Palaeointensity just at the onset of the Cretaceous normal superchron

摘要: Abstract A combined palaeomagnetic and geochronologic study has been conducted on a basaltic lava sequence of five flows at Jianchang in Liaoning Province, northeastern China. Radiometric 40 Ar/ 39 Ar dating indicates that the volcanism occurred about 119 Ma within marine anomaly C34n Cretaceous normal superchron (CNS). Rock magnetic investigations show pseudo-single domain (PSD) or mixture single multi-domain grain titanium-poor titanomagnetite is dominant studied flows. Both stepwise thermal alternating field demagnetizations isolate well-defined characteristic remanent magnetization (ChRM) independent with mean direction D / I  = 11.1°/57.2° ( α 95  = 5.7°). Palaeointensity was determined using Coe modified Thellier method systematic partial thermoremanent (pTRM) checks. All together, 61 samples from 286 positive pTRM tail checks yield palaeointensities ranging between 10.1 73.6 μT. To further check effects alteration palaeointensity results, new ratio M rs,480°C rs,25°C , where are room temperature saturation isothermal magnetizations for raw sample product after 480 °C treatment, respectively) proposed. Results changes significantly even those check. We put forward results acceptable 0.9 1.1. With this criteria, 17 consistent an average 20.0 ± 3.2 μT. This value corresponds to virtual dipole moments (VDM) (3.5 ± 0.6) × 10 22  Am 2 which half present field. finding, other available suggests Earth just onset CNS characterized by weak palaeointensity.