Zircon U–Pb age, geochemical, and Sr–Nd–Hf isotopic constraints on the origin of mafic dykes in the Shaanxi Province, North China Craton, China

Abstract

Mafic dolerite dykes form a series of swarms that are widespread across the North China Craton (NCC). We present U–Pb zircon ages, geochemical data, and Sr–Nd–Hf isotopic data for representative samples of the Shaanxi dolerite dykes in the southern NCC. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon analyses for three samples yield ages ranging from 448.1±1.2 to 489.6±0.9Ma (i.e., Cambro-Ordovician). The dolerites are characterised by a wide range of rock compositions. They display enrichments in light rare earth element (LREEs) and large ion lithophile element (LILE) (i.e., Ba, U, K and Pb), as well as depletion in high field strength (HFSE) (Nb, Ta, Hf and Ti). The mafic dykes have relatively uniform (87Sr/86Sr)i ranging from 0.7049 to 0.7076, (176Hf/177Hf)i from 0.282187 to 0.282236, low εNd (t) values, from −4.1 to −4.9, εHf (t) values of between −7.9 and −9.8, for zircon, and high hafnium model ages (TDM1=1366–1535Ma, TDM2=1924–2081Ma). These results suggest that the dykes are derived by partial melting of an enriched, lithospheric mantle source. The magmas underwent fractionation of olivine, pyroxene, plagioclase, and Ti-bearing phases (rutile, ilmenite, titanite), along with crustal contamination. The formation of the Shaanxi Province, NCC mafic dykes can be attributed to the collision between the NCC and south Qinling Block. Specifically, these magmas formed as a result of crustal thinning in response to extension that followed this collision.