Petrogenesis and tectonic implications of the late Neoproterozoic mafic dykes in the South Qinling Belt, China

Abstract

The mafic dykes exposed in the Suizhou area are crucial for understanding the late Neoproterozoic tectonic evolution of the South Qinling Belt. However, it remains unclear whether these rocks were formed in a continental rift or a back-arc tectonic regime. In this contribution, we present a series of new results of zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Pb isotopes on mafic dykes in the Suizhou area. Three mafic dyke samples give zircon U–Pb ages of 635 ± 2 Ma, 631 ± 3 Ma and 630 ± 2 Ma, respectively, indicating that they were emplaced during the late Neoproterozoic. The mafic dykes are tholeiitic in composition and have low concentrations of SiO2 (44.26–49.08 wt%), varying TiO2 (0.50–1.71 wt%) and MgO (6.43–14.24 wt%) with Mg# values of 53 to 74. They are enriched in LREEs with (La/Yb)N ratios of 2.10–5.20 and display E-MORB-like trace element patterns. Isotopically, the mafic dykes show negative εNd(t) values of −5.09 to −7.15 and low ratios of 206Pb/204Pb (16.77 to 16.92), 207Pb/204Pb (15.39 to 15.41), and 208Pb/204Pb (37.23 to 37.51). These characteristics illustrate that the mafic dykes were mainly derived from the enriched lithospheric mantle, since the negligible crustal contamination has been assessed by the energy-constrained assimilation and fractional crystallization simulation of Sr-Nd isotopes. In addition, the mafic dykes have ΔNb values of −0.46 to + 0.15 with zircon εHf(t) values of −1.5 to + 10.0 previously reported, indicating that the depleted mantle source had participated in their formation. Furthermore, the non-modal batch melting calculation infer that, a mixing of melts from the asthenospheric mantle (18% extent of partial melting) at garnet-stable depth with peridotites from the lithospheric mantle could fit well with geochemical composition of the mafic dykes. The correlation of incompatible elements reveals that these rocks are akin to the intra-continental mafic rocks. Combined with the coeval magmatism in the study area, we conclude that the Suizhou mafic dykes were formed in a rifting continental margin during the late Neoproterozoic.