Formation of mafic magmas through lower crustal AFC processes — An example from the Jinan gabbroic intrusion in the North China Block

Abstract

The Jinan gabbroic intrusion from the North China Block (NCB) experienced complex differentiation from a reduced and Si-undersaturated magma to an oxidized and Si-saturated melt, with a mineral assemblage of olivine+pyroxene+plagioclase+biotite+ilmenite. New dating of baddeleyite and zircon yield a consistent U–Pb age range from 130 to 134Ma, confirming emplacement during the early Cretaceous. In situ oxygen analyses on olivine show relatively homogeneous O isotopic compositions, and the olivine separates give a limited δ18O range from 6.0 to 6.1‰, higher than that of olivine from normal mantle peridotites. The zircons have even higher δ18O (6.9–7.3‰) than olivine and clinopyroxene separates (δ18O=6.2–6.9‰) and show clear O isotope disequilibrium with olivine/clinopyroxene. The δ18O of the primary magma for olivine and clinopyroxene crystallization was increased by 1.2–1.4‰ to form the evolved melt for zircon crystallization. This δ18O variation points to assimilation or contamination by high-δ18O crustal components during magmatic evolution. Systematic decreases in whole-rock εNd(t) and εHf(t) are accompanied by increases of δ18O, Ba/Nb, Ba/Th and Zr/Nb and decrease of Th/La, along with a change in oxygen fugacity of the magma. Modeling results suggest that the Jinan gabbroic intrusion experienced assimilation-fractional crystallization (AFC) with felsic material at lower crustal levels. Combined geochemical and Sr–Nd–Hf–O isotopic features suggest that the primary melts were derived from the NCB lithospheric mantle that had been further metasomatized by a small volume of carbonatitic melt, possibly derived from the subducted Yangtze crust.A compilation of the published mineral O isotope data shows that most Mesozoic mafic intrusions in NCB exhibit O isotope disequilibrium between zircon and olivine/clinopyroxene, with higher δ18O of zircon than olivine and clinopyroxene, indicating their crystallization from 18O-richer differentiated melts due to crustal contamination or assimilation. Therefore, δ18O of zircon as a tracer of mafic magmas should be used with caution, particularly for primary Si-undersaturated magmas.