Magnetic study of mafic granulite xenoliths from the Hannuoba basalt, north China

Abstract

The composition and structure of the lower continental crust are of great interest in understanding the dynamic processes of the Earth interior. In this study, we examined relationships between the magnetic properties and opaque mineralogy of a set of mafic granulite xenoliths (derived from ∼24–41 km) collected from the Hannuoba region, north China (a region well known for its exposures of lower crustal rocks). These mafic granulite xenoliths are weakly magnetic with an average magnetic susceptibility (χ, mass specific) and saturation isothermal remanent magnetization (SIRM) of 5.87 × 10−7 m3 kg−1 and 31.53 × 10−4 Am2 kg−1, respectively. The opaque minerals identified by electron microprobe (EMP) analysis in mafic granulite xenoliths are mainly paramagnetic Mg‐rich ilmenite, which significantly contributed the room temperature χ. In contrast, SIRM are controlled predominately by pyrrhotite and magnetite, which were clearly identified by both scanning electron microscopy (SEM) and rock magnetic analyses. Our results show some relationships between these magnetic properties and the ratio of concentrations of the trace rare earth elements (La/Yb), which is sensitive to degree of fractional crystallization. More significantly, the Mg content in ilmenite is highly related to the fractional crystallization. These observations bring new insights into the complexities of the magnetic properties and the mineral composition (e.g., ilmenite) during the dynamic processes of mafic granulite xenoliths formation. However, the exact relationship between the rock magnetism and the fractional crystallization process needs further studies.