Paleointensity study of the middle Cretaceous Iritono granite in northeast Japan: Implication for high field intensity of the Cretaceous normal superchron

Abstract

Paleointensity of the Cretaceous normal superchron (CNS) has been studied using the middle Cretaceous Iritono granite of the Abukuma massif in northeast Japan. Our previous study [Wakabayashi, K., Tsunakawa, H., Mochizuki, N., Yamamoto, Y., Takigami, Y., 2006. Paleomagnetism of the middle Cretaceous Iritono granite in the Abukuma region, northeast Japan. Tectonophysics 421, 161–171] indicates that the Iritono granite retains a stable and primary component of high blocking temperatures and high coercivities which is characterized by the shallow inclination and carried mainly by single-domain magnetite. Applying Coe's version of the Thellier method and the LTD–DHT Shaw method, we have obtained 16 successful results with an average of 58.4 ± 7.3 μT. However, an effect of long cooling time of the granite on the paleointensity measurement should be taken into account. An estimate of cooling time to acquire the primary component ranges in 4 × 10 4 to 1.4 × 10 7 years from a thermal diffusion model of the granite body and the difference between 40Ar– 39Ar biotite age of 101.9 ± 0.2 (1 σ) Ma and U–Pb zircon age of 115.7 ± 1.9 (1 σ) Ma. From single-domain theory, thermoremanent magnetization (TRM) of the Iritono granite samples in nature is estimated to be about 1.5 times as strong as the laboratory TRM. Applying this correction factor, the corrected paleointensity is 39.0 ± 4.9 μT and the virtual dipole moment (VDM) is calculated to be 9.1 ± 1.1 × 10 22 A m 2. This VDM can be interpreted as representative of the middle CNS geomagnetic field since the individual granite samples with long cooling time can average out the paleosecular variation. The obtained VDM is a few times higher than mean VDMs averaged for 0–5 Ma (3.6 × 10 22 A m 2) and 0–160 Ma (4.8 × 10 22 A m 2) except for the CNS by previous studies. This suggests that the geomagnetic field intensity was high in the middle CNS.