Early Cretaceous Wulong intermediate-mafic dike swarms in the Liaodong Peninsula: Implications for rapid lithospheric delamination of the North China Craton

Abstract

The North China Craton (NCC) underwent a dramatic change in the lithospheric architecture, and more than 100 km of the ancient and refractory lithospheric mantle was removed. However, the timing and mechanism of the lithospheric thinning remain controversial. New zircon UPb ages and whole-rock/in-situ geochemical and isotopic data of the Early Cretaceous intermediate-mafic dike swarms in the Liaodong Peninsula have been carried out to decipher the timing and mechanism of lithospheric thinning of the NCC. The dike swarms are chemically subdivided into the ca. 126 Ma diorite porphyry and coeval (ca. 119 Ma) low-Ti (TiO2 < 1.20 wt%, Ti/Y < 375) and high-Ti (TiO2 > 1.90 wt%, Ti/Y > 580) diabase dikes. The diorite porphyries exhibit a moderate silica (56.71–61.13 wt%) and high-K calc-alkalic signature, with low TiO2 and total Fe2O3 and high Cr and Ni compositions. They are enriched in large ion lithophile elements (LILEs) (e.g., Rb, K, Pb, and Sr) and depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, Ti, and P). They have high radiogenic Sr isotopes (whole-rock (87Sr/86Sr)i = 0.711553–0.714284, in-situ (87Sr/86Sr)i of plagioclase = 0.71392), low εNd(t) values (−19.9 to −14.5), and old Nd model ages (1.81–2.25 Ga). Meanwhile, some plagioclase phenocrysts have reverse zonings with Na-rich cores (An: 35–48) and Ca-rich rims (An: 48–57). These features imply that the diorite porphyries were generated by mixing between magmas from the Archean–Paleoproterozoic lower crust and the lithospheric mantle. The low-Ti diabase dikes are characterized by medium to high-K calc-alkaline series with high Al2O3 but low TiO2 contents. They show arc-like trace element signatures, such as enrichment in LILEs (Rb, K, Pb, and Sr) and depletion in HFSEs (Nb, Ta, Zr, Hf, Ti, and P). They also have relatively lower initial 87Sr/86Sr ratios (0.706331–0.708315 and 0.70837 for the whole-rock and in-situ Sr isotopes, respectively) compared to the porphyries and negative εNd(t) values (−13.6 to −5.0), indicating that they were derived from partial melting of an enriched lithospheric mantle. Their variable Th/Yb and Th/Nb ratios with constant Sr/Nd and U/Th values further suggest that the mantle source might have been previously modified by the subduction-related melt. In contrast, the high-Ti diabase dikes belong to the high-K calc-alkaline to shoshonitic series, with higher TiO2 and total Fe2O3 but lower Al2O3 contents. They have OIB-like trace elements (e.g., slight enrichments of Nb and Ta with no HFSE depletion, high Nb/U values (42 ± 5)) and SrNd isotopic compositions (e.g., positive whole-rock εNd(t) values (+3.1 to +4.3)), suggesting that they were generated from an asthenospheric mantle source. The coexisting Wulong low-Ti and high-Ti mafic dikes with similar formation ages (ca. 119 Ma) may indicate a transition from lithospheric to asthenospheric mantle source. In combination with previous studies on the Mesozoic–Cenozoic mafic rocks in the Eastern Block of the NCC, we propose that the lithospheric thinning beneath the Liaodong Peninsula initiated at ca. 119 Ma and this process beneath the NCC lasted for a narrow duration (ca. 13 Ma). The unsteady mantle flows due to the rapid subduction and rollback of the Paleo-Pacific plate might trigger the thickened, modified lithosphere to lose its gravitational stability, eventually leading to lithospheric delamination and thinning of the NCC.