Initiation of continental breakup documented in evolution of the magma plumbing system of the ca. 925 Ma Dashigou large igneous province, North China

In the Wilson cycle, continental rifting leads to ocean basin formation, but the relationship between rift-related magmatism and the initiation of seafloor spreading and continental drift remains unclear. A recently recognized large igneous province (LIP), the ca.925 Ma Dashigou LIP of the North China Craton, possibly represents initial of continental breakup. The magmatic center of this LIP is thought to be located in the Xuhuai basin, where two generations of sill complexes (945–920 Ma Niutishan and 915–890 Ma Pingshan) occur. Both complexes are dominantly tholeiitic, with minor alkaline rocks in the Pingshan complex. The Niutishan complex compositionally varies from gabbro (SiO₂ = 48.0–49.0 wt%, MgO = 4.0–6.0 wt%) to quartz dolerite (SiO₂ = 53.0–56.0 wt%, MgO = 2.0–3.0 wt%), while the Pingshan complex is comprised of more primitive gabbro (SiO₂ = 47.0–48.5 wt%, MgO > 6.0 wt%) with only a few quartz syenite veins (SiO₂ = 58.5–59.8 wt%, MgO = 1.0–1.2 wt%). Trace elements and isotopic compositions indicate a relatively minor continental signature in the Niutishan parental magma (⁸⁷Sr/⁸⁶Sr_initial = 0.7040–0.7048, εNd_initial = −0.84 to +1.91, Nb/La = 0.8–1.2), while Pingshan exhibits transitional to MORB compositions and shows distinct geochemical signals of seawater alteration (⁸⁷Sr/⁸⁶Sr_initial = 0.7060–0.7079, εNd_initial = −1.34 to −0.23, Nb/La = 0.6–0.7). The compositional ranges of both units are consistent with varied degrees of partial melting in the source regions followed by varied degrees of fractionation in the magma chambers/plumbing systems. Sr isotopic characteristics indicate a possible increasing in alteration of oceanic fluids through time. Coherent geochemical features show that the far-field radiating Dashigou dyke swarm and Niutishan events belong to the plumbing system of a single LIP in a pre-drift environment. The Pingshan complex is probably related to the growth of an inner subaerial seaward-dipping syn-rift reflector sequence. The plumbing system evolved from a sill complex (Niutishan) near the magmatic center at ca. 945 Ma to a far-field radiating dyke swarm (Dashigou) at ca. 925 Ma, which then reverted back to a sill complex (Pingshan) near the magmatic center at ca. 915 Ma. The Dashigou LIP may be correlated with the Bahia-Gangila LIP of the São Francisco-Congo Craton, and the evolution of their magmatic plumbing systems is interpreted as an indication of rift-to-drift transition tectonics, which resulted in the initiation of seafloor spreading and consequent drift.