Neoarchean arc basaltic magmatism and associated sulfide mineralization in the North China Craton: Evidence from the Taoke mafic-ultramafic complex in Shandong Province

The Neoarchean Taoke mafic-ultramafic complex is located in the eastern block of the North China Craton. We report the first geochronological and geochemical data for the complex, and use these data to unravel the fundamental tectonic-magmatic processes involved in the formation of this complex. The Taoke complex comprises a large elongated gabbronorite intrusion, a small sulfide-mineralized olivine websterite body in the center, and many dolerite dikes. The U-Pb isotope ages of magmatic zircon from the mafic and ultramafic intrusions are 2520.3 +/- 6.5 Ma and 2537 +/- 12 Ma, respectively, which are contemporaneous with abundant granitoids and boninitic basalts but similar to 620 myr older than the collision-related, granulite-facies metamorphism in the region. Like the coeval boninitic basalts, the Taoke mafic-ultramafic rocks are characterized by light-REE enrichments and negative Nb-Ta anomalies, which are characteristic of modern arc basalts. The epsilon(Nd)(t) values of the Taoke mafic-ultramafic rocks are from 1.0 to 3.3, within the range of the coeval basalts in the region. The epsilon(Hf)(t) values of zircon from the Taoke mafic-ultramafic complex are from 0.2 to 2.3. The integrated Nd-Hf isotope data indicate that crustal contamination is absent to negligible. No correlation between Th/Nb ratios and epsilon(Nd)(t) values are present, implying that the negative Nb-Ta anomalies of these rocks are not the result of crustal contamination but are original features of the magma. Our new data support the interpretation that the complex is a Neoarchean arc-type mafic-ultramafic complex. Liquid inversion using the La/Yb and Gd/Yb ratios of the associated dolerite dikes and coeval boninitic basalts indicates a garnet-bearing lherzolite mantle source. The depth of partial melting is estimated to be > 85 km, which is within the range of mafic magma generation in modern arc settings. The results from this study demonstrate that by Neoarchean the style of arc basaltic magmatism already closely resembled the modern counterpart. Based on the presence of sulfide mineralization in the Neoarchean arc-type mafic-ultramafic complex at Taoke and the association of major magmatic Ni-Cu sulfide ore deposits with post-Archean arc-type mafic-ultramafic complexes elsewhere in the world, we suggest that exploration for this type of deposit in the Neoarchean arc-type mafic-ultramafic complexes including the Taoke complex in western Shandong, should continue.