Dutse

This is a nearly circular complex covering about 29 km2 but, apart from a single exposure of basement granite on the western margin, the contacts are obscured by aeolian sands. It comprises three types of peralkaline granite, syenite and minor volcanic rocks. The volcanics are preserved in three small hills and consist of agglomerates and ignimbrites, the latter with some quartz crystals and containing fiamme. The syenite lies along the southwestern margin and possibly represents a remnant of a ring-dyke. It contains perthite phenocrysts, occasionally with plagioclase cores, ferroaugite, fayalite (Fa94-95) rimmed by ferro-actinolite or ferro-edenite, magnetite, allanite and aggregates of biotite and possible richterite; quartz and alkali feldspar form an interstitial granophyric groundmass. In some marginal areas of the syenite aggregates of aegirine and Fe-Ti oxides occur that may be a metasomatic development resulting from intrusion of later peralkaline granite. The main rock type of the complex is a coarse-grained granite of alkali feldspar, quartz, aenigmatite, ferrorichterite-arfvedsonite, aegirine, which may rim amphibole or form sheaves of acicular crystals, with accessory Fe-Ti oxide, astrophyllite, fluorite and zircon; possible pyrochlore has been found. An earlier porphyritic granite occurs near the centre of the complex and is found as xenoliths in the main granite; it contains aegirine and arfvedsonitic amphibole. In the north the main granite is intruded by an aegirine-albite microgranite containing a little arfvedsonite. Analyses of fayalite and pyroxene will be found in Bennett et al. (1984).