Prolonged mantle metasomatism in the Neoproterozoic continental arc: Insights from mafic magmatism in the western Yangtze Block, South China

Abstract

Mafic rocks generated from subduction settings have recorded valuable source information about the mantle source. In this study, we present a comprehensive analysis of zircon U–Pb dating, whole-rock major and trace elements, and Sr–Nd isotopic data for the mafic gabbro located in the Yumen area, on the western part of the Yangtze Block, South China, aiming to constrain the processes of mantle metasomatism within subduction settings. U–Pb dating results for zircon yield crystallization ages of 800 Ma for type 1 mafic gabbro and 753–734 Ma for type 2 mafic gabbro. Type 1 mafic gabbro exhibits higher SiO₂ (44.13%–48.93%) and Al₂O₃ content but lower total Fe₂O₃ and MgO content than type 2 gabbro (SiO₂: 41.02%–43.28%). These gabbros display a high-Mg^# signature (52.50–62.81 for type 1, 50.89–57.04 for type 2), while they are enriched in significant large-ion lithophile elements (LILEs: Rb, Ba, Sr, K) and depleted in high-field-strength elements (HFSEs: Zr, Hf, Nd, Ta, Ti), which indicates an arc-like element signature. The positive whole-rock εNd(t) values (type 1: 3.5–4.4, type 2: 5.6–6.3) combined with a narrow range of (⁸⁷Sr/⁸⁶Sr)_i (type 1: 0.7035–0.7043, type 2: 0.7035–0.7036) of both gabbro types suggest a depleted lithospheric mantle origin. Therefore, these mafic rocks may derive from a metasomatized spinel lherzolite mantle source (with amphibole) due to the interactions of the deep mantle source and subduction fluid materials. We propose that the long-term metasomatism recorded by mafic gabbro in this study supports the fact that the subduction during the Neoproterozoic contributed to the formation of a metasomatized mantle source in the Yumen area, western Yangtze Block, South China.