西藏改则洞错地区白垩纪火山岩锆石U-Pb年代学、Hf同位素组成及对班公-怒江洋俯冲闭合的制约

班公湖—怒江缝合带是青藏高原内一条重要的缝合带,其俯冲极性和闭合时限一直存在着争议,这无疑限制了我们对青藏高原演化历史的认识。本文对仲岗安山玄武岩和一套新发现的晚白垩世安山岩进行研究,获得了其锆石U-Pb年龄分别为123.75±0.92 Ma和74.23±0.76 Ma。仲岗安山玄武岩锆石的ε_(Hf)(t)值为-7.3~+4.4,具有岛弧玄武岩特征,指示班公湖—怒江洋盆在该地区仍然继续向北俯冲;晚白垩世安山岩锆石的ε_(Hf)(t)值为+3.1~+11.1,其可能是亏损地幔混熔了部分的陆壳物质而形成的,且不整合在蛇绿岩之上。结合区域资料本文认为班公湖—怒江洋盆在改则地区的闭合时限在100~75 Ma之间。

Zircon U-Pb geochronology and Hf isotopic composition of the Cretaceous volcanic rocks and constraint of the collision age of Bangong-Nujiang suture zone in Dongco area,Gaize, Tibet

Bangong-Nujiang is an important suture zone of Tibet Plateau. However, its subduction polarity and closure time are still remaining controversial, which undoubtedly limits our understanding of the evolution of the Tibet Plateau. To constrain some information regarding this aspect, this research was carried out on Zhonggang andesitic basalt and a newly discovered andesite. The obtained zircon U-Pb ages of these two units are 123.75±0.92 Ma and 74.23±0.76 Ma respectively. The obtained zircon ε_Hf(t) values of Zhonggang basalt ranging between -7.3 and +4.4, which is identical with the island arc basalts, indicate that Bangong-Nujiang oceanic basin continues its northward subduction in research area. The ε_Hf(t) values of the Late Cretaceous andesite are +3.1～+11.1, indicative of depleted mantle melting continental crust. The Late Cretaceous andesite is unconformably overlying the ophiolite. Therefore, the Bangong-Nujiang ocean basin has already closed at that moment. Combined with regional data, this study concludes that the closure time of Bangong-Nujiang ocean basin was between 100 Ma and 75 Ma in the Gaize area.