Ras ed Dom migrating ring complex: A-type granites and syenites from the Bayuda Desert, Sudan

Abstract

A major cluster of anorogenic A-type, subvolcanic ring complexes of Silurian to Jurassic age is emplaced into Proterozoic rocks of the basement complex in the Bayuda Desert of northern Sudan. The complexes belong to the Younger granite association which is widespread throughout the Sudan and environs. The Ras ed Dom complex is a striking example of migrating centres in which intrusive activity shifted eastwards and declined in strength with time. Intrusive rocks predominate and consist of metaluminous and peralkaline syenites and granites. Field relations indicate that the sequence metaluminous syenite-peralkaline syenite-peralkaline granite is a recurrent fractionation series representing high level fractionation of a parental metaluminous syeno-monzonitic magma. The metaluminous syenites contain rare early-formed plagioclase (oligoclase-andesine) but otherwise the suite is typically hypersolvus. The metaluminous rocks are characterised by an early pyroxene zone from ferrohedenbergite to aegirine-augite and enclosed in ferroedenitic amphibole. Microprobe analysis reveals a broad range of amphibole compositions in the peralkaline rocks, comprising ferrorichterite-arfvedsonite-riebeckite solid solutions. Late aegirine, exhibiting marked enrichment in the acmite component, typically replaces the arfvedsonite, especially in the granites. Extremely pure albite accompanies aegirine and the development of these phases is attributed to limited interaction between the rocks and late sodic fluids. Geochemical variation in the suite includes marked depletions of CaO, Ba and Sr with increasing silica, consistent with fractionational removal of alkali-feldspar. The peralkalinity of the later rocks has been enhanced by sodic autometasomatism, reflected in their high contents of REE and HFS elements such as Zr, Nb and Ga, and by the erratic distribution of these elements. LREE-enrichment in the suite is marked and suggests equilibrium with a residual HREE-rich phase such as garnet in the source rocks. The Younger Granite magmas are attributed to melting of a lower crustal mafic-rich source such as garnet-eclogite. The heat for this anatectic event is thought to have come from mantle-derived basalt magma, the uprise of which was largely checked in the lower crust, only minor amounts reaching the surface.