班公湖-怒江缝合带中段东巧地区早白垩世岩浆作用——对大洋演化和地壳增厚的指示
Early Cretaceous magmatism in Dongqiao, Tibet: Implications for the evolution of the Bangong-Nujiang Ocean and crustal growth in a continent-continent collision zone
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摘要: 对班公湖-怒江缝合带内的岩浆作用进行LA-ICP-MS锆石U-Pb测年和地球化学分析,在辉绿岩中获得138.7±1.0Ma的206Pb/238U年龄加权平均值,在流纹岩中获得了110.4±0.4Ma的谐和年龄,表明区内岩浆作用具有2期成因。地球化学研究认为,辉绿岩是地幔熔融的产物,花岗闪长岩为岩石圈地幔熔融的产物,而流纹岩显示2类不同的岩石地球化学特征,低Sr流纹岩为古老岩石圈地幔熔体经历分离结晶作用的产物,高Sr流纹岩具有埃达克岩的特征,为增厚下地壳熔融的产物。综合已有的研究,早白垩世岩浆作用在缝合带两侧均有展布,其中早期岩浆岩为班公湖-怒江洋双向俯冲的产物,末期岩浆岩是碰撞后俯冲洋壳前缘断离形成的。早白垩世班公湖-怒江洋经历了双向俯冲到大洋闭合的演化过程,并在早白垩世末期发生了俯冲洋壳的断离事件。同时,高Sr流纹岩的发现表明,早白垩世末期班公湖-怒江缝合带已经发生了明显的地壳增厚作用。Abstract: The Dongqiao area is located across Bangong Co-Nujiang River suture zone (BNSZ) and the southern Qiangtang terrane. The study area has widely exposed diverse rock types such as diabases, rhyolites and granodiorites. In this paper, the authors report the LA-ICP-MS zircon U-Pb age and whole-rock major and trace element composition data of the diverse Early Cretaceous magmatic rocks from Dongqiao. The diabase sample yielded a zircon U-Pb age of 138.7±1.0Ma, and the zircons from rhyolite yielded an age of 110.4±0.4Ma, indicating that the magmatic rocks in Dongqiao formed in two periods of magmatism. According to geochemical characteristics of the rocks, the diabases were produced by partial melting of the mantle, and the granodiorites by partial melting of ancient lithospheric mantle that had been modified by subduction-related components. In addition, the geochemical data indicate that rhyolites can be divided into two types of high Sr and low Sr rhyolites. Low Sr rhyolites were formed by partial melting of ancient lithospheric mantle, and the melt subsequently underwent intense fractional crystallization. High Sr rhyolites had an affinity with adakites, which were derived from partial melting of thickened lower crust. The new data obtained by the authors, together with recently published data, led the authors to develop a model of bidirectional subduction and subsequent slab break-off of the lithosphere of the Bangong-Nujiang Ocean which can explain the two magmatic events in the region from BNSZ to the southern Qiangtang terrane. Research on high Sr rhyolites indicates that the extensive magmatism and continent-continent collision contributed significantly to the crustal growth after the closure of Bangong-Nujiang Ocean in Early Cretaceous.
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