地学前缘 ›› 2016, Vol. 23 ›› Issue (2): 190-205.DOI: 10.13745/j.esf.2016.02.019

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喜马拉雅早古生代造山作用:来自尼泊尔帕朗花岗质片麻岩锆石U - Pb年代学和Hf同位素证据

王晓先,张进江,王盟   

  1. 1. 中国地震局 地壳应力研究所 地壳动力学重点实验室, 北京 100085
    2. 北京大学 地球与空间科学学院; 造山带与地壳演化教育部重点实验室, 北京 100871
  • 收稿日期:2015-03-09 修回日期:2015-06-11 出版日期:2016-03-15 发布日期:2016-03-15
  • 作者简介:王晓先(1986—),男,博士,助理研究员,构造地质学专业。E-mail:xiaoxianwang@pku.edu.cn
  • 基金资助:

    中国地震局地壳应力研究所中央级公益性科研院所基本科研业务专项(ZDJ2014 - 09);国家自然科学基金项目(41402175,41172176)

Early Paleozoic orogeny in the Himalayas: Evidences from the zircon U - Pb chronology and Hf isotope compositions of the Palung granitic gneiss in Nepal.

  • Received:2015-03-09 Revised:2015-06-11 Online:2016-03-15 Published:2016-03-15

摘要:

尼泊尔帕朗花岗质片麻岩是加德满都逆冲席体的一部分,其主要矿物组成为石英、斜长石、钾长石、微斜长石和白云母。片麻岩中的锆石发育核边结构,由继承碎屑锆石核和韵律环带的岩浆锆石边组成。LAICPMS UPb定年结果显示,边部岩浆锆石的加权平均年龄为(485.5±1.4) Ma和(455.1±3.1) Ma,指示片麻岩原岩为早古生代早期的花岗岩,并记录了两期岩浆作用。锆石边部εHf(t)值变化范围为-8.7~-3.5,Hf同位素两阶段模式年龄TCDM为2.01~1.69 Ga,结合岩石学特征并对比大喜马拉雅和小喜马拉雅变质沉积岩的Hf同位素成分,认为原岩花岗岩来自大喜马拉雅变泥质岩的部分熔融。帕朗花岗质片麻岩的研究结果和现有的年代学数据表明,喜马拉雅地区存在早古生代造山事件,这一事件可与相邻的拉萨地体、羌塘地体以及青藏高原东南缘的保山腾冲地体内同一时代的构造事件对比,是早古生代早期原特提斯洋岩石圈沿冈瓦纳大陆北缘俯冲的安第斯型造山作用的产物,而与冈瓦纳大陆内部块体聚合过程中陆陆碰撞的泛非造山作用无关。

关键词: 喜马拉雅造山带, 花岗质片麻岩, LA - ICP - MS锆石U - Pb定年, Hf同位素, 早古生代造山作用

Abstract:

The Palung granitic gneiss in Nepal is a part of the Kathmandu thrust sheet of which the major mineral compositions are: quartz, plagioclase, Kfeldspar, microcline and muscovite. Zircons from the granitic gneiss have corerim structures with inherited detrital cores and magmatic rims showing concentric oscillatory zoning. LAICPMS zircon UPb dating results of the rims gave weighted mean ages of 485.5±1.4 Ma and 455.1±3.1 Ma, hinting that the protolith of the gneiss was the early Paleozoic granite recording two episodes of magmatism. The εHf(t) values range from -8.7 to -3.5 and the calculated TCDM(twostage model age) vary from 2.01 Ga to 1.69 Ga. Combined with the petrological characteristics and compared with Hf isotope compositions of the metapelites and orthogneisses in the Greater Himalayan Complex(GHC)and the metasedimentary rocks in the Lesser Himalayan sequence(LHS), we propose that the source of granite was possibly generated from the partial melting of the metapelite in the GHC. Zircon UPb ages of the Palung granitic gneiss and the published geochronological results of the CambrianOrdovician granites/granitic gneisses demonstrate that the early Paleozoic orogeny was existed in the Himalayas. The early Paleozoic tectonic events preserved in the Himalayas are well compared with the contemporaneous ones in Lhasa terrane, Qiangtang terrane and BaoshanTengchong terrane located in the southeast of the Tibetan Plateau. Integrating the results from previous studies, we suggest that it is resulted from the Andeantype orogeny formed through the subduction of the protoTethys Ocean lithosphere along the northern margin of the Gondwana, rather than the PanAfrican orogeny which was related with the continentcontinent collision during the Gondwana assembly.

Key words: Himalayan orogen, granitic gneiss, zircon LAICPMS UPb dating, Hf isotope, early Paleozoic orogeny

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