Paleomagnetic and paleointensity study of an Oligocene-Miocene lava sequence from the Hannuoba Basalts in northern China

A combined geochronologic, paleomagnetic and paleointensity investigation has been performed on a lava sequence from the Hannuoba Basalts in northern China. Potassium-argon (K-Ar) dating indicates that the lava sequence is of late Oligocene to middle Miocene age (∼24-15 Ma). Rock magnetic measurements of thermomagnetic curves, hysteresis properties, saturation isothermal remanent magnetization (SIRM) and back-field demagnetization on representative samples from each lava flow in conjunction with reflected light microscopy observations show the primary carriers of remanence to be fine-grained pseudo-single domain titanomagnetite with various titanium contents. Stepwise thermal and alternating field demagnetization successfully isolated stable characteristic remanent magnetization from 40 out of the 43 lava flows, which record 24 paleomagnetically independent directions. Data (excluding directions regarded as abnormal/transitional) yield a mean direction of D/I = 6.8°/56.3°, with α95 = 5.2° and κ = 51.6, corresponding to a paleomagnetic pole at 232.3°E, 83.7°N with dp = 5.4° and dm = 7.5°, consistent with previously published paleomagnetic poles for the North China Block. The modified Thellier method with systematic partial thermoremanent magnetization (pTRM) checks and pTRM-tail checks was conducted on 72 selected samples. Reliable paleointensity estimates from four lava flows yield a mean virtual dipole moment of (5.6 ± 1.6) × 1022 A m2, suggesting moderately low intensity of the geomagnetic field. This is in good accord with reliable paleointensity datasets for the late Oligocene to middle Miocene (30-15 Ma), but lower than the average for middle Miocene to Pliocene age (15-2 Ma). It thus appears that at some point in the mid-Miocene there was a transition where the field intensity changes between these two states, although many more data are needed to confirm this.