Geomagnetic palaeointensity just prior to the Cretaceous normal superchron

A combined geochronologic (K-Ar) and palaeomagnetic study has been conducted on abasalt 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 MO). 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 (K-T) show evidence of low-titanium magnetite and a Curie temperature of 560-585 degreesC with generally good reversibility of heating and cooling curves. Hysteretic properties, ratios of M-rs/M-s and Her/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 degrees with alpha (95) = 2.3 degrees. 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 X 10(22) to 5.27 x 10(22) A m(2) with an average of (3.96 +/- 0.07) x 10(22) A m(2). 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 occurred just prior to the Cretaceous normal superchron (CNS). (C) 2001 Elsevier Science B.V. All rights reserved.