Mesozoic lithospheric mantle evolution in South China: Evidence from Ar-Ar dating, geochemistry and Sr-Nd-Pb isotopes of lamprophyres

Abstract

Understanding the Mesozoic lithospheric mantle beneath South China is challenging, primarily because mantle-derived magmatic rocks are rare, whereas granitic magmatism is far more abundant. In this study, we present new ⁴⁰Ar/³⁹Ar geochronology, major and trace elemental composition, and Sr-Nd-Pb isotopic data for lamprophyres from northern and central Guangxi Province, South China. Our results reveal two intrusion phases: an earlier phase at 207.7 ± 1.2 Ma, and a later phase between 95.6 ± 0.6 Ma and 102.3 ± 0.7 Ma. The earlier phase, represented by the Danyang lamprophyres, is marked by low Cr (330–355 ppm) and Ni (241–258 ppm) contents and strongly fractionated REE patterns with (La/Yb)n ratios of 57.5 to 62.8. The later phase, represented by the Leidong, Nongchang, and Longchang lamprophyres, shows higher Cr (490–781 ppm) and Ni (237–377 ppm) contents, with less fractionated REE patterns and (La/Yb)n ratios of 20.2 to 27.8. Despite their temporal differences, the lamprophyres share similar isotopic characteristics, including high initial ⁸⁷Sr/⁸⁶Sr (0.707940–0.721310), negative ε_Nd(t) (−9.19 to −5.31), and radiogenic Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb: 18.200–18.331, ²⁰⁷Pb/²⁰⁴Pb: 15.62–15.72, ²⁰⁸Pb/²⁰⁴Pb: 38.53–38.84). These features suggest derivation from low-degree partial melting of a phlogopite-bearing, refractory peridotite lithospheric mantle, modified by olivine and clinopyroxene fractionation. This refractory mantle was metasomatically enriched by Proterozoic subduction processes beneath the Yangtze Craton. By synthesizing published Sr-Nd-Pb isotopic data, we propose that the lithospheric mantle beneath South China shifted from enriched to depleted mantle between ∼220 Ma and ∼ 100 Ma, possibly due to extension associated with the subduction and rollback of the Paleo-Pacific Plate.