A Preliminary Study On The Petrogenesis Of Neoproterozoic Mafic Diabases From The Xuzhou Area In The North China Block: Plume Products During The Break-up Of The Rodinia?

Neoproterozoic diabases occurring in the Xuzhou region of the North China Block can be compositionally subdivided into three types: the high-Ti (TiO2=3.67-3.76 wt %), transitional-Ti (TiO2=2.45-2.56 wt %) and low-Ti groups (TiO2=1.24-1.55 wt %). The high-Ti diabases have OIB-type trace element signatures (e.g., low La/Nb and Zr/Nb and highest Nd isotope data). The low-Ti series show variable degrees of Nb-Ta depletion in primitive mantle-normalized spidergrams with slightly enriched to depleted Nd isotopic compositions, whereas the transitional series exhibit similar geochemical features to EMII-OIB and have transitional characteristics between the high-Ti and low-Ti rocks. All three groups show no continuous shift in major and trace elements and cannot be regarded as different phases during magma differentiation process.On the basis of normalization to MgO=8%, the high-Ti diabases have the lowest Sig, Nag and highest Tig and Peg. Such features can be ascribed to lower degrees of partial melting and greater melting depths. In particular, the decoupling between the lowest Nag and highest Tig tends to indicate residual jadeite during melting process so that Na behaved as a moderately compatible element to lower the NaaO concentration in these rocks. This is consistent with the REE patterns in the high-Ti diabases that have the highest (Gd/Yb)cN ratios than other two groups, which suggest a more important role of residual garnet in generation of the primary magmas. The low-Ti diabases, however, have the highest Sig, lowest Peg and Tig as well as intervenient Nag, indicating the largest melting degrees and shallowest melting depths, as suggested by their lowest (Gd/Yb)cN values. The transitional group of diabases have intervenient Sig, Feg, Tig, indicative of in-between melting degrees and melting depths. The highest Nag in these rocks tend to suggest that Na behaved as a moderately incompatible element during magma generation, thus preclude the possibility for residual jadeite to host Na.All of the abovementioned facts suggest that the Neoproterozoic diabases from the Xuzhou area of the North China Block were derived from Nd isotopically-depleted mantle sources with variable enrichment in fertile components and produced through variable degrees of melting at different P-T conditions. The simplest and feasible explanation of these rocks is that they were generated under the framework of plume-asthenosphere-lithosphere interaction wherein the plume head impinged into the base of the lithosphere. Taken along with the Neoproterozoic breakup of theRodinia Supercontinent and the close relationship between the coeval magmatism and the hypothesis of a superplume, it is reasonable to contribute the emplacement of the Neoproterozoic diabases in the Xuzhou area to the geochemical fingerprints of the contemporaneous mantle plume activity preserved in the continental margin of the North China Block.