Discovery of Neoproterozoic adakitic rocks in the Eastern Tianshan (NW China) of the southern Altaids

The Paleo-Asian Ocean, as the processor of the Altaids, was a long-lived ocean, and its subduction gave rise to substantial continental growth in the Central Asian continent. However, the location of the major oceanic basin and its evolution history are debatable, impeding our better comprehension of the early accretionary tectonics of the Paleo-Asian Ocean and its link to the reconstruction of the Rodinia. Here, we report our newly discovered Neoproterozoic adakitic rock (Xingxingxia gneissic granite) in the Eastern Tianshan of the southern Altaids to address the above issues. The Xingxingxia gneissic granite displays high Sr/Y ratios (47-114) and strongly fractionated rare earth element (REE) patterns with low concentrations of Y (3.00-6.36 ppm) and Yb (0.23-0.62 ppm), indicative of the adakite affinity. The adakitic rock is depleted in Nb, Ta, P and Ti, and possesses low Mg# (37-42), comparatively high K2O/Na2O ratios (0.75-1.08). Zircon U-Pb dating reveal that the adakitic rock was formed at 869 +/- 9 Ma, which has relatively depleted Hf isotopic compositions (epsilon(Hf)(t) = + 1.9 to + 4.7). Their geochemical features are consistent with those of an origin from a thickened lower arc crust. Our work suggests that subduction of the Paleo-Asian oceanic slab beneath the Central Tianshan Arc occurred at least in the Neoproterozoic. Therefore, our study provides a solid and key evidence to show that the tectonic position of the major Paleo-Asian Ocean was located in the South Tianshan, where the paleo-ocean evolved into its mature stage following the subduction beneath the Central Tianshan at ca. 870 Ma. This work also shed light on the reconstruction of Rodinia and the interaction of the Paleo-Asian Ocean and the Jiangnan-North Tarim Ocean.