Sr-Nd-Pb isotopes of the Early Paleozoic mafic-ultramafic dykes and basalts from South Qinling belt and their implications for mantle composition

摘要

Late Early Paleozoic mafic-ultramafic dykes and volcanic rocks from the South Qinling belt are characterized by ε_(Nd)( t) = +3.28- +5.02, (~(87)Sr/~(86)Sr)_i = 0.70341 - 0.70555, (~(206)Pb/~(204)Pb)_i = 17.256-18.993, (~(207)Pb/~(204)Pb)_i = 15.505-15.642, (~(208)Pb/~(204)Pb)_i 37.1 25 - 38.968, Δ8/4=21.18-774.43, Δ7/4 = 8.11-18.82. These charac- teristics suggest that they derived from a Middle Neoproterozoic mantle with isotopic compositions of mixed HIMU, EMU and minor EMI components. We interpret that these rocks were melting products of depleted mantle modified by subducted ancient oceanic crust and continental margin sediments along the northern margin of Yangtze block during Early Neoproterozoic.