Early Permian continental arc magmatism in the Zhaojinggou area of the northern North China Craton: Implications for crust-mantle interactions during southward Paleo-Asian plate subduction

Abstract

Early Permian magmatism in the Zhaojinggou area of Inner Mongolia, northern China produced intrusive gabbroic diorite and granites including biotite granite, K-feldspar granite and fine-grained granite. We present zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Hf isotopic compositions of these rocks to resolve complex crust–mantle processes beneath the northern margin of the North China Craton (NCC). Zircon U–Pb dating reveals that the rocks were emplaced in the Early Permian, with gabbroic diorite (291.1 ± 1.8 Ma) forming ~10 Myr earlier than the granites (biotite granite, 280.0 ± 1.6 Ma; K-feldspar granite, 279.7 ± 2.1 Ma; fine-grained granite, 282.2 ± 2.9 Ma). Geochemical data indicate that the rocks have calc-alkaline to high-K calc-alkaline and metaluminous to peraluminous affinities, with enrichment in light rare-earth elements (LREE) and large-ion lithophile elements (LILE), and depletion in high-field-strength elements (HFSE), typical of continental arc magmatic rocks. The gabbroic diorite is characterized by low–moderate Mg# values (38–59, mean 50), moderate average Cr (45.0 ppm) and Ni (36.5 ppm) contents, and moderately enriched Sr–Nd–Hf isotopic compositions with (87Sr/86Sr)i values of 0.704719–0.705497; εNd(t) values from −7.8 to −7.0; and εHf(t) values from −11.2 to −6.7. These features, combined with highly variable Rb/Y, Ba/La, Th/Yb, Th/Ce and Th/Sm ratios and Ba contents, indicate that the gabbroic diorite was derived from subduction-related enriched sub-continental lithospheric mantle (SCLM), previously metasomatized by slab fluids and ~5% sediment melts. Weakly negative to strongly positive Eu anomalies among gabbroic diorite samples indicate no residual plagioclase accumulation or fractionation during magma evolution. The granites have similar ages and enriched isotopic features (87Sr/86Sri = 0.703611–0.707427; εNd(t) = −11.4 to −9.2; εHf(t) = −12.8 to −7.2) with old Nd and Hf model ages of 1980–1801 Ma and 1730–2088 Ma, respectively, indicating that they originated through partial melting of ancient mafic lower crust. Both biotite granite and K-feldspar granite exhibit weakly negative to highly positive Eu anomalies and relatively low Zr saturation temperatures of 618–648 °C. In contrast, the fine-grained granite has negative Eu anomalies and relatively high Zr saturation temperatures (692–709 °C). These features imply that the magma source of the fine-grained granite (with plagioclase accumulation) was deeper than that of the biotite granite and K-feldspar granites (without plagioclase accumulation). Different mantle and crustal sources involved in the generation of continental arc magmatism in the Zhaojinggou area have implications for extensive and complex crust–mantle interactions beneath the northern margin of the NCC related to southward subduction of the Paleo-Asian oceanic crust during the Early Permian.