Palaeomagnetism and Geochronology of the Harrats Lunayyir and Khaybar Lava Fields, Saudi Arabia

Abstract

To better constrain the tectonic history of the Arabian craton in the Tertiary, we carried out a combined paleomagnetic and 40Ar/39Ar geochronological study on volcanic rocks from the Khaybar and Lunayyir Harrats plus a site of sediments deposited below the Miocene rocks in the former area. Progressive thermal or alternating field demagnetization successfully isolated stable characteristic magnetizations (ChRM) that are consistent with a primary magnetization only in the Late Quaternary lava flows from Harrat Lunayyir. The Harrat Lunayyir paleomagnetic data set of 11 flow-mean directions (D = 0.31°, I = 36.9°, α95 = 10.5) is statistically indistinguishable from the present field and the virtual geomagnetic poles (VGP: 214.1°E, 85.1°N; A95 = 12.3°) which indicate a negligible rotation (R = −1.98 ± 10.49o) with respect to the coeval African pole position. The paleomagnetic signal of the Miocene lava flows from the Harrat Khaybar area appear to be contaminated by the effect of lightning and weathering and consequently no tectonic/plate movement significance may be attributed to the large CCW rotation shown from 2 sites with antipodal directions. The direction of the high coercivity chemical remanent magnetization (CRM) isolated after thermal cleaning from the Pre-Miocene siltstones (D = 169.6°, I = −44.8°; α95 = 5.4°) is consistent with the few existing paleomagnetic results from Arabia. The associated VGP (314.4°E, 80.6°N) is close to the Pliocene VGP of the Arabian Plate and CCW rotated (R = 14.86 ± 6.38°) with respect to the Oligocene African VGP. The results imply that the whole rotation of the Arabian Plate took place during the last phase (4–5 Ma) of the opening of the Red Sea, corresponding with the true sea floor spreading as already noted in the past by other authors. Whole rock 39Ar/40Ar step-heating analyses yield whole-rock plateau ages of 12.8 to 16.3 Ma for the alkaline lava flows from the Khaybar area, which is consistent with the estimated age range of the region-wide late Cenozoic alkaline volcanism in Saudi Arabia.