Magmatic evidence for middle-late Permian tectonic evolution on the northern margin of the North China Craton

Late Paleozoic tectonic evolution of the northern margin of the North China Craton (NCC) remains controversial, especially for the mechanism and time during the late-stage oceanic consumption of the Paleo-Asian Ocean (PAO). We address this issue through geochronological, geochemical and Lu-Hf isotopic studies on felsic-mafic volcanic rock suites and granite plutons from the Damao area in the western segment of the NCC's northern margin. Zircon U-Pb dating reveals that the volcanic suites, with eruptive unconformities in-between, erupted primarily at ~275–260 Ma while the granites emplaced later at ~255 Ma. The volcanic rocks contain rhyolites and dacites having an A-type affinity with high FeOt/MgO, (K2O + Na2O)/CaO and Ga/Al ratios, enriched Th, U, Zr, Hf and depleted Ba, Sr and Eu contents. The basaltic rocks display low MgO and TiO2 contents, negative Nb, Ta, Ti anomalies, as well as negative ƐHf(t) values. We suggest that the ~275–260 Ma volcanic suites were generated in a post-collisional setting, with the dominant rhyolites and dacites derived from the mixing between mantle and crustal magmas and subordinate basalts from the partial melting of enriched sub-continental lithospheric mantle. The sudden and intensive occurrence of these ~275–260 Ma felsic-dominant bimodal post-collisional magmatism and their sudden increase of zircon ƐHf(t) values imply a significant mantle contribution to their sources, possibly due to the slab breakoff of the subducting PAO plate. The ~255 Ma granites also have an A-type affinity and a large spread of negative ƐHf(t) values (−3.6 to −30.5), indicating a prominent involvement of continental crust materials that was likely related to crustal reworking after the slab breakoff. Overall, these ~275–255 Ma magmatic suites document a significant post-collisional crustal extension at the western segment of the northern NCC. In this region the southward subduction of the PAO likely ceased prior to ~275 Ma on the basis of a regional comparison.