Klinghardt Mountains (Klinghardtsberge)

In the Klinghardt Mountains a volcanic field of coulees, tholoids (domes), composite volcanoes, minor lava flows and ignimbrites, pyroclastic breccias and rare dykes is represented by over 100 occurrences, most of which are phonolitic in composition. Within a 15 km radius of the mountains three minor intrusions of olivine melilitite and nephelinite occur (Marsh, 1987; Kaiser, 1926, Map 12). The phonolite bodies cut dolomites, quartzites and phyllites of the Pan-African Gariep Group which overlie late Precambrian gneisses. The most abundant forms taken by the phonolite are endogenous domes, but contacts with the country rocks are usually obscured by blown sand. The largest are several kilometres in diameter but the biggest bodies, notably Kokerboom and Quellkuppe in the north, are considered by Lock and Marsh (1981) to be coulees involving more lateral movement as thick lava flows. True flows are rare but where identified are associated with airfall tuffs and form part of composite volcanoes in one of which, Glasrucken, welded ash-flow tuffs are also present. The clearest example of a composite volcano is Hochster, which is built largely of air-fall pyroclastic rocks cut by numerous conduits filled with lava or breccia. Several low-lying pans west of the mountains appear to be underlain by pipes filled with breccia and pyroclastic materials (Lock and Marsh, 1981). The inclusions comprise immediate country rocks together with syenite, phonolite and nephelinite and xenocrysts of diopside, nepheline and olivine. The phonolites vary from aphyric varieties to rocks with phenocrysts up to 2 cm in diameter and constituting as much as 40% of the rock. Phenocrysts include alkali feldspar, nepheline, occasionally sodalite, aegirine-augite, sometimes Ti-magnetite and titanite and rarely amphibole, which is pargasite to kaersutite and mantled by sodic pyroxene. The groundmass is of alkali feldspar, nepheline and sodic pyroxene with aenigmatite in all except amphibole-bearing varieties. Analyses of the main mineral phases are given by Marsh (1987) who also presents major and trace element data for a range of rocks, including 87Sr/86Sr values, which are used for petrogenetic modelling.