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

The volcanic rocks of Aden, Little Aden, and Ras Imran, heredesignated as belonging to the Aden Volcanic Series, were eruptedthrough central-vent, strato-volcanoes about 5 m.y. ago. Inits major element chemistry the Aden Volcanic Series is intermediatebetween the alkaline and tholeiitic associations, and this isdemonstrated by comparing it with the alkaline suite of Hawaiiand the tholeiitic series of Thingmuli, Iceland. It is proposedthat the most acceptable ‘parental’ magma is a mildlyalkaline olivine basalt which, on fractionation, produced aseries ranging from trachybasalts through trachyandesites andtrachytes to rhyolites. These rhyolites are peralkaline as themolecular proportion of alumina is less than that of the combinedalkalis, and are comenditic as the series is poor in normativefemic constituents. Trace element data suggest that the peralkalinesilicic eruptives are chemically comparable with those of MayorIsland, New Zealand, where a mildly alkaline olivine basaltparent 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 geomorphologicalcriteria, an ⁸⁷Sr/⁸⁶Sr versus ⁸⁷Rb/⁸⁶Sr isochron plot suggeststhat the series is related to a thermal event some 20-30 m.y.older than the age of eruption. As this earlier age correspondsdirectly to the age of the previous magmatic episode, the eruptionof the Yemen Trap Series, the upper part of which is petrologicallysimilar to the Aden Volcanic Series, and as the initial ⁸⁷Sr/⁸⁶Srratios suggest that the magma originated in the mantle, it isproposed that the most acceptable petrogenetic scheme, whichwould also explain the anomalously old Rb-Sr age, is: (a) Partialfusion in the upper mantle giving rise to the alkaline YemenTrap Series, (b) After the cessation of surface activity, alarge 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 furtherthermal activity, resulted in the partial remelting of thisfractionated plutonic sequence and the liquids so formed reachedthe surface without significant mixing or chemical fractionation.