Magnetic properties of archean gneisses from the northeastern North China Craton: The relationship between magnetism and metamorphic grade in the deep continental crust

Magnetic mineralogy of crustal rocks has important implications for understanding continental crustal evolution and origin of regional magnetic anomalies. However, magnetic properties of the deep continental crust are still poorly understood. In this paper, measurements of density (ρ), mass-specific magnetic susceptibility (χ), natural remanent magnetization (NRM) and magnetic hysteresis loops, temperature-dependent magnetic susceptibility (χ-T), chemical and mineral analyses were conducted on Archean gneiss samples from the Jixian petrophysical section in the Precambrian terrain, northeastern North China Craton, with the aim of refining understanding of magnetic phase transformations in the deep crustal rocks. Results show that density and rock magnetic properties change distinctly with metamorphic facies. The dominant magnetic mineral is magnetite, while a little hematite is present in a few samples. Together with geochemical and mineralogical compositions, it is inferred that progressive increase in metamorphic grade from east to west is the major cause for magnetic enhancement of the lower crust in the studied section. Therefore, we conclude that study of magnetic phases of deep crustal rocks can offer important insights into the history of high metamorphic grade terranes.