Late Jurassic adakitic granodiorite in the Dong Co area, northern Tibet: Implications for subduction of the Bangong-Nujiang oceanic lithosphere and related accretion of the Southern Qiangtang terrane

In this paper we present new major and trace element chemical data, zircon U-Pb age data, and zircon Hf isotopic compositions for the Dong Co granodiorites (DGs) that were emplaced directly in the Dong Co ophiolites (DOs) in the middle segment of the Bangong-Nujiang suture zone (BNSZ), northern Tibet. We use these new data to discuss the genesis of the DGs and implications for the evolution of the region. The DGs have high contents of SiO2, Al2O3, Na2O, and Sr, high values of Mg#, low contents of Yb and Y, and markedly high Sr/Y ratios, indicating an adakitic affinity, and we show that they were derived from the partial melting of subducted oceanic crust. The DGs contain large numbers of inherited zircons that have similar shapes and peak ages to the detrital zircons in the accretionary wedge graywackes that surround the DGs, suggesting that many of these sediments were assimilated during the formation of the DGs. Five DG samples yield the youngest zircon U-Pb ages of 155-160 Ma, indicating that the DGs formed during the Late Jurassic. Based on the present results and regional geological data, we infer that the Bangong-Nujiang oceanic lithosphere was subducted northwards beneath the Southern Qiangtang terrane during the formation of a continental arc system during the Late Jurassic. The Bangong-Nujiang oceanic lithosphere retreated rapidly after its initial subduction, and the Southern Qiangtang terrane to the north of the Bangong-Nujiang Ocean acquired a massive amount of new oceanic material in the form of an accretionary wedge. This wedge, which included the DOs and the Mugagangri Group, became sufficiently large (both in size and width) for arc-type magmas (DGs) to develop within it during the Late Jurassic.