勉略构造带庄科岩片洋壳型变玄武岩锆石U-Pb年龄和Lu-Hf同位素特征:新元古代勉略洋盆存在的直接证据

庄科岩片位于南秦岭南缘勉略构造带。前人系统的岩石地球化学研究表明,庄科岩片变玄武岩属拉斑玄武岩系列,整体表现出N-MORB特征,为勉略洋盆残余,但一直未获得高精度的测年数据。本文首次获得庄科岩片洋壳型变玄武岩LA-ICP-MS锆石U-Pb年龄为999±4Ma(MSWD=0.08,n=25)、991±6Ma (MSWD=1.50,n=21),具有正的ε_Hf(t)值(+11.00-+16.56)和较小的单阶段Hf模式年龄(t_DM1为872-1108Ma),符合典型基性岩浆锆石特征,表明庄科岩片变玄武岩的结晶时代为新元古代早期。结合最新区域研究成果认为,勉略洋盆的存在时限为新元古代早中期(约1000-800Ma),并非长期被认为的晚古生代—早中生代。

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.