学会

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    西秦岭三叠纪大河坝组砂岩构造背景与物质来源

    Tectonic Setting and Provenance of Sandstones from Triassic Daheba Formation in the West Qinling Orogenic Belt

    • 摘要: 大河坝组是西秦岭夏河–碌曲地区出露最晚的一套海相地层,记录了印支期海退事件。笔者基于大河坝组层序特征、砂岩主微量元素成分和碎屑锆石U-Pb年龄基础上,探索其物质来源特征和构造背景指示意义。大河坝组主体为砂板岩复理石建造,包括长石砂岩、岩屑长石砂岩、含砾岩屑长石砂岩等。其中,砂岩具有中等SiO2(64.11%~68.27%,平均为65.45%),较高Al2O3(13.61%~15.19%,平均为14.51%),较低Fe2O3(0.82%~1.72%,平均为1.25%)、CaO (2.39%~3.32%,平均为3.06%)、MgO(0.06%~0.10%,平均为0.08%)特征。稀土元素总量较高,ΣREE值为184×10−6~434×10−6,平均为252×10−6。球粒陨石标准化图解中显示Eu负异常,具有与活动大陆边缘环境相似的Th、Zr、Ti、Co、Ni等微量元素含量。砂岩中碎屑锆石U-Pb年龄主要为2204 ~2520 Ma、1536 ~2067 Ma、253.3~448.7 Ma等3个阶段,在2445 Ma、1986.4 Ma、1757.6 Ma、448.5 Ma、278.2 Ma等5个年龄段出现峰值,锆石最小年龄为253.3 Ma。砂岩主微量元素分析和碎屑锆石U-Pb年代学谱系显示,大河坝组源区构造环境为活动大陆边缘,碎屑物质来自上地壳中性火成岩,主要物源区为研究区北部北秦岭构造带、祁连造山带东段和华北板块南缘基底,少部分碎屑来自西秦岭同期岩浆岩和多旋回沉积物。

       

      Abstract: The Daheba formation is the latest strata exposed in the Xiahe-Luqu area of the West Qinling Orogenic Belt (WQOB), with recording the Indosinian regression event. In this study, based on the stratigraphic sequence characteristics, major and trace element compositions and zircon U-Pb dating of the sandstone, we traced the origin of the sandstone and explored the tectonic settings of the Daheba Formation. The Daheba Formation consists of feldspar sandstone, lithic feldspar sandstone, conglomerate clastic feldspar sandstone. The SiO2 content of sandstone range from 64.11% to 68.72% with an average of 65.45%. The sandstones are also characterized by high Al2O3 (13.61%~15.19%), lower Fe2O3 (0.82%~1.72%), CaO (2.393%~3.32%) and MgO (0.06%~0.1%) contents. They have high rare earth elements (REE) range from 184.13×10−6 to 434.06×10−6, and show negative Eu anomaly in the chondrite normalized diagram. They also have similar Th, Zr, Ti, and Co, Ni compositions with that of active continental margin sediments. The detrital zircon U-Pb ages from the sandstones are mainly concentrated in 253.3~448.7 Ma, 1536~2067 Ma and 2204~2520 Ma, and show five age peaks at 278.2 Ma, 448.5 Ma, 1757.6 Ma, 1986.4 Ma and 2445 Ma, with the youngest ages at 253.3 Ma. These indicates that the Daheba Formation was deposited in an active continental margin setting, and the detritus originated from the upper crust igneous rocks. The main sources for the detritus is possibly of the basement of the northern Qinling, the eastern part of the Qilian, and the southern edge of the North China Plate in the northern parts of the study area with small parts from the magmatic rocks and polycyclic sediments in the West Qinling.

       

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