Palaeomagnetism and K-Ar dating of Cretaceous basalts from Mongolia

We report a combined geochronology and palaeomagnetic study of Cretaceous igneous rocks from Shovon (44.4(-)N, 103.8(-)E) and Arts-Bogd (44.3(-)N, 102.2(-)E) localities in the Gobi Desert, south Mongolia. K-Ar dating based on seven rock samples, with two independent measurements for each sample, allows us to propose an age of 94.7 1.3 Ma for Shovon locality and a 98.2 1.4 to 118.3 1.7 Ma age range for Arts-Bogd. Stepwise thermal and AF demagnetization generally isolated a high temperature component (HTC) of magnetization for both Shovon and Arts-Bogds basalts, eventually following a low temperature component (LTC) in some samples. The HTC directions display normal polarity, consistent with the Cretaceous Long Normal Superchron. Rock magnetic analysis identifies fine-grained pseudo-single domain (PSD) magnetite and titanomagnetite as primary carriers of the remanence. Mean HTC palaeomagnetic direction is Dm D 8.2(-), Im D 63.7(-) (n D 8 flows, k D 41.1, fi(95) D 5.5(-)) for Shovon and Dm D 12.1(-), Im D 66.4(-) (n D 27 flows, k D 53.0, fi(95) D 3.9(-)) for Arts-Bogd. Because of their similar ages, we combine data from Shovon and data previously obtained from Khurmen Uul (92.0 +/- 4.0 Ma), recomputed in geographic coordinates, and not in tilt-corrected ones as in our previous interpretation, at the Shovon locality. The combined final average palaeomagnetic direction for Shovon-Khurmen Uul is Dm = 7.4(-), Im = 62.7(-) (n D 23 flows, k D 41.4, fi(95) = 4.8(-)). The corresponding palaeopoles computed from these HTC lie at , D 84.7(-)N, ' D 195.0(-)E, dp/dm D 5.8=7.5 for Shovon-Khurmen Uul (average age: 93.4 2.6 Ma) and , D 80.5(-)N, ' D 159.0(-)E, dp/dm D 5.2=6.3 for Arts-Bogd (average age: 104.6 6.6 Ma). These poles are consistent with those from the European apparent polar wander path (APWP) at 90, 100 and 110 Ma, and other published pole from the Mongol-Okhotsk suture zone, Amuria and North China blocks. This confirms the lack of a discernable latitudinal motion between Amuria and Siberia since their final accretion by the Late Jurassic-Early Cretaceous, and reinforces the idea that Europe APWP can be used as a reference for Siberia by the mid-Cretaceous. We finally propose a mid-Cretaceous mean palaeomagnetic pole for the Siberia-Amuria-North China Block assemblage which lies at: D = 86.4(-)N, D = 191.1(-)E(n D 10, k D 74.9, A(95 D) 5.8(-)).