The Peralkaline Volcanic Suite of Aden and Little Aden, South Arabia

Abstract

The volcanic rocks of Aden, Little Aden, and Ras Imran, here designated as belonging to the Aden Volcanic Series, were erupted through central-vent, strato-volcanoes about 5 m.y. ago. In its major element chemistry the Aden Volcanic Series is intermediate between the alkaline and tholeiitic associations, and this is demonstrated by comparing it with the alkaline suite of Hawaii and the tholeiitic series of Thingmuli, Iceland. It is proposed that the most acceptable ‘parental’ magma is a mildly alkaline olivine basalt which, on fractionation, produced a series ranging from trachybasalts through trachyandesites and trachytes to rhyolites. These rhyolites are peralkaline as the molecular proportion of alumina is less than that of the combined alkalis, and are comenditic as the series is poor in normative femic constituents. Trace element data suggest that the peralkaline silicic eruptives are chemically comparable with those of Mayor Island, New Zealand, where a mildly alkaline olivine basalt parent has also been postulated. Although the age of eruption of c. 5 m.y., given by K-Ar measurements, is entirely consistent with an age deduced from geomorphological criteria, an 87Sr/86Sr versus 87Rb/86Sr isochron plot suggests that the series is related to a thermal event some 20-30 m.y. older than the age of eruption. As this earlier age corresponds directly to the age of the previous magmatic episode, the eruption of the Yemen Trap Series, the upper part of which is petrologically similar to the Aden Volcanic Series, and as the initial 87Sr/86Sr ratios suggest that the magma originated in the mantle, it is proposed that the most acceptable petrogenetic scheme, which would also explain the anomalously old Rb-Sr age, is: (a) Partial fusion in the upper mantle giving rise to the alkaline Yemen Trap Series, (b) After the cessation of surface activity, a large body of magma existed in the upper mantle and this magma, on crystallizing, fractionated to produce a layered sequence, (c) About 5 m.y. ago some event, either pressure relief or further thermal activity, resulted in the partial remelting of this fractionated plutonic sequence and the liquids so formed reached the surface without significant mixing or chemical fractionation.