A palaeomagnetic and rock magnetic study of the northern Rajmahal Volcanics, Bihar, India

Abstract

A new palaeomagnetic study of mid Cretaceous lavas from 23 localities in the northern part of the Rajmahal Hills gives a mean virtual geomagnetic pole at 10.4°N, 296.6°E, confirming the results of earlier workers. Stepwise thermal and alternating field demagnetisation indicates that 21 out of 23 sites are normally magnetised, whereas two sites in the northwest of the area have recorded two different transitional directions. We have measured the strong-field thermomagnetic behaviour, DC and AC hysteresis parameters, and the variation at low temperature of low-field susceptibility of these and other samples from the Rajmahal Volcanics. These rock magnetic studies reveal that the basaltic lavas contain titanomagnetites which have hardly been oxidised at high temperature, but have undergone low temperature oxidation. The reddened lavas of Gandeswari Hill, on the other hand, show evidence of extensive high temperature deuteric oxidation of titanomagnetite.