Abstract
The interaction of Paleo-Pacific slab-derived recycled materials with mantle peridotites is an important mechanism for causing the intensive lithospheric thinning in Eastern China during the Early Cretaceous. However, the definitive geochemical observation recording the Paleo-Pacific plate subduction at the peak time (~125 Ma) of lithospheric thinning has not been clearly identified so far. Here, we report an Early Cretaceous (~123 Ma) mafic dyke in Eastern China, the Daixi diabase, which is characterized by OIB-type elemental affinities and depleted Nd–Hf isotopes [εNd(t) = 5.4–6.9; εHf(t) = 9.6–12.1]. Its exceptionally high Nb/Nb*, Ti/Ti*, Eu/Eu* and Sr/Sr* ratios are well correlated with Hf/Sm, Th/LREE, Ce/Pb, and εHf(t) values, indicating that recycled basaltic, gabbroic oceanic crusts and terrigenous sediments were involved as source components. These recycled oceanic crustal materials (as eclogites) have contributed to the formation of lithologically distinct pyroxenite within the mantle peridotite matrix by melt-rock interaction. This is revealed by the elevated δ56Fe value (0.21‰ ~ 0.35‰) of the diabase and its good correlations with Fe/Mn, FC3MS (FeO/CaO-3*MgO/SiO2, all in wt%), Nb/Nb*, Sr/Sr*, εHf(t), and (206Pb/204Pb)i ratios. High U/Pb (μ = 25.0–33.4) and Th/Pb (ω = 85.6–118) ratios but unexpectedly unradiogenic Pb isotopic compositions [(206Pb/204Pb)i = 17.863–18.084, (207Pb/204Pb)i = 15.499–15.517, (208Pb/204Pb)i = 37.605–37.904] suggest that the subducted young oceanic crust is possibly recycled into the source ca. 180 Ma ago and most likely derived from the Paleo-Pacific oceanic plate. This is consistent with the conclusion drawn from the coupled NdHf isotopic systematics. Our study thus provides compelling geochemical evidence for the Paleo-Pacific subduction at the peak time (125 Ma) of the lithospheric thinning in Eastern China.
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