Tectonics, cooling rates and temperatures during emplacement of the Rajmahal traps, India

Abstract

Paleomagnetism is used to decipher the emplacement setting of the Rajmahal traps. Sampling was restricted to flows with known age. Three magnetic minerals are identified, titanomaghemite, Ti-poor magnetite, and possibly Mg–Al-rich titanomagnetite. Detailed analysis of thermomagnetic curves revealed that the emplacement temperature for Rajmahal traps was lower than 600 °C. Two magnetization components are distinguished. The low coercivity component, probably chemical remanent magnetization, is erased in fields up to 3.1 mT. In contrast, the high coercivity thermal remanent magnetization is more stable, sometimes throughout the demagnetization. The paleomagnetic results show that the sites have suffered up to 11° of clockwise and 17° of counter-clockwise rotation. As there is no field evidence of faulting and pervasive deformation, we propose that this rotation is probably due to penecontemporaneous joints or faults. These structures could have acted as conduits for fluids that increased the cooling rate.