Geochronological, Geochemical and Sr–Nd–Hf Isotopic Constraints on the Neoproterozoic Mafic Magmatism in the Zhongza Terrane, SW China: Implications for Petrogenesis and Tectonic Setting

Abstract The study of Neoproterozoic mafic magmatism in the western margin of the Yangtze Terrane is crucial for reconstructing the Rodinia Supercontinent. However, the origin and tectonic significance of these widespread Neoproterozoic magmatic suites remain debated. The key point of the debate lies in whether these magmatic rocks are related to subduction-related island-arc magmatism or mantle-plume-driven rifting. The Shigu and Judian Groups in northwestern Yunnan are distributed in the Shigu-Judian area, and belong to the Zhongza Terrane which was situated at the westernmost margin of the Yangtze Terrane during Neoproterozoic. Two sets of metamorphic basic rock interlayers were identified respectively in the Yangpo and Tacheng Formation of the Shigu and Judian Groups in the Weixi County area. Zircon U–Pb dating has constrained their formation ages at 867 and 796 Ma, confirming the Neoproterozoic deposition of both groups. Whole-rock geochemistry and Sr–Nd–Hf isotope analyses indicate that these volcanic rocks were derived from a mantle source that had been modified by subduction-related components (sediments or slab melts). The primary magma likely originated from a low degree of partial melting of a spinel–garnet lherzolite mantle source and subsequently underwent plagioclase and clinopyroxene fractional crystallization during ascent. The geochemical and isotopic signatures are consistent with a subduction‑modified mantle source, and with a low degree of partial melting (spinel-garnet lherzolite source) followed by fractional crystallization of plagioclase and clinopyroxene. Collectively, the Neoproterozoic mafic magmatism of the Shigu and Judian groups in the Zhongza Terrane is compatible with long-lived oceanic-lithosphere subduction beneath the Zhongza Terrane, with subduction persisting until at least ~796 Ma.