Early Cretaceous magmatism in Dongqiao, Tibet: Implications for the evolution of the Bangong-Nujiang Ocean and crustal growth in a continent-continent collision zone

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.