A Detailed Magnetic and Opaque Petrologic Study of A Thick Tholeiite From the Rajmahal Volcanics, India

Abstract

Summary A 22 m thick Rajmahal tholeiite basalt lava flow shows wide variations in magnetic properties which have been explained by opaque petrologic studies. Microscopic ore evidence clearly demonstrates that the upper one-third of the lava is dominated by Fe-rich titanomagnetites from high temperature (deuteric) oxidation and that the lower two-thirds are dominated by unoxidized Ti-rich titanomagnetite except for a thin low temperature oxidation zone. the natural remanent magnetization (NRM) intensity appears to be related to the composition of magnetic minerals, giving rise to high values for Fe-rich titanomagnetites and lower values for their Ti-rich counterparts. Stable magnetization is confined to two narrow zones in the flow where the titanomagnetite grains are intensively subdivided by lamellar ilmenites or by wide cracks. the initial susceptibility and the concentration of magnetic oxide grains both show little variation, suggesting the dependence of the former on the latter. Although the relation of hysteresis properties and opaque mineralogy is usually complex, the ratio of remanent magnetization to the saturation remanent magnetization, JRS/JS, is chiefly dependent on the effective grain size and the coercive force Hc, tends to be higher for anisotropic grain fabrics resulting from lamellar ilmenite formation in titanomagnetite. the Q-values of the specimens suggest that the remanent magnetization is significant only in the high temperature oxidation zone of the lava.