Magnetic study of the UHP eclogites from the Chinese Continental Scientific Drilling (CCSD) Project

To establish the relationships between rock magnetism of eclogites and the corresponding retrograded metamorphic processes, this study integrated both rock magnetism and metamorphic petrology of 171 eclogite samples with different degree of retrograded metamorphism at the depth range of 100-2050 m of the main hole of the Chinese Continental Scientific Drilling (CCSD) Project, located in the Sulu ultrahigh pressure (UHP) metamorphic belt, eastern China. Results show that the average density is a suitable indicator for the degree of the retrograded metamorphism. Opaque minerals in variable eclogites are mainly magnetite, ilmenite, pyrite, and hematite. With increasing the metamorphic retrogression, the concentration of opaque minerals generally increases; however, magnetic properties of eclogites first increase from well-preserved eclogite to partially retrogressed eclogites because of the neoformation of coarse grained magnetite particles, and then decrease for the completely retrogressed eclogites because of the consumption of the magnetite particles and the formation of weakly magnetic minerals (e.g., hematite). These processes can be summarized as garnet + omphacite + rutile + Si0₂ + H₂0 > amphibole + plagioclase + magnetite + ilmenite > amphibole + epidote + hematite + ilmenite. The retrograded metamorphic eclogites with elevated magnetic properties take up 25% of all rocks and thus are one of major magnetic rocks in the CCSD main hole located in the Sulu UHP area. Therefore the significant volumes of retrograded eclogites may account for the magnetic anomalies flanking the northeastern part of the Sulu UHP metamorphic belt.