Mesozoic slab-derived magmas from mid-eastern China: Responses to a ridge-transform fault-ridge subduction system

Distinct thermal states of mid-ocean ridges and adjacent transform faults on subducting plates tend to produce distinct magmatic responses on overlying plates. The origins and spatial distribution of the widespread late Mesozoic adakitic rocks in mid-eastern China may provide critical insights into understanding magmatic re-sponses under various thermal states of the underlying Pacific plates. Among them, the intrusive rocks from the Xu-Huai (Xuzhou-Huaibei) region are spatially important but their petrogenesis remains controversial. These intrusions were proposed to be formed by foundering of an 'eclogitic' lower continental crust represented by the mafic xenoliths hosted by these intrusions. By eliminating abundant tiny xenoliths that potentially affect chemical and isotopic compositions of the host rocks, our new data reveal a positive correlation between Sr and Nd isotopic ratios defining two end-members on the EM-I and EM-II arrays respectively, which cannot be explained by a delaminated lower crustal source alone. The radiogenic Pb isotopic ratios of the Xu-Huai in-trusions are as high as those of the oceanic crust and marine sediments. Meanwhile, these rocks are sodic with high Na2O/K2O, and have relatively low (La/Yb)N but high (Dy/Yb)N, Sr/La and Sr/Y, resembling those of oceanic adakites. Given the contemporaneous subducting oceanic ridges under mid-eastern China, the Xu-Huai intrusions were most likely derived from partial melting of a hot subducted oceanic crust that was close to the mid-ocean ridge. Consequently, a sandwich-like distribution pattern of Mesozoic adakitic rocks in mid-eastern China can be recognized, and subduction of a ridge-transform fault-ridge system is proposed to explain this observation.