徐淮地区早白垩世丰山和蔡山高镁埃达克质岩:拆沉下地壳熔融的熔体与地幔橄榄岩的反应

Early Cretaceous Fengshan and Caishan high-Mg~# adakitic rocks from the Xuzhou-Huaibei area,central China: Interaction between mantle peridotite and melt derived from partial melting of delaminated lower continental crust

徐淮地区丰山花岗闪长斑岩和蔡山石英闪长玢岩的岩石地球化学、Sr-Nd-Hf同位素和石榴石的矿物化学研究对探讨华北克拉通东南缘早白垩世高镁埃达克质岩的岩石成因和构造演化具有重要意义。结果表明,丰山花岗闪长斑岩和蔡山石英闪长玢岩具高SiO_2(60.98%～67.88%)、富Al_2O_3(14.37%～15.04%)以及高的Na_2O/K_2O比值(1.58～2.24)和Mg~#值(57～66)的特征;富集LILE(Rb、Ba、Sr)和LREE,亏损HFSE(Nb、Ta、Ti)和HREE,具有Pb正异常和弱的Eu异常;结合高的Sr含量(579×10^(-6)～778×10^(-6))以及Sr/Y(33～69)和(La/Yb)_N比值(8.63～13.7),低的Y(10.5×10^(-6)～17.8×10^(-6))和Yb含量(0.74×10^(-6)～1.17×10^(-6)),暗示它们属于高镁埃达克质岩。丰山和蔡山埃达克质岩石的初始^(87)Sr/^(86)Sr比值介于0.7079～0.7086之间,ε_(Nd)(t)值变化于-10.77～-7.18之间,t_(DM2)=1504～1793Ma;岩浆锆石的ε_(Hf)(t)值为-14.2～-5.3,t_(DM2)=2101～2898Ma。徐淮地区早白垩世利国、班井、夹沟高镁埃达克质岩石中岩浆锆石的ε_(Hf)(t)值分别介于-13.4～-7.0、-13.4～-7.9和-15.9～-4.5之间,它们的t_(DM2)分别变化于2248～2825Ma、2331～2824Ma和2030～3048Ma之间。徐淮地区丰山和蔡山高镁埃达克质岩的Sr-Nd-Hf同位素组成和丰山花岗闪长斑岩中铁铝榴石残留晶的存在,结合它们高的Pb同位素组成和继承锆石U-Pb年代学暗示,丰山和蔡山高镁埃达克质岩浆主要起源于俯冲断离的扬子克拉通石榴辉石岩相下地壳物质熔融的熔体与地幔橄榄岩的反应,同时有拆沉的华北克拉通基底物质的参与,形成于华北克拉通东部岩石圈减薄的伸展构造背景。

Whole-rock geochemistry, Sr-Nd-Hf isotopes and mineral chemistry of the garnet for the Fengshan granodioritic porphyrys and Caishan quartz dioritic porphyrites in Xuzhou-Huaibei area are carried out in present study, which are of significance for understanding the petrogenesis of the Early Cretaceous high-Mg# adakitic rocks and tectonic evolution in the southeastern margin of North China Craton. Geochemically, these rocks have high SiO2 （60.98%-67.88%） and Al2O3 contents （14.37%-15.04%）, high Na2O/K2O ratios （1.58-2.24） and high Mg# values （57-66）. They are enriched in large iron lithophile elements （e.g., Rb, Ba, and Sr） and light rare earth elements, depleted in high field strength elements （e.g., Nd, Ta, and Ti） and heavy rare earth elements, and display prominent positive Pb anomalies and weakly Eu anomalies. The above geochemical characteristics, combined with their high Sr contents （579×10-6-778×10-6）, high Sr/Y （33 to 69） and （La/Yb）N ratios （8.63 to 13.7）, low Y （10.5×10-6-17.8×10-6） and Yb contents （0.74×10-6-1.17×10-6）, suggest these rocks are high-Mg# adakitic rocks. Additionally, these adakitic rocks have the （87Sr/86Sr）i values of 0.7079-0.7086, have the εNd（t） values and tDM2 ages of -10.77--7.18 and 1504-1793Ma, have εHf（t） values and tDM2 ages of -14.2--5.3 and 2101-2898Ma, respectively. The εHf（t） values of the Liguo, Banjing and Jiagou high-Mg# adakitic rocks in Xuzhou-Huaibei area range from -13.4 to -7.0, -13.4 to -7.9 and -15.9 to -4.5, and the corresponding tDM2 values range from 2248 to 2825Ma, 2331 to 2824Ma and 2030 to 3048Ma, respectively. The above isotopic features and the existence of Almandine residual crystals in Fengshan granodiorite porphyrys, combined with their high Pb isotopes compositions and U-Pb geochronology of inherited zircons, imply that the Fengshan and Caishan high-Mg# adakitic rocks were derived from partial melting of a delaminated garnet pyroxenite phase lower continental crust and subsequent interaction with the mantle peridotites, geodynamically related to extensional environment in response to lithospheric thinning.