Al Wahbah is a large (~2.2 km diameter, ~250 m deep) maar crater in the Harrat Kishb volcanic field in western Saudi Arabia. It cuts Proterozoic basement rocks and two Quaternary basanite lava flows, and is rimmed with an eroded tuff ring of debris from the phreatomagmatic explosion that generated the crater. A scoria cone on the northern wall of the crater was dissected by the explosion and exposes a dolerite plug that was intruded immediately prior to crater formation. The dolerite plug yields a 40 Ar/ 39 Ar age of 1.147 ± 0.004 Ma. This is the best possible estimate of the time Al Wahbah crater formed. It is a few tens of thousand years younger than the age of the lower and upper basalt flows, 1.261 ± 0.021 Ma and 1.178 ± 0.007 Ma respectively. A dolerite dyke exposed within the basement in the wall of the crater is dated at 1.886 ± 0.008 Ma. This is the most precise age so far determined for the initiation of basaltic volcanism of Harrat Kishb, and confirms that it is significantly younger than the other post-rift volcanic provinces in the region. This study provides constrains the timing of humid climatic conditions in the region and suggests that the Quaternary basaltic volcanism that stretches the length of the western side of the Arabian peninsula may prove to be useful for establishing palaeoclimatic conditions.
Α series of samples of the system Coι-xΖnxFe2O4 (x = 0.0, 0.3, 0.4, 0.5 and 0.8) have been prepared by the usual ceramic technique. X-ray analysis shows that they are cubic spinel (single phase). The lattice parameter a and the bulk density D are measured for the samples. The substitution experiments replacing Coe+ by Ζn2+ furnish new significant fact concerning the magnetic behaviour of ferrites. The magnetic strength of the magnetic ions on the Band A-sites can be varried relative to each other by the substitution of Ζn2+ The increase of density of the composition increases the magnetic interaction of the dipole moment at the B-sites.
The Loch Doon, Cairnsmore of Fleet and Criffell granites and aureoles show considerable variation in their magnetic mineralogy and fabrics. The Loch Doon area has magnetite as the dominant magnetic mineral, while the Cairnsmore of Fleet area shows magnetite and hematite in different areas. The Criffell granite and immediately adjacent aureole are characterized by magnetite, but hematite predominates in the outer aureole and country rocks. Despite such differences, the magnetic fabric only shows effects of granitic emplacement within the thermal aureole, mostly within 200–300 m of the contact. Considerable variation in magnetic fabric is observed over small distances, but the granitic contacts are generally marked by a strong foliation parallel to the granite contact.
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