Early–Middle Permian southward subduction of the eastern Paleo-Asian Ocean: Constraints from geochronology and geochemistry of intermediate-acidic volcanic rocks in the northern margin of the North China Craton

Abstract

The northern margin of the North China Craton (NCC) resulted from complicated subduction-accretion process of the eastern Paleo-Asian Ocean (PAO), thus late Paleozoic magmatic records in this region could place conclusive constraints on the final-stage subduction processes of the eastern PAO. Here we present petrological, geochronological, geochemical, and in situ zircon Hf isotopic data of the late Paleozoic intermediate-acidic volcanic rocks, and compiled age and geochemical data of the Permian magmatic rocks along the northern margin of the NCC, with the aim of elucidating their ages, petrogenesis and tectonic implications. Zircon UPb ages indicate that these rocks formed in the early–middle Permian (275–267 Ma). Lithologically, these volcanic rocks are composed mainly of andesites, with minor rhyolites and dacites. The studied andesites contain 53.94–64.87 wt% SiO2, 1.86–3.05 wt% MgO and 0.47–3.56 wt% K2O, and belong to low-K calc-alkaline to shoshonite series. Coupled with variable zircon εHf(t) values of −14.57 to +12.57, we propose that the parental magma was derived from partial melting of heterogeneous lithospheric mantle resulted from metasomatism of subduction-related fluids. In contrast, the rhyolite-dacite suites are distinguished by higher SiO2 (65.94–74.77 wt%) and lower MgO (0.15–1.60 wt%) contents, indicating an affinity with highly fractionated I-type granite originated by partial melting of lower crust. These intermediate-acidic rocks are characterized by enrichment in large-ion lithophile elements (LILEs; e.g., Rb, Th, and K) and depletion in high-field-strength elements (HFSEs; e.g., Nb, Ta, P, and Ti), suggesting that they formed in an active continental margin setting. These geochemical features and lithological assemblage, together with the East-West-trending Permian magmatic belt with arc affinities, indicate that their generation was related to southward subduction of the Paleo-Asian oceanic lithosphere beneath the NCC. Synthesized data from this and previous studies lead us to favor a double-sided, early–middle Permian subduction model, with the PAO not closed prior to the middle Permian.