Bidirectional subduction of the Middle Tethys oceanic basin in the west segment of Bangonghu-Nujiang suture, Tibet: Evidence from zircon U-Pb LAICPMS dating and petrogeochemistry of arc granites
西藏班公湖-怒江缝合带西段中特提斯洋盆的双向俯冲:来自岛弧型花岗岩锆石U-Pb年龄和元素地球化学的证据

Through geological surveys in Tibet, the authors have discovered arc granite batholiths both in the north side of Shiquanhe-Gerze-Dongcuo ophiolite zone and in the south side of Laguocuo ophiolite zone in the western segment of Bangonghu-Nujiang suture zone. These granite bodies are lithologically dominated by mid-grain granodiorite, and petrochemically rich in large-ion incompatible elements(LILE, e.g. Rb, Th, U, K, Pb)and depleted in high field strength elements (HFSE, e.g. Nb, Ta, Ti), and show essential characteristics of arc magmatic rocks, indicating there were bidirectional subduction of the Bangonghu Middle Tethys oceanic basins. Zircon U-Pb LAICPMS dating results show these granite bodies have different ages. In the north side of Shiquanhe-Gerze-Dongcuo ophiolite zone, the zircon 206Pb/238U weighted average ages of Galale and Gerze-north bodies are 155.6±1.1Ma(MSWD=1.7)and 142.15±0.35Ma(MSWD=2.9), respectively. And in the south side of Laguocuo ophiolite zone, the age of Zhabuye-north body is 134.07±0.77Ma(MSWD=1.8). These ages show that the northward subduction of the Bangonghu Middle Tethys oceanic basin occurred in Late Jurassic, whereas the southward subduction occurred in Early Cretaceous, with an about 8Ma delay of the latter. The arc granitic magma was all generated by partial melting of mantle wedges, and the magmatic sources have undergone metasomatism of sedimental melts from subducted plates. Proportions of sediment melts involved in different granite bodies are variable. The Zhabuye-north body has the highest proportion of from 12%~16%, followed by the Galale body (9%~13%), and the Gerze-north body is the lowest (5%~10%).