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现代地质 ›› 2019, Vol. 33 ›› Issue (03): 487-500.DOI: 10.19657/j.geoscience.1000-8527.2019.03.03

• 矿床学与岩石学 • 上一篇    下一篇

西藏纳木错早白垩世流纹岩锆石U-Pb年龄、地球化学及其构造意义

魏永峰1,2(), 肖倩茹2, 李有波1,2, 罗巍1,2, 冉杰1, 林美英1   

  1. 1.四川省地质矿产勘查开发局 区域地质调查队,四川 双流 610213
    2.成都理工大学 地球科学学院,四川 成都 610059
  • 收稿日期:2018-03-21 修回日期:2018-11-30 出版日期:2019-06-23 发布日期:2019-06-24
  • 通讯作者: 肖倩茹
  • 作者简介:肖倩茹,女,博士研究生,1987年出生,矿物学、矿床学、岩石学专业,主要从事岩石地球化学研究及教学工作。Email: 18628183715@163.com
    魏永峰,男,高级工程师,硕士研究生,1974年出生,矿物学、矿床学、岩石学专业,主要从事区域地质矿产调查、矿产勘查及研究工作。Email: 393875930@qq.com
  • 基金资助:
    中国地质调查局项目“西藏班公湖—怒江成矿带地质矿产调查”(1212011221083)

Zircon U-Pb Age,Geochemical and Tectonic Significance of Early Cretaceous Namtso Rhyolite in Tibet,China

WEI Yongfeng1,2(), XIAO Qianru2, LI Youbo1,2, LUO Wei1,2, RAN Jie1, LIN Meiying1   

  1. 1. Regional Geological Survey Party,Sichuan Bureau of Geology and Mineral Resources Development,Shuangliu,Sichuan 610213,China
    2. College of Earth Sciences,Chengdu University of Technology,Chengdu,Sichuan 610059,China
  • Received:2018-03-21 Revised:2018-11-30 Online:2019-06-23 Published:2019-06-24
  • Contact: XIAO Qianru

摘要:

西藏纳木错流纹岩构造上处于北冈底斯南缘,地层角度不整合于中—晚侏罗世拉贡塘组之上,归属于卧荣沟组。其中流纹岩的LA-ICP-MS锆石U-Pb年龄为120~112 Ma,时代为早白垩世中期。岩石具高SiO2、富Al2O3和K2O,贫TiO2、MgO、CaO特征,A/CNK=0.94~1.91(平均1.16),属于高钾钙碱性-钾玄岩系列过铝质岩石。(La/Yb)N为1.49~11.08,δEu=0.12~0.55,球粒陨石标准化稀土元素分布模式显示为略右倾V字形。大离子亲石元素及高场强元素Rb、K、Th、Ta、Ce、Zr、Hf相对富集,Nb相对亏损,Ba、Sr、P、Ti强烈亏损,成岩岩浆源自地壳,可以与多尼组酸性火山岩对比。流纹岩具同碰撞型火山特征,不具传统岛弧环境特征,研究认为在早白垩世中期(120 Ma左右),北冈底斯发生了一次重大的构造事件,即班公湖—怒江新特提斯洋俯冲已经结束,冈底斯带与羌塘地块陆-陆碰撞开始,碰撞过程中下地壳增厚并部分发生熔融,纳木错流纹岩是碰撞后部分熔融的产物。

关键词: 锆石U-Pb年龄, 早白垩世, 流纹岩, 卧荣沟组, 纳木错, 北冈底斯带, 中国西藏

Abstract:

The Namtso rhyolite is located in the southern margin of the North Gangdese Belt. It belongs to the Woronggou Formation and overlies the Middle-Late Jurassic Lagongtang Formation along an angular unconformity.The rhyolite is LA-ICP-MS zircon U-Pb dated to be Early Cretaceous(120-112 Ma). The rocks are characterized by high SiO2, Al2O3 and K2O, and low TiO2, MgO and CaO, A/CNK=0.94-1.91(average 1.16). The rhyolite is high-K calc-alkaline and peraluminous. The chondrite-normalized patterns show a gently right-inclining ((La/Yb)N=1.49 to 11.08) “V” shape and weakly negative Eu anomalies(δEu= 0.12 to 0.55). The elements of Rb, K, Th, Ta, Ce, Zr and Hf are enriched, while Nb and Ba, Sr, P and Ti are strongly depleted, suggesting a crustal inheritance. The geochemistry can be compared with the Duoni Group felsic volcanic rocks. The Namtso rhyolite does not have the characteristics of typical island arc rhyolites, but mimics those of collisional felsic volcanic rocks. We considered that during the middle Early Cretaceous(ca.120 Ma), a major tectonic event may have occurred in the North Gangdese Belt, i.e., subduction of Neo-Tethys Ocean in the Bangong Lake and Nujiang River region had ended, and the continent-continent collision in the Gangdese and Qiangtang region had begun. During the collision, the lower crust may have thickened and partially melted, forming the Namtso rhyolite.

Key words: zircon U-Pb age, Early Cretaceous, rhyolite, Woronggou Formation, Namtso, North Gangdese belt, Tibet, China

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