Jubal Asma

The Jubal Asma Mountains in the north of the Bayuda Desert cover some 15x40 km and consist mainly of extrusive rocks but with two windows in which underlying plutonic rocks are exposed. In the central area a ring structure occurs which, because of the rapid weathering, has negative relief. It is about 6 km in diameter and comprises peralkaline syenite and granite enclosed by ring-dykes of syenite porphyry and quartz porphyry with tangential quartz porphyry dykes in the southeast. A 1 km diameter window of similar syenite occurs in the southeast of the area and a further syenite body outcrops among superfical deposits some 7 km to the east-southeast of the ring intrusion. The superstructure of the occurrence consists of a lower sequence of mainly pyroclastic rocks including agglomerates and tuffs, but trachytic lava flows are also present. Younger rhyolite flows are confined to the northeast. To the east of the complex is the north-northwest-striking dyke swarm of Jebel Barqat Abu Hashim. The dykes include quartz porphyries, trachytes and trachyte porphyries. There are a number of small basaltic plugs at the periphery of the mountains, particularly in the north and south.