The Control of Metamorphic Grade on the Rock Magnetism and Magnetic Anisotropy of Crustal Rocks: A Case Study From Chhotanagpur Granite Gneissic Complex, East Indian Shield

Rock magnetic properties, magnetic anisotropy and magnetic fabrics of crustal rocks are controlled by lithology and crustal processes like metamorphism, which causes a systematic variation in the rock magnetic properties and magnetic fabrics. Such variations of rock magnetic properties and magnetic fabrics due to lithology and grade of metamorphism are still to be unravelled in detail. In this study, rock magnetic and anisotropy of magnetic susceptibility (AMS) studies are conducted on the metamorphic rocks belonging to granulite facies (quartzofeldspathic rocks/migmatite and granite gneiss), amphibolite facies (meta-gabbro and meta-norite) and associated igneous intrusions of the Proterozoic Chhotanagpur Granite Gneissic Complex (CGGC) of the East Indian Shield. The main aim is to determine the control of the metamorphic grade on the rock magnetic properties and the magnetic fabrics of the rocks. The main magnetic mineral identified is magnetite, with certain interferences of sulphide. Rock magnetic properties and magnetic fabrics show that the volcanic rocks exhibit the strongest magnetization due to their high concentration of stable single domain (SSD) magnetite. Magnetite dominates the magnetic mineralogy of both volcanic and metamorphic rocks; within the metamorphic sequence, SSD magnetite increases from amphibolite to granulite facies but decreases in migmatites owing to quartzofeldspathic enrichment. This integrated approach provides more robust magnetic proxies for metamorphic grade. Consequently, rock magnetic and magnetic fabric analyses together offer valuable insights into the metamorphic signatures of crustal basement rocks.