Zircon U–Pb geochronology, whole-rock geochemical, and Sr–Nd–Pb isotopic constraints on the timing and origin of Permian and Triassic mafic dykes from eastern North China Craton

Abstract

This work reports an important episode of extensional, mafic magmatism that impacted the North China Craton (NCC) during the Permo-Triassic and influenced the evolution of this Craton. We sampled 30 representative mafic dykes cropping out in eastern NCC, within the Hebei Province, China. New zircon LA-ICP-MS U–Pb ages, geochemical, and Sr–Nd–Pb isotopic data for these mafic dykes identified them as typical dolerites formed between 211.9 ± 1.3 and 263.9 ± 2.6 Ma. The mafic rocks fall into the alkaline and calc-alkaline series. They are enriched in light rare earth elements, some large ion lithophile elements (e.g., Rb, Ba, and Sr), K, and Pb, and depleted in Nb, Ta, and Ti. The dykes have high initial 87Sr/86Sr ratios (0.7034–0.7178), negative eNd (t) values (−2.2 to −5.9), and relatively constant initial Pb isotopic ratios, that are EM1-like: (206Pb/204Pb)i = 16.39–16.74, (207Pb/204Pb)i = 15.22–15.24, and (208Pb/204Pb)i = 36.66–36.86, respectively. Our results indicate that the Hebei Province dolerites were likely derived from magma generated through low-medium degree partial melting (3.0–20%) of an EM1-like garnet-lherzolite mantle source. The parental magmas fractionated olivine, clinopyroxene, and Ti-bearing phases with obvious crustal contamination during rapid ascent and dyke emplacement. Since the possible influence of subduction of the Yangtze Plate has been excluded, we propose a model for the origin of the investigated mafic rocks, involving the Permian collision between the Siberian Block and the NCC. The mafic dykes of eastern NCC within Hebei Province formed during a period of crustal thinning in response to extension after the early Permian collision.