Two-stage crust-mantle interactions from oceanic subduction to post-collisional extension in the northern margin of the North China Craton: Insights from Paleozoic to Mesozoic magmatism

Abstract

The northern margin of the North China Craton experienced prolonged tectono-magmatic evolution during the late Paleozoic−early Mesozoic in response to the southward subduction and closure of the Paleo-Asian Ocean. However, details about the subduction process and the timing of the tectonic transition from subduction to post-collision are still poorly constrained. Here, we identify two-stage crust-mantle interactions in the Wulashan area and report new geochronology, geochemistry, and Sr-Nd-Pb-Hf isotopic data for magmatic rocks that record such processes following the subduction and closure of the Paleo-Asian Ocean. The early Carboniferous Xiguanjing pluton features a bimodal suite of gabbro (ca. 333 Ma) and syenogranite (ca. 331 Ma). The gabbros have arc-like geochemical affinities, with low Nb/La (0.31−0.40) and La/Ba (0.04−0.09) ratios, and variable Rb/Y (1.22−2.94) ratios, as well as enriched, mantle-like Sr-Nd-Pb (87Sr/86Sri = 0.7046−0.7047; εNd(t) = −3.8 to −3.5; 206Pb/204Pbi = 17.078−17.141) and enriched to depleted Hf (εHf(t) = −4.5 to +6.2) isotopic values. Such geochemical signatures indicate that they were derived from partial melting of the subcontinental lithospheric mantle that was metasomatized by slab-derived fluids, with minor involvement of asthenospheric components. In contrast, the contemporaneous syenogranites are characterized by lower negative εNd(t) (−13.5 to −12.1) and εHf(t) values (−16.3 to −8.2), which suggests that they were formed by partial melting of the lower crust. Late Triassic Shadegai and Xishadegai plutons are mainly composed of enclave-bearing syenogranite, and both mafic microgranular enclaves and syenogranites crystallized at ca. 233−231 Ma. The mafic microgranular enclaves have geochemical features similar to those of the early Carboniferous gabbros, and also have moderately enriched isotopic compositions (εNd(t) = −9.7 to −8.4; εHf(t) = −9.2 to −0.3), which suggests that they originated from interaction between mantle-derived magma and overlying crust-derived magma, with minor additions of asthenospheric melts in their sources. Field and petrological observations, coupled with the similar ages of the host granites and mafic microgranular enclaves, suggest a magmatic mingling process. Isotopic mixing models suggest that minor amounts (∼10%−20%) of lower crustal materials were mixed during the formation of the mafic microgranular enclaves. The host syenogranites display calc-alkaline to alkalic and metaluminous to weakly peraluminous compositions, and negative εNd(t) (−15.0 to −12.1) and εHf(t) values (−16.4 to −9.8), which indicates that they were mainly derived from partial melting of the lower crust and experienced the injection of deep mantle-derived magmas. Our new data, along with previously published data for magmatic rocks in the northern margin of the North China Craton, suggest that the early Carboniferous bimodal intrusive rocks formed in a localized back-arc extensional regime that was probably triggered by slab rollback of the Paleo-Asian Ocean. However, the Late Triassic plutons formed in a post-collisional extensional regime in response to slab breakoff or lithospheric delamination. Temporal variations of Nd-Hf isotopes for the magmatism in the northern margin of the North China Craton suggest that tectonic switching from advancing to retreating subduction to post-collisional extension occurred during the late Paleozoic to early Mesozoic. We propose that a tectonic transition from subduction to post-collisional extension may have occurred during the Early−Middle Triassic, marking the final closure of the Paleo-Asian Ocean, which most likely took place at ca. 250−235 Ma.