High-Mg Dioritic Magmas Generated via Fractional Crystallization: Insights from Early Cretaceous Complex in the Handan-Xingtai District, North China Craton

Abstract

High-Mg diorites with high Sr/Y ratios are commonly associated with metal mineralization. But their petrogenesis still remains controversial. Here we present a case study of the Early Cretaceous high-Mg dioritic intrusions (Fushan intrusive complex) in the Handan-Xingtai district, central North China Craton (NCC). Samples from the Fushan Complex (123±1 Ma) have SiO2 of 53.3–62.6 wt%, relatively variable K2O of 0.5–3.6 wt%, and high Mg# values (44–63). They show enrichment in light rare earth elements and depletion in heavy rare earth elements and high field strength elements, with high Sr/Y ratios and insignificant Eu anomalies, similar to those of adakitic rocks. For the Sr-Nd isotopes, they have whole-rock initial 87Sr/86Sr ratios and εNd(t=123 Ma) values ranging from 0.706564 to 0.707546 and from −16.1 to −12.8, respectively, which are distinct from those in the lower continental crust but similar to those in the contemporaneous mafic rocks in the study area. Because of these characteristics, together with their zircon δ18O values of 6.6‰±0.4‰, we suggest that the Fushan high-Mg dioritic complex was most likely formed by factional crystallization of mantle-derived basaltic magma. The high Sr/Y ratios could be achieved by the fractionation of amphibole crystals plus significant volumes of feldspars from a mafic magma. We suggest that late Mesozoic strong lithospheric extension and thinning, caused by the subduction of the paleo-Pacific slab in the east of the NCC, might have triggered the melting of the metasomatic lithospheric mantle to generate these intrusive complexes in the Handan-Xingtai district.