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

Assimilation serves as a significant part in magma's evolution. Because of rapid cooling of ultramafic‐mafic magma, assimilation between mantle‐derived magma and crustal rocks (wall‐rocks) is often neglected. The Beidaihe diabase dikes in North China Craton suggest that assimilation of the rapidly cooled mantle‐derived magma can not be ignored. The diabase dikes are very thin (0.5 to 2 m in width) and discordantly intrude into Ordovician limestone. Center of dyke is porphyritic texture while chilled margin of it is aphanitic texture, suggesting a rapid crystallization process. Moreover, limestone close to the dyke was transformed into marble because of contact‐thermal metamorphism. From the center to margin of the dykes, contents of SiO₂ (48.9→ 41.6 wt.%) and MgO (8.3 → 4.0 wt.%) are obviously decreased, but contents of CaO (6.4→8.3 wt.%) and CO₂ (0.8 → 5.2 wt.%) are distinctly increased. Meanwhile, calcites in the wall rocks adjacent to and far from the dykes are also greatly different. The calcites close to the dykes have high MgO and FeO contents but low CaO and CO₂ contents. All these suggest an assimilation process of the mantle‐derived magma with wall‐rock limestone. Calculating CO₂ contents in the dyke and wall rock suggest that the extent of assimilation is thought to reach up to 8 ‐ 12 %. Therefore, when the wall rocks of the mantle‐derived magma are carbonate rocks, its components are obviously influenced by assimilation, although it often rapidly chilled. Simultaneously, this study provides possible explanation for some mantle‐derived rocks in carbonate areas with high LOI.