THE ALKALINE MEIDOB VOLCANIC FIELD (LATE CENOZOIC, NORTHWEST SUDAN)

The Meidob volcanic field (MVF) forms part of the Darfur Volcanic Prov ince and developed from 7 Ma to 5 ka as indicated by K/Ar, thermolumin escence and C-14 ages. It is situated in an uplifted high of the Pan-A frican basement, which consists of greenstones, high-grade gneisses an d granites, and which is covered by Cretaceous sandstone. The MVF basa ltic lavas, which originated from more than 300 scoria cones, formed a lava plateau of 50x100 km and up to 400 m thickness in the time betwe en 7 and < 0.3 Ma. Young phonolitic mesa flows, together with rare tra chytic-benmoreitic lava flows, trachytic pumice fallout deposits, igni mbrites and maars, form the central part of the field. The total amoun t of volcanic rocks is between 1400 and 1800 km(3), with 98 vol.% bein g basaltic rocks, which results in an integrated magma output rate of similar to 0.0002 km(3) a(-1). A combination of age data of the lavas with erosional features yields uplift rates for the Darfur Dome of sim ilar to 30 m Ma(-1) in the MVF area. Magma was generated by 3-5% melti ng of predominantly asthenospheric mantle with a HIMU contribution. Fr actionation of olivine, pyroxene, An-poor plagioclase-anorthoclase, ma gnetite and apatite leads to a differentiation from basanite to phonol ite. Assimilation of crustal rocks near the top of the phonolitic uppe r crustal magma chambers - facilitated by volatile enrichment - produc ed magmas which gave way to benmoreitic and trachytic lavas, as well a s to trachytic ignimbrites and pumice fallout deposits. Ultramafic cum ulate xenoliths indicate the existence of major magma reservoirs at th e crust-mantle boundary during MVF activity. Magma ascent occurred in a tensional regime, which changed its orientation at around 1 Ma. Earl y during MVF development, west-east and subordinately northeast-southw est trending lineaments were active whereas volcanic activity younger than 1 Ma took place along northwest-southeast and northeast-southwest trending systems. The Central African Fault Zone, a transcontinental, lithospheric shear zone, played an important role for the rise of mag mas in the Darfur Dome. (C) 1997 Elsevier Science Limited.