Physics of the Earth and Planetary Interiors 128 (2001) 207–222 Geomagnetic palaeointensity just prior to the Cretaceous normal superchron

A combined geochronologic (K–Ar) and palaeomagnetic study has been conducted on a basalt lava sequence at Zhuanchengzi in Liaoning province, northeastern China. The new K–Ar age obtained from these lava flows is 120.93 ± 0.88 Ma (marine anomaly M0). Detailed investigations were conducted on two samples from each lava flow to determine their rock-magnetic properties. The temperature dependences of the saturation magnetization (J–T) and the initial susceptibility (κ–T) show evidence of low-titanium magnetite and a Curie temperature of 560–585◦C with generally good reversibility of heating and cooling curves. Hysteretic properties, ratios of Mrs/Ms and Hcr/Hc, indicate that pseudo-single-domain (PSD) grains of magnetite are dominant in these lavas. Stepwise thermal demagnetization isolated well-defined characteristic magnetization (ChRM) in all the 20 lava flows with a mean direction of D/I = 174.9/−60.1 ◦ with α95 = 2.3◦. The modified version of the Thellier and Thellier palaeointensity method (Coe, 1967a) with systematic partial thermoremanent magnetization (pTRM) checks was used. Forty-two of 71 samples (59%) yielded reliable palaeointensity results. Virtual dipole moment (VDM) values range from 1.71 × 1022 to 5.27 × 1022 A m2 with an average of (3.96 ± 0.07) × 1022 A m2. This low dipole-field intensity value is approximately half the present day VDM, consistent with older results obtained from the Caribbean submarine basalt glasses for the period of 125 ± 1 Ma (Pick and Tauxe, 1993). This suggested that a weak geomagnetic field probably occurre