Spatial extent of the influence of the deeply subducted South China Block on the southeastern North China Block: Constraints from Sr–Nd–Pb isotopes in Mesozoic mafic igneous rocks

Abstract

Spatial variations in whole-rock Sr–Nd–Pb isotopic compositions were investigated for Early Cretaceous mafic igneous rocks from western Shandong in the southeastern edge of the North China Block (NCB). The results are used to constrain the spatial extent of the influence of the deeply subducted South China Block (SCB) on the collided NCB. Five localities of mafic rocks were analyzed in this study, which are the Feixian basalt, the Shangyu gabbro–diorite, the Tietonggou high-Mg diorite, the Jinling high-Mg diorite, and the Jinan gabbro. The new data are combined with previous analyses on the other localities of Mesozoic mafic rocks (the Fangcheng basalt, the Jiaoyu diorite, the Yinan gabbro–diorite, and the Zouping gabbro). All these mafic rocks have SiO2 contents of 48.29 to 59.75%, Mg# values of 56 to 76, and Na2O/K2O ratios of 0.45 to 4.17. They are characterized by enrichment of LREE and LILE (e.g., Rb, Ba, Th, and U), depletion of HFSE (e.g., Nb, Ta, Zr, Hf, and Ti), and prominent positive Pb anomalies. Sr–Nd–Pb isotopic data indicate that the Jinan and Zouping gabbro and the Jinling high-Mg diorite, which are located far away from the Tan–Lu fault zone, have initial 87Sr/86Sr ratios of 0.7047 to 0.7054, εNd(t) values of −3.92 to −10.47, 206Pb/204Pb ratios of 16.671 to 17.111, 207Pb/204Pb of 15.320 to 15.378, and 208Pb/204Pb of 36.562 to 37.095. In contrast, the Shangyu gabbro–diorite, which are located close to the Tan–Lu fault zone, along with the contemporaneous Feixian and Fangcheng basalts, have initial 87Sr/86Sr ratios of 0.7096 to 0.7117, εNd(t) values of −13.04 to −16.60, 206Pb/204Pb ratios of 17.181 to 17.649, 207Pb/204Pb of 15.477 to 15.591, and 208Pb/204Pb of 37.536 to 38.472. The Tietonggou and Jiaoyu high-Mg diorites, which are located at intermediate distances from the fault zone, have initial 87Sr/86Sr ratios of 0.7067 to 0.7075, εNd(t) values of −7.82 to −13.30, 206Pb/204Pb of 17.466 to 17.667, 207Pb/204Pb of 15.442 to 15.472, and 208Pb/204Pb of 37.615 to 37.812. The Early Cretaceous mafic rocks in western Shandong show spatial variations in Sr–Nd–Pb isotopic compositions: from southeast to northwest, there is a decrease in initial 87Sr/86Sr, 206Pb/204Pb,207Pb/204Pb, and 208Pb/204Pb ratios, and an increase in εNd(t) values. Together with the Pb isotopic compositions of the contemporaneous Xuzhou–Huaibei adakitic rocks, the geochemical property of magma sources for the Mesozoic mafic rocks is inferred in view of the Pb isotopic difference in the NCB–SCB distinction. It is suggested that the magma sources for the Feixian, Fangcheng, Shangyu, Tietonggou, and Jiaoyu mafic rocks were mainly modified by the SCB basement. In contrast, the magma sources for the Jinan, Zouping, and Jinling intrusions, which are located far from the Tan–Lu fault zone, were mainly modified by melts derived from the delaminated NCB lower crust. These inferences are consistent with a tectonic model that outlines northwestward subduction of the SCB beneath the NCB during the Early Mesozoic. Along with data on the Sr–Nd–Pb isotopic compositions of coeval mafic–ultramafic rocks from southern Liaoning and southern Jilin provinces, it is concluded that the lateral extent of the influence of the deeply subducted SCB on the adjacent NCB lithospheric mantle is about 200km in its southeastern margin.