Post-collisional ultramafic complex in the northern North China Craton: implications for crust-mantle interaction

The post-collisional ultramafic intrusions within the northern margin of the North China Craton (NCC) preserve important imprints of crust-mantle interaction. Here we investigate ultramafic intrusions from the Luojianggou complex composed of pyroxenite and hornblendite associated with serpentine, with a view to gain insights into the nature of orogenic lithospheric mantle in this major continental collision zone. Zircon UPb data from the ultramafic suite define different age populations, with Paleoproterozoic ages (>2.2 Ga and 1.82 Ga) representing xenocrystic grains captured from the basement. The magmatic zircon grains range in age from 872 Ma to 458 Ma, and are possibly related to multiple magma emplacement associated with the Paleo-Asian Ocean closure. The youngest age of ca. 230 Ma is related to a period of post-collisional extension in the northern margin of the NCC, and this inference is further supported by apatite UPb ages of ~207 Ma. Phase equilibrium modeling suggests temperatures of 700–800 °C and pressures of 11–13 Kbar. The pyroxenite and hornblendite show similar geochemical features and REE patterns, indicating the same magma source and formation through differentiation and accumulation. The arc-like geochemical features of the rocks with enrichment of LILE (Rb, Th and La) and LREE, but depletion of HFSE (Nb, Zr and Hf), possibly formed at the boundary of sub-continental lithospheric mantle (SCLM) and crust through metasomatic reaction of peridotite with felsic melts derived from subduction-related components. The arc-like features, zircon rare earth element patterns and obvious positive Pb anomaly in primitive mantle-normalized trace element spidergrams also indicate the mixing of continental materials in the magma source. The post-collisional extensional setting is correlated to the tectonics associated with the assembly of the Mongolian arc terranes within the northern NCC during the Triassic.