Late Cretaceous Transtension in the Eastern Tibetan Plateau: Evidence From Postcollisional A‐Type Granite and Syenite in the Changdu Area, China
Late Cretaceous Transtension in the Eastern Tibetan Plateau: Evidence From Postcollisional A‐Type Granite and Syenite in the Changdu Area, China
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DOI：
10.1029/2018jb017072
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发表时间：
2019-07
期刊：
影响因子：
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通讯作者：
Xin-Song Wang;A. Williams-Jones;X. Bi;R. Hu;Jia‐Fei Xiao;Mingliang Huang
中科院分区：
文献类型：
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作者：
Xin-Song Wang;A. Williams-Jones;X. Bi;R. Hu;Jia‐Fei Xiao;Mingliang Huang
晚白垩世是青藏高原重要的地质时期，恰逢构造形态由拉萨—羌塘碰撞向印亚拼合转变的时期。然而，构造体制变化的性质和控制却很少受到约束。在本文中，我们报告了对青藏高原东部昌都地区两次入侵的研究结果。锆石 U-Pb 测年表明，这两个侵入体形成于大约 1997 年。 77.6–74.3 Ma。邦达岩体具有A型花岗岩亲和力和过铝质特征，而鲁多岩体是准铝质正长岩。两种岩体具有非常相似的微量元素组成、稍富集的锆石 εHf(t) 值（-9.3 和 -1.7）以及类似 EM-2 的 Sr-Nd-Pb 同位素比。两个岩体的这些特征表明，它们的岩浆来源于富碱玄武岩下地壳的部分熔融和一小部分地幔熔融。碱性侵入的发生与青藏高原东部晚白垩世的伸展作用一致。基于本研究结果和前人资料，我们提出了板内伸展构造模型，其中拉萨-羌塘碰撞后，青藏高原东部存在NS-NNW向的晚白垩世转张。这种延伸被解释为是由班公湖-怒江板片在110 Ma左右的断裂引发的，并由新特提斯洋壳的远场俯冲驱动。
The Late Cretaceous is an important geological time interval for the Tibetan Plateau because it corresponds to the period when the tectonic regime changed from Lhasa‐Qiangtang collision to Indo‐Asian assembly. However, the nature of and controls on the change in tectonic regime are poorly constrained. In this paper, we report results of a study of two intrusions in the Changdu area of the Eastern Tibetan Plateau. Zircon U‐Pb dating shows that both intrusions formed at ca. 77.6–74.3 Ma. The Bangda intrusion has A‐type granite affinity and a peraluminous character, whereas the Ruduo intrusion is a metaluminous syenite. Both intrusions have very similar trace element compositions, slightly enriched zircon εHf(t) values (−9.3 and −1.7), and EM‐2‐like Sr‐Nd‐Pb isotope ratios. These features of the two intrusions indicate that their magmas were derived from partial melting of an alkali‐rich basaltic lower crust and a small proportion of mantle melt. The occurrence of alkaline intrusions is consistent with Late Cretaceous extension in the Eastern Tibetan Plateau. Based on the results of this study and previous data, we propose an intraplate extensional tectonic model, in which there was NS‐NNW‐directed Late Cretaceous transtension in the Eastern Tibetan Plateau following the Lhasa‐Qiangtang collision. This extension is interpreted to have been triggered by the Bangong‐Nujiang slab break‐off at around 110 Ma and driven by the far‐field subduction of the Neo‐Tethys oceanic crust.