PALEOMAGNETIC STUDY OF THE RAJMAHAL TRAPS
The east Indian shield is endowed with a 117 Ma-old tholeiitic lava flow which isknown as the Rajmahal traps. These traps cover an area of about 4100 sq. km and are known to erupt from the Kerguelen hotspot. Accessory opaque minerals like magnetite and ilmenite impart magnetic properties to the volcanic rocks of the trap. With time, primary and secondary magnetization has taken place within them in the form of natural remanent magnetization (NRM). It can be measured with the help of instruments like the molspin magnetometer, spinner magnetometer and the astatic magnetometer. Removal of the secondary components and isolation of primary components is done with the help of processes like alternating field demagnetization, thermal demagnetization, etc. These processes are also called 'magnetic cleansing' and 'thermal cleansing'. After each step of cleansing, the remanence is measured with the help of a suitable magnetometer. Isothermal remanent magnetization (IRM) measurements are conducted not only to establish the coercivity spectra and saturation remanence of the samples, but are also useful in finding out the mineralogy of the samples. The Lowrie-Fuller Test is useful in categorizing the minerals present in the sample as single domain (SD), multi domain (MD) and pseudo single domain (PSD). In this test the sample is given saturation remanent magnetization and then is demagnetized in number of steps. These entire tests are useful in Paleomagnetic and rock magnetic studies.
Keywords: Primary magnetization, Secondary megnetization, Natural remanent magnetization, Magnetic cleansing, Thermal cleansing, Isothermal remanent magnetization, Coercivity spectra