Jabal As Sawda

This volcanic province extends over 6,000 km2, but originally was probably in excess of 10,000 km2 (Busrewil and Esson, 1991), and consists essentially of a plateau of basaltic lavas, up to 100 m thick, within which are the remnants of seven small shield volcanoes 2-11 km in diameter and numerous volcanic cones of 1 km or less. Intrusions are represented by plugs within the volcanoes and dykes and sills cutting the lavas. The extrusive rocks are dominantly alkali olivine basalts but modal nepheline can sometimes be identified, although commonly it is zeolitized. The intrusive rocks are generally gabbroic, but some varieties contain alkali feldspar while interstitial nepheline is sometimes abundant. By chemical as well as mineralogical criteria Woller and Fediuk (1980) classify the more alkaline rocks as theralite, olivine theralite and nephelinitic basanite and present a range of average rock analyses, including a broad range of trace elements. A palaeomagnetic study of the flows is available (Ade-Hall et al., 1975a).