Magneto-tectonic framework of the East Indian Shield: The present state of knowledge

The present study discusses the magnetic fabrics of the East Indian Shield (EIS) and associated rocks and relates them to different geological events. The orientation of the magnetic fabric elements and the values of Anisotropy of Magnetic Susceptibility (AMS) parameters from the different units of the EIS such as Singhbhum Granite (SBG), Iron Ore Group (IOG), Jagannathpur and Malangtoli lavas, North Singhbhum Mobile Belt (NSMB), Dalma lavas, Chhotanagpur Granite Gneissic Complex (CGGC) and lower Gondwana sedimentary rocks are discussed. The different AMS parameters include mean magnetic susceptibility (K_m) magnetic foliation (F), magnetic lineation (L), corrected degree of magnetic anisotropy (P_j) and shape parameter (T). These parameters along with the orientation of the magnetic foliation and magnetic lineation is used to address some holistic problems in the EIS and associated rocks. These include lava flow patterns and AMS facies (i.e., whether the magnetic fabric is foliation dominant or lineation dominant), which in turn is controlled by the conditions of eruption of the volcanic rocks, deformation related magnetic fabric development in the deformed and metamorphosed rocks and palaeocurrent vis-à-vis depositional environment in younger sedimentary rocks. Primary magnetic fabrics are recorded in the volcanic and the sedimentary rocks. Such primary fabrics are used to determine the flow patterns and AMS facies in the volcanic rocks and depositional sedimentary environment in the younger sedimentary rocks. The Jagannathpur and Malangtoli lavas display flow direction towards SW to WSW with weakly lineated AMS facies, whereas, the Dalma lavas show a random flow direction with foliated and lineated AMS facies. The primary magnetic fabrics of the sedimentary units of the lower Gondwana Supergroup indicates that although the palaeocurrent direction is due SW in both the Talchir and Barakar Formations, the depositional environment was glacio-fluvial in the Talchir Formation and fluvial in the Barakar Formation. The metamorphic (and deformed) rock units like the IOG, SBG, NSMB or the CGGC have complex magnetic fabrics. The magnetic fabrics of the IOG fossilises both folding events in it as well as explains the coeval relationship of the faulting event and the second-generation folds. Like the IOG, the magnetic fabrics of the CGGC also records two generations of deformation suffered by these basement rocks and the time relationship between deformation of the basement and the intrusion of the pegmatite veins. The magnetic fabrics of the meta-sedimentary rocks of the NSMB are controlled by the second (D₂) deformational event with some deviations due to ferrimagnetic control over magnetic fabric and thrusting events.