A Transitional Basalt-Pantellerite Sequence of Fractional Crystallization, the Boina Centre (Afar Rift, Ethiopia)

Volcanological and petrological evidence, 87Sr/86Sr data, the linear correlation between pairs of residual elements (e.g. Th, U, Zr, Hf, La, Ce, Ta) indicate that the rock series from mildly alkaline (transitional) basalt to pantellerite erupted in recent Quaternary times at the Boina volcanic centre, can be entirely explained in terms of fractional crystallization at shallow depth. The fractionation process has been reconstructed by referrinġ to variation diagrams of major and trace elements reported as a function of the fraction (f) of the initial composition formed by the residual liquid, evaluated from the distribution of residual elements and by estimating the composition of the parent magma. The main crystal phases involved in the differentiation are, in the order of appearance: olivine, plagioclase, clinopyroxene, Fe-Ti oxides, alkali feldspar. Crystallization of Fe-Ti oxides occurs only at an advanced stage of fractionation in iron rich liquids (ferrobasalts). The transition to the peralkaline field (near f=0·20) occurs without passing through a 'true' trachytic (low-silica) salic stage and is determined by the 'plagioclase effect'. Fractionation within the peralkaline field is dominated by alkali feldspar. Evidence is given for a 'low-temperature zone' of the oversaturated mildly peralkaline system running along a line of constant alkali-ratio. Po2 variations are recorded during the evolution of the Boina series as suggested by petrological data and distribution curves of total iron, Fe++/Fe+++and europium. Po2 values calculated from europium distribution in feldspar and whole rocks agree with published Po2 mineralogical calculations in pantellerites from other localities. A crucial stage is recognized near the transition to the peralkaline field, with a sudden Po2 drop during the crystallization of the oxides probably provoking the precipitation of apatite, followed by a rapid Po2 increase at f=0·30. This limited oxygen unbuffered zone is important in the basalt-pantellerite evolution, as it determines marked chemical variations in a restricted crystallization interval. It is suggested that such a crucial stage occurs also in the other known pantellerite series, such as Pantelleria. It may also account for the scarcity of rocks frequently reported at this stage ('Daly gap'). Data obtained from Boina and comparisons with other volcanic series indicate that many peralkaline rhyolites are genetically related to transitional basalts and that their nature is mainly controlled by the composition of the parent basaltic magma. The association is characteristically found in zones of extension of both continental and oceanic environments. The views of Coombs (1963) are confirmed and the rocks series from transitional basalts to comendites and/or pantellerites should be distinguished from the alkalic (undersaturated) igneous rock suites produced by differentiation of alkali basalts.