No abstract

The Ghirian volcanic province comprises three distinct units. An early (50-55 my.) phase of extensive plateau lava (basaltic hawaiite) eruption was followed by the emplacement of small phonolite domes ( N 40 m.y.). After a long period of inactivity, a rejuvenation phase (< 12 m.y.ago) led to the formation of scattered minor volcanic centres, generally of basanitic composition. and carrying lherzolite fragments. 39 new analyses illustrate the distinctive chemical characteristics of each episode. Differences in volume and composition between early and late basic volcanicity can be related to regional episodes of strong mantle convection in the early Tertiary and weak convection in the late Tertiary. The phonolites may be fractionates of an intervening injection of basic magma trapped at moderate depth. Continuing structural control was exerted on a regional scale by the zone of uplift and fracturing connecting the Ghirian area with Tibesti.

Large ignimbrite dykes continuous with an overlying sheet of rhyolitic ignimbrite have been found at two localities in the centrally-subsided block of the Sabaloka cauldron. There is good evidence that these dykes fed ash-flow eruptions. Other possible feeder vents occur along the marginal fracture zone of the cauldron but evidence for the origin of some of these structures is ambiguous. Ignimbrites within the dyke-shaped feeders contain a very strong eutaxitic foliation oriented parallel to the contacts and this feature is thought to result from inwardly-directed pressures exerted by the dyke walls during a collapse which followed eruption of the ash-flows. Vents of this type are believed to originate by permissive intrusion through the roof of a shallowseated magma chamber, in contrast to forcefully injected diatreme vents. A broad genetic classification of these bodies is suggested, based on their mode of emplacement.

In the northern Red Sea Hills. of Sudan, deformation is largely concentrated into a network of shear zones. Several of these zones strike north-south. hut the important Nakasih and Sol Hamid zones are northeast-trending, and there are also minor shear zones which strike east-west. We distinguish between massive shear zones, such as that of Nakasih. and more diffuse braided shear zones. typified by the Oko zonc. The massive type owes its character to an origin a s a reactivated oceanic suture. whereas the braided type was characterized by strikc-slip shearing from its inception. Most of thc shear zone rocks are mylonites formed under greenschkt facies conditions. but early shearing along the Oko zone took place at higher temperature and resulted in gneissose mylonites with amphiholitc facics mineralogy. The northeast-trending. Nakasib shearing appears to have preceded a phase of batholithic intrusion, whereas north-south shearing in the Oko and Abirkitib zones is younger than the batholithic intrusions and is i n turn post-dated by emplacement o f bimodal granite-gabbro complexes. Thcsc cvcnts cannot be dated precisely as yet, but took place between 95 Ma and 700 Ma. and probably towards the end of that interval. The pattern of shear and suturc zoncs in northeast Sudan suggests that the northeast-trending Hijaz and Asir arcs. recognized in Saudi Arabia. terminate to the west against a north-south suture which was later rejuvenated to form the Ahirkitib shear zone. West of this suturc was an immaturc volcanic arc which may have lain along the margin of the African continent. The rejuvenation which caused northeast and north-south strike-slip shear was probably a consequence o f soft collisions in East Arabia.

Sabaloka is one of the best exposed and most accessible of a large number of Younger Granite complexes in Sudan. These complexes have close affinities with the Younger Granites of western Africa and like them range widely in age. Sabaloka itself probably dates from the Proterozoic or early Palaeozoic. The paper includes a detailed map and description of the complex and presents the results of 20 new whole-rock chemical analyses. Of the two main centres at Sabaloka, the large Cauldron Complex comprises a subsided block of basement overlain by up to 2 km of volcanic rocks and circumscribed by a polygonal zone of ring-fracturing. The fracture system was intruded by a ringdyke of porphyritic microgranite after eruption of the volcanic rocks, and at about the same time a boss of mica granite with associated tin-tungsten mineralization was injected into the subsided block. There is also gravimetric evidence of subsurface granite intrusions in both the north and south of the cauldron, but no indications of any large mass of basic rock. Nearly all of the volcanic and intrusive rocks of the Cauldron Complex are thoroughly acidic, but a thin group of basaltic lavas lies at the base of the volcanic succession and a few minor intrusions are of basic and intermediate composition. The acidic rocks include metaluminous and subaluminous types, but peralkaline rocks are either absent or very minor in amount and altered beyond recognition. Lavas dominate the lower part of the volcanic succession whereas rhyolitic ignimbrites compose most of the upper part. Of the two main episodes of subsidence which formed the cauldron the first followed upon eruption of the lavas and produced a structural basin centred on the eastern margin of the present complex. Subsequent establishment of the ring-fracture system appears to have been consequent upon an extension of the magma chambers to the north, and was accompanied by voluminous ash-flow eruptions and the formation of a caldera. The second major subsidence post-dated all the volcanic rocks still preserved, and was probably followed by resurgent doming in the north, though the evidence on this point is not conclusive. The Cauldron Complex is classified as a ‘Valles type’ of caldera volcano. The much smaller Tuleih Complex lies north of the Cauldron and includes a boss of quartz-syenite and subacid granite together with a plexus of smaller intrusions which include peralkaline intermediate and acidic rocks of comenditic character. The age of these intrusions relative to the Cauldron Complex is not known. The chemistry of these various rock types reflects in many respects their close similarity to the Younger Granite association of western Africa, although the rocks of the Cauldron Complex are somewhat poorer in soda than most analysed acidic rocks from the Nigerian Younger Granites.