Messum

Messum probably contains a greater range of rock types than any other igneous complex in Namibia including as it does tholeiitic, granitic and silica undersaturated alkaline rocks. The complex is approximately circular and 23 km in diameter. About half is obscured by sand and scree but, in the outer part, there are arcuate chains of mountains and hills which are separated by a ring of unexposed ground from an inner ring of mountains which enclose a central group of domes (Korn and Martin, 1955). Around the complex is a sequence of volcanic rocks that Milner and Ewart (1989) consider had their source in the Messum complex. The central complex consists essentially of two parts which are separated by a system of ring-faults. The outer part, which comprises the greater part of the complex, consists of inward-dipping gabbro sheets of which, in the eastern half of the complex, there are almost a dozen but in the western half only two. The western gabbros, on the inner margins of which anorthosites occur, are intersected by many outward-dipping granitic dykes, while the eastern gabbros are separated by granitic, monzonitic and dioritic rocks that contain zones of large, disorientated basic inclusions. The core of the complex consists solely of alkaline rocks, namely a central foyaite which intrudes and greatly alters tuffs and agglomerates which constitute an outer zone to the core. Basic dykes, mainly orientated radially, intrude all zones of the complex. That Messum was formerly a large volcano is indicated by the volcanic rocks at the margin of the core and in the peripheral hills, which have a total thickness of 4-5000 m, with higher horizons being preserved in the core, which suffered the greatest down faulting. The structural geology and volcanic history are described in some detail, and illustrated by a large, segmented block diagram, by Korn and Martin (1955). The lavas of the volcanic phase are all tholeiitic and oversaturated with calcic plagioclase but poor in olivine (Mathias, 1956). The acid lavas and pyroclastic rocks are generally strongly metasomatised so that their original nature is difficult to determine; they appear to have included quartz porphyries, rhyolites, tuffs and agglomerates. The gabbroic sheets of the outer part of the complex comprise olivine eucrites, which grade upwards into anorthosites, and hypersthene gabbros rich in biotite. Like the basic volcanics all of these rocks are tholeiitic. The granitic rocks that cut the basic lavas and the gabbroic sheets include microgranites and granophyres, some of which contain aegirine-augite and fluorite, minerals which also occur in some of the intrusive granites. The alkaline rocks of the core area lie within a ring fault along which, and associated concentric and radial fissures, tinguaite and syenite were intruded. The tinguaite, which is mainly confined to the ring-dyke, contains phenocrysts of microperthite or orthoclase in a groundmass of the same mineral plus oligoclase, nepheline, sodalite, patchy analcime, ferrohastingsite, aegirine-augite, biotite and accessories including fluorite, eudialyte and calcite. The syenite, which forms the radial and concentric dykes, contains phenocrysts of plagioclase (An38-30), some mantled by K-feldspar, in a groundmass of K-feldspar, oligoclase, quartz, amphibole, biotite and accessories. At the centre of the core area is a circular, three km-diameter intrusion of foyaite which has fenitised all the surrounding rocks particularly acid tuffs and agglomerates and basic lavas and dykes. The foyaite consists of alkali feldspar, relatively minor oligoclase, 20-30% nepheline, ferrohastingsite, a little pale green clinopyroxene and minor biotite, ore, apatite and sodalite. Mathias (1956) could distinguish two groups of fenites, a melanocratic group involving metasomatism of xenoliths, dykes and sheets of basic rocks, and a leucocratic group generated by alteration of acid agglomerates and tuffs. The melanocratic fenites are highly variable rocks showing a great range of alteration from slight, towards the margin of the core area, to rocks completely transformed to types referred to as theralite by Mathias (1956). Most involve the introduction of orthoclase, nepheline, a hastingsitic amphibole, sometimes aegirine-augite and some sodalite and analcime. The leucocratic fenites include what are called by Mathias 'foyaite fenites' which occupy a small area close to the foyaite, and syenite fenites which form a broad ring around the foyaite and foyaite fenites. The foyaite fenites only differ marginally from the foyaites, while the syenite fenites consist of about 80% perthite, about 5% quartz, a greenish-brown amphibole that shows alteration to arfvedsonite, aegirine-augite and usually biotite. Some fenites were mobilised. For a fuller account of these complicated rocks the reader is referred to the papers already cited. The closing stage of activity at Messum involved the emplacement of radial dykes of nephelinite and olivine tephrite. Olivine nephelinite is the most abundant type and contains phenocrysts of augite, olivine, nepheline and an opaque phase in a groundmass of the same minerals, excluding olivine, plus biotite, apatite and analcime. The nephelinites grade into tephrites with the addition of hastingsitic amphibole, biotite and occasional poikilitic groundmass K-feldspar and sodic labradorite. The occurrence of three dykes of carbonatite in the Goboboseb Mountain region about 9 km north-northeast of Messum is recorded by Milner and Ewart (1989). The largest is 1-2 m wide and 70-100 m long. It is a breccia consisting of fragments of volcanic rocks and quartzite in a matrix of carbonate. Analyses of 28 rocks are given by Mathias (1956 and 1957) and discussed in some detail in the later reference. A further 21 whole rock analyses with O isotope data are in Harris (1995). Details of gravity and magnetic surveys of the complex are given by Aldrich (1986).