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现代地质 ›› 2018, Vol. 32 ›› Issue (04): 704-717.DOI: 10.19657/j.geoscience.1000-8527.2018.04.07

• 岩石学 • 上一篇    下一篇

西秦岭甘青交界一带晚三叠世火山岩岩石成因及构造指示意义

刘伯崇1(), 李康宁1,2(), 史海龙1, 蒲万峰1, 汪宏涛1, 王舒恒1   

  1. 1.甘肃省地质矿产勘查开发局 第三地质矿产勘查院,甘肃 兰州 730050
    2.中国地质大学(北京) 地球科学与资源学院 北京 100083
  • 收稿日期:2017-12-09 修回日期:2018-02-28 出版日期:2018-08-10 发布日期:2018-09-19
  • 通讯作者: 李康宁
  • 作者简介:李康宁,男,工程师,硕士,1986年出生,岩石学专业,从事地质矿产调查与找矿预测工作。Email:379607468@qq.com
    刘伯崇,男,高级工程师,1965年出生,地质矿产勘查专业,长期从事地质矿产调查评价与技术管理工作。Email:lbchym@126.com
  • 基金资助:
    中国地质调查局项目“甘肃省夏河-合作地区金矿整装勘查区矿产调查与找矿预测”(121201004000160901-60);“青海省同仁县多哇地区I48E005001、I48E005002两幅1∶50000区域地质矿产调查”(12120114018220)

Petrogenesis and Tectonic Implications of Late Triassic Volcanic Rocks at the Gansu-Qinghai Junction in the West Qinling Mountains

LIU Bochong1(), LI Kangning1,2(), SHI Hailong1, PU Wanfeng1, WANG Hongtao1, WANG Shuheng1   

  1. 1. The Third Institute Geological and Mineral Exploration of Gansu Provincial Bureau of Geology and Mineral Resources,Lanzhou,Gansu 730050,China
    2. School of Earth Sciences and Resources,China University of Geosciences,Beijing 100083,China
  • Received:2017-12-09 Revised:2018-02-28 Online:2018-08-10 Published:2018-09-19
  • Contact: LI Kangning

摘要:

西秦岭甘肃、青海交界一带中生代火山岩较为发育,火山岩组成以安山质、流纹质为主,均为高钾钙碱性火山岩,主、微量元素含量变化较大。SiO2的质量分数为56.67%~78.17%,平均69.78%, Al2O3含量较高(12.77%~17.29%,平均14.64%)。TiO2含量少(0.02%~0.75%,平均0.22%),Na2O/K2O比值较小(0.03%~2.68%,平均0.65%)。轻稀土元素中等富集,重稀土元素相对亏损,稀土元素配分曲线明显右倾,具Eu负异常。安山质火山岩与流纹质火山岩稀土元素球粒陨石配分曲线中重稀土型式明显不同,暗示二者来自不同的源区。微量元素中P、Nb、Ti、Ta等高场强元素(HFS)相对亏损,Th、 Ba、K、Rb等大离子亲石元素相对富集。研究区安山岩LA-ICP-MS锆石U-Pb同位素定年结果为(236±1.2)Ma,代表了火山岩的喷发年龄。它们形成于隆务峡蛇绿岩洋壳向南俯冲的活动大陆边缘弧环境,为俯冲洋壳在地幔深部发生高程度部分熔融作用的产物,并在上升过程中受到陆壳物质作用。在岩浆演化过程受部分熔融和分异结晶作用的控制,但安山质火山岩主要受控于分异结晶作用,而流纹质火山岩受部分熔融作用较大。

关键词: 西秦岭, 钙碱性, 火山岩, 洋壳俯冲

Abstract:

Volcanic rocks are developed in the West Qingling Mountains, near the border between Gansu and Qinghai Provinces. The rocks are dominantly andesitic and rhyolitic, and are all high-K calc-alkaline volcanic rocks.These volcanic rocks differ greatly in their major and trace element contents. The SiO2 contents range from 56.67% to 78.17% (average 69.78%). The Al2O3 are high (12.77% to 17.29%,average 14.64%) and the TiO2 contents are low (0.02% to 0.75%, average 0.22). The Na2O/K2O ratios are low (0.03% to 2.68%,average 0.65%). Light rare earth elements (LREEs) are enriched whereas heavy rare earth elements (LREEs) are depleted. REE patterns are obviously right inclining with distinct negative Eu anomalies. The chondrite-normalized HREE patterns of the andesitic and rhyolitic volcanic rocks are markedly different, which implies that the both volcanic rock types were derived from different magma sources. The volcanic rocks are depleted in high field strength elements(HFSEs) such as P,Nb,Ti, and Ta, but are enriched in large ion lithophile elements (LILEs) such as Th,Ba,K, and Rb. LA-ICP-MS zircon U-Pb dating of the andesitic volcanic rock yielded (236±1.2) Ma,which represents the volcanis mage.These rocks were likely oceanic subduction products with high-degree partial melting in the deep mantle, and the magma is also influenced by fractionation and crustal assimilation during its ascent. The volcanism may have formed in an active continental margin arc setting, related to the south-dipping subduction of the Longwu Gorge Ocean(remnants of which preserved now as ophiolite). Magmatic evolution of these volcanic rocks was likely controlled by partial melting and fractional crystallization: the andesitic magma was mainly controlled by fractional crystallization, whereas the rhyolitic magma was largely dominated by partial melting.

Key words: West Qinling Mountain, calc-alkaline, volcanic rock, subduction of oceanic crust

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