Migrating younger granite ring-complexes in the Bayuda desert, northern Sudan

A5 Some of the shear zones are partially lined by metamorphosed basic and ultrabasic rocks, perhaps representing dismembered ophiolites. Within-block structure is characterized by westward-verging, subhorizontal folds trending N--S. The blocks show differences in lithology, structure and metamorphic grade, probably indicative of structural, rather than stratigraphical, juxtaposition. The intruding batholiths display the full calc-alkaline range from gabbro to mildly alkalic granite, with granodiorites predominating. Many of these rocks are weakly foliated, and some are strongly foliated orthogneisses. The batholiths are cut by numerous calc-alkaline dykes, and are locally affected by zones of intense shearing. Evidently, the whole region has undergone at least two major episodes of shear deformation, the first related to ophiolite emplacement and the second post-dating emplacement of the batholiths. Later still, calc-alkaline activity was concentrated into centred complexes, and since there is evidence that these are now exposed at more than one structural level, it is likely that there was uplift and deep erosion during this interval. Finally, following stabilization, magmatism switched over to the alkaline-granite--syenite association characteristic of the Younger Granites of northern Africa. Most of these complexes are aligned along the N--S Wadi Onib lineament. The shear-defined block structure which dominates the structural pattern is presumably a less chaotic lateral equivalent of the "ophiolitic melange" of Shackleton et al. (1980). The authors regard the high-grade rocks at the western end of the traverse as a reworked sialic basement to the Lower Proterozoic 'Greenschist assemblage', but since we have not so far found undoubted equivalents east of the junction with the low-grade rocks, it may be that the Abu Hamed area lay near to a Lower Proterozoic continental margin. On the other hand, the very large volume of the granitoid batholiths may indicate a contribution from subadjacent sial.

MIGRATING YOUNGER GRANITE RING-COMPLEXES BAYUDA DESERT, NORTHERN SUDAN

IN THE

D.C. ALMOND 1, P: CURTIS 2, M. McCORMAC:, D. O ' H A L L O R A N ' and J.R. VAIL 2

'Kingston Polytechnic, Kingston upon Thames (Great Britain) 2-Portsmouth Polytechnic, Portsmouth (Great Britain)

Five adjacent igneous ring-complexes have been examined in detail and fifty component intrusive and extrusive units identified, the geology being displayed on a 1:100 000 scale map. The complexes intrude quartzofeldspathic gneisses of the Proterozoic Basement complex, and comprise ringdykes, cone-sheets, radiating dyke swarms and stocks together with downfaulted calderas. Granites and syenites predominate, and early emplaced peraluminous biotite-granites are distinguishable from later subaluminous (hedenbergite) and peralkaline (aegirine-arfvedsonite-riebeckite) types. The plutonic rocks are intrusive into early-formed volcanic succesSions preserved in subsided cauldrons. Compositionally two sequences may be recognized in the volcanic rocks, an early hawaiite-trachyte sequence, and a more extensive acid sequence with ignimbritic and rhyolitic lavas and pyroclastic rocks. Thirty distinct intrusive foci are present, defining an irregular migration path over a distance of 137 kin, within which a progressive reduction in average diameter of the ring-dykes, from more than 20 km to less than one km can be recognized. Preliminary age data suggest emplacement of the intrusions occurred over a span of about 160 Ma. Professor Almond is now at the University of Khartoum. Dr. Curtis is with Geosurvey. M. McCormac is with U.K. Institute of Geological Sciences. Dr. O'Halloran is with Liverpool University.