Geochronological, geochemical and Sr–Nd isotopic fingerprinting of Neoproterozoic mafic dykes in the western margin of the Yangtze Block, SW China: implications for Rodinia supercontinent breakup

Abstract

The Neoproterozoic magmatic suites in South China have been central to the debate over the assembly and disruption of the Rodinia supercontinent. In this study, we report LA-ICP-MS zircon U-Pb ages, as well as mineralogical, geochemical and Sr-Nd isotopic data on the Mid-Neoproterozoic mafic dykes in the Xichang area along the western margin of the Yangtze Block. The zircon U-Pb data yield ages in the range of 795–743 Ma. Based on available age data, the magmatism along the western margin of Yangtze Block during this period can be divided into three episodes: Episode-I (800–790 Ma), Episode-II (790–780 Ma) and Episode-III (780–730 Ma). Episode-I was widespread and dominated by High Ti-Alkaline (AL-HT) type of mafic–ultramafic rocks, which possibly formed during “active” rifting or the pre-rift stage. This type is characterized by OIB-like geochemical features, higher clinopyroxene crystallization temperature and some pyroxenite (eclogite) component in source, implying picritic source triggered by a mantle plume with involvement of minor pyroxenite (eclogite). Episode-II marks magmatic quiescence with only minor mafic rocks. The mafic dykes in Episode-III display a rang from OIB (AL-HT type) to E-MORB/N-MORB (Enriched/Normal Mid-Ocean Ridge Basalts; Transitional type and TH-LT, Low Ti-Tholeiite), suggesting possible syn-rift setting, and involved mixing between enriched (OIB-like) and depleted sources (asthenospheric mantle), followed by variable degrees of partial melting. Based on the temporal evolution and geochemical features of the mafic–ultramafic rocks, we propose that the 800–730 Ma magmatic events in the western margin of Yangtze Block involved the entrainment of a dense eclogite-derived material generated through subduction processes during the Sibao orogenesis at the base of mantle, together with instabilities caused by thermochemical mantle plume. This process also marks the transformation from intercontinental rift to the opening of oceanic basin, ultimately leading to the breakup of the Rodinia supercontinent.