Geochronological and geochemical constraints on the petrogenesis and tectonic significance of Paleozoic dolerite dykes in the southern margin of Alxa Block, North China Craton

Dolerite dykes are common in the southern margin of Alxa Block which is the westernmost part of North China Craton. The ages and petrogenesis of the dolerite dykes, which are important for a better understanding of tectonic-magmatic evolution in the region, are uncertain. This paper reports the results of an integrated geochronological and geochemical study of the dolerite dykes in the Jinchuan area. Previously, these dolerite dykes were considered to be coeval with the ∼831 Ma Jinchuan ultramafic intrusion by some researchers, despite a cross-cutting relationship for the dykes. Our new zircon U-Pb dating indicates that the dolerite dykes were emplaced at ∼424 Ma, >400 Ma younger than the Jinchuan intrusion. The Jinchuan dolerite dykes contain 5-9 wt.% MgO and 47-51 wt.% SiO2, and are characterized by light REE (rare earth elements) enrichments relative to heavy REE, pronounced negative Nb anomaly, elevated initial 87Sr/86Sr from 0.7056 to 0.7068, negative εNd from −2 to −5, and zircon εHf from −2 to −20. The trace element and isotope data are consistent with an asthenosphere-derived parental magma that experienced 10-25% crustal contamination prior to final emplacement. Since the Jinchuan dolerite dykes are 20-40 Ma younger than the subduction-related basaltic volcanic rocks, blueschists and eclogites in the nearby North Qilian orogenic belt to the south, we propose that the Jinchuan dolerite dykes are the products of basaltic magmatism induced by delamination of previously subducted oceanic lithosphere and the associated asthenosphere upwelling in a post-subduction setting. The occurrence of ∼424 Ma post-subduction dolerite dykes in the Jinchuan area and the ∼450 Ma arc basalts to the south indicate that the southwestern margin of North China Craton was an active plate margin in Early Paleozoic.