Acid volcanism on the south Arabian coast

Abstract

Along the south coast of Arabia, between Aden and the southern entrance to the Red Sea, there are six central vent volcanoes of probable Pliocene age. All are characterised by the interstratification of basic and acidic extrusives, the formation of large central calderas at a late stage in the volcanic cycle and the subsequent infilling of these calderas with horizontal acidic ignimbrites and basic lavas.

Lying 60 miles to the west of Aden and of particular interest is Jebel Khariz, the largest and best preserved of the six volcanic centres, covering a roughly circular area of about 100 square miles and rising to a height of 2,766 feet. The volcanic sequence of Jebel Khariz is broadly divisible into two suites: a) alkali-rich rhyolites and trachytes which occur as flows and pyroclastic horizons and form about 80 per cent of the volume of the cone, and b) effusives of basaltic composition that occur in the caldera, locally on the south-east and south-west flanks and in a small parasitic cone on the northern flank.

The alkali-rich acidic suite includes lavas, ash-flow and ash-fall rocks as well as vent and flow breccias, Generally, all rocks of this suite have phenocrysts of anorthoclase, and may contain phenocrysts of fayalitic olivine, aegirine-augite, magnetite and/or quartz. The fine grained matrix is composed of the same minerals with skeletal riebeckite and, in some cases, cossyrite. The basaltic suite is characteristically porphyritic, the phenocrysts being of calcic plagioclase, clinopyroxene, olivine and magnetite in a fine-grained mesostasis of plagioclase, olivine, clinopyroxene and ore. The plagioclase, on initial investigation, appears to lie in the labradorite-bytownite range, the olivine is commonly replaced by iddingsite and the clinopyroxene is most commonly a pale mauve titanaugite.

Near the centre of the volcanic pile, as exposed in the caldera wall, masses of rhyolitic composition can be seen to form over half of the volcanic sequence. These masses are markedly lenzoid in cross-section normal to the flow direction and display intricate flow folding; they are considered to have been extruded as viscous lava. Further from the volcanic centre, these acidic extrusives become less markedly lenzoid until in the distal areas of individual units, some 5 miles from the caldera, they have spread out to form sheet-like masses covering as much as 10 square miles to a uniform thickness rarely exceeding 25 feet. The presence of agglomeratic bases, hard compact central sections and less compact upper divisions, together with the ubiquitous presence of columnar jointing and occasional shard textures suggest that these distal parts of each extrusive unit have been formed by an ash-flow/ash-fall mechanism.

It is postulated that the majority of the Jebel Khariz volcanic pile was formed by emission of acidic material, effusive in the central area, but deposited mainly by an ash-flow mechanism around the flanks of the cone. This could be due to either the synchronous eruption of viscous lava from the central vent with ash flow eruptions on the flanks; or, more probably, to the progression of an individual volcanic episode through an initial ash-flow phase followed by the effusion of viscous lava, all emanating from the central vent.