东昆仑造山带塔妥地区奥陶纪辉绿岩年代学、地球化学及构造意义
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1.长安大学地球科学与资料学院;2.青海省第五地质矿产勘查院

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国家自然科学基金项目(面上项目,重点项目,重大项目)


Zircon U-Pb geochronology and geochemistry of Tatuo mafic dykes in East Kunlun orogen, and its geological significance
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1.Chang'2.'3.an University;4.No. 5 Geological Survey Institute of Qinghai Province

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    摘要:

    东昆仑造山带出露有一套奥陶纪基性岩墙,对研究东昆仑造山带早古生代活动大陆边缘与原特提斯洋相关的俯冲过程具有重要意义。本文对出露于东昆仑东段塔妥地区的基性岩墙进行了详细的锆石U-Pb年代学、全岩地球化学及Sr-Nd同位素等研究。两件辉绿岩样品的锆石U-Pb测年结果表明该基性岩墙锆石U-Pb形成年龄为486~483Ma,形成时代为早奥陶世。全岩地球化学结果表明该套基性岩墙具低SiO2(50.06%-51.68%)、中等MgO(5.08%~6.77%)和TiO2(0.99%~1.20%)含量特征,属于拉斑玄武岩系列。微量和稀土元素方面,岩石呈现大离子亲石元素(Rb、Sr、Ba、Th、K)富集和高场强元素(Nb、Ta、Ti)负异常的特点;球粒陨石标准化稀土元素配分图呈现轻稀土元素弱富集、重稀土元素亏损以及弱的Eu正异常的特征。样品具有高的(87Sr/86Sr)i值(0.7126-0.7133)和相对低的εNd(t)同位素组成(-9.80~-4.31),综合研究表明该基性岩浆起源于俯冲带流体交代的富集地幔,形成于大洋俯冲带上盘岩浆弧(伸展)构造环境。在综合区域地质资料基础上,本文认为东昆仑原特提斯洋在早古生代早期存在向北俯冲,并于早奥陶世俯冲大洋板块后撤导致俯冲带上盘出现伸展构造背景。

    Abstract:

    Early Ordovician mafic dykes exposed in the North Kunlun are of great significance to the subduction processes and dynamic background of the paleo-active continental margin. In this paper, we presented detailed petrology, zircon U-Pb chronology, whole-rock geochemistry and Sr-Nd isotopes of the mafic dykes. Zircon U-Pb dating results show that the zircon U-Pb ages of these dykes range from 486 to 483 Ma, suggesting Early Ordovician. Whole-rock geochemical results show that the mafic dykes have low SiO2 (50.06%-51.68%), medium MgO (5.08%-6.77%) and TiO2 (0.99%-1.20%) content, and belong to the tholeiitic basalt series. For the trace and rare earth elements, the rocks are enriched in large-ion lithophile elements (e.g., Rb, Sr, Ba, Th, K) and depleted high-field-strength elements (e.g., Nb, Ta, Ti); The chondrite-normalized REE diagrams are featured by wild enrichment of LREE and depletion of HREE, with weak Eu anomaly. In addition, the dykes show the high (87Sr/86Sr)i (0.7126-0.7133) and relatively low εNd(t) isotopic composition (-9.80~-4.31) of the enriched lithospheric mantle. The petrogenesis research indicates that the basic magma was originated from the enriched lithospheric mantle ever metamorphosed by fluids in the subduction zone, and then experienced fractional crystallization mainly of olivine and clinopyroxene during the diagenetic processes. Based on the regional geological data, we argued that the Kunzhong Ocean subducted northward in early stage of Early Paleozoic and formed a magmatic arc system. It may have experienced the retreat of the subducting oceanic slab at 486Ma, leading to an extensional tectonic setting in the upper plate of the subduction zone and then generation of mafic dykes.

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  • 收稿日期:2022-10-24
  • 最后修改日期:2022-12-31
  • 录用日期:2022-12-31
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