U–Pb zircon age, geochemical and Sr–Nd isotopic data as constraints on the petrogenesis and emplacement time of the Precambrian mafic dyke swarms in the North China Craton (NCC)

Abstract

Precambrian mafic dykes in the North China Craton (NCC) consist of dolerite. Geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses were performed on this suite of mafic dykes to characterise their ages and petrogenesis. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) U–Pb zircon analyses yield consistent ages ranging from 837.9±4.8Ma to 2510±18Ma for seven of the mafic dykes (SHS01, TDG01, LJZ01, LQS02, HB02, WJ01, and DST01) from NCC. Based on these, the mafic dykes can be divided into three groupings: ~2.4–2.5Ga, ~1.8–1.9Ga and 0.8–1.0Ga. The studied dykes belong to the alkaline and sub-alkaline magma series in terms of K2O+Na2O contents (2.6–6.2wt.%), and to the Tholeiitic, calc-alkaline and high-K calc-alkaline series based on their K2O contents (0.1–2.1wt.%). The mafic dykes are further characterised by low and variable light rare earth elements (~2.4–2.5Ga mafic dykes, (La/Yb)N=3.83–10.4; ~1.8–1.9Ga mafic dykes, (La/Yb)N=1.45–2.35); 0.8–1.0Ga mafic dykes, (La/Yb) N=0.66–4.27, show variable Eu anomalies (δEu=0.94–1.35, 0.77–1.21; 0.92–1.28, respectively), positive anomalies in Ba, U, La and Pb, and are depleted in Rb and high field strength elements (Nb, Ta, Ti, and minor Zr and Hf). In addition, the studied mafic dykes all display relatively low radiogenic Sr [(87Sr/86Sr)i=0.6747–0.6921; 0.7017–0.7025; 0.7005–0.7049, respectively for the ~2.4–2.5Ga, ~1.8–1.9Ga and 0.8–1.0Ga mafic dyke groups] and large εNd (t) (13.2–13.5; 10.4–11.8; 6.5–8.5, respectively). These results suggest that all the mafic rocks were derived from a depleted mantle source hybridised by foundered lower crust. The parent mafic magmas likely experienced fractional crystallisation of olivine, pyroxene, hornblende, plagioclase and Fe–Ti oxides (e.g., rutile, ilmenite, titanite, etc.) during dyke ascent, with negligible evidence for crustal contamination prior to emplacement at a high crustal level.