The Sabaloka igenous complex, Sudan

Abstract

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.