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Palaeomagnetism and ~(40)Ar/~(39)Ar age from a Cretaceous volcanic sequence,Inner Mongolia,China:Implications for the field variation during the Cretaceous normal superchron

【摘要】:正An integrated palaeomagnetic and ~(40)Ar/~(39) Ar dating study was carried out on an Early Cretaceous volcanic lava sequence from the Suhongtu section,Inner Mongolia,to determine the field behavior within the Cretaceous normal superchron(CNS).~(40)Ar/~(39) Ar ages were obtained from 12 lava flows,indicating that the studied lava was formed around 114.1±0.3 Ma for the lower interval and 110.6±0.1 Ma for the upper interval.Rock-magnetic experiments and electron microprobe analyses indicate that the primary Fe-Ti oxides are the main magnetic carriers.All lava flows carry normal palaeomagnetic directions,which can be grouped into 31 units by an F-test,with a Fisher mean characteristic remanent magnetization(ChRM) of D/I= 12.8/58.6°(α_(95) = 2.3°).The corresponding palaeomagnetic pole is located at 80.3°N and 200.3°E (A_(95) = 3.2°,K=64.4),which is indistinguishable at the 95%confidence level from the Eurasia pole derived from the apparent polar wander path for the early Cretaceous.Using a modified Thellier palaeointen-sity method with stringent acceptance criteria,we obtained two time-series of palaeointensity records from 15 independent palaeomagnetic units(total 136 samples).The virtual dipole moment(VDM) values varied from 2.53×10~(22) Am~2 to 9.92×10~(22) Am~2.The mean VDMs for the upper and lower intervals are (5.38±2.06)×10~(22) Am~2 and(4.61±2.67)×10~(22) Am~2,respectively.The observed time-series of palaeointensity, together with the previously available data for the CNS,suggest that magnetic field strength during the CNS fluctuated significantly with time.

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