2.09 Ga mafic dykes from Western Shandong, Eastern block of North China Craton, and their tectonic implications

Abstract

A group of mafic dykes was discovered in the Menglianggu area, the Western Shandong Province located in the Eastern block of the North China Craton. The dykes are chlorite–actinolite schists, which commonly underwent greenschist facies metamorphism. LA-ICP-MS zircon U–Pb analysis reveals that the magmatic precursors of these dykes were emplaced at 2084 ± 15 Ma and preserved a group of ∼2.5 Ga inherited zircons, which are consistent with the monzogranitic gneisses that surround the dykes.The dyke samples are characterized by geochemical compositions that are similar to the extrusive counterparts of tholeiitic basalts and have high MgO content (Mg# values of 70–82), negative Eu anomalies (δEu = 0.55–0.60), slight light rare-earth element enrichment (La/YbN = 1.45–2.70 and Gd/YbN = 0.98–1.34), and small negative anomalies in the high-field-strength elements. They show slightly negative to positive zircon εHf (t) values (−0.26 to +3.89) and negative εNd (t) values (−4.87 to −0.53).Evidence of dyke petrogenesis suggests that they most likely derived from the partial melting of a depleted mantle source caused by asthenospheric upwelling at shallow levels in the upper mantle. They then experienced 3%–12% crustal assimilation and initial fractional crystallization of olivine, hypersthene, and plagioclase during magma ascent through the continental crust. These dykes have geochronological features that are consistent with the 2200–2000 Ma Hengling magmatic belt (HMB), which indicates that Paleoproterozoic magmatic events extended to the Western Shandong Province (the interior of the Eastern block in the North China Craton).