Tonian crustal melting triggered by subduction along the Rodinia periphery: Evidence from the Liujiaping batholith, NW Yangtze Block, South China

Abstract

Preserving vital insights into deep-crustal processes and the tectonic evolution of the Tonian northwestern Yangtze Block, the Liujiaping intrusive complex remains enigmatic regarding its precise petrogenesis and tectonic context. Herein, we present new data on petrography, zircon –b geochronology, zircon Hf isotopes, whole-rock major and trace elements, whole-rock Sr–Nd isotopes and mineral chemistry of the Xiangfengkou granodiorite, the Maoping granite and the Chenjiagou granite from the Liujiaping batholith. LA–ICP–MS zircon U–Pb dating reveals their crystallization ages at ca. 802–796 Ma in the Tonian. The Xiangfengkou granodiorite is characterized by high A/CNK ratios of 0.90–0.98 and molar (Fe + Mg) values of 0.08–0.11. Zircons exhibit ε_Hf(t) values of −0.39 to +6.79, while the whole rocks have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.708720–0.709944 and ε_Nd(t) values of −1.07 to +0.55. The Maoping and Chenjiagou granites show similar geochemical compositions (A/CNK = 1.00–1.10, molar Fe + Mg = 0.03–0.05), with zircon ε_Hf(t) values ranging from +1.26 to +7.93, initial ⁸⁷Sr/⁸⁶Sr ratios of 0.706810–0.707015, and ε_Nd(t) values of 0.00 to +1.85. All samples display a pronounced negative correlation between A/CNK and Fe + Mg, indicative of the typical high-mafic I-type granitoid characteristics. Combined mineralogical and geochemical data suggest that these granitoids were mainly generated by the partial melting of a newly formed mafic lower crust. The notably high Fe, Mg, Ti and Ca contents further imply the entrainment of Fe-Mg-Ti-Ca-rich minerals during melt segregation. Strong positive correlations between Ti and Ca contents with maficity, as well as a negative correlation between A/CNK and maficity, indicate that a peritectic assemblage entrainment process involving transitional minerals (e.g., clinopyroxene, plagioclase and ilmenite) occurred during biotite-hornblende coupled melting. The geochemical, isotopic and mineralogical evidence collectively support the view that the Liujiaping granitoids formed in a subduction-related active continental margin setting. Together with previous studies, these results further demonstrate that the northwestern to western margin of the Yangtze Block was part of a long-lived subduction-related active continental margin, consistent with its tectonic position along the periphery of the Rodinia supercontinent.