Assimilation of the mafic‐ultramafic magma: A case study of diabase dyke at the Beidaihe, North China Craton

Abstract

Assimilation serves as a significant part during magma's evolution. Because of rapid cooling of mafic–ultramafic magma, assimilation between mantle‐derived magma and crustal rocks (wall rocks) is often neglected. The Beidaihe diabase dyke in North China Craton suggests that assimilation of the rapidly cooled mantle‐derived magma cannot be ignored. The diabase dyke is very thin (~1 m in width) with chilled margin. The dyke centre is porphyritic texture while the chilled margin is aphanitic texture, suggesting a rapid crystallization process. From the centre to margin of the dyke, contents of SiO2 (from 48.12–49.94 wt.% to 38.29–46.63 wt.%), MgO (from 7.46–9.17 wt.% to 3.39–4.71 wt.%), Ni (from 91.5–118 ppm to 6.5–45.8 ppm), and Cr (from 243–306 ppm to 1.7–80.5 ppm) are obviously decreased, but contents of Fe2O3T (from 8.71–9.39 wt.% to 9.21–12.30 wt.%), CaO (from 5.67–7.08 wt.% to 6.91–10.26 wt.%), and CO2 (from 0.25–1.92 wt.% to 4.52–6.58 wt.%) are distinctly increased. The dyke intruded discordantly into Ordovician limestone. Limestone far away from the dyke mainly consists of micro calcites with low MgO (0.55–0.77 wt.%) and FeO (0.07–0.09 wt.%) but high CaO (53.82–55.51 wt.%) and CO2 (44.46–45.02 wt.%) contents. Limestone close to the dyke metamorphosed into marble, which mainly comprises of recrystallized calcite/dolomite and diopside, with high MgO (1.85–2.35 wt.%) and FeO (0.17–1.36 wt.%) but relatively low CaO (52.30–54.15 wt.%) and CO2 (38.77–41.04 wt.%). All these suggest an assimilation process of the mantle‐derived magma with wall rocks limestone. Based on the chemical balance at the interface between the diabase and limestone, CO2 exchange model is used to estimate the degree of assimilation. The calculated assimilation degree is about 8–12% at the dyke margin. There is obvious elements exchange between magma and wall rocks, and the transfer ability of Mg is greater than that of Ca. Meanwhile, Fe oxides quickly precipitated at the margins of the dyke for the increase of fO2. Therefore, even for mantle‐derived magma which chilled rapidly, it may also be obviously influenced by assimilation, especially when wall rocks are carbonate sediments. Simultaneously, this study provides a possible explanation for some mantle‐derived rocks in carbonate areas with high LOI.