Zircon U-Pb age and Lu-Hf isotopes of the oceanic metabasalts from Zhuangke tectonic slice, Mianlue tectonic zone: direct evidence of Neoproterozoic Mianlue ocean

The Zhuangke tectonic slice crops out in the Mianlue tectonic zone at the southern margin of the South Qinling Orogen. It is widely accepted as a relict of the Mianlue ocean basin based on the tholeiitic and N-MORB (normal mid-ocean ridge basalt)-like geochemical features of its metabasalts. However, the formation age of Zhuangke metabasalts is not well understood due to the lack of high-precision geochronological constraints. Here we report, for the first time, zircon U-Pb ages of 999 ± 4 Ma and 991 ± 6 Ma for the Zhuangke metabasalts obtained by LA-ICP-MS method. Besides, our combined in-situ Lu-Hf isotopic analysis shows that these zircon separates have depleted εHf(t) of +11.00~+16.56 and single-stage Hf mode ages of 872~1108 Ma. These results clearly suggest that the metabasalts from Zhuangke tectonic slice formed in Early Neoproterozoic, and were sourced from a depleted mantle. In the framework of regional geology, we argue that the lifespan of Mianlue Ocean is Early-Middle Neoproterozoic (about 1000 to 800 Ma), rather than the long-held hypothesis of Late Paleozoic-Early Mesozoic. © 2021, Science Press. All right reserved.