藏南冲巴淡色花岗岩锆石U-Pb、白云母40Ar-39Ar年代学及其地质意义
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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)


Zircon U-Pb and muscovite 40Ar-39Ar chronology of the Chongba leucogranites in south Tibet and its geological implications
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    摘要:

    本文采集藏南冲巴淡色花岗岩样品并进行系统的锆石LA-ICP-MS U-Pb和白云母40Ar-39Ar年代学分析。锆石U-Pb定年结果显示,冲巴淡色花岗岩年龄为12.4±0.4 Ma,处于前人划分的新喜马拉雅阶段与后喜马拉雅阶段分界处。结合淡色花岗岩沿藏南拆离系分布的特征,可将其归入新喜马拉雅阶段。冲巴淡色花岗岩为同构造侵位花岗岩,是藏南拆离系活动导致的构造减压熔融的产物,12.4±0.4 Ma的锆石U-Pb年龄代表了研究区藏南拆离系的活动时代。然而,这一活动时代明显滞后于喜马拉雅中西部地区,呈现自西向东启动时代和停止活动时代逐渐变晚的趋势;白云母40Ar-39Ar年代学分析表明,冲巴淡色花岗岩冷却年龄分别为9.11±0.25 Ma和9.62±0.10 Ma。锆石U-Pb和白云母40Ar-39Ar年代学计算表明,冲巴淡色花岗岩体从12.4 Ma到9.11 Ma发生了快速冷却剥露,冷却速率高达137~162℃/Ma,这一结果与前人通过变质P-T-t研究得到的快速折返的结论相吻合。综合前人研究成果,认为12.4~9.11 Ma的快速冷却事件可能与研究区藏南拆离系的大规模伸展拆离导致的构造剥露有关。

    Abstract:

    The Chongba leucogranites, located in the east part of the Greater Himalayan leucogranite belt, are exposed along the South Tibet detachment system (STDS). Their emplacement and cooling ages are significant for evaluating the time span of the magmatism in Himalayan orogeny, constraining the timing of the STDS and revealing the history of the uplift-exhumation-cooling. In this paper, the authors collected leucogranite samples from Chongba area and carried out systematic LA-ICP-MS zircon U-Pb and muscovite 40Ar-39Ar dating. The results of zircon U-Pb dating show that the Chongba leucogranites were emplaced at 12.4±0.4 Ma. This age lies on the border between Neo-Himalaya and Post-Himalaya subdivision based on previously published data. However, taking into account the characteristics of the distribution along the STDS,the authors classify it as Neo-Himalaya. The Chongba leucogranites were generated through decompression-dehydration melting of the metapelite in Greater Himalayan Crystalline complex (GHC) triggered by the STDS activity. Thus, the emplacement age of 12.4±0.4 Ma represents the activating timing of the STDS and apparently postdates the timing in the middle and west part of the Himalayan orogeny, which shows a regularity of getting later from west to east. Muscovite 40Ar-39Ar cooling ages of the Chongba leucogranites are 9.11±0.25 Ma and 9.62±0.10 Ma, respectively. Combined with the ages and the close temperatures of zircon and muscovite, the authors hold that the rapid cooling exhumation might have occurred between 12.4 Ma to 9.11 Ma, whose cooling rate was up to 137℃/Ma to 162℃/Ma. These calculated results are in accord with previous conclusion obtained by metamorphic P-T-t of the granulite from GHC in Chongba area. It is considered that the rapid cooling event during 12.4~9.11 Ma was probably associated with the structural exhumation caused by the extension and detachment of the STDS in the study area.

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王晓先,张进江,闫淑玉,刘江,2016,藏南冲巴淡色花岗岩锆石U-Pb、白云母40Ar-39Ar年代学及其地质意义[J].岩石矿物学杂志,35(2):242~251. WANG Xiao-xian, ZHANG Jin-jiang, YAN Shu-yu, LIU Jiang,2016,Zircon U-Pb and muscovite 40Ar-39Ar chronology of the Chongba leucogranites in south Tibet and its geological implications[J]. Acta Petrologica et Mineralogica,35(2):242~251.

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  • 收稿日期:2015-02-06
  • 最后修改日期:2015-08-11
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  • 在线发布日期: 2016-03-28
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