Origin and Geodynamics of Neoarchean Mafic Dikes in Western Shandong, Northeastern North China Craton: Geochronological, Geochemical, and Nd-Hf Isotopic Evidence

The North China Craton (NCC) witnessed major extensional tectonic events at the close of the Neoarchean with anorogenic magmatic pulses, which have important implications for understanding the evolution of Precambrian continental crust. Here we study a suite of mafic dikes from western Shandong in the northeastern NCC to discuss the nature, timing, and source of the rift-related mafic magmatism, using zircon U-Pb data, whole-rock geochemistry, and Nd-Hf isotopes. Zircon U-Pb dating of four dike samples by laser ablation ICP-MS yields weighted mean 207Pb/206Pb ages in the range of 2509±6.1–2537±6.2 Ma. The dikes are classified as alkaline dolerites, on the basis of their K2O+Na2O contents (6.78–7.21 wt%), and belong to the shoshonitic series according to their K2O contents (3.23–3.36 wt%). They show low concentrations of light rare earth elements ((La/Yb)N=10.0–18.2); negligible Eu anomalies (Eu/Eu*=0.89–1.20); positive Rb, Ba, U, K, Pb, and Sr anomalies; and depletions in high-field-strength elements (Nb, some Ta, P, and Ti). In addition, the dolerites have low initial 87Sr/86Sr (0.6969–0.7022) and positive εNd(t) (0.2–0.8) and εHf(t) (0.5–8.6) values. Collectively, these data suggest that the mafic dikes were derived from the partial melting (5.0%–10%) of a depleted garnet-lherzolite mantle source that was hybridized through interaction with foundered lower-crustal material. The parental mafic magmas of these dikes underwent fractional crystallization of clinopyroxene and Ti-bearing oxides (e.g., rutile, ilmenite, titanite) during ascent, with negligible evidence for crustal contamination before emplacement at a higher crustal level. The mafic magmatism in the form of dike swarms is considered to be a response to widespread lithospheric extension, which affected the NCC at ca. 2.51–2.54 Ga, during the Neoarchean.