The geology of the Ras Ed Dom and Abu Dom igneous ring-complexes Bayuda Desert, Sudan

O'Halloran, Desmond Anthony (1982) The geology of the Ras Ed Dom and Abu Dom igneous ring-complexes Bayuda Desert, Sudan. (PhD thesis), Kingston Polytechnic,


Ras ed Dom and Abu Dom are two anorogenic ring-complexes, of Triassic age, emplaced into the Precambrian gneisses of the central Bayuda Desert in northern Sudan. The two complexes are closely similar lithologically and are composed of early-formed subaluminous syenites, and later peralkaline syenites and granites. The intrusions are emplaced as sub-volcanic stocks and ring-dykes and both complexes are partly girdled by acid cone-sheet swarms. Minor occurrences of acid volcanic rocks are also present. Ras ed Dom is a striking example of a migrating ring-complex, and in it the focus of magmatism migrated eastwards, and declined in strength, with time. Field relations indicate that the sequence early syenite -> alkali syenite -> alkali granite constitutes a recurrent fractionation Series in the complexes. The early syenites occasionally contain early-formed plagioclase, but in general the suite is typically hypersolvus. Feldspar bulk compositions cluster close to Or38 Ab62, and the most calcic composition (An 7 mol%) is from an early syenite. Pyroxenes in the subaluminous units are zoned from cores of ferrohedenbergite to rims of aegirine-augite, and are enclosed within later ferroedenite. Amphiboles in the peralkaline rocks range from ferrorichterite through arfvedsonite compositions to riebeckite. Late aegirine typically replaces alkali amphibole, especially in the granites. Aenigmatite is an additional mafic phase in some of the syenites . Minor late albite is common in the peralkaline rocks. The principal cryptic variation in the mafic phases is the substitution of Na and Fe{sup]3+ for Mg, Ca and Fe[sup]2+. Major element variation in the rocks supports the proposed fractionation sequence, and it is apparent that the main changes with fractionation are enrichment in SiO[sub]2 and Fe[sub]2O[sub]3 relative to FeO, together with an increase in alkalinity due to A1[sub]2O[sub]3 depletion relative to the alkalies. Relations to alkali-rich analogues of the 'granite system' confirm a magmatic origin, and trends in terms of SiO[sub]2 - A1[sub]2O[sub]3 - (Na[sub]2O + K[sub]2O) suggest that fractionation of alkali-feldspar is the major process in the evolution of the rock-series. Major depletions in Sr and Ba support this hypothesis, as does the variation in Rb/Sr ratios. In common with other alkaline provinces the Bayuda rocks exhibit strong enrichment in incompatible elements such as Zr, Nb, Rb, Y and the LREE group. Concentrations are highest in the granites and this may reflect in part the overprinting by late sodic residual fluids. Partial REE trends are characterised by LREE enrichment, and are consistent with a cogenetic origin for the rocks. The parental melt is believed to have been similar in composition to the early syenites, and is considered to have been generated by lower crustal partial melting. Ras ed Dom and Abu Dom, together with some eighteen or so similar ring-complexes in the Bayuda area, belong to the Younger Granite Association which occurs widely throughout the Sudan, and shows many features in common with the Nigerian Younger Granite province.

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