Vein-plus-wall rock melting model for the origin of Early Paleozoic alkali diabases in the South Qinling Belt, Central China

Abstract

Early Paleozoic mafic dykes are widespread in the South Qinling Belt, central China. In this study, we present new major and trace element, zircon U–Pb age and Sr–Nd–Hf isotopic results of Early Paleozoic diabases dykes in the South Qinling Belt to explore the nature of their mantle source. The zircon U–Pb dating yielded ages of 455.9 ± 1.5 Ma and 446.2 ± 1.1 Ma. The South Qinling Belt diabases had low SiO2 (42.1–49.5 wt%), high TiO2 (2.89–5.17 wt%) and variable MgO (4.0–9.4 wt%) contents. In primitive mantle normalized multi-element diagrams, all samples were strongly enriched in the majority of incompatible trace elements but showed systematic depletion in Rb, K, Pb, Zr and Hf. The negative K and Rb anomalies, together with high TiO2 and high Na2O/K2O, suggests magma was derived from a source rich in amphibole. Partial melting modeling indicated that 20%–36% partial melting of amphibole-clinopyroxene-phlogopite veins with subsequent dissolution of ~30% orthopyroxene from the wall-rock peridotite within the spinel stability field can produce the observed diabase compositions. Additionally, the South Qinling Belt diabases were characterized by moderately depleted Nd (εNd(t) = +2.2 to +3.3) and Hf (εHf(t) = +6.2 to +7.2) isotopic compositions without pronounced isotope decoupling, indicating mantle metasomatism occurred prior to Early Paleozoic magmatism. We propose that low-degree silicate melts released from the asthenosphere infiltrated and solidified within the lithospheric mantle, forming non-peridotitic lithologies rich in amphibole, clinopyroxene and phlogopite. Subsequent lithospheric extension caused the melting of the most easily fusible material in the lithosphere, which gave rise to the Early Paleozoic alkaline magmatism in South Qinling.