Palaeomagnetism and K-Ar dating of cretaceous basalts from Mongolia

Abstract

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 18 flows, k D 41.1, fi95 D 5.5-) for Shovon and Dm D 12.1-, Im D 66.4- (n D 27 flows, k D 53.0, fi95 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 D 7.4;, Im D 62.7- (n D 23 flows, k D 41.4, fi95 D 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, A95 D 5.8-). © 2007 The Authors Journal compilation © 2007 RAS.