Geochronology and geochemistry of mafic dykes in the Helanshan complex: implications for Mesozoic tectonics in the North China Craton

Abstract

The Helanshan tectonic belt (HTB) is a major tectonic divide between the Alxa and Ordos blocks in the North China Craton. The geochronology and petrogenesis of the mafic dykes in the northern HTB are keys to understanding the tectonic evolution of this belt. The mafic dykes, intruded into the Neoarchean–Paleoproterozoic metamorphic basement, are mainly composed of diabase with a mineral assemblage of plagioclase (45%–60%), pyroxene (25%–35%), minor quartz and Fe–Ti oxides. The LA-ICPMS U–Pb analysis of zircon grains from representative dykes yield a weighted mean age of 206 ± 1.9 Ma, which represents the crystallization age of the dyke. The diabases show high contents of Fe2O3T (11.88–17.55 wt.%), low contents of SiO2 (45.65–50.95 wt.%) and MgO (3.31–5.50 wt.%) with low Mg# (=100 × MgO/(MgO + FeO) atomic ration) of 33–44. They are characterized by enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs) (e.g., Rb, Ba and Pb), and slight depletion of high field strength elements (HFSEs). These features suggest that the magma has undergone extensive fractionation of olivine and pyroxene but only minor crustal contamination during its evolution. Their high Sm contents and La/Sm ratios, and low Sm/Yb ratios indicate that magma from which the dykes formed was derived from low degree (about 5%) partial melting of an enriched garnet + spinel lherzolite mantle source. Together with regional geology, these geochemical and geochronological data suggest that the mafic dykes in the HTB were formed in an intracontinental extensional setting during the late Triassic.