Early Neoproterozoic Mafic Dykes in the South Qinling Belt, China: Geochemistry, Zircon Ages and Tectonic Implications

Abstract

The mafic magmatism during the Middle-late Early Neoproterozoic period (ca. 830–720 Ma) is frequently construed as indicative of plume magmatism, facilitating the breakup of the supercontinent Rodinia and establishing a climatic backdrop conducive to the onset of the Sturtian glaciation. Nevertheless, the tectonic models proposed to explain the formation of the latest Tonian magmatism in the Yangtze Block remain a subject of debate. In this study, four mafic dyke samples yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 733 ± 14 Ma, 722 ± 8 Ma and 734 ± 6 Ma, 733 ± 5 Ma, respectively, indicating their crystallisation age in the Neoproterozoic era. These mafic dykes are characterised by a subalkaline tholeiitic composition, with relatively low concentrations of SiO₂ (43.04–46.75 wt%) and MgO (5.50–7.86 wt%), and Mg^# values ranging from 48 to 64. Furthermore, the mafic dykes are also marked by relatively high FeO^T (9.26–12.28 wt%), TiO₂ (1.20–1.62 wt%). They exhibit a slight enrichment in LREE with (La/Yb)_N ratios of 2.99–3.35 and demonstrate trace element patterns that are typical of continental tholeiite. Moreover, the mafic dykes display εNd (t) values ranging from −3.7 to +4.4, εHf(t) values ranging from −12.33 to 4.41, along with low ratios of (²⁰⁶Pb/²⁰⁴Pb)_i (16.70–17.46), (²⁰⁷Pb/²⁰⁴Pb)_i (15.37–15.70) and (²⁰⁸Pb/²⁰⁴Pb)_i (36.67–38.00). The combination of these characteristics and clinopyroxene compositions provides evidence that the source region of the mafic dykes was created through the partial melting of the depleted asthenosphere mantle mixed with subduction fluid metasomatic lithospheric mantle during the transition of the spinel-garnet facies peridotite within an extensional setting. The congruities in formation chronology, petrogenesis, source attributes and tectonic settings among the mafic rock samples within the Douling Complex, the Yaolinghe Group and the Wudang Block suggest that the South Qinling Belt was in an extension setting during the Middle-late Early Neoproterozoic, and presumably witnessed the rifting of the Rodinia supercontinent.